Optical Properties of ZincOxide and Strontium TitanateThin Films by Reflectometryand Ellipsometry

Dionne A. MillerGraduate School and City CollegeCity University of New YorkJune 11, 2007Mentor: Prof. Glen R. Kowach

Outline

Objectives and Motivation Introduction to the Optical Constants Reflection of Light Reflectometry and Ellipsometry Techniques Zinc Oxide and Strontium Titanate Thin Films

Crystal Structure and Properties Film Preparation Results and Discussion

Summary

Objectives and Motivation

Objectives1. Develop a reliable model to determine the optical properties of zinc oxide (ZnO) and strontium titanate (SrTiO3 or STO) thin films.2. Determine how optical constants are affected by deposition conditions and substrates.

Motivation1. Need for reliable data on optical constants of thin films for use in device applications.2. Very few systematic studies of how deposition temperatures and substrate type affect optical properties of ZnO, STO thin films

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Wavelength (nm)

% R

efl

ec

tan

ce

R

T

I

d Newton’s rings: concentric ring pattern ofrainbow colors because different wavelengthsof light interfere at different thicknesses

Rainbow patterns of color on the waferdue to the thickness variation of the ZnOfilm. (substrates: Si, left; Pt on Si, right)

Optical Constants

When light is incident on a plane-parallel interfacebetween two media it may be reflected or refracted

The second medium is characterized by a complexindex of refraction: Ň = n -ik

sol.sci.uop.edu/.../incidentreflectrefract.jpg

Optical Constants

n: (real) index of refraction; inverse measure of the phasevelocity of light in the material

k: extinction coefficient; measure of how rapidly intensitydecreases as the light passes through the material

ε: dielectric function; degree to which a material may bepolarized by an applied electric field ε = ε1 + iε2 ; ε1 = n2 – k2; ε2 = 2nk

Both n and k are functions of wavelength; optical constantspectra show n and k as functions of wavelength or photonenergy – dispersion spectra

Reflection of Light

Light incident on the sample has two components ofpolarization: in plane (p-waves) and perpendicular (s-waves) to the plane of incidence

Reflectance (R) :ratio of intensity of outgoing light toincoming light – measured in reflectometry

Ratio of the amplitudes of outgoing and incoming light (r)is also of interest – measured in ellipsometry. For a singleinterface, this ratio is called the Fresnel ReflectionCoefficient and is based on Snell’s Law.

Reflectance, Rp = │Rp│2

raab

raab

pNN

NNr

!!

!!

cos~

cos~

cos~

cos~

+

"=

Fresnel Reflection Coefficient for single interface:

Ratio of amplitude of outgoing resultant wave to amplitudeof incoming wave is defined as the total reflectioncoefficient. For a single film (two interfaces) this is:

R

c

a

T

b

I

φb

φc

φa

d

!

!

2

2

1iP

bc

P

ab

iP

bc

P

abP

err

errR

"

"

+

+=

bi !

"#$ cos))((2

bbkn

d%=

d = film thickness; nb= index of refraction of layer b;kb = extinction coefficient of layer b

Reflectance,Rp=│rp│2

Delta, Δ = δ1 – δ2 is the phase shiftinduced by reflection in the p ands-waves (0 – 360° or -180 to +180°)

Phase shift also induces an amplitudereduction for both p- and s-waves: tan ψ = |Rp| / |Rs| (ψ: 0 to 90°)

ρ = Rp/Rs = tan ψeiΔ or tan ψeiΔ = Rp/Rs

(fundamental equation of ellipsometry)

Ellipsometers measure Δ and ψ: fromthese quantities optical constants andthickness can be calculated using atheoretical model.

Ellipsometry

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N K

Wavelength (nm)

Layer #1

Index Of Refraction (N)

Extinction Coefficient (K)

4

2

2

10)(

!!!

nnnn ++=

Cauchy Function

Lorentz Oscillator Model

)exp(0

'

2

2

tieExdt

dx

dt

xdm !"# =++

Zinc OxideZnO

Zinc Oxide Structure Hexagonal wurtzite structure – non-

central symmetry Alternating planes of Td coordinated

zinc and oxide ions stackedalternately along the c-axis

Positively charged (0001)-Zn2+ andnegatively charged (0001 )-O2- polarsurfaces result in a normal dipolemoment and spontaneouspolarization along the c-axis

Uniaxial with optical axis being thehexagonal c-axis – gives rise to twoindependent refractive indices, n0and ne

(0001) - Zn2+

( 0001 )- O2-

Zinc Oxide Properties andApplications

II-IV, n-type semiconductor (band gap 3.3 eV)

High excitonic binding energy (60 eV) - optically pumped UVlasing at room temperature demonstrated by Yu et al andBagnall et al1

Due to the non-central crystal symmetry, ZnO is piezoelectric:useful for electromechanical coupled sensors, varistors andtransducers2

Transparent conductive oxide, exhibiting high transmittance inthe visible region and low electrical resistivity: ideal material forsolar cells and flat panel displays3

1. Yu et al, 23rd Intl. Conf. On Physics of Semiconductors, M. Scheffler and R. Zimmermannn, Eds., World Scientific 1996. Bagnall, D.M., et al., ApplPhys Lett, 1997, 70(17): p. 2230-2232.

2. Rebien, M., et al., Appl Phys Lett, 2002. 80(19): p. 3518-3520. Wang, Z.L., Nanostructures of Zinc Oxide, in Materials Today. 2004. p. 26-33.

3. Liu et al, Thin Solid Films, 2006. 510(1-2): p. 32-38.

Zinc Oxide Thin Film Preparation

Films were deposited using radio frequency (RF) planarmagnetron sputtering using a 3 inch diameter Zn targetreactively sputtered in Ar-O2 (1:1) plasma discharge

Substrates were placed parallel to the target surface in asputter down geometry and heated with quartz lamps totemperatures varying from 200°C to 600°C

substrateheater

3 inch target

RF

shutter

tovacuum

Ar

O2

95 cm

Schematic of ANELVA SPF-332H sputtering system

Data Analysis

Multi-parameter regression analysis:

Build model of the thin film structure: estimate thickness, opticalcharacteristics

Determine actual change in reflection/polarization over a widespectral range

Measured change is compared to the calculated change andregression analysis based on selected model parameters isperformed until a ‘best fit’ is achieved

Best fit measured by minimizing the root mean square error (RMSE)

Models of Film Structure

Silicon substrate

ZnO thin film (L1)

EMA Layer

Platinum film

voidZnO

Sample Data: ZnO on Si

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Wavelength (nm)

% R

efl

ecta

nce

Figure 1: Measured (-) and simulated (--) 0 and 70 degreereflection data for ZnO film on Si substrate deposited at 500C.

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Wavelength (nm)

Delt

a (

deg

rees)

Figure 2: Measured (-) and simulated (--) delta ellipsometric data for the film shown in Fig. 1

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Wavelength (nm)

Psi (d

eg

rees)

Figure 3: Measured (-) and simulated (--) psi ellipsometric data for the film shown in Fig.

o Quality of fit: RMSE 1.437o Strong interference oscillations below the band edge due to multiple internal reflections between the film and the substrateo Model is able to fit data above and below the band gap

Discussion Optical constant spectra show

strong peaks which may beattributed to interbandtransitions (375 nm) as well asa smaller peak (338 nm)usually attributed to freeexcitonic and exciton-photoncomplex transitions1

n exhibits typical semi-conductor dispersion: ndecreases with increasing λand resonant frequency closeto visible region (~370 nm)

ZnO film is transparent inregion 400 – 920 nm

1. Washington et al, Appl. Phys. Lett. 1988, 72, 25, 3261-63. Liu et al, Thin Solid Films, 2006, 510, 32-38.

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N K

Wavelength (nm)

Layer #1

Index Of Refraction (N)

Extinction Coefficient (K)

Refractive index and extinction coefficient obtainedusing SCI TM dispersion model for the film in Fig. 1

1.437RMSE

0.0000Extinction coefficient @ 632 nm

1.99575Refractive Index @ 632 nm

10.83 nmThickness Layer 2

540.14 nmThickness Layer 1

Extracted Data

Discussion – Si Substrate

n – indicator of packingdensity and filmstoichiometry. Increasingtemperature results inincreased surface mobility ofimpinging species and amore regular lattice structure.

Index also increases withO: Zn ratio – film is nearlystoichiometric.

EMA layer has no correlationto temperature – real

Refractive index (n) vs temperature (°C) for ZnO (Sisubstrate) for films of similar thickness (650 nm ± 30 nm)

EMA layer thickness (nm) vs temperature (°C) for ZnO (Sisubstrate) for films of similar thickness (650 nm ± 30 nm)

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Temperature/deg C

EM

A l

ay

er

thic

kn

es

s/n

m

1.98

1.99

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2.03

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Temperature (deg C)

R

efr

acti

ve In

dex, n

Discussion – Pt Substrate

Refractive index (n) vs temperature (°C) for ZnO (Ptsubstrate for films of similar thickness (640 nm ± 40 nm)

EMA layer thickness (nm) vs temperature (°C) for ZnO (Ptsubstrate for films of similar thickness (640 nm ± 40 nm)

n increases with substratetemperature

EMA layer has no correlationto temperature – real. FESEMimages confirm significantlyhigher roughness in RT, 550°Cfilms

Low temperature films on bothsubstrates exhibit low n andhigh EMA thickness, i.e., arerough and not very dense.

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Substrate temperature/deg C

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n

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Temperature (deg C)

EM

A layer

thic

kn

ess/n

m

Discussion: Refractive Index Bulk refractive index @ 600

nm ~ 2.00(1.99 on sapphire1;1.9985 singlecrystal2 (ord. ray); 1.997 uniaxial thinfilm material3 (ord. ray)

Measured values are close toordinary bulk values (1.99 -2.03 for ZnO/Si and 1.96 –2.01 ZnO/Pt) indicating a highquality film

Comparison of substratesindicate high quality films canbe grown on both materials

1. Postava et al, J. of Appl. Phys., 2000, Vol. 87, No. 11, 7820.2. Bond et al, J. Appl. Phys., 1965, 36, 16743. Jellison et al, Phys. Rev. B, 1998, 58, 7, 3586

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Temperature/deg C

Re

fra

cti

ve

in

de

x,n

Platinum

Silicon

Strontium TitanateSrTiO3

Structure and Properties Perovskite crystal structure Important dielectric material

with incipient ferroelectric transition(tetragonal cubic) at 110K andbandgap of 3.22 eV1

High dielectric constant, excellentoptical properties and stablestoichiometry at RT offersprospective applications inmicroelectronics and optoelectronicsin thin film morphology

Isotropic optical behavior as a resultof cubic crystal structure

Sr Ti O

1. M.-S. Zhang et al, Applied Physics A, 76, 2003, 1105-8. J.H. Ma et al, J. Appl. Phys. 99, 2006, 033515.

Thin Film Preparation

Films were deposited using radio frequency (RF)planar magnetron sputtering using a 3 inchdiameter SrTiO3 target (IC Mechanics 99.9%) inAr-O2 (4:1) plasma discharge in an Anelva SPF-332H sputtering system

Substrates were placed parallel to the targetsurface (target to substrate distance of 4.5 cm) ina sputter down geometry and heated with quartzlamps to temperatures varying from roomtemperature to 800°C

Sample Data: SrTiO3 Film

Analysis using multi-anglereflectometry

Graded model Quality of fit: RMSE 1.638 Strong interference

oscillations due to multipleinternal reflections betweenthe film and substrate

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Simulated @ 0o Measured @ 0o

Simulated @ 70.775o Measured @ 70.775o

Wavelength (nm)

% Reflection

Measured and simulated 0° and 70° reflection data for SrTiO3film on silicon substrate (deposition temperature 700°C)

1.638RMSE

0.0000Extinction coefficient @ 632 nm

2.32958Maximum Refractive Index @ 632 nm

564.88 nmThickness Layer 1 (STO)

Extracted Data

Discussion Data analysis indicates a

graded refractive index – nincreases linearly in thenormal direction, reaching amaximum at the film surface

Graded index occurs only infilms deposited at 400°Cand above

Applications: multi-bandrugate filters, anti-reflectivecoatings, edge and dichroicfilters

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N K

Substrate SurfaceThickness (nm)

Layer #1

N @ 632 nm

K @ 632 nm ( 1x10

-5 )

Graded refractive index and extinctioncoefficient spectra at 632 nm obtained for film

lower index film

higher index film

substrate

EMA layer

STO film

Discussion

1.84401

1.9894

2.27359

2.31942

2.28099

2.32958

2.366842.38458

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Substrate Temperature (deg C)

Maxim

um

Refr

acti

ve In

dex, n

Clear trend of increasing nwith increasing depositiontemperature: increasedpacking density

Significant increase in nabove 300°C (1.989 to 2.274at 400°C)

Bulk refractive index ofSrTiO3 is 2.394 at 620 nm;maximum value of 2.385 at800°C which is very close tothe bulk values, indicatingvery high crystallinity andfilm quality.

Maximum refractive index versus substratedeposition temperature for SrTiO3 films onsilicon substrate

Summary

High quality ZnO and SrTiO3 films can beproduced via sputtering on Si and Pt substrates

Film density, as measured by an increase inthe refractive index, increases with increasingsubstrate temperature during deposition

For ZnO films, model is able to fit data both aboveand below the band gap

For STO films, a marked change in crystallinity ofthe deposited films occurs above 300°C; films alsoexhibit a graded index at higher depositiontemperatures

DETECTOR

SAMPLE

LIGHTSOURCE

BEAMSPLITTER

Reflectometry Instrumentation

Intensity of a monochromaticbeam of light is measuredbefore and after it reflects fromthe sample

Absolute reflectance of thesample can be calculated fromabsolute reflectance of thestandard (bare Si wafer)

I sample_________I standard

Rrelative =

Ellipsometry Instrumentation

Measurement of the change in polarization state ofa beam of light upon reflection from the sample

Lightsource detector

samplepolarizer analyzer

Detector signal is measured as afunction of time then Fourieranalyzed to obtain psi and delta