Chapter 3 UNDOPED ZnO THIN FILMS -...

Chapter 3

UNDOPED ZnO THIN FILMS

The contents of this chapter is published in

1. Nanda Shakti, P.S. Gupta, Structural and Optical properties of sol-gel

prepared ZnO thin film, Applied Physics Research 2(1), ( 2010) 19.

2. Nanda Shakti, P.S.Gupta, Structural and photoluminescence of ZnO thin

film and nanowire, Optoelectronic and Advanced Material - Rapid

Communication, 4(5), (2010) 662.

3. Nanda Shakti, P.S. Gupta, Study of Carrier concentration and mobility with

multilayer ZnO film, Processing and Fabrication of Advanced Materials-

XVII, I.I.T. Delhi, Vol II, (2008) 346.

CHAPTER.3: UNDOPED ZnO THIN FILMS

59

3. Introduction

Zinc oxide is an inexpensive n-type semiconductor having direct band gap of 3.3 eV

which crystallizes in hexagonal wurtzite structure (c = 5.025 Å and a = 3.249 Å) [1].

Due to large exciton binding energy of 60 meV , they have potential applications in

Optoelectronic devices such as in solar cells [2],Optical wave guide [3], Light emitting

diodes (LED) [4]. Zinc oxide thin films are applied in Thin Film Transistors (TFT) [5]

and have been recognized as spintronic material [6].Various gas, chemical and

biological sensors were based on ZnO thin film [7]. Thin films of Zinc oxide can be

prepared by various techniques; among them are Sputtering [8], Chemical Vapor

Deposition (CVD) [9], Laser ablation [10], Sol-gel [11], Spray pyrolysis [12]. M.

Inoguchi et al. [13] have studied structural & optical properties of nanocrystalline ZnO

thin films derived from clear emulsion of monodispersed ZnO nanocrystals.

Properties of ZnO thin films show dependence on the synthesis technique used. Apart

from doping, to increase the functionality of ZnO thin film, the effect of preparation

conditions on the properties have to be considered for its effective technological

applications. Relatively few works [14-15] have been done in this direction for ZnO

film prepared by sol-gel process. In this chapter we have studied the synthesis and

characterization of undoped ZnO thin films by sol-gel spin coating process [16]. Zinc

acetate dihydrate was used as the precursor material. The Sol-gel process has the

advantages of controllability of compositions, simplicity in processing and is cost

effective. We have studied the effect of annealing the ZnO thin films at three different

temperatures (400oC, 500oC and 6000C) on its structural, optical and electrical

properties. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) with

Eenergy dispersive scattering (EDS) attachment were used for structural and

morphological characterization respectively. UV-VIS-NIR spectrometry,

Photoluminescence and spectroscopic ellipsometry were used for optical

characterization. The electrical characterization of the films was done using four-probe

method.


60

The second part of the chapter deals with the investigation of quantum confinement

effect in sol-gel prepared ZnO thin films. Till now an exclusive investigation on

quantum confinement effect in sol-gel prepared ZnO thin films has not been reported.

We have prepared ZnO thin film by sol-gel method with annealing the samples at five

different temperatures viz. 250ºC, 300ºC, 350ºC, 400ºC and 450ºC.The shift of optical

band gap with annealing temperature have been observed.

3.1 Synthesis and characterization of ZnO thin film by sol-gel

spin coating method

3.1.1 Experimental method

ZnO thin films were prepared on fused quartz substrates (2.5×2.5 Cm2) by sol-gel spin

coating method. The quartz substrates were cleaned with soap solution followed by

ultrasonication in acetone for 5 min. Then it was degreased in ethanol for final cleaning.

For sol preparation 10% solution of zinc acetate dihydrate Zn(CH3COO)2.2H2O was

prepared in boiling iso-propanol. The turbid solution was cleared by adding 10 drops of

diethanolamine by a 5ml dropper. The clear sol was further boiled for 30 min. The sol

was then allowed to cool to room temperature. For film preparation, the quartz substrate

was spin coated by a spin coater (SCU 2005 Apex Inst.Co.) while spinning rate was

kept at 3000 rpm. The wet films were dried at 100oC for 10 min and subsequently

annealed for 1 hour. The process was repeated to obtain the workable thickness of the

film. Multilayer films post annealed at 400oC, 500oC and 600oC for 1hour were

prepared to study the effect of annealing.

The structural properties of the prepared films were studied by x-ray diffraction

measurements (Panalytical Xpert pro, with CuKα radiation (λ= 1.54059 Å)). Scanning

electron microscope (Jeol; JSM-6390LV) was used to record Energy dispersive

scattering (EDS) and surface morphology.


61

UV-VIS-NIR Spectrophotometer (HR 4000 Ocean Optics) was used to record the

transmission spectrum of the films in the wavelength range from 250 nm to 600 nm.

Ellipsometer (Nano View; SE MG-1000UV) was used to measure refractive index n,

extinction coefficient k and thickness of the films. Photoluminescence of the films was

measured by Spectroflurophotometer (Hitachi).

3.1.2 Structural Properties

The crystal structure of ZnO films was investigated through X-ray diffraction (XRD).

The X-ray diffraction spectrum of ZnO film annealed at 400ºC, 500ºC and 600ºC with

prominent reflection planes is shown in figure 3.1.

The peaks in the XRD spectrum correspond to those of the bulk ZnO patterns from the

JCPDS data (Powder Diffraction File, Card no: 36-1451) [17], having hexagonal

wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å.The presence of

prominent peaks shows that the film is polycrystalline in nature. The lattice constants

‘a’ and ‘c’ of the Wurtzite structure of ZnO can be calculated using the relations (3.1) &

(3.2) given below [18].

a = θ

λ

sin3

1 (3.1)

c = θ

λ

sin (3.2)

For (002) plane calculated values are a= 3.15 and c= 5.29 which agrees with the JCPDS

data.


62

20 30 40 50 60 70 80

(101)

(002)

(100)

(201)

(112)

(103)

(110)

(102)

600oC

500oC

400oC

Inte

nsity

2θ (degrees)

Fig. 3.1 XRD spectrum of ZnO thin film annealed at 400ºC, 500ºC and 600ºC.


63

Since the total breadth of a XRD peak can be separated into contributions from the

finite crystallite size and the presence of strain in the crystal. The Williamson-Hall plots

[19] for the Gaussian peak shape have been used to determine crystallite size and strain

in the ZnO thin films.

Bt(2θ).cosθ = 0.9λ/D + 4ε.sinθ (3.3)

where λ = 1.54059Å, is the wavelength of X-ray used, D is the crystallite size, ε is the

microstrain and Bt(2θ) is the full width at half maximum, at diffraction angle 2θ in

radians. A straight line fit in the plot of Bt(2θ).cosθ and sinθ yields the values of

crystallite size as well as microstrain. This is plotted for the samples as shown in figure

3.2.

0.25 0.30 0.35 0.40 0.45 0.50

0.0040

0.0045

0.0050

0.0055

0.0060

0.0065

0.0070

400oC

Bt(2θθ θθ).

Co

sθθ θθ

Sinθθθθ

y= 0.00323 + 0.00496 x

0.25 0.30 0.35 0.40 0.45 0.50

0.0035

0.0040

0.0045

0.0050

0.0055

y= 0.0017 + 0.00803 x

Bt(2θθ θθ).

Co

sθθ θθ

Sinθθθθ

500oC

0.25 0.30 0.35 0.40 0.45 0.50

0.0034

0.0036

0.0038

0.0040

0.0042

0.0044

0.0046

0.0048

0.0050

0.0052

y = 0.00174 + 0.00677 x

600oC

Sinθθθθ

Bt(2θθ θθ).

Co

sθθ θθ

Fig. 3.2 Williamson-Hall plots for ZnO film at different annealing temperatures.


64

Table 3.1(a) and 3.1(b) gives the values of crystallite size and microstrain respectively,

as obtained from the Williamson-Hall plots (figure 3.2) for ZnO films annealed at

different temperatures.

Table 3.1: Crystallite size (3.1a) and microstrain values (3.1b) for ZnO films annealed

at 400°C, 500°C and 600°C.

Table 3.1a:

Annealing temperature (OC) Crystallite size (nm) 400 42.9 500 81.6 600 79.6

Table 3.1b:

Annealing temperature (OC) microstrain 400 0.0012 500 0.0020 600 0.0017

From the table 3.1 we observe that for film annealed at 500°C, the crystallite size is

maximum and on further increase of annealing temperature to 600°C the crystallite size

decreases. This may be due to the fact that on heat treatment, the flow of fluid near the

substrate interface coagulates, to reorganize the grain size to increase. The coagulation

process discontinues on further increasing the heat treatment temperature with threshold

around 500°C [20]. Further the microstrain value corresponding to the annealing

temperature of 500 °C is also maximum indicating the increased tensile strain in the

crystallite of the film sample.


65

3.1.3 Surface Morphological Characterization

The micro structural analysis of the ZnO film annealed at 400ºC was carried out using

Scanning electron microscope (SEM) with energy dispersive scattering (EDS)

attachment. Figure 3.3 shows the surface morphology of the ZnO film annealed at

400ºC.

Fig. 3.3 SEM micrograph of ZnO film annealed at 400ºC.


66

Fig. 3.4 EDS of ZnO film annealed at 400ºC

The film surface is granular in nature with grain size of the order of nm. The EDS

analysis of the film (Fig 3.4) shows the presence of Zinc, Oxygen, Silicon and Gold.

The Gold in the ZnO film is from the Gold coating of the sample ZnO film for SEM

analysis.


67

3.1.4 Optical Properties

Figure 3.5 shows the transmission spectra of the ZnO films deposited at different

annealing temperatures.

250 300 350 400 450 500

0

20

40

60

80

100

Tra

ns

mit

tan

ce

%

Wavelength (nm)

400OC

500OC

600OC

Fig. 3.5 Transmission spectra of ZnO thin film annealed at 400ºC, 500ºC and 600ºC.

The spectra show interference fringes which has its origin in the interference of light

reflected between air-film and film-substrate interface. The appearance of interference

fringes indicates smooth reflecting surface of the film and low scattering loss at the

surface. The films annealed at 400ºC and 600ºC exhibit good transparency in visible

region (>90%).From the spectrometric Ellipsometry, the thicknesses of the ZnO films

annealed at 400ºC, 500ºC and 600ºC are measured to be 120.4 nm, 163.3 nm and 64.7

nm respectively. The variation of refractive index n and extinction coefficient k with


68

photon energy for ZnO films annealed at different temperatures is shown in figures 3.6

and 3.7.

1.5 2.0 2.5 3.0 3.5

1.85

1.90

1.95

2.00

2.05

2.10

2.15

2.20

2.25

2.30

600oC

500oC

n

hν (eV)

400oC

Fig. 3.6 Plot of refractive index n as a function of Photon energy for ZnO

film at different annealing temperature.


69

1.5 2.0 2.5 3.0 3.5

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

500oC

600oC

k

hν (eV)

4000C

Fig. 3.7 Plot of extinction coefficient k as a function of Photon energy for ZnO

films at different annealing temperature.


70

In figure 3.6 the refractive index increases with increasing photon energy, with peak at

about 3.25eV. This can be attributed to the band gap of ZnO ~ 3.3eV.It may be noted

that the refractive index n is almost unaffected by the variation in the annealing

temperature upto the peak, that is, 3.25 eV, after which it shows a small dependence on

the annealing temperature. Figure 3.7 shows k to build up only after 3 eV and attains a

peak at 3.4 eV. It also shows a dependence on annealing temperature above 3 eV.

The absorption coefficient α and the extinction coefficient k are related by the formula [21]

k = π

αλ

4 (3.4)

The Optical energy gap Eg and absorption coefficient α are related by the equation

α = ( )βυυ

gEhh

k−

(3.5)

where k = a constant

h = Planck’s constant

hν = The incident photon energy and β is a number which characterizes the

nature of electronic transition between valance band and conduction band [22]. For

direct allowed transitions β =1/2 and it is known that ZnO is a direct band gap

semiconductor. Therefore the formula used is

α = ( ) 2/1

gEhh

k−

υ

υ

which gives

( ) ( )gEhCh −= ννα

2 (3.6)


71

where C is a constant. The variation of (hνα)2 vs. hν as obtained using equation (3.6) is

shown in figure 3.8 for ZnO films annealed at different temperatures.

Fig. 3.8 Plot of (αhν)2 vs. Photon energy hν for ZnO films at different annealing

temperatures.

1.5 2.0 2.5 3.0 3.5

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

400oC

500oC

(αhν)

2 n

m-2(e

v)2

hν (eV)

600oC


72

The energy gap Eg of the samples was evaluated from the intercept of the linear portion of

the each curve for different annealing temperature with the hν in X-axis. Table 3.2 gives

values of Eg of the films deposited at different annealing temperatures.

Table 3.2: Band gap Eg for ZnO films at different annealing temperature as estimated

from Fig.3.8

The value of Band gap as calculated above agrees nearly with band gap of bulk ZnO (3.37 eV).

It is assumed that the absorption coefficient α near the band edge shows an exponential

dependence on photon energy for many materials. This dependence is given by [23]

α = αoexp

uE

hυ (3.7)

where αo is a constant and Eu is Urbach energy which is the width of the tails of the

localized state associated with the amorphous state in the forbidden band. The plot of

lnα vs. photon energy hν plots for ZnO thin films annealed at 400°C, 500°C and 600°C

is shown in figure 3.9.

Annealing

temperature (ºC)

Band gap Eg (eV)

400 3.216

500 3.212

600 3.216


73

1.5 2.0 2.5 3.0 3.5

-12

-11

-10

-9

-8

-7

-6

-5

-4 600oC

500oC

ln α

hv (eV)

400oC

Fig. 3.9 Urbach plot for ZnO film annealed at 400°C, 500°C and 600°C.


74

From the plot the value of Urbach energy at the band edge ~3.21 eV for films annealed

at different temperatures is shown in table 3.3.

Table 3.3: Urbach energy for ZnO thin films annealed at different temperatures.

From the table, the increase in Urbach energy between 400°C and 500°C may be

attributed to the increase thermal induced structural disorder of the film within this

temperature range [24].

The room temperature Photoluminescence (PL) of ZnO film annealed at different

temperatures is shown in figure 3.10.

Annealing temperature (°C) Urbach energy Eu (meV)

400 96.6

500 101.0

600 101.0


75

350 400 450 500 550 600

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

500oC

400oC

Inte

nsity

Wavelength (nm)

600oC

Fig. 3.10 Room temperature Photoluminescence spectrum of Zn0 films

annealed at different temperatures.


76

At the excitation wavelength of 315 nm the Photoluminescence spectrum consist of a

prominent peak at about 380 nm responsible for UV emission and suppressed peak at

about 500 nm responsible for green emission. The reason of small intensity at 500 nm

may lie in the fact that with increasing annealing temperature, both the amount of grown

ZnO and the specific surface area of the grains decreases, which jointly weakens the

green emission. The band ~ 380 nm correspond to the band edge emission due to free

exciton while at ~ 500 nm attributed to the presence of defects, non-stoichiometry and

crystal imperfection [14]. From the spectra we note UV emission depends on the

annealing temperature. This type of variation can be attributed to the polycrystalline

nature of ZnO film.

3.1.5 Electrical properties

The electrical conductivity of the samples were measured using four-probe method.

Figure 3.11 shows the plot of electrical conductivity of ZnO thin film as a function of

annealing temperature.

The decrease in conductivity of the samples is attributed to the reason that with increase

in annealing temperature the number of imperfections in the atomic lattice structure

increases which hampers electron movement and also due to scattering from grain

boundaries.


77

Fig. 3.11 Plot of electrical conductivity vs Annealing temperature of ZnO films.


78

3.2 Investigation of Quantum Confinement effect in sol-gel

prepared ZnO thin films

The central theme of nanoscience is, how scale affects the properties of materials.

Quantum confinement effect is one of them. It is the effect of size on energy band gap

of the material. The electronic energy levels and density of states determine the optical

and electronic properties of materials. In nanostructured materials the energy levels and

density of states changes with change in size, resulting in dramatic changes in material

properties. The energy level spacing increases with decreasing dimension and this is

known as Quantum size confinement effect. It can be explained using particle- in- a- box

problem in quantum mechanics. We have prepared ZnO films annealed at 250°C,

300°C, 350°C, 400°C and 450°C by sol-gel spin coating method and investigated the

effect of crystallite size on the band gap of the prepared ZnO films.

3.2.1 Structural properties

The structural properties of the films prepared by varying annealing temperatures were

studied by X-ray diffraction. Figure 3.12 shows the X-ray diffraction of the ZnO films

annealed at 250°C, 300°C, 350°C, 400°C and 450°C. From the diffractogram it is seen

that the intensity of the characteristic peaks of ZnO increases as annealing temperature

increases, showing increasing crystallinity.


79

Fig. 3.12 X-ray diffraction of ZnO films annealed at five different temperatures.


80

The crystallite size of the ZnO films annealed at different temperatures was also

calculated by using Scherrer’s formula [25].

D = θβ

λ

cos

9.0 (3.8)

Where D is the crystallite size, wavelength of the X-ray used is λ =1.54059Å, β is the

broadening of diffraction line measured at the half of its maximum intensity in radians

and θ is the angle of diffraction.

Table 3.4 gives the average crystallite size of the films with their corresponding

annealing temperature.

Table 3.4: Average crystallite size of ZnO films calculated using Scherrer’s formula .

Annealing Temperature (°C) Average crystallite Size (nm)

250 6.33

300 3.77

350 7.92

400 23.49

450 29.49

From table 3.5 we observe that the crystallite size increases with annealing temperature

of ZnO films, except at 300°C where it decreased from 6.33 nm to 3.77 nm, on heat

treatment the flow of fluid near the substrate interface coagulates, to reorganize and

increase of grain size. The coagulation process discontinues on further increasing the

heat-treatment temperature with a threshold around 300°C. More over the crystallite

size <10 nm for annealing temperature upto 350°C, indicating nanocrystals in

embedded in the amorphous phase.


81

3.2.2 Optical properties

Figure 3.13(a) shows the transmittance of ZnO film prepared on glass substrate,

annealed at five different temperatures. The films show interference fringes in the

transmission spectrum due to the interference of light reflected from air-film and film-

substrate interface. This also indicates that the film surface is smooth.

200 400 600 800 1000

0

20

40

60

80

100

Tra

nsm

itta

nce

%

wavelength (nm)

250oC

300oC

350oC

400oC

450oC

Fig. 3.13(a) Transmission spectrum of ZnO films annealed at five different

temperatures.


82

Figure 3.13(b) shows the absorbance of ZnO films prepared on glass substrate, annealed

at five different temperatures. From the figure we observe that the edge of the

absorbance peak of ZnO films does not coincide at the same point for different

annealing temperatures. There is a shifting of edges in the absorbance spectrum of the

films.

350 420 490

0.0

0.5

1.0

1.5

Ab

so

rba

nce

Wavelength (nm)

250oC

300oC

350oC

400oC

450oC

Fig. 3.13(b) Absorbance of ZnO film annealed at five different temperatures.


83

The band gap of the ZnO films annealed at five different temperatures is estimated by

the variation of (hνα)2 vs. hν as obtained using equation (3.6) is shown in figure 3.14.

3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1

0.0000.000

0.025

0.050

0.0750.075

(αhν)

2 (

nm

-2e

V2)

Photon energy (eV)

250oC

300oC

350oC

400oC

450oC

Fig. 3.14 Plot of (αhν)2 vs. Photon energy hν for ZnO films annealed at five

different temperatures.

Table 3.5 displays the estimated values of band gap of ZnO films corresponding to

their annealing temperatures. We observe that as the annealing temperature increases

the band gap of ZnO film decreases. Indicating the size effect that comes into play.


84

Table 3.5: Estimated band gap of ZnO films annealed at five different temperatures.

Annealing temperature (°C) Band gap (eV)

250 3.83

300 3.80

350 3.78

400 3.77

450 3.30

The Photoluminescence (PL) of ZnO films annealed at five different temperatures is

shown in figure 3.15.

375 400 425 450 475 500 525

PL In

tensity (

a.u

.)

Wavelength (nm)

250oC

300oC

350oC

400oC

450oC

Fig. 3.15 Photoluminescence of ZnO films annealed at five different temperatures.


85

Figure 3.15 displays the shift in the emission peak of of ZnO film due to band edge

emission towards the longer wavelength side with increase in annealing temperature,

Conforming the narrowing of band gap.

After Gaussian fitting of the PL peaks the band edge emission wavelength of the films

were found and tabulated in table 3.6.

Table 3.6:Band edge emission peaks of ZnO films annealed at five different

temperatures.

Annealing Temperature (°C) Band edge emission peak (eV)

250 3.2219

300 3.2197

350 3.2024

400 3.1831

450 3.1309

From table 3.6, we observe that as annealing temperature increases the band edge

emission energy decreases, clearly indicating the narrowing of band gap.

The emission of the nanocrystallite structured ZnO films will have the same quantum

size effect as the quantum dot and can be described by the following equation,

E��,�� =E��,�� +��ℏ�� × � �

��∗+ �

�!∗" − 0.248E��∗ (3.9)

With The bulk band gap E(gap,bulk) = 3.2 eV


86

The bulk exciton binding energy ERy* = 60 meV

The electron and hole effective masses are taken as me*=0.24m0 and mh

*=2.31m0

h is Plank’s constant and R is radius of ZnO nanocrystal [26].

Figure 3.16 shows the plot of nanocrystal band gap E(gap,nanocrystal) versus the nanocrystal

radius R. The solid curve is the theoretical fit of equation (8) while the symbols are the

grain sizes estimated from XRD data and their corresponding band gaps measured from

Photoluminescence spectrum.

Fig. 3.16 Plot of E(gap,nanocrystal) versus the nanocrystal radius R for ZnO films annealed

at different temperatures.


87

From figure 3.16 it is observed that the band gaps measured from PL follow the

theoretical curve especially between crystallite size 5nm to 25 nm. It is also observed

that the band gap variation is not much as the crystallite size changes, corresponding to

different annealing temperature. Thus the above curve indicates that the PL emission is

due to the nanocrystals by quantum confinement effect. The decrease in crystallite size

indeed exhibits quantum confinement effect.

3.2.3 Surface morphological properties

The surface morphology of the ZnO films annealed at five different temperatures are

shown in figures 3.17 to 3.21. The surface of all the films are granular in nature. The

grain size of the films vary between 50-100 nm. This value is much larger than the

crystallite size obtained from XRD measurement. This is due to the fact that these

grains are itself made from still smaller grains which is detected by FESEM. The

surface porosity of the films is also increased as the annealing temperature is increased.

CHAPTER.3: UNDOPED

Fig. 3.17 F

: UNDOPED ZnO THIN FILMS

88

ig. 3.17 FESEM images of ZnO film annealed at 250

d at 250oC


89

Fig. 3.18 FESEM images of ZnO film annealed at 300°C


90



91



92


CHAPTER.3: UNDOPED

The Energy dispersive sca

were recorded by an attac

confirmed and their atomic

the computer generated

films annealed at five diffe

Fig. 3.22 EDS of


93

ispersive scattering of ZnO film annealed at five differe

by an attachment to FESEM. The presence of Zinc an

their atomic weight percent were measured. Figures 3.22

generated EDS spectrum with quantity of Zinc and oxygen

at five different temperatures recorded using JEOL FESE

EDS of ZnO film annealed at 250°C (S1-eds) and 30

t five different temperatures

Zinc and Oxygen were

Figures 3.22 to 3.26 displays

c and oxygen present in ZnO

g JEOL FESEM.

) and 300°C (S2-eds).


94

Fig. 3.23 EDS of ZnO film annealed at 350°C (S3-eds) and 400°C (S4-eds).

CHAPTER.3: UNDOPED

Fig. 3.2

From The EDS measurem

follow non uniform varia

induced defects in the crys

ZnO film has highest Oxyg

3.3 Conclusions

We have studied the eff

temperatures (400oC, 500

properties. X-ray diffracti

Eenergy Dispersive Scat


95

24 EDS of ZnO film annealed at 450°C (S5-eds)

S measurement we see that the film annealed at differe

variation of oxygen or Zinc content, this may

ts in the crystal lattice. However at the highest annealing

highest Oxygen content.

died the effect of annealing the ZnO thin films

C, 500oC and 6000C) on its structural, optica

ray diffraction (XRD) and Scanning electron microsco

persive Scattering (EDS) attachment were used for

eds).

led at different temperatures

this may due to heating

est annealing temperature the

at three different

optical and electrical

ron microscopy (SEM) with

re used for structural and


96

morphological characterization respectively. UV-VIS-NIR spectrometry,

Photoluminescence and spectroscopic Ellipsometry were used for optical

characterization. The electrical characterization of the films was done using four-probe

method. The band gap of the films were calculated and were in agreement with bulk

ZnO. It was found that the ZnO films were emitting in Ultra violet region. The

conductivity of film slowly decreased with annealing temperature indicating scattering

of mobile electrons due to heating induced defects.

The investigation of Quantum confinement effect in sol-gel prepared ZnO thin films

were also performed. We prepared ZnO thin film by sol-gel method with annealing the

samples at five different temperatures viz. 250ºC, 300ºC, 350ºC, 400ºC and 450ºC.The

blue shift of optical band gap with annealing temperature have been observed. The

quantum confinement effect in these systems was found.


97

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