Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 51-55
doi:10.4236/jsemat.2011.12008 Published Online July 2011 (
Copyright © 2011 SciRes. 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.
Received May 16th, 2011; revised June 13th, 2011; accepted June 22nd, 2011.
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 105
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
105 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
Copyright © 2011 SciRes. JSEMAT
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
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
Copyright © 2011 SciRes. JSEMAT
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
010002000 3000
Wavelength (nm )
Transmittance (%)
Figure 3. Spectral behaviour of transmittance with wave-
Figure 4. Variation of ‘nand ‘kwith 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
Copyright © 2011 SciRes. JSEMAT
Table 2. Well defined Maxima and Minima in variati on of
n’ and ‘k’.
Composition (x)
(µ) Maxima Minima
n’ ‘k’ ‘n’ ‘k
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
0.4 - 0.076 1.84 -
0.9 1.96 - - 0.019
1.2 - 0.035 1.92 -
1.6 1.97 - - 0.010
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 - 0.078 1.84 -
0.9 1.96 - - 0.017
1.1 - 0.031 1.93 -
1.6 1.97 - - 0.010
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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