The Optical Parameters of Zn<sub>x</sub>Cd<sub>(1-x)</sub>Te Chalcogenide Thin Films

doi:10.4236/jsemat.2011.12008

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Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 51-55

doi:10.4236/jsemat.2011.12008 Published Online July 2011 (http://www.SciRP.org/journal/jsemat)

The Optical Pa ramete rs o f ZnxCd(1-x)Te

Chalcogenide Thin Films

Umeshkumar P. Khairnar1, Sulakshana S. Behere2, Panjabrao H. Pawar2

1Department of Physics, S.S.V.P.S. ACS College, Shindkheda, Dhule, India; 2Thin Film Laboratory, Department of Physics, Zulal

Bhilajirao Patil College , Dhule, India.

Email: [email protected]

Received May 16th, 2011; revised June 13th, 2011; accepted June 22nd, 2011.

ABSTRACT

A procedure to make optical quality thin films of ZnxCd(1-x) Te by use of thermal evaporation of the ternary compound

has been investigated. Structural and optical properties of ZnxCd(1-x) Te solid solution with x = 0.1 to 0.5 were synthe-

sized, from the resulting ZnTe and CdTe composition used in preparation of thin films. Structural investigation indi-

cates they have polycrystalline structure. Composition was confirmed from EDAX while SEM picture shows homogene-

ity in films. Plots o f (αhν)2 versus (hν) yield straight line indicating direct transition occurs with optical band gap ener-

gies in the range 1.7 - 2.3 eV. It is also found with increase Zn content the band gap of the films increases. Refractive

indices and extinction c oefficients have been evaluated in the spectral range (200 - 2500 nm).

Keywords: Thermal Evaporation, EDAX, XRD, Optical Band Gap

1. Introduction

Solid solution formation in semiconductors has been of

interest for a number of years. An important question

regarding ternary zinc-blende compound semiconductors

is concerned with the structural and dynamic changes

that can occur upon replacement of either cations or ani-

ons in the binary base material The II-VI compounds

semiconductors and solid solutions based on them are

promising source for various types of thin film devices

such as thin film transistors [1], Solar cells [2] and pho-

toconductors [3].

Thi n fil ms of Z nxCd(1-x)Te were prepared by variety of

techniques, such as, two source vacuum evaporation [6],

molecular beam epitaxy [7], chemical vapour deposition

and closed space vapour transport [8,9], physical vapour

transport (PVT) [10], vertical Bridgman growth [11]. In

the recent study ZnxCd(1-x)Te thin films are deposited by

thermal evapo ration at subs trate te mperature (373 K) and

the films are annealed and then characterized by energy

dispersive X-ray analysis (EDAX) and scanning electron

microscopy (SEM) technique for composition and sur-

face morphology of the films. Optical properties of the

films were studied by optical transmittance and reflec-

tance measurement.

2. Experimental Details

For the preparation of ternary semiconductors, ZnxCd(1-x)

Te the constituent compounds ZnTe (Purity - 99.999%,

Aldrich Co. Make, USA) and CdTe (Purity - 99.99+%,

Aldrich Make, USA) have been taken in molecular stio-

choimetry proportional weights and crushed and mixed

homogenously. The different sets of samples of varying

compositions (x = 0.1 to 0.5) were deposited via subli-

mation of the compound in vacuum higher than 10−5

mbar under controlled growth conditions of various

compositions onto the amorphous precleaned glass sub-

strates at the temperature of 373 K. The thicknesses of

films were controlled by using quartz crystal thickness

monitor model No.DTM-101 provided by Hind-High

Vac. The deposition rate was maintained 10-15 Å/sec

constant throughout sample preparations. The source to

substra t e distance was kept constant (15 cm) and sub-

strate was kept at constant temperature (373 K). Depos-

ited samples were kept under vacuum overnight. All the

samples are deposited under the similar optimized condi-

tion. These samples were annealed at reduced pressure of

10−5 mbar for the duration of 3 hours at the temperature

of 573 K and maintain carefully. These samples were

then used for various characterizations. X-ray diffraction

(XRD) studies were carried out using a Rigaku, Miniflex,

The Opti cal Parameters of ZnxCd(1-x)Te Chalcogenide Thin Films

Japan X-ray diffractometer. The XRD patterns were re-

corded in the 2θ range of 20˚ - 80˚ glancing angle 30˚

using CuKα ra diation (λ = 1.5418 Å). The morphology of

the ZnxCd(1-x)Te thin films was examined using Scan-

ning Electron Microscope (SEM) (model 501, Philips,

Holland with EDAX atta chment) using acceleration volt-

age variable from 1.6 KV to 30 KV. For this pur p o se t hin

layer of gold (50 Ǻ) was deposited on the film using

physical vapour deposition. UV-Vis spectra of the sam-

ples were recorded on a HITACHI-MODEL-330 UV-Vis

spectrophotometer in the wavelength range 200 - 2500

nm.

3. Results and Discussion

All the ZnxCd(1-x)Te films prepared by the above tech-

nique were polycrystalline of multi phase structure indi-

cating preferential of the film crystallites corresponding

to textured (100)H and (220)C growth [14].

From the Figure 1 X-ray diffractograms of various

compositions it is observed that for x = 0.1 and x = 0.2

there are only two prime peaks which corresponds to

(100) plane of hexagonal CdTe and (220) plane of cubic

ZnTe [14]. Diffraction anal ysis su ggest s that all the sam-

ples of various compositions are polycrystalline nature.

However the charactertics peak of hexagonal CdTe and

cubic ZnTe changes their angular position and relative

inten sities for different compositions suggesting multi

phases and inhomogeneity in the growth of films. The

samples of compositions (x = 0.1, 0.2 and 0.4) exhibit

Figure 1. X-ray diffractograms of various ZnxCd(1-x)Te

structures .

predominant diffraction lines corresponding to (100)

plane of CdTe (H) may be attributed to the characteristics

growth with (100) reflecting plane as a preferred orienta-

tion. While the sample (x =0.3 and 0.5) exhibits pre- do-

minant diffraction lines corresponding to (111) plane of

ZnTe (C) is again attributed to the charactertics growth

with the (111) reflecting plane as preferred ori- entation.

The shifting of peak positions of these promi- nent dif-

fraction lines suggests the formation of solid so- lution

corresponding to ZnxCd(1-x)Te material from the basic

starting compounds CdTe and ZnTe.

From the scanning electron micrographs, it is found

that the thicknesses of the films samples are too small to

observed structure patterns. However films surfaces are

very smooth. In all t he co mposition s it has b een observed

that t he reflec ti vi t y o f t he f il m gr ad ua ll y i ncreases, which

is quite natural t hat the reflec tivit y of the fil m is expected

to increases with the increase of ‘x’ composition. The

same observation as above shows the straitions, which

are exactly parallel equidistance and extend from one end

to another end. These straitions indicate the oscillatory

gro wth [1 5], whic h indicate at the one end and terminate

at the other end. It may be said that the oscillatory

growth in the film which is manifested by straitions has

been initiated by the presence of small composition of Zn

(x = 0.1) in Figure 2 and these oscillatory growth has

been found to reduce successively with the composition x

= 0.3 and x = 0.4. It is about reduce with composition of

x = 0.5. The composition of starting basic ingredients and

film compositio n comparison is presented in the Table 1

and expressed in atomic percentages. The atomic per-

centage of basic ingredient seems to be in agreements

with that obtain from EDAX analysis. From this table it

is remarkable point to note tha t for composition (x = 0.3)

the atomic percentage of basic ingredient taken is very

close to atomic percentage obtain from EDAX spectra.

The reflectance and transmittance spectra of these sam-

Figure 2. EDAX of ternary compound ZnxCd(1-x)Te Thin

Film. (x = 0.1).

The Opti cal Parameters of ZnxCd(1-x)Te Chalcogenide Thin Films

Table 1. EDAX data for ZnxCd(1-x)Te composite thin fil ms.

Basic Ingredient Taken EDAX Composition

‘x’ At% Zn At % Cd At% T e At% Z n At% Cd At% T e

0.1 5 45 50 7.07 40.52 52.41

0.2 10 40 50 13.42 37.78 48.81

0.3 15 35 50 15.59 34.46 49.96

0.4 20 30 50 28.20 25.90 45.90

0.5 25 25 50 31.49 20.95 47.56

ples were recorded using Hitachi Spectrophotometer

model-330 in spectral region 200 - 2500 nm. Using these

data, the absorption coefficient α has been calculated by

applying the relation [12]. Absorption coefficients have

been evaluated using percentage transmittance data as a

func tion of wavelength presented in Figure 3 for the

samples of different compositions. The plot of (αhν)2

versus hν are plotted and shows clearly linear depend-

ence for the value of p = ½. This is attributed to an al-

lowed and d irec t transitio n wi th dir ect b and ga p e ne rgies.

The evaluated band gap energies are 1.7 eV, 2.05 eV, 2.2

eV, 2.3 eV and 1.5 eV for the samples of compositions x

= 0.1 to 0.5 respectively clearly indicating dependence

on compositions of films. Band gap energies are to be

composition dependent. Band gap energy increases with

the inc reasin g ‘x’, this is as expected as band gap energy

for ZnTe is 2.26 eV and band gap energy for CdTe is 1.5

eV [16].

CdZnTe thin films of thickness 450 - 1400 nm have

been evaporated under vacuum onto unheated glass sub-

strates, using a multilayer method [1 7]. He reported vari-

ation of optical band gap between 1.16 and 1.63 eV.

Band-to-band transitions which give rise to the optical

absorption in the visible region of the spectrum may be

interpreted in terms of direct allowed transition with the

band gap in the range of 2.05 - 1.92 eV. [18]. The band

gap energy of the films measured by optical trans-

mittance measurement is 1.523 eV [14]. Po lyc ry stalline

thin films of CdZnTe and CdMnTe have been grown by

molecular beam epitaxy and metal-organic chemical va-

por deposition [19]. He reported with band gaps of 1.65 -

1.75 eV for the top of a two-cell tandem design. P-i-n

cells were fabricated and tested using Ni/p+-ZnTe as a

back contact to the ternary films. CdTe cells were also

fabr icate d using both growth techniques.

Near normal incidence reflectance and transmittance

data have been used to determine optical constant ‘n’ and

‘k’ [13]. The variation of refractive indices and extinct-

tion coefficients as a function of wavelength as repre-

sented in Figure 4 for the samples of compositions

100

120

010002000 3000

Wavelength (nm )

Transmittance (%)

(x=0.1)

(x=0.2)

(x=0.3)

(x=0.4)

(x=0.5)

Figure 3. Spectral behaviour of transmittance with wave-

ngth.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

00.5 11.5 22.5 3

hν(eV)

0.02

0.04

0.06

0.08

0.1

0.12

Figure 4. Variation of ‘n’ and ‘k’ with wavelength for

ZnxCd(1-x)Te (x = 0.1).

(x = 0.1) It is found that variations in refractive indices

and extinction coefficients are oscillatory in nature. Se-

condly variation in n and k seems to be complementary

i.e. maxima of one and minima of the other at the same

wavel ength as estimated in Table 2.

Polycrystalline Cd0.96Zn0. 04Te thin films [20] we re de-

sited onto well-cleaned glass substrates kept at room

temperature by vacuum evaporation. Optical properties

of thin films were studied by optical transmittance meare-

The Opti cal Parameters of ZnxCd(1-x)Te Chalcogenide Thin Films

Table 2. Well defined Maxima and Minima in variati on of

‘n’ and ‘k’.

Composition (x)

(µ) Maxima Minima

‘n’ ‘k’ ‘n’ ‘k’

0.1

0.6 - 0.10 0.055 -

0.9 1.23 - - 0.038

1 - 0.044 1.10 -

1.2 1.57 - - 0.021

1.5 - 0.044 1.11 -

2.1 1.56 - - 0.021

0.2

0.4 - 0.076 1.84 -

0.9 1.96 - - 0.019

1.2 - 0.035 1.92 -

1.6 1.97 - - 0.010

0.3

0.4 - 0.065 1.86 -

0.9 1.96 - - 0.019

1.2 - 0.034 1.93 -

1.7 1.97 - - 0.010

0.4

0.4 - 0.078 1.84 -

0.9 1.96 - - 0.017

1.1 - 0.031 1.93 -

1.6 1.97 - - 0.010

0.5

0.9 1.95 - - 0.021

1.1 - 0.046 1.90 -

1.7 1.99 - - 0.0007

ment and spectroscopic ellipsometry (SE). The spectra of

various optical constants obtained from the SE ε(E) data

(ε1, ε2, R, n, k and α) revealed three distinct critical

points (E1, E1+Δ1 and E2). The band gap energy of the

films determined by transmittance measurement was

1.523 eV at room temperature. He reported refractive

indices ‘n’ varies between 2.4 to 2.6 and extinction coef-

ficient ‘k’ varies 0.5 to 1.

Optical properties of ZnxCd(1-x)Se films [21]. He re-

por ted var iation in n a nd k in the wavele ngth r ange 6 00 -

100 0 nm is ver y close matc hing wi th pre sent work. T hey

also reported the band gap energy increases with in-

creasing Zn component in ZnxCd(1-x)Se films. At higher

wavelengths the experimental results T and R satisfy the

relationship T + R = 1. T his indica tes that neit her ab sorp-

tion nor scattering of light occurs beyond the absorption

edge. The appearance of maxima and minima results

from interference effect and their number increases with

increases film composition. These results are satisfactory

and a s theoretically expected.

4. Conclusions

• Ho moge ne ous p o l ycr ys tal l ine of mult i p has e structure

of the thin films of ZnxCd(1-x)Te have been success-

fully deposited by thermal evaporation technique us-

ing basic ingredient ZnTe and CdTe elemental start-

ing materials.

• EDAX composition seems to be closely matched with

starting basic ingredients.

• The dependence of the optical parameters of the films

on the l ight energy supports the direct character of the

interband transition through an optical band gap in the

range 1.7 - 2.3 eV.

• The variation in optical constants as a function of

wavelength is osc illatory in nature having well de-

fined maxima and minima, which depends on the

composition of the thin films.

5. Acknowledgements

The authors are thankful to P rof. Dr. M. V. Patil, Princi-

ple, S. S. V. P. S. ACS College, Shindkheda. The authors

are also grateful to Prof. Dr. P. P. Patil, Head, Depart-

ment of Physics, North Maha rashtra Universit y, Jalgaon.

One of the authors U. P. K. thanks to the University

Grants Commission, New Delhi, for financial supports

through the U.G.C. Minor Research Scheme No.F. 47 -

1275/09 (WRO) Pune.

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