Zinc Oxide- Silicon Heterojunction Solar Cells by Sputtering

by

Jeanne-Louise Shih

A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Engineering

Department of Electrical & Computer Engineering McGill University Montreal, Quebec

Canada

© November, 2007

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Abstract

Heterojunctions of n-ZnO/p-Si solar cells were fabricated by RF sputtering

ZnO:Al onto boron-doped (1 00) silicon (Si) substrates. Zinc Oxide (ZnO) films were also

deposited onto soda lime glass for electrical measurements. Sheet resistance

measurements were performed with a four-point-probe on the glass samples. Values for

samples evacuated for 14 hours prior to deposition increased from 7.9 to 10.17 and 11.5

0/o for 40 W, 120 and 160 Win RF power respectively. In contrast, those evacuated for

2 hours started with a higher value of22.5 0/o, and decreased down to 7.6 and 5.8 0/o.

Vacuum annealing was performed for both the glass and the Si samples. Current-voltage

measurements were performed on the ZnO/Si junctions in the dark and under illumination.

Parameters such as open-circuit voltage, V0 e; short-circuit current, Ise; fill factor, FF; and

efficiency, 1J were determined. A maximum efficiency of 0.25% among all samples was

produced, with an Ise of 2.16 mA, Voe of 0.31V and aFF of 0.37. This was a sample

fabricated at an RF power of 80 W. Efficiency was found to decline with vacuum

annealing. Furthermore, interfacial state density calculated based on capacitance-voltage

measurements showed an increase in the value with vacuum annealing. The results found

suggest that the interface states may be due to an interdiffusion of atoms, possibly those

of Zn into the Si surface. The Electron Bearn lnduced Current (EBIC) method was used to

determine diffusion length to be at a value ~40-80 ~-tm and therefore a minority carrier

lifetime calculated of 3 ~-tsec. It was also used to determine the surface recombination

velocity (SRV) of the fractured surface of the Si bulk from the fabricated solar cells. An

SRV of ~500 cm/sec was determined from the fractured Si surface, at a point located at

30 and 20 ~-tm away from the junction interface.

Résumé

Des cellules solaires composées de hétérojonctions n-ZnO/p-Si ont été fabriquées

par pulvérisation irradiation à fréquence radio (RF). Les échantillons ont été fabriqué en

pulvérisant du ZnO:Al sur des substrats de silicium dopés de bore. Du ZnO a aussi été

pulvérisé sur des substrats de verre pour faire des études. Les résistances par carré ont été

mesurées sur ces derniers avec une sonde à quatre points. Les résultats mesurés pour les

échantillons qui ont été évacuées dans la chambre à vide pendant 14 heures avant la

pulvérisation ont augmenté d'une valeur de 7.9 à 10.17 et ensuite 11.5 0 /o pour une

puissance RF de 40W, 120W et 160W respectivement. Tandis que, ceux qui ont été

évacués pendant seulement 2 heures ont commencé avec une valeur de 22.5 0 /o, et ont

diminuées à 7.6 et 5.8 0/o. Les substrats en verre et en silicium ont subi des recuits dans

le vide. Les caractéristiques courant-tension ont été réalisées dans le noir et dans la

lumiére. Les paramètres de tension circuit-ouvert (Voe), courant court-circuit (Ise), facteur

remplissage (FF) et efficacité (YJ) ont été établis. Une efficacité maximum de 0.25 %

parmi tous les échantillons a été réussie avec un Ise de 2.16 mA, Voe de 0.31 V et un FF de

0.37. Cet échantillon a été fabriqué avec une puissance RF de 80 W. L'efficacité a baissé

avec les recuits dans le vide. La densité état d'interface a été calculées par les

caractéristiques capacité-tension et montre une augmentation avec recuit dans le vide. Les

résultats suggèrent que les états d'interface pourraient être dû à une interdiffusion

d'atomes, et ceux du zinc vers la surface du slicium. La longueur de diffusion a été

mesuré par la méthode de courant induit par faisceau électronique (EBIC). Cette valeur se

trouve à être ~ 40-80 JLm. La durée de vie des porteurs minoritaires calculée a une valeur

de 3 f.Lm. La vitesse de recombinaison superficielle sur la surface fracturée du silicium sur

11

les cellules fabriquées ont été mesurées deux fois par la méthode EBIC à des point situés

à des distances de 30 et de 20 pm. Les valeurs calculées sont semblables et reviennent à

~500 cm/sec.

lll

Acknowledgements

1 would like to thank Dr. C. K. Jen, Dr. 1. Shih for their continued help and

support throughout the course of my study. 1 would also like to thank the graduate

students in the Electronic Deviees and Materials lab at McGill University for their

encouragement and friendship. I'd finally like to acknowledge the support of my family

for whom this would not have been possible.

lV

Table of Contents

Abstract ................................................................................................................................ i

Résumé ................................................................................................................................ ii

Acknowledgements ............................................................................................................ iv

Table of Contents ................................................................................................................ v

Chapter 1 INTRODUCTION .............................................................................................. 1

1.1 ZnO-Si based photovoltaics ................................................................................ 1 1.2 Theory ofheterojunction solar cells ................................................................... 2

1.2.2 Built-in electric field .......................................................................................... 3 1.2.3 Op ti cal absorption .............................................................................................. 3 1.2.4 Electron ho le generation .................................................................................... 4 1.2.5 Charge separation and collection ....................................................................... 5 1.2.6 Open circuit voltage, short circuit current, fill factor and conversion efficiency

····································································································································· 5 1.2. 7 Recombination ................................................................................................... 8

1.3 Effects of series and shunt resistances ................................................................ 9 1.4 Effects of interface states .................................................................................. 10 1.5 Objective and Outlines ofthe thesis ................................................................. 11

Chapter 2 ZnO FILM DEPOSITION AND GROWTH ................................................... 12

2.1 The princip le of RF sputtering ................................................................................. 12 2.2 The effects of pressure and RF power on the deposition rate and film quality ...... 15

2.2.1 Crystallinity and grain size .............................................................................. 15 2.2.2 Crystallinity vs. op ti cal properties ................................................................... 16 2.2.3 RF power effects .............................................................................................. 17 2.2.4 Argon Pressure effects ..................................................................................... 17

2.3 Sample preparation ................................................................................................. 18 2.4 Principles of film growth ........................................................................................ 21 2.5 Experimental steps ofthe film deposition .............................................................. 23 2.5.1 Experimental conditions ...................................................................................... 24 2.6 Stoichiometric content of ZnO films deposited by RF magnetron sputtering ........ 25 2.7 Experimental results ................................................................................................ 26

2.7.1 Resistivity ........................................................................................................ 26 2.7.2 Effects ofannealing ......................................................................................... 27 2. 7.3 Resistivity measurements for annealed samples .............................................. 28

2.8 Conclusions ............................................................................................................. 32

Chapter 3 1-V CHARACTERIZATION ........................................................................... 33

3.1 Heterojunction barrier formation ............................................................................ 33

v

3.2 Current transport mechanisms ................................................................................ 3 7 3.3 Previous work on ZnO-Si heterojunctions .............................................................. 42 3.4 Effects of Substrate Annealing ............................................................................... 44

3.4.1 Experimental results ......................................................................................... 44 3.5 Conclusions ............................................................................................................. 53

Chapter 4 C-V CHARACTERIZATION .......................................................................... 54

4.1 Capacitance of a heterojunction .............................................................................. 54 4.2 Princip les of interface state density calculations .................................................... 63 4.3 Experimental results ................................................................................................ 64 4.4 Conclusions ............................................................................................................. 82

Chapter 5 ELECTRON BEAM INDUCED CURRENT MEASUREMENTS ................ 83

5.1 Principles ofthe Electron Bearn Induced Current method ..................................... 83 5.2 Principles of diffusion length and surface recombination velocity measurements. 84 5.3 Experimental results ................................................................................................ 89

5.3.1 Characterization ofZnO-Si heterojunctions .................................................... 89 5.3.2 Diffusion length measurements ....................................................................... 90 5.3.3 Surface recombination velocity measurements ................................................ 97 5.5 Conclusions ....................................................................................................... 100

Chapter 6 CONCLUSIONS ............................................................................................ 101

References ....................................................................................................................... 105

VI

Chapter 1

INTRODUCTION

This chapter gives an introduction to the ZnO/Si heterojunction structure and

properties for heterojunction solar cells. Topics such as carrier generation, charge

separation and collection are discussed. As well, the method of characterizing the

performance of a solar cell with parameters such as open circuit voltage, short circuit

current, fill factor and efficiency are presented.

1.1 ZnO-Si based photovoltaics

Zinc Oxide (ZnO), a group II-VI semiconductor is widely applied and has useful

properties to act as a window layer in a heterojunction solar cell. It is considered among

other transparent and electrically conducting films (TCO) as having high conductivity,

low optical absorption and as being resistive to high energy radiation [1.1]. ZnO has a

direct wide band gap whose value is ~3.3 eV at 300 K and a large exciton binding energy

[ 1.1]. Its crystalline structure varies between the zinc-blende, hexagonal wurtzite and the

rocksalt. The thermodynamically stable phase in the ambient is the wurzite whose unit

cell is hexagonal with two lattice lengths 'a' and 'c' and a 'c/a' ratio of 8/3 112• Epitaxial

growth of ZnO films is performed by methods such as magnetron sputtering, chemical

vapor deposition (CVD), molecular bearn epitaxy (MBE) [1.1], spray pyrolysis [1.2], sol

gel, as well as thermal oxidation [1.3]. Magnetron sputtering has been of interest due to

its low cost, simplicity and low operating temperatures [ 1.1]. It is also able to grow highly

oriented films i.e. films of high quality [1.3]. The majority of work related to ZnO films

1

used as solar cells involves the study of n-type ZnO deposited on a p-type material other

than ZnO. The growth of reproducible p-type ZnO films is difficult [1.4] and materials

such as Si, GaN, AlGaN, CdTe, GaAs, etc are used to form the heterojunction with the n­

type ZnO instead. The topic of this work is focused on n-type ZnO:Al deposited by the

RF magnetron sputtering technique onto p-type Si.

When producing n-type ZnO, doping it with aluminum (Al) has excellent benefits.

The Znü's optical bandgap widens in proportion to the Al doping concentration.

Furthermore, the material's conductivity, charge carrier density and mobility are

improved [1.5]. It has been reported that maximum conductivities and the highest

mobilities are achieved at Al-doping concentrations of 2-3 at. %. Furthermore, these

parameters are optimized for maximum crystalline sizes [ 1.5].

1.2 Theory of heterojunction solar cells

In a semiconductor heterojunction, one semiconductor is grown onto of a different

semiconductor to form a junction. Each has its own lattice structure with different lattice

constants and are chosen based on their individual and common properties. Conduction

and valence bands are therefore misaligned and when joined form one of the three types

of alignments: straddled, staggered, or broken. Heterojunction energy bands are

characterized by discontinuities at the conduction and valence band edges which

determine carrier transport mechanisms in the deviee. This topic will be further discussed

in chapter 3.

2

1.2.2 Built-in electric field

No matter whether the semiconductor materials being joined are the same or

different, a built-in electric field will result when an n-type and p-type are brought

together to make a junction. The large amount of electrons in the n-type region will have

a tendency to diffuse into the p-type region, giving rise to a diffusion current. The same

occurs for holes in the p-type region. As electrons leave the n-type region, they willleave

behind a fixed, positive, ionized charge in its place. Similarly, holes will leave behind in

the p-type region, fixed, negative, ionized charges. At a certain point, equilibrium is

reached and the donor ions in the n-type region close to the interface will no longer be

compensated by electrons, and acceptor ions in the p-type region also close to the

interface will not be compensated by holes. A charged region, called the depletion region

results in an electrostatic potential barrier being created, and hence an electric field. This

built-in electric field will thereafter help newly generated carriers to drift across the

junction and into the extemal contacts of the solar cell.

1.2.3 Optical absorption

Carriers are generated in the solar cell by photons. Photons have different levels of

energy and wavelengths. When light shines onto a solar cell, sorne of the energy will

reflect off and sorne will be absorbed. In the case of a ZnO/Si heterojunction solar cell,

photons will tend to travel through the highly doped ZnO at the front surface and be

absorbed in the depletion region within the Si. This is convenient since most of the

depletion region, hence the built-in electric field, is in fact contained at the interface, but

within the Si. This is in fact the reason why the highly conductive ZnO is chosen as a

3

"window layer" to be deposited onto Si when fabricating a solar cell. Si has a high

absorption coefficient while ZnO has a low one in the visible region. The absorption

coefficient is therefore a parameter which describes to what extent incident light can be

converted into electricity. Since the goal is to minimize the photon absorption in the

window layer (ZnO) and to maximize it in the Si, it is best to achieve a thin deposition of

ZnO (~0.1 pm). The absorption coefficient of crystalline Si varies between 2xl06 and 1

cm-1 in the spectral range between 300 and 1150 nm [1.6]. The absorption coefficient of

ZnO is in the range of 1-4 cm-1• The absorption coefficient is determined by two factors:

the excitation of an electron from the valence band to the conduction band and the

transitions within a band. The former which is known as the fundamental absorption

occurs when a photon with energy slightly lower or equal to the energy band gap of the

semiconductor transfers energy to an electron in the valence band. The latter type, which

is known as the free carrier absorption, occurs when a photon with much larger energy is

involved. In this case, one electron-hale pair is generated and the excess energy will be

dispersed as thermal energy, becoming a loss. The absorption coefficient o(J...) is the sum

of the fundamental absorption coefficient and the free carrier absorption coefficient.

1.2.4 Electron hole generation

Absorption of light generates electron-hale pmrs via the mechanisms just

described. For a planar slab, a photon absorbed will generate g(x)* ~x electron-hale pairs

in a layer at a depth x- x+ ~x. The generation function g(x) is described by Equation

1.1.

4

g(x) = a(À)exp[-a(À)·x] (1.1)

The generation rate is denoted as Gand is related to g(x) by G = g(x)/A where Ais the

illuminated area of the sample.

1.2.5 Charge separation and collection

Electron-hole pairs generated at a rate G have a finite lifetime 7. In order for the

generated carriers to be carried off into the extemal circuit, an electric field must be

present in order for the free carriers to be separated and drawn out through the front and

back contacts of the solar cell. This is the purpose and reason for the solar cell's built-in

potential and is wh y the conversion of sunlight into an electrical current is possible with a

structure like the p-n junction. The electric field will attract the newly generated electrons

from the p-type region (Si) towards the n-type region (ZnO) and out the front contact and

the newly generated holes from the n-type into the p-type and out the back contact,

creating an extemal current flow. The heterojunction solar cell's top window layer, ZnO,

is a thin, highly doped material, whereas the lower bulk Si is thick and less doped. High

doping in the thin layer of ZnO achieves a high conductivity for separated carriers to be

drawn out through the front contact.

1.2.6 Open circuit voltage, short circuit current, fill factor and conversion efficiency

The equivalent circuit for an ideal solar cell is represented by a current source,

connected in parallel with a rectifying diode. The 1-V characteristic described is shown in

5

Equation 1.2, where Imum is the photogenerated current, k is the Boltzmann constant, T is

the temperature, q is the electronic charge and 10 is the dark saturation current.

(1.2)

Under thermal equilibrium conditions, the diffusion current and the drift current due to

both electrons and holes cancel each other out so that when the solar cell is not

illuminated, no current is induced. Taking a closer look at the diffusion mechanism, only

the electrons in the n region which have enough energy to overcome the built-in barrier

will diffuse across and recombine with a hole in the p region. Under equilibrium, the

recombination process is characterized by an exponential relationship with the built-in

potential. The recombination current is shown in Equation 1.3 where Vbi is the built-in

potential.

- (-q~iJ-lrecomb - lo exp kT - lgen_dark (1.3)

The recombination current, Irecomb, is being compensated by the generation of electrons in

the p-type region. These thermally generated electrons under equilibrium conditions make

up the dark generation current, Igen_dark· It is independent of the barrier height and is

determined by the amount of minority carriers on the p-side. Once the solar cell is

illuminated, the electron concentration in the p-region will increase, increasing the

magnitude of the initial dark generation current. In order for charge neutrality to be

maintained, the recombination current will increase to compensate for the generation

6

current. The electrostatic potential will be lowered by an amount of Yoe, known as the

open-circuit voltage. The generated current expression is shown in Equation 1.4 and is

made up of the dark thermal generation current, lgen_dark and the generated current under

illuminated conditions, lgen_illum·

I I - I (- q(Vbi- Voc)) gen _ dark + gen _ mum - o exp kT (1.4)

The ideal equation relating the photo-generated current with Yoe is shown in Equation 1.5.

(1.5)

The short circuit current Ose) is the measure of the induced current at zero voltage

as a result of electron-hole generation from light. Both the Yoe and Ise are measures of the

solar cell's performance. The standard expressions goveming open-circuit voltage

behavior of n + -p junctions, relating it to the short circuit current are presented in the

following equations,

(1.6)

(1.7)

where, A is the cell area, ni is the intrinsic carrier concentration, DN is the electron

diffusion constant in the bulk, LN is the electron diffusion length in the bulk, p is the

7

-- equilibrium hole concentration at the front of the bulk and q is the electron charge. The

fill factor (FF) is also a very good indicator to the quality in performance as it is the ratio

of the maximum power incurred by the sol ar cell at a voltage V max and current Imax and an

ideal maximum power characterized by the V oc and Ise· A good FF is dependent on a high

mobility of free charge carriers, and low traps. Efficiency is another measure of

performance and is calculated by dividing the maximum power generated by the solar cell

(V max *Imax) over the incident power of the source oflight.

1.2.7 Recombination

There are several ways to categorize recombination mechanisms. One of which

distinguishes between bulk and surface recombination. The other between band-to-band

recombination and recombination that is aided by defect levels within the band gap. Other

methods involve differentiating between radiative and non-radiative recombination.

Surface recombination is related to the defects at the surface or interface. It affects

in particular the dark saturation current and the efficiency of the solar cell. Two methods

of optimizing performance may be undertaken. These are to passivate the surface or

improve the window layer. It is important to note that solar cells may not be completely

passivated with oxide since the current would not be extractable. Interestingly, it has been

reported that during RF sputtering of ZnO onto Si, it has been previously reported that a

thin native oxide layer, with a thickness of a few Angstroms is unintentionally grown.

The effect of this layer during ZnO growth in the sputtering chamber has been previously

discussed by other authors [1.7, 1.8, 1.9, 1.10]. The discussion in this work therefore

8

- extends to understanding the growth process of ZnO during sputtering and how the

stoichiometric content between Zn and 0 is affected at different deposition conditions.

Retuming to the topic of recombination at the surface or interface of a

semiconductor, the investigation of the process is important as it affects the minority

carrier lifetime of the deviee. The lifetime is the time a minority carrier is able to exist in

a sea of majority carriers before it recombines. The diffusion length is the length at which

it can travel within that time. The relation between these is shown in Equation 1.8, where

D is the diffusion coefficient, L is the diffusion length and 7 is the minority carrier

lifetime.

L =-fi).; (1.8)

1.3 Effects of series and shunt resistances

The effect of series resistance is to lower the fill factor value from its ideal. Series

resistance is composed of resistance along the mean current path of the collected carriers.

These may be bulk resistance, front and back contact resistance, grating resistance, etc.

Series resistance may also be due to leakage from junction defects. The fill factor

relationship with series resistance is shown in Equation 1.9, where FF0 is the ideal fill

factor whose expression is shown in Equation 1.10. FF0 is the fill factor ofthe solar cell

with the ideal I-V characteristic described in Equation 1.2 [1.3].

FF = FF0 (l- rs) (1.9)

9

Voc -In(Voc +O. 72)

voc +l (1.10)

Fill factor may also reduce due to low shunt resistance. It has been observed in the

laboratory with the HP parameter analyzer that low shunt resistance decreases the open

circuit voltage whereas high series resistance decreases the short circuit current.

Furthermore, these resistance values may be easily observed from the I-V characteristic.

Specifically, the series resistance is the inverse of the slope of the I-V curve in the 1 st

quadrant whereas the shunt resistance is that in the 3rd quadrant.

1.4 Effects of interface states

Interfacial states or surface states are caused by defects at the interface of a

junction or at the surface of the semiconductor. Under ideal conditions, where interfacial

states do not exist, the junction will ohey the current superposition principle after series

resistance corrections. The presence of interfacial states will results in non adherence and

changes the electrical characteristics of the heterojunction such as a reduction in the open

circuit voltage. The effect of interfacial states may be mode led by band diagrams and may

be observed by performing capacitance measurements. This will be discussed in chapter 4.

Since epitaxial ZnO has a crystalline structure slightly different than that of Si, interfacial

states are likely to exist after ZnO sputtering. ZnO's wurzite structure is characterized by

unit size lengths a= 0.324 nm and c = 0.519 nm whereas Si's has a length a= 0.543 nm

and it is easy to guess the presence of effects from lattice mismatch.

10

1.5 Objective and Outlines of the thesis

The object of this work is to perform fabrication and characterization of n-ZnO/p­

Si solar cells that enables the understanding of film growth and its properties which

contribute to energy conversion. The method of ZnO deposition is by RF sputtering since

it is considered a relatively simple procedure that may be performed at low temperatures

and at lower costs. The objective is to present data that may support understanding of the

growth of ZnO via RF sputtering, the effect that sputtered growth has on the junction's

stoichiometric content and how the latter may affect solar cell performance. Further work

related to determine the diffusion length and the surface recombination velocity from a

method called Electron Bearn lnduced Current (EBIC) will also be discussed.

This document is organized as follows. Each chapter starts with an overview of

studies and work done by other authors on a specifie topic. It continues with a description

of measurements performed on fabricated samples of ZnO/Si solar cells and provides

discussions and interpretations. To start, Chapter 2 relates to the method of fabrication of

the ZnO/Si solar cells through RF sputtering. It describes the method of deposition and

the growth action of ZnO. Chapter 3 discusses current-voltage measurements performed

on samples that subsequently go through annealing steps under vacuum conditions. The

topic of stoichiometry is presented. Chapter 4 presents capacitance-voltage and

capacitance-frequency measurements performed a:fter each annealing step and a

discussion on interfacial states. Chapter 5 relates to diffusion length and surface

recombination velocity measurements taken via the EBIC method on samples that have

already gone through all annealing steps.

11

Chapter 2

ZoO FILM DEPOSITION AND GROWTH

This chapter describes the method and principle of ZnO film deposition by RF

magnetron sputtering performed on Si and soda lime glass substrates. The experimental

steps taken for fabricating the n-ZnO/p-Si solar cell are also described here. Previous

work done related to the study of crystallinity and film quality with respect to the change

in sputtering conditions are mentioned in this chapter. Resistivity measurements

performed under different conditions on the ZnO/glass samples are presented. The

purpose is to introduce material that is useful in interpreting results related to the growth

and oxidation of RF magnetron sputtered ZnO:Al films.

2.1 The principle of RF sputtering

Sputtering designates the action of atoms being ejected out from a target as a

result of bombardment from source particles. The purpose is to deposit a layer of the

target material onto the substrate. This is performed under vacuum conditions. The source

of particles is a supply of argon atoms whose flow rate into the vacuum chamber is

controlled by the mass flow controller as shown in Fig. 2.1. Argon atoms pumped into the

chamber will receive energy from the electric field that is being supplied by the RF power

source connected to the target. The target, which is composed of sintered ZnO doped with

Alz03 is held by a water-cooled support. lt acts as the cathode whereas the substrate,

which is suspended above the target and clipped to a grounded Al support, acts as the

anode. An electric field is induced between these two by the RF power source. As Ar

12

-- atoms are pumped into the chamber, collisions result and produce Ar+ ions and electrons

creating a soft purple glow within the chamber. The positive argon ions will be attracted

to the cathode target. The electric field will accelerate these particles onto the target

whereas the electrons will be attracted towards the anode. As the positive argon ions

bombard the surface of the ZnO:Al target, the atoms of the sintered target are knocked

loose. Electrons are sputtered out and are prompted by the electric field to deposit onto

the substrate. It is in this manner that a layer of ZnO:Al is grown onto the substrate.

13

Aluminum support

Vacuum chamber

Target

Mass flow

controller for Argon c=[> .___..JI9'---t

Cooling water c=[> L...----.--------1

RF Power

Diffusion pump Backing pump

Fig. 2.1 A schematic diagram of the RF magnetron sputtering system.

14

2.2 The effects of pressure and RF power on the deposition rate and film quality.

Sputtering conditions may be controlled during deposition to affect deposition rate

and film quality. For example, it is known that RF power regulates the sputtering yield

rate since it controls the rate of bombardment of Ar+ ions onto the ZnO target. [2.1]. In

fact, the higher the power, the higher is the yield rate. In this work, films are deposited at

different RF powers and therefore for higher power levels, the amount of time for which

deposition takes place is shortened. For example, films which were deposited at 40 W

were sputtered for 16 hours, whereas films which were deposited at 80 W were sputtered

for 6 hours.

2.2.1 Crystallinity and grain size

Crystallinity as weil as grain size increase with substrate temperature during

deposition [2.2]; however, at a given substrate temperature, the appropriate growth rate

for which an optimum crystalline morphology can be achieved may be controlled by the

RF power [2.1]. Setting the optimum combination of RF power and deposition

temperature has been shown to yield the highest level of c-axis oriented columnar

structures [2.3]. For example in the work presented by Kim et al. [2.3], among the ZnO

films deposited at 550 oc the optimum power required was 80 W to produce a highly c-

axis oriented columnar structure. They have used sintered ZnO (99.999%) targets with 2

inch diameters [2.4], and a mixed plasma of Ar and 0 2 for the sputtering. It was observed

that with an increase or decrease in power, crystallinity was se en to have degraded. If the

RF power was lower than the optimum value, the sputtered atoms did not have enough

15

energy to migrate to the substrate surface. If the RF power was higher than the optimum

value, the decrease in crystallinity was said to be due to the possibility that the power was

too high and therefore gave insufficient time for atoms to find a stable site for stable grain

growth. It has been concluded that growth quality of ZnO via RF magnetron sputtering is

affected by two variables. The first is the energy supplied by the RF power source as it

regulates bombardment and sputtering. The second is the amount of thermal energy

supplied to the system during the deposition process. An optimum lev el of thermal energy

during deposition at a given RF power being supplied encourages stable deposition and

larger grain size which greatly improves optical properties in the deviee. In the current

work, substrate temperature was not regulated; however the points mentioned above are

important in understanding the changes in film characteristics for changes observed after

post-deposition annealing. Post-deposition annealing is performed under vacuum

conditions for fabricated samples in this work. Annealing is known to achieve lattice re­

ordering and increased crystalline quality [2.5].

2.2.2 Crystallinity vs. optical properties

It is interesting to note that optimum quality in terms of crystallinity is not an

indicator of optimum optical performance. In Kim et al. 's work [2.3], although films

deposited at 80 W showed the highest level of crystallinity they demonstrated lower

photoluminescence properties (PL spectra) than those that were deposited at 120 W.

Crystallinity had degraded at 120 W and polycrystalline tendencies were said to have

resulted. These are said to have had an influence on the formation of defects such as

dislocations, vacancy and interstitial defects. The diffusion of defects during the growth

16

process increased. The result was that the density of defects was said to have been

reduced inside the columns themselves as a result of higher power, which improved the

optical photoluminescence property. If deposition temperature was increased to 600 °C,

with power maintained at 120 W, both crystallinity and optical property were

simultaneously improved. The suggestion was made that this would be due to enough

thermal energy being supplied, subsequent! y helping to lead atoms to move to stable sites.

As well, impurities would move to grain boundaries and defect density within the

columnar structures themselves would decrease, resulting in improved PL properties.

2.2.3 RF power effects

At an optimum temperature of 300 oc reported by Das et al. and high pressures,

for an increase in RF power, stronger sputtering action results in better stoichiometry

which transpires into a higher resistivity value [2.6]. Lower power creates oxygen

deficiency and non-stoichiometric films. In this case, oxygen vacancies or interstitial zinc

atoms will act as donor electrons thereby decreasing resistivity. Higher conductivity is

therefore a result of non-stoichiometry and of doping [2. 7].

2.2.4 Argon Pressure effects

Higher pressure serves to decrease ZnO film quality for the purposes of a solar

cell. According to Y oo et al. [2.2], lower gas pressure pressure results in higher

conductivity. In addition, according to Jeong et al. [2.8], combining low pressure (<2

mTorr) with a high substrate temperatures (> 573 K) results in a denser and more

17

compact morphological structure with effective light-trapping capabilities. On the other

hand, when the pressure is increased, ZnO film becomes less crystalline, evidenced by its

increase in resistivity [2.9, 2.2]. An explanation given by Das et al. [2.6] is that precursors

tend to collide severa! times with other gas molecules before they arrive at the substrate.

Other effects that are reported were that the deposition rate of the film was found to

decrease from 105 to 60 A/min with an increase in argon pressure from 0.04 to 1.33 Pa

[2.2]. Grain size as weil has been noted to decrease with increase in argon pressure [2.9].

Overall, a lower pressure is said to produce a more ordered film. As a result, in this work,

pressure has been maintained at a low 5 mTorr for each deposition performed.

2.3 Sample preparation

The heterojunction solar cell sample was prepared on a singled-side-polished p­

type (1 00), 1.5 0 -cm Si wafer. The wafers were eut with a diamond scriber into ~4 x 2

cm2 sample sizes. The samples went through the following cleaning procedure. They

were immersed in acetone for 2 minutes, in de-ionized (DI) water for 2 minutes, in

buffered hydrfluoric acid (BHF) for 10 seconds and then in DI water for 10 seconds.

Samples were then spun dry at 3000 rpm for 20 seconds and then loaded into the

sputtering chamber for ZnO deposition. After ZnO deposition was performed on the front

substrate surface, the samples were prepared for the evaporation of Al contact grids. A

thin flexible mask containing the contact grid pattern was mounted on an Al support as

shown in Fig. 2.2. The Al support was then clipped onto the ZnO/Si samples and loaded

into the vacuum chamber. 6 pieces of 1 mm diameter Al wires with a length of about 2

cm were placed on the tungsten heating filament. Evacuation was performed for a

18

minimum of 2 hours. Once evaporation was performed, the samples were removed and 6

drops of photoresist AZ-1827 was deposited onto the ZnO and contact surface. The

photoresist was spun at 3000 rpm for 20 seconds and soft-baked at 90 oc for 10 min. The

photoresist was then exposed through the transparency-printed mask as shown in Fig. 2.3

to an Ultra-Violet source for 6 minutes at 150 W. The samples were then immersed in

concentrated developer for a few seconds and in DI water for 1 minute. Etching was then

performed to remove ZnO from the peripheries and was done with a solution of 10% HCl

diluted in 90% DI water for 2-3 minutes. The samples were then rinsed in DI water for 1

minute and light1y swabbed with a q-tip while immersed in DI water. The samp1es were

then rinsed in DI water' for 10 minutes. Photoresist was then removed by immersing the

samples in acetone. The samples were again rinsed in DI water for 2 minutes and spun

dry at 3000 rpm for 20 seconds. The resulting sample is shown in Fig. 2.4.

19

Fig. 2.2 A photograph of the flexible contact grid mask taped to the Al support.

Fig. 2.3 A photograph of the window mask printed on a transparency

20

Fig. 2.4 A photograph of a finished sample with three n-ZnO/p-Si solar cells and their respective Al contact grids

2.4 Principles of film growth

It has been found by Song et al. [2.1 0] through transmission electron microscopy

(TEM) that during RF magnetron sputtering deposition, an unintentional thin layer of

oxide is grown between the Si and the ZnO layer. In Song et al.'s work, the ZnO:Al/n-Si

heterojunction was formed by RF magnetron sputtering with an RF power of 150 W, Ar

pressure of 0.5 Pa and a substrate temperature of 250 °C. TEM high-reso1ution bright-

field images showed that the thickness of the amorphous Si oxide layer at the interface

was about 12 A. The native oxide growth occurring during RF magnetron sputtering of

ZnO has been observed by other authors and is often referenced [2.11, 2.12, 2.13].

21

It would be a good guess that an oxide layer grown between two non-compatible

materials such as Si (a= 0.543 nrn) and epitaxial ZnO (a= 0.324 nrn and c = 0.519 nrn)

would serve to decrease the high defect density resulting from the lattice mismatch

between these two materials. However, it has been noted in Song et al.'s work [2.10] that

despite it, a rather high density of active defects within the Si depletion layer exists which

is evidenced by a large n, ideality factor. The high ideality factor is the basis for which

the forward bias current transport mechanism has been reported as being dominated by

trap-assisted multistep tunnelling. Current transport mechanisms of ZnO-Si

heterojunctions are discussed in chapter 3.

It has been docurnented that film growth of ZnO tends to an orientation of

crystallites perpendicular to the substrate surface during sputtering. Along this c-axis, the

axis or direction for which there exists the highest rotational syrnmetry, a conductive

channel is predestined [2.14]. It has been shown through lattice images that strictly c-axis

oriented ZnO:Al crystals may be grown on a Si wafer despite the amorphous native

surface oxide at the interface [2.11]. The conclusion is that orientation is not affected by

the crystallographic structure of the substrate and is rather a result of a self-ordering effect

caused by the minimization of the crystal surface free energy and by the interaction

between the deposit material and the substrate surface. This so-called self-ordering effect

is known as self-texturing. It is important to note that during the sputtering process,

conditions must remain stable to achieve a highly ordered and therefore highly conductive

film since it is still possible to change the crystal structure during growth and produce a

highly resistive polycrystalline film by changing the sputter conditions [2.14].

22

Growth of crystalline films depends on the energy conditions of the reactive

components prior to bombardment. It also depends on the surface and the substrate. Three

growth phases have been noted for crystalline films: nucleation, horizontal growth of the

nuclei to a closed layer and a vertical growth of the closed layer. High resistivity

polycrystalline films may be described as largely dominated by growing mechanisms

such as nucleation and horizontal growth of the nuclei to a closed layer such is the case

for Moeller et al.'s reports [2.14] of films with resistivities ranging between 8x1010 and

2x 1011 0 -cm. It may th en be inferred that low resistivity, crystalline films are dominated

by the vertical growth.

2.5 Experimental steps of the film deposition

RF magnetron sputtering of ZnO was performed under vacuum conditions. The

vacuum chamber is connected to a diffusion and backing pump for evacuation, and a

sputtering gun for argon pumping. The sintered target is made of 99.9999 % pure ZnO

and doped with 2 wt % of Ah03 and is held in a metal holder. The target' s diameter

measures 7 cm. The metal holder is attached to a water-cooled support. Prior to vacuum

chamber loading, the Si substrates were cleaned as described in section 2.3. The cleaned

Si substrates and four-finger-probe soda lime glass substrates were then clipped side-by­

side onto the Al substrate holder and placed face down at a 45 degree angle from the edge

of the target as shown in Fig. 2.1. The approximate distance between the substrate and the

target is 4-5 cm. A glass slide with four-fingered Au probes were clipped onto the Al

substrate holder along side the Si substrates during deposition. The vacuum chamber was

first evacuated ovemight. Following this, the evacuated chamber is then filled with pure

23

argon. The RF power is then tumed on at 40W for 20 minutes and argon pressure is

maintained at 20 mtorr. This step is done to rem ove contamination from the surface of the

target and ensure that the system is stable prior to actual film deposition. Actual

deposition is performed thereafter under the conditions listed in Table 2.1.

2.5.1 Experimental conditions

Compiled in Table 2.1 is a list of samples that were characterized in this study.

The first set of ZnO films were deposited on four-fingered soda lime glass. They are

identified by an alpha numeric number which describes the evacuation time prior to

deposition and the RF power used during deposition. The second set of ZnO films were

deposited on p-type (1 00) Si wafers. These are identified by a letter showing the length of

time during which dry oxidization was performed prior to ZnO deposition. Round 3

samples were exposed to air for ~3 weeks prior to ZnO deposition.

Table 2.1 A summary of samples fabricated with RF power, deposition time and pressure listed.

Sample no. RFpower(W) Sputtering Pressure (mtorr) Remarks Time (h)

ZnO:Al/soda lime glass

A-80 80 6 5 More uniform than B B-80 80 6 5

A-120 120 4 5 More uniform than B B-120 120 4 5 A-160 160 4 5 More opaque than B B-160 160 4 5

ZnO:Al/p-Si

Round 1-a 40 16 5

24

Round 1- b 40 16 5 Round2-a 120 2 5 Round2- b 120 2 5 Round 3- a 80 6 5 Round3- b 80 6 5

*Ail samples were deposited w1th an Ar pressure of 5 mTorr. **Prior to deposition as described above, Ar pressure was set to 16 mTorr and RF power to 40 W for 20 minutes to decontaminate the target and stabilize the system.

A = evacuated for 14 hrs B = evacuated for 2 hrs a = not oxidized b = oxidized for 15 min

2.6 Stoichiometric content of ZnO films deposited by RF magnetron sputtering

A reported useful method of detecting film quality in terms of stoichiometric

content in Znü is through photoluminescence spectra. If visible emission in PL spectra is

suppressed without suppression in the ultraviolet emission spectra, that is an indication of

a reduction of oxygen vacancies and zinc-interstitial-related defects [2.8]. This is the case

reported by Jeong et al. [2.8] for a Znü film deposited by RF magnetron sputtering using

a ZnO target (99.999%), at a Si substrate temperature of 400 oc and an increase in

lm et al. [2.15] showed that UV luminescence intensity changes are more strongly

dependent on stoichiometry rather than on the crystal's microstructural quality. A

dependency nevertheless exists with respect to the latter. Conclusions resulting from this

work show that with increasing deposition temperature, the grain of c-axis oriented

texture increases in size. Furthermore, it was observed that UV emission was only

observed for ZnO deposited at 550 oc which means that samples deposited at this

25

temperature were found to have the best stoichiometric quality. What is meant by good

stoichiometric quality is that the film con tains as close as possible, a 1: 1 Zn to 0 content

in the film. Interestingly, even though stoichiometric film quality was best for Znü

deposited at 550 oc, the best photocurrents that were observed through 1-V measurements

were for those deposited at 480 °C. This is an interesting result since it shows that

photocurrent depends on something other than or in addition to stoichiometry.

2. 7 Experimental results

2.7.1 Resistivity

Resistivity depends on the crystalline volume fraction in the film, as weil as on the

carrier concentration and mobility [2.9]. For Znü:Al films, two types of donors are

considered. The first originates from Zn interstitials or oxygen vacancies. The second

exists due to substitutional Al atoms. Mobility depends upon impurity and grain boundary

scattering. Steady increase in concentration with doping was observed by Y oo et al. [2.9]

and caused a decrease in mobility. Similarly, for a grain size decrease, grain boundary

density increases, increasing scattering effects thereby lowering mobility. A lower

mobility or a lower carrier concentration results in a lower resistivity film. It has been

noted by Jeong et al. [2.7] that resistivity is also dependent on target-substrate distance as

weil as Al(OH)3 content in the target. When the distance is below 50 mm, resisitivity

remains constant. The lowest resistivity obtained in their study (5.0x10-1 n -cm) was for a

distance of 45 mm. With increasing Al content from 0 to 4 wt%, resistivity decreases

from 5.0 x 10-1 ü -cm for pure Znü to 9.8 x 10-2 ü -cm. According to Jeong et al. [2.13],

Znü deposited on glass substrates showed increase in resistivity with decrease in layer

26

thickness at values below 100-300 mn. The reason for this is possibly due to the different

growth and morphology in early stages ofthe film growth [2.13].

2.7.2 Effects of annealing

Annealing has been performed in previous studies [2.16, 2.17]. For reports of

ZnO:Allayers deposited onto soda lime glass by DC planar magnetron sputtering [2.16],

annealing was performed under vacuum conditions. Resistivity was seen to decrease after

annealing at 250 oc and was attributed to the increase in carrier concentration. Increase in

carrier concentration is a result of the outgassing of oxygen from the film and into the

vacuum ambient. Outgasing of oxygen severs bonds, creating oxygen deficiencies in the

film and frees electrons. The Hall mobility that was measured after the 250 oc annealing

step decreased, as is expected when an increase in carrier concentration is observed. The

resistivity decrease was therefore confirmed to be attributed to the increase in carrier

concentration and dominated by grain boundary scattering. Between annealing

temperatures of 250 and 450 oc, resistivity was reported to continue to decrease, but in

this case, Hall mobility increased. The possible explanation for this is that creation of new

vacancies would have started to cease and instead, the lattice was being changed by the

annealing temperature in such a way as to improve crystallinity, thereby increasing

mobility and hence lowering resistivity. In essence, the measure of resistivity with

annealing has shown to be effective in detecting two parameters of the film: the

stoichiometric content of Zn vs. 0 in the film and the crystallinity via the decrease and

increase in Hall mobility.

27

Interesting is the case of ZnO:Al deposited by an evaporation method where

annealing was performed at temperatures within the range of 27 to 427 oc in air and

vacuum. Resistivity increased by two orders of magnitude for those annealed m air

whereas it decreased by five orders of magnitude for th ose annealed in vacuum.

Also interesting is resisitivity measurements performed for samples deposited at

different deposition temperatures [2.18]. For low temperature, low resistivity values were

obtained with a higher carrier concentration. For 500 oc, resistivity increases and the

compound reaches optimum stoichiometery and the carrier concentration starts to

decrease. At higher temperatures, resistivity starts to decrease and ZnO becomes oxygen

deficient. Two depositions were performed at a substrate temperature of 500 °C. The first

was at an Ar/02 oxygen supply ratio of 4:1 and the second at 6:1. Zn/0 stoichiometry was

observed to be similar between the two with a slight difference in resistivity. lt may be

said that either oxidation of zinc is easier at a higher temperature since oxygen is more

reactive or it is possible that at a higher temperature, the native oxide is thicker and

becomes a source of 0 to the ZnO being sputtered and therefore a source of SiOx defects.

2. 7.3 Resistivity measurements for annealed sam pies

Sheet resistance measurements performed on ZnO sputtered onto soda lime glass

for this work are shown in Fig. 2.6 and 2.7. These glass samples have been sputtered at

the same time as their Si counterpart and have been annealed under vacuum conditions, as

well at the same time. Fig. 2.5 shows the sheet resistance measurement setup performed

after each annealing step, where step 0 represents measurements taken for as-deposited

ZnO on the glass. Measurements were taken with a four-point-probe and the HP 4145A

28

parameter analyzer as shown in Fig. 2.5. Equation 2.1 shows the relationship between V

and I taken from the parameter analyzer, where Rs is the sheet resistance and p is the

resistivity of the Znü film. S is the spacing between the contacts and Le is the length of

the contact fingers. S has been measured with a microscope to be 0.2 mm and Le is 5 mm.

Rs = Le . V = Le . p (Q. 1 sq) s 1 s

Glass with ZnO layer

(2.1)

1

Fig. 2.5 Four-point-probe measurement method for the resistivity of Znü sputtered onto soda lime glass.

29

In Fig. 2.6, the trends for 40 W and 120 W are similar; however, for the 40 W

sample, sheet resistance slightly decreases after the 200 oc annealing step and increases

steadily thereafter. As noted above, a decrease is indicative of oxygen outgassing. The

sample fabricated at 120 W increases in sheet resistance with each annealing step. The

trend for the ZnO deposited at 80 W on the other hand has a much larger decrease in sheet

resistance until the annealing step at 300 oc.

In Fig. 2. 7, sheet resistance was also measured for samples that were deposited at

different RF powers and were evacuated in the vacuum chamber prior to ZnO deposition

either for 2 hours or 14 hours. The sheet resistance trend for samples which were

evacuated for 2 hours shows a decrease with RF power, whereas those which were

evacuated for 14 hours show an increasing trend. Since a decrease in sheet resistance has

been noted to be associated with an outgassing of oxygen during vacuum annealing, it

may be qualitative! y said that the sample which was evacuated in the vacuum chamber for

only 2 hours had higher oxygen content. In other words, an RF power increase results in

an increase in the sheet resistance of samples which have lower oxygen content. The

opposite is true for samples which appear to have higher oxygen content.

Furthermore, the difference in resistivity measurements might be an indication

that the native oxide, which has been mentioned in other reports to grow during RF

sputtering, is affected by the amount of time that evacuation is performed. This has not

been verified in this work.

30

Sheet resistance for ZnO on soda lime glass

70

60

_.,_ ZnO deposited at 40 W _.,._ ZnO deposited at 80 W __,._ ZnO deposited at 120 W

0 +-------~--------~--------~-------,

0 2

Annealing step

3 4

Fig. 2.6 Sheet resistance for ZnO deposited at 40, 80 and 120 Won soda lime glass. Each annealing step lasts 30 minutes and is performed under vacuum conditions. Step 0 is the as-deposited sample. Step 1 annealing was performed at 200 °C, step 2 at 300 oc, step 3 at 350 oc and step 4 at 400 oc.

Sheet Resistance vs RF power 24~------------------------------~

22

20

(1) 18 u_ c: (1) 16 ~ ... 1i) ~ 14 ·- :::1 t/) C" 12 (1) t/)

o::: "E 10 -J:: (1) 0 8 .ê-(1) 6

4

Ill

2

0~~--~--~~--~--~~--~--~~ 70 80 90 100 110 120 130 140 150 160 170

RF Power(W)

.....-sputtering time: 6h- Evacuated for 2h -111- Sputtering ti me: 6h- Evacuated for 14h --.- Sputtering ti me: 4h- Evacuated for 2h

"' Sputtered ti me: 4h- Evacuated for 2h -•- Sputtering ti me: 4h- Evacuated for 14h -1!1- Sputtering ti me: 4h- Evacuated for 14h

Fig. 2.7 Sheet resistance for soda lime glass evacuated in the vacuum charnber for 2 or 14 hours before ZnO deposition at 40, 80 and 120 W.

31

2.8 Conclusions

It has been shown that resistivity has increased continuously for ZnO sputtered

onto soda lime glass at high RF power (120 W) with vacuum annealing. Film deposited at

80 W showed a large initial decrease in resistivity after 200 and 300 oc annealing and

then a continued increase thereafter. The film deposited at 40 W showed a very slight

initial decrease after the 200 oc annealing step and a graduai increase in resistivity,

similar in trend to the film deposited at 120 W.

It has also been shown that resistivity measurements of ZnO films sputtered onto

soda lime glass samples are characterized by an increase in sheet resistance with an

increase in RF power for samples which were evacuated for 14 hours prior to deposition.

The opposite is true for samples which were evacuated for only 2 hours prior to

deposition. The decrease in sheet resistance for samples only evacuated for 2 hours prior

to deposition is probably due to outgassing of O. The difference in resistivity trends is a

possible indication that the amount of time spent on evacuation prior to ZnO deposition

affects the growth of native oxide during reported by others for the sputtering process.

Specifically, it is possible that traces of water and oxygen reside on the surface and a

longer evacuation period is required to remove these. It would be interesting to examine

native oxide thickness via Transmission Electron Microscopy (TEM) and to examine the

change in quality with annealing via Hall mobility and resistivity measurements with

different evacuation conditions prior to sputtering.

32

Chapter 3

1-V CHARACTERIZATION

This chapter describes the current-voltage (1-V) characterization results of n­

Znü/p-Si solar cells with the ZnO layers prepared by the RF magnetron sputtering

method. It describes previous work done by others and also presents results achieved in

this work. The purpose is to understand transport mechanisms in the deviee. As well, it is

to understand the changes in the 1-V curves and therefore the photoelectrical performance

due to high temperature annealing under vacuum conditions.

3.1 Heterojunction barrier formation

The energy band dia gram of an ideal ZnO/Si heterojunction is shown in Fig. 3 .1.

Features of the band alignment are determined based on the Anderson energy-band model.

It is worth noting that the Anderson model neglects parameters such as lattice mismatch

and interface traps; however it is useful in sketching the heterojunction band alignment

showing ideal band edge differences. Known also as the electron affinity model, the

Anderson model has been used to model ZnO/Si and ZnO/SiC heterostructures [3.1, 3.2]

and is based on the energy equation x1 - ~Ec- Xz =O. This represents the fact that the

energy of an electron moving from the vacuum level to the conduction band of

semiconductor "l" to the conduction band of semiconductor "2" and back to the vacuum

level must be zero. The conduction band discontinuity is therefore the difference between

the two electron affinities, ~Ec = Xzno - Xsi where Xzno and Xsi are the electron affinities of

ZnO and Si. Similarly, the valence band discontinuity is ~Ev= ~Ec + ~Eg, where ~Eg is

33

the energy band gap difference between ZnO and Si. Two sources have quoted values for

L1Ec and L1Ev to be 0.4 eV and 2.55 eV respectively [3.3, 3.4] following the above

equations for conduction and valence band discontinuities, where the ZnO work function

is assumed to be 4.45 eV [3.1, 3.3], though other sources have stated other values [3.2]. In

fact, work function and electron affinity for ZnO have shown to vary in the literature.

Sundaram et al. have determined values of work function of sputter deposited ZnO films

via the Shottky barrier mode! for ZnO/p-Si and ZnO/n-Si junctions, including the

assumption of the presence of a very thin Si02 layer at the interface. The values they

found were to be between 4.45 and 4.5 eV. For the purposes ofFig. 3.1, a value of 4.45 is

used.

With respect to ZnO films, work function is assumed to be nearly equal to the

electron affinity [3.3]. This is due to the fact that doped ZnO films are highly conductive.

The assumption is that the Fermi leve! coïncides with the conduction band edge in the

ZnO, hence work function is considered almost equal to the electron affinity [3.3]. The

electron affinity therefore also has a value ofranging between 4.45 and 4.5 eV, according

to Sundaram et al.'s evaluations. In the case of Si, the electron affinity is weil known to

be 4.05 eV [3.3]. L1Ec, according to the above equation is therefore equal to ~0.4 eV and is

schematically pointed out in Fig. 3 .1. L1Ev is of a much larger value of 2.55 eV which is a

great indication of wh ether current density is dominated by electron or ho les during non­

illuminated conditions. This point will be discussed shortly.

There are two shortcomings with the Anderson mode! [3.4]. Specifically related to

the modeling of ZnO/Si junction, the first is of the lack of interface trap and lattice

mismatch considerations to determine band edge differences. This has been addressed by

34

Ruan and Ching [3.5]. According to them, the valence band discontinuity is changed due

to the formation of the junction interface, which they have modeled as a newly formed

effective dipole at the interface. Band discontinuities ~Ec and ~Ev are affected by this

dipole effect. Interestingly however, for lattice mismatched junctions, the dipole effect

induced by electron transfer is said to be destroyed or severely altered and reduced when

there is a large amount of localized interfacial defects, such is assumed to be the case for

the ZnO/Si heterojunction. Furthermore, deviations in electron affinities due to dipole

layers has also been previously noted to be small (1 %) by Shay et al [3.4]. The second

shortcoming with the Anderson model is that it neglects electron correlation effects.

Correlation effects arise when an electron is moved into the vacuum level. The

surrounding electrons will rearrange themselves to reduce the total energy of the electron

system.

Considering the fact that there is very high lattice mismatch between ZnO and Si

and therefore that a high defect density is assumed to exist, the dipole theory is neglected

here. Whether correlation effects are small for ZnO/Si is not determined in this work;

however, according to [3.4], the magnitude of correlation effects is generally small.

Anderson modeling is therefore validated for determining band edge differences in the

case ofhighly mismatched ZnO/Si heterojunction.

In terms of the heterojunction band lineup shown in Fig. 3.1, ZnO/Si is

characterized as a type II structure [3.4, 3.6]. This means that the conduction and valence

bands of ZnO and Si are staggered with respect to each other as is shown in Fig. 3.1. The

staggered structure clearly demonstrates the location and magnitude of the conduction

band and valence band discontinuities ~Ec and ~Ev.

35

As discussed previously, the conduction band discontinuity is much smaller

compared to that of the valence band discontinuity. The result of this is that majority

carriers (electrons) in the ZnO will not be as greatly impeded from flowing across the

junction as holes from the Si are during forward biasing. Under non-illuminated

conditions, the forward bias current density is therefore largely dominated by electrons.

Once the junction is illuminated, specifically by low energy photons, photo-generated

electron-hale pairs will be created in the depleted p-Si region. The depletion region is

extremely narrow for the ZnO region since ZnO is highly doped and highly conductive.

As discussed, the Fermi leve! is assumed to coïncide with the conduction band edge.

Generated electrons will start to flow down the potential barrier whereas generated holes

will have a harder time doing so towards the ZnO region as is shown in Fig. 3.1. It is clear

that under illuminated forward biasing, two competing carrier flows are present, one

resulting from the forward bias, the other from the photogeneration. In fact, the

illuminated I-V curves, as may be seen in Fig. 3.2 a, show two oppositely directed current

densities. Positive current appearing on the I-V curve is largely the result of the injection

of electrons coming from the n-ZnO, overcoming the potential barrier and diffusing into

the p-Si. Negative current on the I-V curve is the result of the electron-ho le generation

occurring in the depleted region of Si, due to low energy photon illumination as is shown

in Fig. 3.1.

Since it has been documented that an unintentional thin oxide layer results from

RF magnetron sputtering deposition of ZnO [3.13.7, 3.8, 3.9], it has been reported that

photocurrent and carrier flow across the junction is affected depending on the thickness

and the quality of the junction. [3.9, 3.10]

36

Ec

Ef Ev

Vacuum Level

\ ~Visible light

r Photo- electron-hole pair generation fê\ - D~

Forward bias electron Dow

1 0-->f---1-.--,_ AEc

'."-® Eg= 1.12 eV

1

l

~\ p-Si

.o. Ev Eg= 3.27 eV

n-ZnO \

XZno = 4.45-4.5 eV

Fig. 3.1 A schematic of the n-ZnO/p-Si band dia gram including the depiction of photo­generation and forward bias electron flow.

3.2 Current transport mechanisms

The dominant current transport mechanism across the n-ZnO/p-Si heterojunction

at intermediate forward bias (~0.25- 0.3 V) has been determined to be via trap assisted

37

multistep tunneling [3.7, 3.11]. This has been proven by the observation of large ideality

factors (~n=3) that are independent oftemperature [3.7, 3.11].

The ideality factor, shown in equation 3.1 is often used to characterize the

relationship between the 1-V curve and transport mechanisms.

(3.1)

An ideality factor of n = 1 is an indication of the presence of mechanisms such as thermal

generation, minority carrier injection and recombination. Current and voltage relationship

for common diode rectification behavior is generally modeled by J a [exp(qV/nkT)]

where k is the Boltzmann' s constant and T is the temperature. Ideality factors for

ZnO/Si junctions have been observed to be larger than 1 and do not vary with temperature

therefore mechanisms mentioned above no longer apply. In fact, the new relationship

between current and voltage is J = J0[exp(Ao·V)] where A0 is a constant independent of

temperature. Whether simple tunneling is a possible dominant mechanism was ruled out

by Song et al. in the case of the n-type Si substrate sample i.e. n--Si/Si02/n+ZnO:Al [3.7]

since the Si is lightly doped (~5x1015 cm-3) which means that the barrier created at the

depleted Si region is considered too thick for simple tunneling to occur. In the case of a p­

Si/n-ZnO heterojunction presented in the work by Song et al. [3.11], multi-step tunneling

is inferred to be the dominant mechanism, the evidence being those mentioned above: an

ideality factor larger than 1 and a forward-bias current-voltage relationship that is

independent of temperature. Simple tunneling is not mentioned by Song et al. for this p

type Si substrate case.

38

At lower forward bias voltages ( <1 00 rn V), Song et al. have reported that current

is due to a shunt resistance of about 5.1 kO cm2 and that it is due to series resistance

effects at large forward voltages (>400 rn V). Dhananjay et al. [3.15] and Song et al. have

reported similar results in ideality factor values. Dhananjay et al. have however grown Zn

films on cleaned Si and thermally oxidized them to create crystalline ZnO films.

Dhananjay et al. have described the low forward voltages to be dominated by

recombination within the space charge region. They have deposited films from a Zn metal

target (50 mm diameter) through DC sputtering inside a vacuum chamber. DC power was

fixed at 35 W and Argon gas was used as the sputter gas. Deposition was carried out at

200 oc at 100 rn Torr. Subsequent oxidation was performed in a tube furnace under 0 2

flow at temperatures ranging from 300 to 500 oc. They have not reported the presence of

an oxide interlayer.

Both Dhananjay et al. and Song et al. show very similar trends in ideality factor.

That is they report an ideality factor (n) of around 2.8 for a temperature of ~350 oc and a

value of n ~ 1.4 for ~ 250 °C. In fact, the description made by Dhananjay et al. that the

change in ideality factor with temperature as being affected by recombination within the

space charge region is equivalent to Song et al.'s description that a shunting effect is

present. In summary, under dark forward bias, sorne electrons that flow from the ZnO

region in Fig. 3.1, up the potential barrier into the Si depletion region will fall into

interfacial traps and recombine. These electrons will not be able to contribute to the dark

forward current and therefore, the recombination of these trapped electrons is equivalent

to current being drawn by a shunt resistor. This is the effect described for low forward

voltages.

39

.~. Song et al. describe three regions in the ideality factor graph. The low forward

bias region characterized by shunting, the mid-forward bias region characterized by

constant n with change in temperature and the high forward bias region where the ideality

factor increases. Song et al. describe the high-forward bias region as being due to the

series resistance effect coming from, bulk resistances in the Si and the ZnO, as weil as the

resistance associated with tunneling through the Si depletion layer and the interfacial

oxide. [3.7]

Interestingly, although Dhananjay et al. and Song et al. have reported similar

ideality factor trends, Dhananjay et al. have not reported the presence of an oxide

inter layer.

Another report of the presence of a thin interlayer of oxide is provided by Sieber

et al. [3.12] whereby ZnO:Al films were prepared by reactive DC magnetron co­

sputtering from Zn and Al targets in an Ar/Oz gas mixture on Si substrates. Si substrates

were cleaned with HF dip prior to deposition. Unlike Dhananjay et al. who have used Ar

as the sputtering gas, Sieber et al. have used Ar/Oz gas mixture. The result is the evidence

of an amorphous Si oxide layer. They state that Si surfaces that are oxygen free such as

those that have H-terminated surfaces also oxidize during the supply of oxygen or oxygen

containing components. Since Dhananjay et al. have deposited Zn films by sputtering

without Oz gas, have performed oxidation after the Zn deposition and have not reported

an oxide interlayer; it is interesting to note that ideality factor trends are still similar to

those by Song et al. for whom an oxide interlayer was reported.

40

The above has characterized the different current transport mechanisms reported

by other authors for the forward bias case. Reverse bias current was shown by Song et al.

to be direct1y proportiona1 to (Vbi-V)112 [3.11] where Vbi is the built-in voltage and Vis

the app1ied voltage, and by extension is directly proportional to the depletion width of the

junction [3.11]. Reverse bias current is therefore dominated by thermal carrier generation.

The location of electron-hole pair generation depends on the photon energy i.e.

whether high or low energy photons are used to illuminate the ZnO/Si junction. In work

developed by Jeong et al. [3.1 ], it was found that low-energy visible photons transmit

through the ZnO layers and are absorbed mainly in the depletion region of the p-Si where

electron-hole pairs are generated. On the other hand, for high energy U.V. light, photons

were absorbed in the depleted n-ZnO region.

It is worth noting that when photon energy increases, the penetration depth of

photons will decrease rapidly. Therefore when the depletion thickness gets larger

compared to the light penetration depth, the photogenerated current level will saturate

which is exactly what is observed on the 1-V curve when a large reverse bias voltage is

applied.

Fig. 3.1 is based on this model. Assuming the presence of a thin layer of oxide at

the interface, in the case where visible photons were involved and generation occurred at

the depleted Si region, photoelectrons were said to encounter the thin oxide barrier on

their way to the front contact whereas holes did not. The total photocurrent density is

therefore dependent on and dominated by hole current density. According to their

equation, it is also dependent on the square root of the potential drop across the depleted

p-Si region. In the case of high energy photons, generation occurs in the depleted ZnO

41

region and photogenerated holes are the ones that encounter the oxide barrier and travel

through the p-Si to reach the back contact. The photocurrent in the high energy case is in

contrast, dependent on the electron density and according to their equation, the voltage

across the n-Znü depleted region. The conclusion is that under the assumption of an

oxide barrier layer at the interface of the ZnO/Si junction, carrier generation occurs in the

depleted region in Si causing current density to be dominated by hole flow when low

energy photons are illuminating the junction whereas high energy photons cause carrier

generation in the depleted region of ZnO resulting in current density being dominated by

electron carrier flow.

3.3 Previous work on ZnO-Si heterojunctions

According to Kim et al. [3.13, 3.14], crystallographic morphology is not the most

influential factor in photoelectrical performance. Poor photoelectric effects have been

observed in [3.3.10] for ZnO films deposited on Si at 550 oc by the sputtering method

that had shown the best stoichiometric quality. Interpretations include the possibility of

defectiveness in the oxide layer since the reaction between Si and oxygen gases is more

probable at higher temperatures. A defective Siüx layer may be the source of large

leakage currents. Lee et al. [3.9] propose that the layer may be a mixture of SiOx-ZnO

that could be acting as light-absorbing media. The conclusion is that for the purposes of a

photodiode, the quality of the diode junction is as important as that of the ZnO film itself.

Furthermore, Kim et al. [3.14] report that since crystallinity is shown to be the highest for

films deposited at 600 °C, and that photocurrent degrades, it is concluded that the overall

photoelectric performance of diodes is not directly proportional to the crystallinity of the

42

ZnO films. Although crystallinity is considered a factor in terms of achieving good

photoelectric performance, it is obviously not sufficient in terms of achieving an optimum

performance. Stoichiometry is the important factor and has been determined to be

improved by optimizing process conditions such as substrate temperature and/or Ar/02

ratio. Increasing the deposition temperature makes the heterojunction cell oxygen

deficient [3 .14]. lt has been shown by Kim et al. that films deposited at 600 oc have the

lowest resistivity values which have been determined to be a direct result of the oxygen

deficiency. According to 1-V measurements shown by Kim et al., low resistivity and

therefore oxygen deficiency may be observed through an increase in dark current leakage.

In fact, Lee et al. [3 .1 0] show that dark leakage current gradually increases with

deposition temperature. The phenomenon is first guessed to be attributed to the formation

of a defective interlayer between n-ZnO and p-Si. Further analysis with X-ray

photoelectron spectroscopy (XPS) shows that the interlayer is Si02 which was clearly

unintentionally grown since the Si substrates were cleaned in buffered HF (HF:H20 = 1:

7) for 1 minute, prior to ZnO deposition. Probing of the junctions by X-ray Photoelectron

Spectroscopy (XPS) shows that the thickness of the insulating layer grows with

deposition temperature [3.10]. The overall conclusions derived from the literature indicate

that an increase in deposition temperature will i) increase the interfacial oxide thickness

and ii) lower resistivity, an indication that the film is oxygen deficient. The latter may

also be observed through an increase in dark current leakage.

43

3.4 Effects of Substrate Annealing

When deposition temperature is increased for an appropriate level of RF power, as

discussed in chapter 2, it supplies thermal energy to the growth process, helps to

encourage stable deposition and larger grain size. Post-deposition annealing provides

thermal energy and in this work, has shown to improve photoelectric performance for

temperatures below and up to 350 oc for Round 2 samples which were deposited at 120

W. A subsequent annealing at 400 °C shows a decrease in photoelectric performance.

This is an interesting result since Chapter 4 will present an increasing trend of interfacial

state density with annealing up to 350 oc for these samples.

3.4.1 Experimental results

In this section, 1-V results obtained in this work from the ZnO-Si heterojunctions

prepared by RF sputtering will be described. It is noted that annealing in this work was

performed under vacuum conditions. The glass and Si heterojunctions were

simultaneously loaded into a glass tube connected to the vacuum system and evacuated

ovemight prior to annealing. Annealing was performed at 200, 300, 350, 400 oc for 30

minute intervals each. Three rounds were performed for ZnO films deposited at different

RF powers, namely, 40 W, 120 W, and 80 Win this order. The Si substrates which were

used to deposit ZnO at 80 W remained in air for an approximate period of three weeks

prior to deposition. Furthermore these fabricated samples were annealed at the 350 oc

round twice, for 30 minutes each.

The 1-V characteristics were measured with a HP 4145A Semiconductor

Parameter Analyzer after each annealing step. The results are shown in Fig. 3.2. The light

44

source used to illuminate the sample during measurements has a power density

approximated to be about 100 mW/cm2• Fig. 3.2 a shows the dark leakage current for the

as-deposited sample to be close to zero. Annealing steps performed at 200, 300 and 350

oc increase the leakage current whereas the subsequent annealing step performed at 400

°C decreases it. Leakage current in the dark reverse biasing situation is generally a result

of thermal generation. Thermally generated electrons are attracted to the n-side whereas

thermally generated holes are attracted to the p-side. Leakage current in the forward

biasing situation results from the trapping of carriers in interfacial states as is described in

page thirty nine of this thesis. Trapped carriers which recombine do not have the

opportunity to contribute to dark forward current. The increase in dark reverse-bias

leakage with annealing is possibly an indication of an increase in trapping effects

resulting from Zn having diffused into the oxide layer. Fig. 3.2 b shows a continued

increase in dark leakage current up until the 400 °C annealing step for the Round 2,

oxidized sample. The interpretation is however qualitative in nature and requires further

experimental analysis.

Table 3.1 lists the measured results of the short circuit current (Ise), open circuit

voltage (V oc), fill factor (FF), efficiency ('YI). Calculations for efficiency were made with

equation 3.2 and for fill factor, equation 3.3 where V max and Imax are the voltage and

current at the maximum power point.

Maximum power density 17 = Power of incident light

45

(3.2)

(3.3)

-$ -0.5 -s:: e ._ ::s 0

Round 2 ZnO-Si not oxidized

2.00E-03

1.00E-03

-4.00E-03

Voltage (V)

Fig. 3.2 (a)

-+- dark at 400 degree anneal

--400 degree anneal

0.5 350 degree anneal

300 degree anneal

_,_ 200 degree anneal

--As-deposited -- dark as-deposited

-- dark at 350 degree anneal

Round 2 ZnO-Si oxidized

2.00E-03

1.00E-03

-2.00E-03

-3.00E-03

-4.00E-03

Voltage (V)

Fig. 3.2 (b)

46

--+- dark at 400 degree anne al -- 400 degree anneal '* 350 degree anneal

300 degree anneal _,._ 200 degree anneal --As-deposited _,._ dark as-deposited

< --c a.t ::::: -0.5 ::::J 0

-<C --c ~ -0.6 ::::s 0

Round 3 ZnO-Si not oxidized

3.00E-03

2.00E-03

1.00E-03

-3.00E-03

Voltage (V)

Fig. 3.2 (c)

--+- dark at 400 degree anneal --- 400 degree anneal

350 degree annealed twice 300 degree anneal

0.5 --200 degree anneal

--As-deposited -- dark as-deposited

Round 3 ZnO-Si oxidized

3.00E-03

2.00E-03

1.00E-03

-3.00E-03

Voltage (V)

Fig. 3.2 (d)

--+- dark at 400 degree anneal --400 degree anneal ~·CM· 350 degree annealed twice

300 degree anneal _,._ 200 degree anneal

--As-deposited " "dark as-deposited"

Fig. 3.2 Plots ofl-V characteristics offabricated samples for which ZnO was deposited at 120 W (a, b) and 80 W (c, d). Illuminated and dark currents are shown.

47

For samples that were deposited at 120 W (Round 2), in the case for the non-

oxidized sample, Ise, FF and 'YI improve with every annealing step as shown in Table 3.1.

On the other hand, for the sample that was oxidized for 15 min prior to ZnO deposition,

Ise and 'YI are highest as-deposited whereas the FF is highest after the 350 °C annealing

step and the V oc is highest after the 200 °C annealing step. Round 3 samples which had

not been oxidized showed a decreasing trend with annealing for ali four parameters.

Th ose which had been oxidized had maximum values for Ise, V oc and 'YI in as-deposited

films; however the FF was best after the 200 oc annealing step. Round 3 samples were

exposed to air for ~ 3 weeks prior to ZnO deposition. Efficiency values for Round 3

samples were of one order of magnitude higher than for Round 2 samples. The highest

values in ali four parameters were achieved for the Round 3 samples which had not been

oxidized prior to deposition.

Table 3.1 A summary oflsc, V oc, FF and 'YI for fabricated samples after each annealing where the maximum values are shaded.

Sample Annealing step: Ise (mA) V oc FF 'YI(%) no.

Temperature COC) Time (V)

Round2 As-deposited 0 0.62 0.16 0.018 not 200 30 1.50 0.16 0.23 0.054

oxidized 300 30 1.49 0.16 0.29 0.069 350 30 0.15 400 30 0.14

Round2 As-deposited 0 0.18 oxidized 200 30

300 30 350 30 400 30

Round 3 As-deposited 0 not 200 30 1.63 0.27 0.34 0.15

oxidized 300 30 1.81 0.22 0.26 0.10 350 30 0.03 0.09 0.31 0.0009

48

,- 400 30 0.01 0.00 0.000008 Round 3 As-deposited 0 0.27 oxidized 200 30 1.22 0.25 • 0.10

300 30 0.68 0.24 0.22 0.04 350 30 0.004 0.07 0.28 0.00 400 30 0.003 0.05 0.29 0.00

The ideality factors shown in Fig. 3.3, for both oxidized and non-oxidized samples

start from a value ofn ~ 1-2 at a bias ofless than 0.1 (shown as q/k:T*V = 5 where kT/q =

0.026) and reaching as high as n ~ 19 for voltages of~ 0.7 V. The most significant

changes in the ideality factors with vacuum annealing is observed at the higher forward

bias region, apparent in Fig. 3.3 b and 3.3 d. Low voltages however display only slight

changes in ideality factor values with annealing, especially so for the non-oxidized

sample. Mid forward bias voltages show a region between ~0.25 - 0.3 V that has lower

ideality factor values than lower or higher voltages which is similar to the results

observed by Song et al. (3.7]. This region has current density that is modeled by the

equation J = J0[ exp(Ao·V)] where Ao is a constant independent of temperature. It can be

initially inferred that prior to any annealing step, current transport mechanism is

dominated by trap assisted multistep tunneling in this forward bias voltage range.

Subsequent annealing will change perhaps the oxide thickness and therefore change

current transport mechanism and the idea that current transport mechanism is dominated

by multi-step tunneling may no longer be fully useful after annealing.

An increase in ideality factor values is nevertheless apparent from Fig. 3.3 b and

3.3 d with annealing. For non oxidized samples, ideality factors increase to maximum

values after the 350 oc annealing step and come back down close to the original values

49

with 400 oc annealing. The oxidized samples however show continued increase in

ideality factor values until the 350 oc annealing where the next 400 oc annealing step

does not show any significant change.

At high forward bias voltages large increases in the ideality factors are observed

with annealing, except after the 400 oc annealing step. The increase may be an indication

of an increase in either one of the following variables: bulk resistances in the Si and the

Znü, as weil as the resistance associated with tunneling through the Si depletion layer

and the interfacial oxide.

Annealing also seems to increase the reverse bias current, indicative of an increase

in leakage current at the junction. As previously mentioned, leakage current has been

reported to be due to a deficiency in oxygen as is the case when deposition temperature is

increased [3.14]. In this work, annealing is performed under vacuum conditions, which

leads to oxygen out-gassing and therefore a deficiency in oxygen. Leakage may be due to

carrier trapping due to increased interfacial state density. This topic will be discussed in

Chapter4.

50

10.0

5.0

::::::-z -5.0 ..J

·10.0

-15.0

·20.0

16.0

14.0

t: 12.0 ,_ 0 10.0 -u CIS

8.0 .... >. ~

6.0 ëij C1l E 4.0

2.0

0.0

0.0 0.1

Round 2 - Oxidized ZnO-Si

1/n factor

q/KT *V

Fig. 3.3 (a)

30.0

" Non ideal: Alter 200 degree anneal

Non ideal: Alter 300 degree anneal

~ Non ideal: Alter 350 degree anneal

---Non ideal: Alter 400 degree anneal

• n=2

Round 2- Oxidized ZnO-Si ldeality factor values

0.2 0.3 0.4 0.5 0.6

Voltage (V)

Fig. 3.3 (b)

51

0.7

· As-deposited

" After 200 degree anneal

After 300 degree anneal

• After 350 degree anneal

_,._ After 400 degree anneal

10.0

-z ....

16.0

14.0

1: 12.0

.... 0 10.0 -u «1

8.0 ~

>-~

'ii 6.0 (1,1

:E 4.0

2.0

0.0

0.0 0.1

Round 2 - Not oxidized ZnO-Si

1/n factor

q/KT*V

Fig. 3.3 (c)

• n=6 ·• n=19

Round 2- Not Oxidized ZnO-Si ldeality factor values

0.2 0.3 0.4 0.5

Voltage (V)

Fig. 3.3 (d)

0.6 0.7

---- As-deposited

-- After 200 degree anneal ' After 300 degree anneal

" After 350 degree anneal _.,_ After 400 degree anneal

Fig. 3.3 Plots ofln(I) vs. 0.026*V for Round 2 oxidized (a) and non-oxidized (c) samples. Plots of n vs. V are shown in (b) and ( d) respective! y.

52

-' '

3.5 Conclusions

Efficiency has been observed to increase for the Round 2 non oxidized sample

with annealing up until the 350 oc annealing step, but decreases with the subsequent 400

oc step. On the other hand, samples that were exposed to air prior to ZnO deposition

showed no efficiency improvement with annealing even if they had been intentionally

oxidized prior to air exposure, perhaps an indication that the samples contained too much

oxygen components. Ideality factors have shown significant increase in the higher

forward bias voltage region with annealing. Only small increases are seen in the low

forward bias voltage region. Ideality factor value curves consistently show a region where

values are lower than both low and high forward voltage. Values increase in this region

with annealing. Annealing has shown to increase the leakage current observable on the I-

V curves of Round 2 samples, up to 350 oc and further decreases with subsequent

annealing steps. Comparing the efficiency trend, it appears that vacuum annealing

increases the leakage current and the efficiency up to the 350 oc annealing step. These

may be associated either with increased oxygen deficiency associated with vacuum

annealing out-gassing. An increase in leakage is also associated with an increase in

interfacial state density. This topic is discussed in the next chapter.

53

Chapter 4

C-V CHARACTERIZATION

This chapter describes the principle behind capacitance-voltage measurements on

heterojunction solar cells of ZnO film deposited on Si by RF magnetron sputtering. The

experimental steps taken for measuring the n-ZnO/p-Si solar cell is described. Previous

work done related to the study of 1/C2- V relationships with deep levels and interfacial

states are reviewed in this chapter. Capacitance measurements were made after each

annealing step. The results are discussed. The purpose is to present material that is useful

in interpreting results related to understanding junction parameters and the capacitance

effects and changes with annealing.

4.1 Capacitance of a heterojunction

Capacitance measurements are useful in determining the built in voltage (Vbi) and carrier

density following equation 4.1 where Eo is the permittivity of free space, Ks is the

dielectric constant, A is the cross-sectional area, NA is the acceptor concentration.

Equation 4.1 is the depletion capacitance equation for a metal-semiconductor, which is a

good approximation and applicable for a highly doped n-ZnO layer sputtered onto a p-Si.

2(Vbi- VA) qNAKs&oA2

54

(4.1)

In the ideal system, plotting 1/C2 vs. V A, the applied voltage, yields a straight line

intercepting at 1/C2 = 0 with the voltage axis. Following equation 4.1, the slope of this

line is proportional to the carrier density and therefore a straight line is indicative of a

uniform doping distribution. Capacitance measurements, which are ideally frequency

independent, indicate the capacitance of the space charge layer. It is however often the

case that these plots are frequency dependent. Two types of plots are possible: curves that

are parallel to each other and therefore having different built-in voltages and curves that

have different slopes, converging to the same built-in voltage. The latter behavior is

attributed to the presence of surface irregularities and/or surface states in the

semiconductor [ 4.1].

C-V measurements are also often used to study deep impurity levels [4.2, 4.3].

Studies have been performed on Schottky barriers by Roberts and Crowell [ 4.3, 4.4]. The

analysis is similarly applicable to the p-n junction. Deep level effects are presented in Fig.

4.1. Deep levels are impurity levels that lie deep in the band gap, as opposed to shallow

levels lying close to the valence band for a p-type semiconductor or close to the

conduction band edge for an n-type. In Fig. 4.1, deep levels are mode led to be in the p-Si

side due to the n-ZnO being heavily doped and highly conductive. As a result of heavy

doping, the depletion region on the n-side is very small and therefore band bending is

minimal when bias voltage changes are applied. The effects of deep levels are however

observed in depletion regions for which band bending is obvious. The absence of deep

levels is modeled on the n-ZnO side in Fig. 4.1. Under high forward biases, when banding

ben ding is less, deep levels in the depletion region of the p-Si lie that lie completely over

the Fermi level are filled and do not contribute to charge storage. When the forward bias

55

is reduced, deep levels closest to the interface will bend downwards, below the Fermi

level, thereby allowing the carriers trapped in these deep levels to jump into the valence

band, creating an ionized charge in its place. The charge from these ionized sites

con tri butes to the overall capacitance measured. It is therefore at the crossing of the deep

level with respect to the Fermi level that a peak in terms of the change in charge density

with the change in barrier bending will be observed

56

0 Filled deep levels + Free holes - lonized levels

Ec ----------------------~

Deep level2 ;-;: oooooooooooooo~

Deep levell ooooooooooooooOê;"c;O\ EfSi--------------------~~:::....>.n

OO<-Xo

p-Si

1

1

~

dP dljfs

<--x-----------;

EfZnO

n-ZnO

Fig. 4.1: Band diagram of p-Si/Si02/n-ZnO with deep level modeling and dp/dl/; schematically plotted as a function of x where x2, x1 represent mid points between peak or demarcations for the different capacitive contributions from different energy levels.

57

Measurements for deep level analysis for p-n junctions are however usually clone

under reverse bias. The reason being that in forward bias, minority carrier injection

produces diffusion capacitance which makes deep level effects hard to detect. Under

reverse bias, band bending increases, depletion width widens and an increasing amount of

deep level sites move under the Fermi level in the depleted p-Si. The number of ionized

acceptors increases as the depletion width widens and the concentration level of free

holes increases in the valence band.

According to Equation 4.1, a non linear relationship between 1/C2 and V are

expected when deep levels are present and exposed and when their sites are ionized.

Concentration levels increase non-linearly as a result of bias changes. Nonlinearity in

1/C2 vs. V curves is therefore a direct result ofthe effect of deep level charge [ 4.3].

Further indication of the presence of deep level effects is observed in a C vs. V

relationship that is dependent on frequency. When an a.c. signal is added to the d.c. signal,

change in the depletion width is observed. During the positive half-cycle, the depletion

width will decrease while the opposite occurs for the negative half-cycle. When the

depletion width increases, deep level sites lying below the Fermi level in the p-Si, will be

ionized, creating a new charge and an increase in the concentration level in free carriers.

When the depletion width shrinks, the ionized site is neutralized by a free carrier from the

valence band. This change in charge is a direct relationship to the capacitance measured

and is inversely proportional to the deletion width as is shown in equation 4.2 where V ac

is the a.c. voltage, Q is the charge in Coulombs, W is the depletion width, A is the cross

sectional area, Eo is the permittivity of free space, and Er is relative permeability. In fact,

since equation 4.2 shows well known relationships that do not take into consideration the

58

effect of deep levels, the depletion width derived from the total capacitance measured

which includes the effect from deep level impurities is not a true value of width. In fact,

when low frequency is used, the depletion width profile is shifted towards the interface;

however, at high frequency, it shifts away from it. At low frequency, deep level sites have

enough time to react and therefore more of them become ionized. More charges are

present and therefore according to equation 4.2, the apparent width Wapp will decrease

towards the interface.

(4.2)

Low-frequency curves help to give properties of deep impurities since the deep

level sites are most likely to be able to follow the a.c. signal and contribute to the

capacitance measured. When the frequency is increased and the deep level sites are no

longer able to follow due to their long time constant for charging and discharging even

though they are below the Fermi level, the contribution from deep level sites is not

detectable on 1/C2 vs. V curves. A value for V bi and dopant concentration is best

measured at high frequency as the time constants of the dopants are substantially shorter

than that of the deep levels. It is clear that if C vs. V measurements change with

frequency, deep level effects are known to be present [4.3].

The change in apparent width with frequency is a result of the total capacitance

being composed of two elements, not properly modeled by the capacitance equation

containing Wapp· The total capacitance is composed of the capacitance due to shallow

dopant impurities and the capacitance due to deep level impurities. This second

59

capacitance term is a summation of the capacitance equations shown as equation 4.3. A

change in bias voltage will result in a non linear change in 1/C2 vs. V when deep level

effects are present. The following is a more detailed explanation of the relationship

between capacitance and a change in bias voltage as described by Roberts and Crowell

[4.4]. Let p(x) be the total charge density at any point x and ql/;8 be the energydifference

between the conduction band and the Fermi level at the interface. When the d.c. bias is

changed, the quantity dp/dl/;s which is shown in Fig. 4.1 will peak at the edge of the

depletion region and at points within the depletion region where deep impurity levels pass

under the Fermi level. The integral of dp!dl/ts over the depletion region is the capacitance

per unit area of the barrier. According to Roberts and Crowell's model, for each value of

d.c. bias, the capacitances may be written as:

(4.3)

Sin ce the midpoints between the dp/dl/;s peaks Xi (i = 0, 1, 2, etc) appearing in Fig. 4.1,

change with applied bias, capacitance values that take into account deep level effects

change with applied bias.

The effect of interface states may also be observed through C vs. V curves.

Surface state density according to Cowley and Sze's study on metal-semiconductor

systems [ 4.5] is a property only of the semiconductor surface. Interface charge appears at

the immediate vicinity of the heterojunction interface. This is an effect that differentiates

the heterojunction from the homojunction. Surface states are produced when foreign

60

atoms are absorbed at the surface and the termination of the periodic structure of the

semiconductor' s crystal is disturbed. A first assumption may be made to assume that the

sputtered ZnO is more likely to be defective than the industrial grade Si wafer, interface

states may be modeled in Fig. 4.2 to be at the interface of the Si02 layer and the ZnO.

Altemately, deep levels discussed previously and modeled in Fig. 4.1 may be present in

the Si side. Evidence of continued change in C-V curves with annealing, specifically a

decrease in V bi derived from the deep regions of the junction, is an indication that it is

more likely that interface states reside on the Si side as is modeled in Fig. 4.3. The results

for this will be presented shortly. Intuitively, this makes sense since ZnO is much more

heavily doped than Si. In this case, during the annealing process, it is possible to expect

that sorne of the zinc atoms may diffuse across from the ZnO si de.

61

Deep level2----------­

Deep levell ---------~ EfSi-------------->.;:----'-::l

p-Si

• Filled surlace state + Empty surlace state

:------------ Ec

EfZnO

-----~--------Ev

n-ZnO

Fig. 4.2 Band diagram of p-Si/Si02/n-Zn0 with interface state modeling on the n-Znü si de.

62

Deep level2--------~..__

• Filled smface state - Empty smface state

Deeplevell-----------~ Ec

EfSi-----------~:----4!!'! f------------ Ef ZnO

p-Si ----------Ev

n-ZnO

Fig. 4.3 Band diagram of p-Si/Si02/n-Zn0 with interface state modeling on the p-Si side.

4.2 Princip les of interface state density calcula ti ons

The density of interface states (Nrs) at the Fermi level in the depletion region may be

calculated via Equation 4.4 using capacitance vs. frequency curves.

N - CIS IS- q

where C18 = CLF- CHF

63

(4.4)

where CLF is the low frequency capacitance, CHF is the high frequency capacitance.

Density of interface states may also be calculated via Equations 4.5 and 4.6 [4.1.50].

Vbi_app = Vbi -V a

qŒ2 V=---=-----

a 2(cnND +&pNA)

(4.5)

(4.6)

Where Vbi_app is the apparent built-in voltage, V bi is the actual diffusion voltage, a is the

number of charged states per cm2, q is the electron charge in Coulombs, Nn is the donor

density, NA is the acceptor density, En is the permittivity of the n-type semiconductor and

Ep is the permittivity of the p-type semiconductor. The value of Vbi may be calculated

using the Equation 4.7 [4.6].

(4.7)

where ~Ec is the conduction band discontinuity, Eg_si is the band gap of Si, Ûv_Si is the

energy between the valence band of Si and the Fermi level, and finally Ûc_zno is the

energy between the conduction band of ZnO and the Fermi level.

4.3 Experimental results

As mentioned in chapter 2, Round 1 samples are characterized by ZnO

depositions performed at 40 W for 6 hours, Round 2 samples at 120 W for 2 hours,

Round 3 samples at 80 W for 4 hours. Dry oxidation was performed at 450 oc for 15

64

minutes on sorne samples prior to Znü sputtering. The following is a list of the V bi and

dopant concentration measured via the 1/C2 vs. V curve with each annealing step on the

heterojunctions fabricated in the present work. Annealing step conditions are shown.

Table 4.1 A summary ofVbi derived from C~V results from high reverse bias regions.

Sample no. Annealing step: vbi(V)

Temperature Time (OC) (min)

*Round 1 As-deposited 0 1.40 not oxidized 200 30 0.10

300 30 0.20 350 30 NIA 400 30 NIA

*Round 1 As-deposited 0 1.40 oxidized 200 30 0.10

300 30 0.15 350 30 NIA 400 30 NIA

**Round 2 As-deposited 0 0 not oxidized 200 30 -0.05

300 30 -0.15 350 30 -0.20 400 30 ~ij~lO

**Round 2 As-deposited 0 0.20 oxidized 200 30 0.10

300 30 -0.05 350 30 -0.15 400 30 -0.20

Round 3 As-deposited 0 1.11 oxidized 200 30 0.41

300 30 0.05 350 30 -0.21 400 30 -0.22

Round 3 not As-deposited 0 1.10 oxidized 200 30 0.40

300 30 0.02 350 30 -0.15 400 30 -0.25

*: values are not accurate due to instrument-use error **: Slopes used are those resulting at high reverse biases

65

The 1/C2 vs. V intercept is shown to decrease with increase in annealing

temperature. This is the case for Round 2 and 3 samples except for the non-oxidized in

Round 2 after an annealing step at 400 °C. Round 1 samples show an increase in Vbi after

the second annealing step, i.e. after an annealing step at 300 °C. The results of Vbi

tabulated in Table 4.1 are shown in Fig. 4.4.

-> -:.c >

1.6

1.4

1.2

0.8

0.6

0.4

0.2

0

-0.2

-0.4

Vbi vs annealing step

Annealing step (30 minutes each)

--Round 2 non-oxidized w Round 2 oxidized

-Round 3 non-oxididized ---Round 3 oxidized -Round 1 non-oxidized

-Round 1 oxidized

Fig. 4.4 Change in V bi after each annealing step performed under the conditions noted in Table 4.1

The behavior of the apparent V bi shown in Fig. 4.4 is an indication of the presence

of surface states which is schematically modeled in Fig. 4.2. Surface states decrease

barrier height c/>b whose expression is shown in Equation 4.8, where Vbi is the built-in

voltage and Vp is the potential difference between the Fermi level (EF) and the valence

energy lev el (Ev).

(4.8)

66

V P may be deduced from impurity concentrations via Equation 4.9 where Nv is the

density of states for the valence band, NA is the impurity concentration and k is the

Boltzmann constant.

(4.9)

A high Vbi is therefore indicative of a higher barrier, one which may be degraded by

interface states. Decrease in barrier height leads to a decrease in open circuit voltage (V oc)

[4.7].

Prior to annealing (annealing step = 0 in Fig. 4.4), Round 1 samples are

characterized by a high and positive Vbi, Round 3 samples are characterized by a slightly

lower Vbi and Round 2 samples are characterized by the lowest Vbi whose values are

negative. This is an indication that surface states are present. Round 2 samples which

were subjected to higher RF power during ZnO sputtering have higher surface states due

to the lowest V bi measured. This is probably due to the bombardment of atoms onto the

surface. Annealing appears to increase the amount of surface states since there is a

decrease in Vbi values with annealing; however, for the Round 2 non-oxidized sample,

after the 350 oc annealing step, the Vbi starts to increase slightly. This is in agreement

with the dark leakage current trend observed in Fig. 3.2 a) whereby the shunt resistance

depicted by the dark leakage decreases up until the 350 oc annealing step and increases

after the 400 °C annealing step. The V bi trend for the oxidized Round 2 sample is also in

agreement with the dark leakage trend.

67

Fig. 4.5 a) and b) show the as-deposited results of C vs. V for the sample that was

oxidized for 15 minutes and the other that was not oxidized prior to ZnO deposition in

Round 2 where ZnO films were deposited at 120 W. The capacitance measurements were

performed at frequencies of 200, 1K, 10K, and 100KHz with the 4247A HP multi­

frequency LCR meter. The curves show that for Round 2 samples, for which ZnO films

were deposited at 120 W in RF power, capacitance generally decreases with increase in

frequency. Round 2 samples are therefore characterized by deep level effects. Fig. 4.6 a)

and b) show as-deposited results of C vs. V for the samples fabricated in Round 1 for

which ZnO films were deposited at 40 W of RF power. Interestingly, the curves do not

vary as much with frequency, indicative of reduced effects from deep levels. 1t is possible

that when RF power is increased, bombardment of atoms onto the substrate surface

creates more deep level defects. Fig. 4.8 a) and b) show results of 1/C2 vs. V for as­

deposited round 2 samples. Fig. 4.7 a) and b) show results of as-deposited round 1

samples. Round 2 samples show high frequency curves for which 3 different slopes are

detectible from high to low reverse bias. Round 1 samples are characterized by one slope

in the high reverse bias voltage range and another smaller slope at low reverse bias.

68

Round 2: Non oxidized ZnO-Si CvsV

-1 .0.9 .0.8 .0.7 .0.6 .0.5 .0.4 .0.3 .0.2 .0.1 0

Voltage (V)

Fig. 4.5 (a)

Round 2: oxidized ZnO-Si CvsV

90 ~--------------------------------~

80 -LL 70 s:::: -60 IV (,J 50 s::::

~ 40

~ 30

~ 20 0 10~~_.~~-+--.--

0 ~~--~--~--~--~--~~--~--~~ -1 .0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0

Voltage (V)

Fig. 4.5 (b)

-+-100KHz -~t--10 KHz

--.k-1 KHz

-+-200Hz

-+-100KHz -~t-10 KHz -+-1KHz -+-200Hz

Fig. 4.5 Reverse bias measurements of capacitance vs. voltage for as-deposited Round 2 samples, which were (a) non-oxidized, and (b) oxidized prior to ZnO deposition. ZnO deposition was performed at 120 W for 2 hours.

69

i:L 8.s

t: 8 -

-1

Round 1: Non oxidized ZnO-Si CvsV

.0.8 .0.6 .0.4 .0.2 0

Voltage (V)

Fig. 4.6 (a)

Round 1: oxidized ZnO-Si CvsV

-+-100KHz

--10KHZ

-.:--1KHz

-+-100Hz

12 ~------------------------------~

10 -LL 1::

- 8 -+-100KHz

-Il-10KHz

-.:--1KHz

-+-100Hz

0 ~----~----~------~----~----~ -1 .().8 .0.6 .0.4 .0.2 0

Voltage (V)

Fig. 4.6 (b)

Fig. 4.6 Reverse bias measurements of capacitance vs. voltage for as-deposited Round 1 samples, which were (a) non-oxidized, and (b) oxidized prior to ZnO deposition. ZnO deposition was performed at 40 W for 6 hours.

70

N < 0 -or-

N < 0 -or-

Round 1: Non oxidized ZnO-Si 1JCA2 VS V 0.03 r-------=-:..~--==--.:....=;.___:;__ ___ ____,

0.025

0.02 --+--100KHz

--10KHz

O.Q15 ---1KHz

0.01

0.005

0.03

0.025

0.02

0.015

0.01

0.005

-1 .0.8 .0.6 .0.4 .0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Voltage (V)

Fig. 4.7 (a)

Round 1: oxidized ZnO-Si 1JCA2 VS V

o~~~-~~~-~~~-~~~;.___:~

-1 .0.8 .0.6 .0.4 .0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Voltage (V)

Fig. 4.7 (b)

--+--100Hz --slope 1

-+-100KHz

--10KHz --.k--1 KHz

~100Hz

--Siope 1

Fig. 4.7 Reverse bias measurements of 1/C2 vs. V for as-deposited Round 1 samples, (a) non-oxidized, and (b) oxidized prior to ZnO deposition.

71

Round 2: Non oxidized ZnO-Si 1/CA2 VS V

0.007

0.006

0.005

N 0.004 < 0 - 0.003 'l""

0.002

0.001

0

-1

0.008

0.007

0.006

N 0.005

< 0.004 0 -~ 0.003

0.002

0.001

0

.().8 .().6 .().4 .().2 0 0.2

Voltage (V) Fig. 4.8 (a)

0.4 0.6 0.8

Round 2: oxidized ZnO-Si 1JCA2 VS V

1

~100KHz

-~t-10 KHz _.,.,_1 KHz

~200Hz

--slope 1 ,,~,Siope 2

-+-100KHz -10KHz ...,..1KHz

-+-200Hz -slope 1

Slope 2

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Voltage (V)

Fig. 4.8 (b)

Fig. 4.8 Reverse bias measurements of 1/C2 vs. V for as-deposited Round 2 samples, (a) non-oxidized, and (b) oxidized prior to ZnO deposition.

72

Round 2 samples for which ZnO films were deposited at an RF power of 120 W

for 2 hours show deviations in 1/C2 vs. v curves from straight lines and is therefore an

indication of a non-uniform doping profile due to deep level effects mentioned previously.

Other possibilities include stray capacitance or high series resistance. Since samples

fabricated in Round 2 and Round 3 show the same initial trend of non linearity, deviations

are most likely due to deep level effects. Linearity appears to be improved with annealing,

meaning the slope in high reverse bias becomes similar to that in low reverse bias. This is

an indication that the states at the near vicinity of the heterojunction interface are either 1)

being remedied by annealing or 2) are overshadowed by impurities in the deeper junction

region. Interestingly, Round 1 samples for which ZnO films were deposited at an RF

power of 40 W for 6 hours show C vs. V relationships that have no dependency on

frequency. The author would like to note that the voltage scale is not complete! y accurate

due to equipment use error. However, the trend of frequency independence is apparent

and an indication of low deep level effects in the samples characterized by low power

ZnO deposition. More specifically, deep levels most probably lie more than 5 kT away

from the Fermi level in neutral bulk [ 4.8] which would therefore mean that during reverse

biasing, the deep levels unlikely reach a level at which they can cross the Fermi level and

have deep level sites ionized. Contribution from the deep levels to the capacitance

becomes less likely and therefore C vs. V curves do not change with frequency. A section

of non-linearity is still however observed for low reverse voltages in the Round 2 curves

for which ZnO films were deposited at 120 W after all annealing steps. Deviation is larger

for non-oxidized samples and become more apparent at high annealing steps. Fig. 4.11 a)

73

,,..-...\ and b) show the results for Round 2 samples annealed after 200 oc degree annealing. Fig.

4.12 a) and b) show their results after 400 oc degree annealing. These non linear tails are

also observed in the Round 3 curves for which ZnO films were deposited at 80 W, but

appear only after 350 and 400 oc annealing. Similar trends of non linear faU offs are

reported in Tavakolian and Sites work with CdS/CulnSe2 [4.9]. The difference is that

their fall offs show an increase in capacitance whereas Round 3 curves show a decrease in

capacitance after each annealing step. Tavakolian and Sites have attributed their non

linear faU offs to interfacial states. Furthermore, beyond a certain threshold in the number

of interfacial states, effects on open-circuit voltage are noticeable such has been presented

by Tavakolian and Sites for their CdS/CulnSe2 ceUs. In this work, it has been shown in

chapter 3 that the highest V oc values for non-oxidized samples are found in as-deposited

samples. They decrease in value with each annealing step. Samples which were oxidized

for 15 minutes prior to ZnO deposition however show a slight increase in V oc after the

200 oc annealing step and then a decrease with subsequent annealing step.

Round 2 samples for which an RF power of 120 W was used show significant

non-linearity at low reverse biasing voltages for low and high frequency curves as shown

in Fig. 4.8, high indication of the presence of deep levels. Round 3 samples for which an

RF power of 80 W was used show non linearity at low reverse biasing voltages only for

low frequency curves as shown in Fig. 4.9 indicating that the film may be ofbetter quality

in terms of deep levelslinterfacial states compared to Round 2 samples. This is in

agreement with conversion efficiency values presented in chapter 3 whereby Round 3

samples were characterized with better Ise, V oc, FF and 'Y/. Round 1 samples for which an

RF power of 40 W was used show only a very slight non-linear portion at low reverse

74

biasing voltages for both high and low frequency curves which follow closely to one

another as shown in Fig. 4. 7. This is an indication that the lower the RF power, the less

interfacial state effects are present. Non-linear tails from Round 1 samples prior to

annealing are most likely not due to deep level effects since the C vs. V curves do not

vary with frequency.

0.012

0.01

~0.008

0 ;::o.oo6

0.004

0.002

Round 3: Non oxidized ZnO-Si 1/CA2 VS V

-1 .0.8 .0.6 .0.4 .0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Voltage (V)

Fig. 4.9 (a)

75

-+-100KHz

--10KHz

-.-1KHz

-+-200Hz

--stope 1

0.012

0.01

N < 0.008

~0.006 oc-

0.004

0.002

Round 3: Oxidized ZnO-Si 1fCA2 VS V

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Voltage (V)

Fig. 4.9 (b)

-+-100KHz

--10KHz .....,.1KHz

-+-200Hz -slope 1

Fig. 4.9 Reverse bias measurements of 1/C2 vs. V for as-deposited Round 3 samples, (a) non-oxidized, and (b) oxidized prior to ZnO deposition.

The number of interface states may be determined via C-V as well as C-F measurements

as discussed previously. Via the C-V method, Equations 4.5 and 4.6 are used to determine

the amount of interface charges present. To determine the actual diffusion voltage,

Equation 4.7 is used and repeated here.

(4.7)

The calculated value for Ôv_sds 0.15 eV assuming NA~lx10 17 cm·3 with a mobility of331

cm2N-sec for the boron-doped (100) Si wafers. For Ôc_zno, an approximate value of

~0.023 eV is calculated, assuming that N0 ~ 1019 cm·3 and ni~ 1.18xto·8 cm·3 which was

calculated with the effective densities of states for Si, since these values were assumed to

be very close to those of ZnO. The calculated V bi is 1.335 V. Using Equation 4.5 and 4.6

and assuming Er for Si is 11.7, Er for ZnO is 8.72, NA= lx1017 cm·3 and N0 =lx1019 cm·3,

76

the calculated parameters used and the calculated interface state densities are shown in

Table 4.2. It is worth noting that equations 4.5, 4.6, 4.7 used for determining the

interfacial state densities follow the assumption that the shallow region doping density is

the same as the deep region density. The deep region is observed at high reverse biasing

whereas the shallow region is observed at low reverse biasing and low forward biasing.

The shallow region may is characterized by low reverse biasing and forward biasing since

the depletion edges remain close to the interface. The deep region is characterized by a

high reverse bias since the depletion edges are deeper into the bulk. Since, in the cases

where there were more than one slopes in the 1/C2 vs. V curves, the slope in high reverse

bias was used to determine the apparent Vbi values, interfacial state densities (a)

calculated and tabulated in Table 4.2 are truly for those in the deep region density.

Table 4.2 A summary of calculated values for the interface state density at -0.18 V bias via C-V measurements.

Sampleno. Annealing step: vbi_app Va a (states-

Temperature Time (V) (V) cm-2

)

(OC) (min) with C-V

*Round 1 As-deposited 0 1.40 0.065 -not oxidized 200 30 0.10 -1.235 1.10x1013

300 30 0.20 -1.135 1.05 x1013

350 30 NIA NIA NIA 400 30 NIA NIA NIA

*Round 1 As-deposited 0 1.40 0.065 -oxidized 200 30 0.10 -1.235 1.10 x1013

300 30 0.15 -1.185 1.08 x1013

350 30 NIA NIA NIA 400 30 NIA NIA NIA

**Round2 As-deposited 0 0 -1.335 1.14 xl013

not oxidized 200 30 -0.05 -1.385 1.16 x1013

300 30 -0.15 -1.485 1.21 x1013

350 30 -0.20 -1.535 1.23 x1013

400 30 -0.10 -1.435 1.18 x1013

**Round 2 As-deposited 0 0.20 -1.135 1.05 x1013

oxidized 200 30 0.10 -1.235 1.09 x1013

300 30 -0.05 -1.385 1.16x1013

77

350 30 -0.15 -1.485 1.21 x1013

400 30 -0.20 -1.535 1.23 x1013

Round 3 As-deposited 0 1.11 -0.225 4.69 x1012

oxidized 200 30 0.41 -0.925 9.51 xl012

300 30 0.05 -1.285 1.12 xl013

350 30 -0.21 -1.545 1.23 x1013

400 30 -0.22 -1.555 1.23 x1013

Round 3 not As-deposited 0 1.10 -0.235 4.79 x1012

oxidized 200 30 0.40 -0.935 9.56 x10 12

300 30 0.02 -1.315 1.13 x1013

350 30 -0.15 -1.485 1.21 x1013

400 30 -0.25 -1.585 1.25 x1013

*: values are not accurate due to instrument-use error **: Slopes used are those resulting at high reverse biases

Interface state densities summarized in Table 4.2 are plotted in Fig. 4.1 O. It can be

observed for Round 2 samples, that the sample which was oxidized for 15 minutes prior

to Znü deposition has a lower deep region interface state density than the sample which

was not oxidized. Similarly, Round 3 samples which were exposed to air for a long period

of time are shown to have much lower interface state densities. The values for interface

state density are similar for oxidized and non-oxidized Round 3 samples with annealing

throughout the annealing processes however after 400 oc annealing, Round 2 samples

which were not oxidized show a decrease in interface state density whereas Round 2

oxidized samples show an increase. The decrease in interface state density for the non-

oxidized sample agrees with the decrease in dark leakage current and by the increase in

vbi at this 400 oc point. It appears that oxidation, whether intentionally performed at the

interface prior to Znü deposition or through exposure to air helps to decrease the

interface state density. Exposure to air however appears to decrease the value

significantly compared to an intentional oxide growth at the interface. The difference is

almost of one magnitude. Interface state density values appear to increase with annealing,

which is a peculiar trend as Si02 thickness is expected to increase. Furthermore, as

78

i---· mentioned earlier, the calculated values are those for the deep regions. Assuming that an

interfacial oxide exists, the increase in interfacial state density with annealing occurs at

the Si/Si02 boundary. The location ofinterfacial state may again be visualized by Fig. 4.3.

According to the increase in linearity for 1/C2 vs. V curves with annealing, interfacial

states at the vicinity of the interface appear to be either removed or overshadowed by the

deep region states. Uniformity in state density from the deeper region to the shallow

region is achieved with annealing though the overall magnitude in density increases.

-~N ·- < tn E s:: u cv-"Ctn cv cv--CQ: CQ:-

- tn tn ..... cv 0 u ... ~~ SE s:: ::::s - s:: -

Interface state density vs annealing step

1.4E+13

1.2E+13

1E+13

8E+12

6E+12

4E+12

2E+12

0

0 1 2 3

Annealing step (30 minutes each)

4

--Round 1 non oxidized _,__Round 1 oxidized

--Round 2 non oxidized

Round 2 oxidized

-- Round 3 non oxidized ---Round 3 oxidized

Fig. 4.10 Change in interface state density with annealing step.

79

N < 0 --

N < 0 --

Round 2: Non oxidized ZnO-Si After annealing at 200 degrees for

30 min

0.0016

0.0014

0.0012

0.001

0.0008

0.0006

0.0004

0.0002

1/CA2 VS V

Voltage (V)

Fig. 4.11 (a)

-+-100KHz

--10KHz ...._1KHz

-+-200Hz ---slope 1

Round 2: Oxidized ZnO-Si After annealing at 200 degrees for 30

min 1JCA2 VS V

0.003

0.0025

0.002 -+-200Hz --10KHz -.-1KHz

0.0015 -+-200Hz ----slope 1

0.001

0.0005

0 -1 .0.9 .0.8 .0.7 .0.6 .0.5 .0.4 .0.3 .0.2 .0.1 0 0.1 0.2

Voltage (V)

Fig. 4.11 (b)

Fig. 4.11 Reverse bias measurements of 1/C2 vs. V after 200 oc annealing for 30 minutes, for Round 2 samples (a) non-oxidized, and (b) oxidized prior to ZnO deposition.

80

Round 2: Non oxidized ZnO-Si After annealing at 400 degrees for

30 min 1/CJ\2 vs V

0.0016 ..--------------------,

0.0014

0.0012

N 0.001 < ~ 0.0008

"l"" 0.0006

N < 0 -"l""

0.0004

0.0002

0.002

0.0018

0.0016

0.0014

0.0012

0.001

0.0008

0.0006

0.0004

0 L-~~~~~~~~~~~--~

-1 -0.9 .Q.8 -0.7 .Q.6 .Q.5 .Q.4 .Q.3 -0.2 .Q.1 0 0.1 0.2

Voltage (V)

Fig. 4.12 (a)

Round 2: Oxidized ZnO-Si After annealing at 400 degrees

for 30 min 1/CA2 VS V

0.0002

0 -1 -0.9 -0.8 .Q.7 .Q.6 .Q.5 -0.4 -0.3 .Q.2 -0.1 0 0.1 0.2

Voltage (V)

Fig. 4.12 (b)

-+-100KHz

--10KHz _._1KHz

-+-200Hz --slope 1

-+-100KHz

-11-10KHz

--1KHz

-+-200Hz

--series5

Fig. 4.12 Reverse bias measurements of 1/C2 vs. V after 400 °C annealing for 30 minutes, for Round 2 samples (a) non-oxidized, and (b) oxidized prior to Znü deposition.

81

4.4 Conclusions

Samples for which ZnO films were sputtered at higher RF power show a higher

amount of interface states. This is probably due to the bombardment of a toms onto the Si

surface as a result of higher power during the sputtering process. It has also been shown

that annealing increases the amount of surface states in the deep to shallow region of the

junction since V bi decreases with annealing; however, for Round 2 non-oxidized samples,

after the 350 oc annealing step, Vbi starts to increase again. On the other hand, samples

which had been intentionally oxidized prior to ZnO deposition show a continued decrease

in V bi with a subsequent annealing. It appears therefore that oxidized samples show a

continued increase in interfacial states with annealing.

Samples for which ZnO films were sputtered at higher RF power also show higher

amount of deep level effects compared to samples for which low RF power was used.

Annealing appears to make uniform the interfacial state density from the deep to shallow

junction regions. A small non-linear tail is however observed in 1/C2 vs. V curves in low

reverse biasing and is not removed with annealing. This has been cited to be due to

interfacial states from other sources. It appears that oxidation, whether intentionally

performed at the interface prior to ZnO deposition or through exposure to air helps to

decrease the interface state density. Long exposure to air appears to lower the initial

interfacial state density measured much more significantly. Overall, the interface state

density increases with annealing. This result suggests that the interface states may be due

to interdiffusion of atoms, likely those of Zn into the Si surface, but possibly also due to

0 diffusion or less likely, Al.

82

Chapter 5

ELECTRON BEAM INDUCED CURRENT MEASUREMENTS

This chapter describes the principle behind the performance of Electron Bearn

Induced Current (EBIC) measurements on heterojunction solar cells. The experimental

steps tak:en for measuring the diffusion length of the n-ZnO/p-Si solar cell fabricated in

this work is described.

5.1 Princip les of the Electron Bearn Induced Current method

The performance of a solar cell depends on the diffusion length of minority

carriers (L) and surface recombination (S) which can are affected by fabrication and

manufacturing steps. The Electron Bearn Induced Current (EBIC) method, performed

with the scanning electron microscope (SEM), has often been used to study and

characterize semiconductor materials and deviees. The method has been successful in

determining carrier lifetime, bulk minority carrier diffusion length, surface recombination

velocities and defect energy levels. The technique involves the collection of carriers that

are generated by an electron bearn and then separated by the electric field in the depletion

region of the p-n junction. Other electron bearn carriers that do not enter the depletion

region are recombined. Once the carriers are collected, they are tumed into a video signal

on the SEM and reveal the location of the junction, as well as recombination centers such

as dislocations and doping inhomogeneities.

Excess carriers are removed via the recombination mechanism, at a rate

that is proportional to the carrier density. Carriers are also removed from a region by

83

mechanisms such as diffusion and drift and they are replenished by generating

mechanisms for steady state equilibrium to be maintained. Equation 5.2 summarizes these

events for an n-type semiconductor region, where G is the generation rate of electron-hole

pmrs.

5.2 Principles of diffusion length and surface recombination velocity measurements

The scanning electron microscope is a system of two cathode ray tubes (CRT),

one of which emits a bearn of electrons onto the sample. Bombardment effects are

captured by a detector. The other CRT, operating in synch with the first, detects the

voltage signal created in proportion to the effects captured by the detector. A point by

point image is then produced of the sample. When the sample is a p-n junction, the

junction's electric field, serves as the detector. When the electron bearn bombards the

sarnple, it generates electron ho le pairs, sorne of which will recombine with one another

most likely with the help oftraps. Other electron-hole pairs which are created in depletion

region, will encounter the junction's electric field, be separated and collected as current in

an extemal circuit [5.1]. Similarly, electron-hole pairs which are created in the n-p-type

bulk region diffuse across the junction and be separated as well into the extemal circuit.

The spatial dependence of EBIC profiles allow for the characterization of the junction,

detection of its diffusion length and determination of the location of recombina ti on sites.

The minority carrier diffusion equation in a p-type semiconductor is shown below

1 flnP G--\IJ --N q r

84

dflnP --=--= 0

dt (5.1)

It describes several mechanisms which are present and required for charge neutrality to

occur. Excess carriers such as those that might be provided by the electron bearn are

removed via the recombination mechanism, at a rate that is proportional to the carrier

density i.e. /':,.np/7. Carriers are also removed from a region by mechanisms such as

diffusion and drift and they are replenished by generating mechanisms, at a rate G for

steady state equilibrium to be maintained. G is related to the generation function g(x)

presented in Chapter 1 by G = g(x)/ A, where A is the illuminated area of the sample. The

Equation 5.1 's solution is of the form presented in Equation 5.2 where K is a constant and

Lp=(DN*7N) 112 is the minority carrier diffusion length. DN is the electron diffusion current

and 7N is the lifetime of an electron.

!J..n = Ke-jr.p p

(5.2)

The induced current from the electron bearn used to measure the diffusion length and the

surface recombination velocity in the EBIC method is proportional to the minority carrier

density, /':,.np.

The generated carrier profile resulting from the electron beam's penetration into

the semiconductor is shown in Fig. 5.1. The contours represent areas with the same

carrier generation rate. What is called "the mushroom" effect by Leamy [5.1] is due to the

scattering of the kilovolt electrons originating from the focused bearn during penetration.

Angular deviations in their original trajectories result in the contour profile shown in Fig.

5.1. Furthermore, it has been previously reported [5.1] that the carrier generation rate

85

decreases with increase in bearn energy while the depth of penetration increases. Reis the

maximum electron range or depth attained for a specified bearn energy.

Electron bearn

w = depletion region

Fig. 5.1 A cross-sectional view of the electron bearn penetration

The diffusion length L is a material parameter which represents the average

distance minority carriers can diffuse among majority carries before they recombine. It is

dependent upon intrinsic properties such as v, the drift velocity and therefore D, the

diffusion coefficient, but it is also dependent on trap density [5.1]. Trap density increases

due to fabrication processes and decreases the diffusion length. The EBIC method has

been extensively used to determine its diffusion length in deviees [5.1, 5.2, 5.3].

It is possible to determine the value for the diffusion length via the EBIC method

by fracturing the sample to expose the junction and then scanning the electron bearn

across it perpendicularly. It is also possible to determine the value by scanning the bearn

86

across the front surface of the heterojunction which is a non-destructive method. It is the

former case that will be dealt with in this work. It is therefore useful to know that the

EBIC peaks at the junction and decays exponentially away from it when the bearn is

scanned perpendicularly across the junction [ 5.1].

Equation 5.1 shows the relationship between lee the induced current, L the

diffusion length and x the distance between the bearn and the junction. The relationship is

valid under the following conditions: that the carrier generation source is a point source

and that the sample width is infinitely large with respect to the diffusion length [5.4, 5.5].

The first assumption is met in practice when a low energy bearn is used whereas the

second is met when the distance between the bearn and the interface is two diffusion

lengths. Equation 5.3 also holds for cases where the surface recombination velocity is

negligible.

(5.3)

where Imax is the current at the depletion edge. Plotting the natural log of the normalized

collected current Ieellmax vs. position x gives an approximate value of the reciprocal of the

diffusion length L. In non ideal cases where the above mentioned assumptions are not met,

the collected current data must be fitted with analytic expressions reported for general

conditions or in the case of Ong et al.'s work, implementing a modified binary search

algorithm to extract the values of diffusion length and surface recombination velocity.

It is worth noting that EBIC images which change with time [5.6] is an indication

of charging at the surface. This effect is expected for Si since the samples used for EBIC

87

measurements had gone through the annealing steps mentioned in previous chapters. An

oxide layer is likely to be present and act as the charger. This has been inferred to be the

case in a previous work done in this lab by Garanzotis [5.7]. It was shown for an Al/Si

Schottky barrier whose EBIC images changed after a few seconds of repeated

bombardment that when the bearn was tumed off for a few seconds, the original EBIC

profile was restored [5.7].

In addition to determining the diffusion length with the EBIC method, it is

possible to determine the surface recombination velocity. Surface recombination velocity

(cm/sec) is the product of trap density per unit are a, capture cross section per trap and the

thermal velocity [ 5.1]. These are part of the set of assumptions made on physical

boundaries and the recombination rate R that are specified to solve continuity equations

in practical situations. Recombination rate at the surface or boundary is proportional to

the surface recombination velocity.

It is commonly reported that actual lifetime for a semiconductor is difficult to

determine due to its relationship with surface recombination effects and those of bulk

recombination [5.8, 5.9]. Nevertheless, the determination of surface recombination

velocity is straightforward with EBIC performed across the fractured surface of the bulk

Si. The expression for the minority carrier flux that is absorbed at a surface recombination

velocity S with the appropriate boundary condition for diffusion to the surface is shown in

Equation 5.4, where z is the surface normal, De is the diffusion coefficient of the minority

carrier in the p-type Si and ~n is the excess electron density.

Sl1n = D 811n e 8zz=O

(5.4)

88

Since the induced current lee is proportional to 8.n, assuming a point source for

which z* ~ Re/3 where Reis the electron range [5.1], the expression shown as Equation

5.5 may be rewritten as Equation 5.6 when z* approaches zero.

alec !cc-l az

limln/cc Z---)-0

Blnlcc az

s

(5.5)

(5.6)

This result shows that with a change in bearn energy, it is possible to deduce from

the trend in induced current lee the value for the surface recombination density.

5.3 Experimental results

5.3.1 Characterization of ZnO-Si heterojunctions

In preparation of EBIC line-scanning across the heterojunction, the sample was

fractured by scribing over the front side and applying pressure with the sample lying on a

glass plate. Fracturing is performed in order to expose the junction. The fracture appeared

relatively clean. Prior to line scanning the deviee, images were grabbed of the junction.

Fig. 5.2 shows the image of one of the ZnO/Si heterojunction samples fabricated, taken

from the secondary electron detcctor of the SEM. The sample used has gone through the 4

annealing steps mentioned in the previous chapters. The image shows the thickness of the

sputtered ZnO film to be about 0.1 pm. The thickness of the ZnO sputtered layer is

89

clearly shown since on the le ft si de of the image, a section of ZnO that has pee led off the

Si is observable. No interfacial, native oxide layer is perceivable on the SEM image,

which is an expected result since documented thicknesses of native oxide growth from RF

magnetron sputtering are in the range of a few nanometers. This is beyond the SEM

resolution range.

Fig. 5.2 Image taken from the SEM secondary electron detector of the ZnO/Si heterojunction fabricated in this work.

5.3.2 Diffusion length measurements

The electron bearn is scanned across the fractured heterojunction and travels

parallel to the barrier as shown in Fig. 5.3. The induced current is preamplified by GW

Electronics preamplifier model 1 03B, and th en measured by GW Electronics Specimen

90

Current Amplifier model 1 03B. The plotting of the induced current was performed

manually by tabulating the induced current value displayed on the LED panel meter and

then plotting vs. time. A snapshot of the section being scanned which includes the

heterojunction interface is displayed on the CRT and saved after measurements are taken.

The images of two different samples that were fabricated under the same conditions and

previously annealed together are shown in Fig. 5.4 (a) and 5.5 (a). The junctions are

magnified at x500 (lcm=lum) and scanned with a 29 KeV bearn energy. The

magnification and actuallength (22.5 cm) ofthe displayed section on the CRT are used to

convert the total scanned time into a position value. A plot of induced current vs. position

is therefore achieved and shown in Fig. 5.4 (b) and Fig. 5.5 (b). Diffusion lengths are

determined according to the slope ofln(lccllmax) asper Equation 5.3. These are determined

from Fig. 5.4 (c) and 5.5 (c). The results are consistent between the two samples.

electron-bearn

w = depletion width Electrode

p-Si

n-ZnO

~-~CRT

LED panel meter

Fig. 5.3 A schematic diagram of the EBIC method performed on the fabricated ZnO/Si heterojunction with the SEM.

91

-c ê< ~­uO -"Cl 111-·- < 't-0 =~ C.X E <(

Fig. 5.4 (a)

lnduced current vs position Round 2 - non oxidized sample 1 -post 15 annealing - n-ZnO/p-Si 14 13 12 11 10 9 8

~ l--1cm=10uml 5 4 3 2 1 o+-~-+~~.-.--.-*~~~~

-1 0 20 40 60 80 100 120 140 160 180 200

Position (um) Fig. 5.4 (b)

92

10

->< ca E 'ù ,g -z ...1

0

LN (lcc/lmax) vs position Round 2 - non oxidized sample 1 - post

annealing - n-ZnO/p-Si

-+-1cm = 10 um -•- Slope 1

20 40 60 80 100 120 140 160 Position (um)

Fig. 5.4 (c)

Fig. 5.4 A CRT snapshot of the first scanned junction (a) for which the plot of the induced current taken from the LED panel meter vs. position is shown in (b ), where zero is the leftmost point of the scanned section appearing on the CRT snapshot. The ln(Icc/Imax) relationship is displayed in (c) where x= 0 is the location ofthe EBIC peak.

Fig. 5.5 (a)

93

-x co E 0 0

::::.. z ...J

10

24 22 20 18

lnduced current vs position Round 2 - non oxidized sample 2 - post

annealing - n-ZnO/p-Si E-beam energy of 29 KeV

l--1cm=10uml

20 40 60 80 100 120 140 160 180 200 Position (um)

Fig. 5.5 (b)

LN (lcc/lmax) vs position Round 2- non oxidized sample 2- post

annealing - n-ZnO/p-Si E-beam energy of 29 KeV

-+-1cm= 10um _,._ Slope 1

0 10 20 30 40 50 60 70 80 90 100 110

Position (um)

Fig. 5.5 (c)

Fig. 5.5 A CRT snapshot of the second scanned junction (a) for which the plot of the induced current taken from the LED panel meter vs. position is shown in (b ), where x =0 is the leftmost point of the scanned section appearing on the CRT snapshot. The ln(Icc/Imax) relationship is displayed in (c) where x'= 0 is the location of the EBIC peak.

94

According to the results of the two scanned samples, the range in values for the diffusion

length is between 40 and 80 /lill. In order to further verify results, a higher magnification

was used to scan across the junction of the second sample to verify whether the slope of

the semi-logarithmic plot was the sarne as that at a lower magnification. Fig. 5.6 shows

that this is the case. Since the diffusion length LN= (DN*7N)112, the approximate minority

carrier lifetime is 3.07 11sec.

10

->< ca E 0 u -z .J

0

LN (lcc/lmax) vs position Round 2- non oxidized sample 2- post

annealing - n-ZnO/p-Si

2 3 4 5 6 7 8 9 10 Position (um)

Fig. 5.6 A plot of ln(Icc/Imax) vs. position for the second scanned sample at a magnification ofxlOOO, where x'= 0 is the location ofthe EBIC peak.

lt has been mentioned previously that Equation 5.3 is most valid for low bearn

energy. The previous samples have been scanned at a relatively high energy of 29 KeV.

Fig. 5.7 shows the result for a lower energy of20 KeV. The semi-logarithmic plot shown

in Fig. 5.7 (b) shows a more linear slope than those previously displayed in Fig. 5.5 and

5.6 showing that there is less scattering occurring for a lower bearn energy. The diffusion

length is still within the sarne range described from the previous scans i.e. 40-80 /lill.

95

-[: ~<( ::::J-uo ...... "0' Q)-·- < ~0 - ...... a. x E <(

8

6

4

2

0

-2

lnduced current vs position Round 2 - non oxidized sample 2 - post

annealing - n-ZnO/p-Si E-beam energy of 20 KeV

!--1cm = 10 umJ

20 40 60 80 1 00 120 140 160 180 200

Position (um)

Fig. 5.7 (a)

LN (lcc/lmax) vs position Round 2- non oxidized sample 2- post

annealing - n-ZnO/p-Si E-beam energy of 20 KeV

10

î ·~. u 1~~~--~~--~~--~~~~ u 1 0 20 30 40 50 60 70 80 90 100

2 ...J

0.1

Position (um)

Fig. 5.7 (b)

Fig. 5.7 A plot of the induced current for a bearn energy of20 KeY, taken from the LED panel meter vs. position (a), where x = 0 is the leftmost point of the scanned section appearing on the CRT snapshot. The ln(Iccllmax) relationship is displayed in (b) where x'= 0 is the location of the EBIC peak.

96

5.3.3 Surface recombination velocity measurements

The surface recombination velocity for the p-type Si has been determined

according to Equation 5.6. Scanning was performed at different bearn energies. Since the

value for the diffusion length has been determined to be ~40 f.J-m, the induced current was

measured at a point located at 30 f.J,m from the interface. The magnification was kept at

x500 (lem = 10 f.J-ID) and re-focusing was performed prior to every scan. The surface

recombination velocity which was determined from the value of ln(Icc) at 0 Ke V in bearn

energy gathered from Fig. 5.8 (c) shows that the measured velocity for the bulk Si which

have been annealed previously is ~520 cm/sec. Since the resistivity of the Si wafers used

were ~0.3-0.5 0-cm, the dopant concentration was estimated to be ~lx1017 cm-3 [5.10],

therefore an electron mobility of ~801 cm2N-sec [5.10] and hence a diffusion coefficient

of ~20.826 cm2 1 sec. Values measured by spectral conductance for wafers with p~0.3 n­

cm were in the range of 105 cm/sec [5.10]. Monocrystalline p-type Si wafers doped to

2x1015 cm-3 are reported to have surface recombination velocity of 5020 cm/sec [5.11].

Palais et al. have also passivated the wafers with an iodine solution which subsequently

lowered the value to 80 cm/sec. The effect of passivating oxides has also been reported to

lower surface recombination velocities in Si [5.12] in sorne cases, with additional

techniques [5.13]. For diffusion lengths above 100 f.J,m, Baek et al. have calculated values

of 500-50 000 cm/sec for Si wafers doped with boron, arsenic, phosphorous or antimony.

The value of ~520 cm/sec found in this work when the induced current was taken at a

point located at 30 f.J-m from the interface has also been repeated for the induced current

taken at 20 f.J-m away from the interface. The results for the latter are shown in Fig. 5.9.

97

Whether this value is an accurate estimation of the surface recombination velocity

appears plausible considering the above, however it has not been full y verified.

4.5

4

...... 3.5 c ~- 3 .... 0 r5 s. 2.5

~ ~ 2 u-::J >< 1.5 "C c

0.5

Fig. 5.8 (a)

lnduced current vs bearn energy

-+- Measured data, 30 um away from the interface

0+-.-.-~--~--------~~~~--~~

0 2 4 6 8 1 0 12 14 16 18 20 22 24 26 28 30

Bearn energy (KeV)

Fig. 5.8 (b)

98

-5

- -10 0 .E -15 -z ...J -20

LN(Icc) vs beam energy

2 4 6 8 10 12 14 16 18 20 22 24 26 28

-+- Measured data, 30 um away from the interface

--Siope 1

-25 ---' -------~v~~~

-30

Bearn energy (KeV) Fig. 5.8 (c)

Fig. 5.8 A CRT snapshot of the scannedjunction (a) for which the plot of the induced current tak:en from the LED panel meter vs. electron bearn energy tak:en at a point 30 f1m away from the interface is shown in (b). The semi-logarithmic plot ofln(Icc) vs. bearn energy is shown in (c).

... c

14

12

cv 10 .__ ... 0 :::l- 8 U..,!...

"C < 6 cvo u-:::l >C 4

"C c

2

lnduced current vs bearn energy

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Bearn energy (KeV)

Fig. 5.9 (a)

99

-+- Measured data, 20 um away from the interface

LN(Icc) vs bearn energy

-5

........ -10 0 .f! -15 -z ...J -20

-25

-30

2 4 6 8 1 0 12 14 16 18 20 22 24 26 28

Bearn energy (KeV)

Fig. 5.9 (b)

--+-- Measured data, 20 um away from the interface

·--Siope 1

Fig. 5.9 A plot of the induced current taken from the LED panel meter is shown vs. electron bearn energy taken at a point located 20 flm away from the interface (a). The semi-logarithmic plot ofln(lcc) vs. bearn energy is shown in (b ).

5.5 Conclusions

The EBIC method has been used to measure the diffusion length and the surface

recombination velocity. The diffusion length measurements were conducted at high bearn

energy (29 KeV) and at a lower one of20 KeV. A diffusion length in the range of 40-80

flm was found. The minority carrier lifetime is therefore calculated to be around 3 flSec.

The surface recombination velocity has been measured in two instances, at 30 and 20 flm

away from the interface. The value is in the range of 500 cm/sec and appears to be low

compared to other reports for Si wafers; however it may be a reasonable range

considering that the sarnples that were scanned were previously annealed at 200, 300, 350

and 400 oc for 30 minutes each step.

100

Chapter 6

CONCLUSIONS

The main objective of this work was to fabricate n-ZnO/p-Si solar cells with the

RF sputtering method and characterize them by performing resistivity, current-voltage,

capacitance-voltage, capacitance-frequency measurements with vacuum annealing. The

diffusion length and surface recombination velocity were also measured with the Electron

Bearn Induced Current (EBIC) method.

Sheet resistance measurements were performed with a four-point-probe on the

glass samples. The values for samples evacuated in the vacuum chamber for 14 hours

prior to deposition increased from 7.9 to 10.17 and 11.5 Q/o for 40 W, 120 and 160 Win

RF power respectively. In contrast, those evacuated for 2 hours respect started with a

higher value of22.5 0/o, and decreased down to 7.6 and 5.8 Q/o for 40 W, 120 and 160

W in RF power respectively.

Since resistivity measurements have been linked to stoichiometric content of Zn/0,

it is likely that the amount of time that evacuation is performed has influence on

stoichiometry and film quality. Water and oxygen molecules diffuse into the surface of

the substrate when exposed to air and may need time to evacuate. A decrease in resistivity

is associated with oxygen outgassing during vacuum annealing and would indicate that

the film evacuated for 2 hours had a higher 0 content. The difference in resistivity trend

is a possible indication that the amount of time spent on evacuation prior to ZnO

deposition affects the growth of native oxide. It would be interesting to examine native

oxide thickness via Transmission Electron Microscopy (TEM) and to examine the change

101

in quality with annealing via Hall mobility and resistivity measurements with different

evacuation conditions prior to sputtering.

Samples which were oxidized for 15 minutes prior to ZnO sputtering showed no

efficiency improvement with annealing. For samples deposited at higher RF power and

which were not oxidized had improved up until an annealing temperature of 350 oc

though interfacial state densities and leakage current had increased up to this point.

Samples which had been exposed to air for a few weeks showed no efficiency

improvement with vacuum annealing, but had the highest magnitude in Ise, V0c, FF and TJ.

This was perhaps an indication that the samples surfaces oxide has formed and outgassing

in vacuum was unable to improve stoichiometry and that a passivation layer had grown at

the surface prior to ZnO deposition. These air-exposed samples had the lowest interfacial

state density ..

Annealing has shown to increase the leakage current observable on the I-V curves

for the Round 2 ZnO/Si samples, for which ZnO was deposited at 120 W, up to an

annealing temperature of 350 °C. The subsequent annealing step performed at 400 oc

decreases leakage. Dark leakage current trends appearing on the I-V curves are a clear

indication of the increases and decrease of interfacial states. Leakage has also been

previously associated with increased oxygen deficiency. It is likely that interdiffusion of

Zn is occurring across the interface with annealing. Interfacial state density increase has

been calculated based on the assumption that the shallow junction region is characterized

by the same density as the deep junction region. The increase indicates that in the vicinity

of the Si/Si02 interface, at the deep regions of the junction, is getting worse in terms of

deep levels/interfacial state density with annealing. Furthermore, annealing serves to

102

make uniform the interfacial state density from the deep to shallow junction regions since

1/C2 vs. V curves become linear. What is interesting is that although evidence shows an

increase in interfacial states in the deep regions of the junction with vacuum annealing,

the measured conversion efficiency, fill factor and short circuit current of the solar cell

increases with annealing up until the 350 oc step.

Round 2 oxidized samples on the other hand show a continued increase in interfacial

states with annealing. 11 and Ise have followed this trend and show a continued decrease

with annealing; however FF has increased up until the 350 oc annealing step, despite the

increasing trend in interfacial state.

It appears that the samples which had been exposed to air for a long period of time

prior to ZnO deposition have the highest values in Ise, V oc, FF and 11 among all samples

fabricated, but also, their values are highest prior to annealing, whether an oxide layer had

been intentionally grown or not. Trends in interfacial state density are very similar

between the two types of samples, that is, deep region interfacial state density increase

with annealing.

Samples for which ZnO films were sputtered at higher RF power show a higher

amount of interface states, likely due to the bombardment of atoms onto the surface

during sputtering. A small non-linear tail, probably due to interfacial states, is however

observed in 1/C2 vs. V curves in low reverse biasing and is not removed with annealing,

perhaps an indication that the very shallow interfacial states are not affected by annealing.

It appears that oxidation, whether intentionally performed at the interface prior to ZnO

deposition or through exposure to air helps to decrease the deep region interface state

density. Long exposure to air appears to lower the initial interfacial state density much

103

more significantly than intentionally growing oxide at the interface at 450 oc for 15

minutes. For these air-exposed samples, the calculated interface state density has been

shown to increase with vacuum annealing, while conversion efficiency decreases with it.

The trend is different from that seen for Round 2 non-oxidized samples whose calculated

conversion efficiency increases with interfacial state density up to an annealing of 350 °C.

The EBIC method has been used to measure the diffusion length and the surface

recombination velocity of the Si bulk. A diffusion length in the range of 40-80 pm was

measured, with a minority carrier lifetime calculated to be around 3 JlSec. It has been

shown that the surface recombination velocity may be measured either at 30 and 20 Jlm

away from the interface. The SRV on the fractured surface of the bulk Si used to fabricate

the solar cells and determined by the EBIC method was calculated to be around 500

cm/sec. It would be worthwhile to measure the surface recombination velocity at the

interface of the ZnO/Si heterojunction and compare the change with annealing to

parameters such as V oc, Ise, FF and 'Y/.

104

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