Preparation and study of the optical properties of as deposited ZnSb

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Optical properties of ZnSb
Investigation of the optical properties of as-deposited ZnSb thin films
Md. Johurul Islama* and M. Sayeedur Rahman Khanb
Department of Applied Physics and Electronic Engineering
University of Rajshahi, Rajshahi-6205, Bangladesh.
a. Tel: 880-1731-002095; Fax. 880-721-750064; e-mail: [email protected]
b. Tel: 880-721-750251; Fax. 880-721-750064; e-mail: [email protected]
Abstract
Zinc-antimonide (ZnSb) thin films have been prepared by using single source resistive heating
vacuum evaporation technique onto glass substrate at a pressure of about 8  10-4 Pa. Optical
investigations of as-deposited ZnSb thin films having thickness of the range 70-95 nm at room
temperature (300K) are carried out in the wavelength range 400-2500 nm. Optical band gap of
these films are determined by using the well-known TDM model. The measured value of optical
band gap is found to be 0.6 eV. The observed nature of the transition is indirect. The values of
dielectric constant vary from 6.8 to 10.8 in the wavelength range 400- 2500 nm. The optical
conductivity of these films varies from 8  1014 to 63  1014 (per sec.). Optical studies show that
the transmittance of these films is approximately zero in the ultraviolet region.
Key words: Thermal evaporation, Optical band gap, Dielectric constant, Optical conductivity.
1. Introduction
Semiconducting thin films play an important role in microelectronics industry. But the traditional
elemental semiconductors Si and Ge cannot address all the emerging photoelectronic
*Author for correspondence
1
applications because the band gap is not correct for the wavelength of interest. Intrinsic carrier
concentration of binary and ternary semiconductors is greater than that of the elemental
semiconductors. This high carrier concentration increases the carrier mobility of the compounds.
These properties are very useful for high-speed devices (Heying et al., 2000). On the other hand
dislocation free or low dislocation density binary and ternary semiconductor crystals are required
for substrates of high performance electronic and optoelectronic devices, because dislocations
existing in substrate have adverse effects on the performance of devices (Miyazaki and Kuroda,
1999).
Recent interest in semiconducting thin films for the use in solar energy application such as
radiation control (Karlsson et al., 1985), photothermal (Pavlovic and Ignatiev, 1986) and
photovoltaic (Abass 1988) conversations has sparked new investigations into their electrical and
optical properties. In this regard the analysis of optical spectra is the most important tools for
understanding and developing the theory of electronic structure in solids. In this paper we
discuss the systematic analysis of the preparation of ZnSb thin films and report its new
information on the optical properties. Optical measurement results such as optical band gap,
dielectric constant and optical conductivity of ZnSb films and their analysis have been presented.
2. Materials and Methods
Thin films of ZnSb (Purity 99.99 %) have been prepared by thermal evaporation from a
molybdenum boat on glass substrate held at room temperature. The material evaporated in an
Edwards E306A vacuum coating unit at a pressure of about 8  10-4 Pa. Films of various
thicknesses have been deposited onto cleaned substrate for optical measurements. Film thickness
2
was measured by Tolansky interference method (Tolansky, 1948) with an accuracy of about 2
nm. Optical measurements such as transmittance and absorbance are measured using
SHIMADZU UV-3100 recording spectrophotometer in the wavelength range 400-2500 nm.
3. Results and Discussion
Optical absorption spectra in the wavelength range 400-2500 nm of ZnSb films are shown in
Fig.1. It is seen from the figure that the absorption is very high (60%) for both the films in the
wavelength range 400-800 nm and sharply decreases to about 15 % for 95nm thick film at 1380
nm and to about 12 % for 70 nm thick film at 1280 nm. Above these wavelengths (>1600 nm)
the absorption is below 5 % for both the film thicknesses. It is also seen from the figure that
higher the film thickness, higher the absorption.
[Fig.1]
To estimate the optical band gap,  opt , the well-known TDM (Tauc-Davis-Mott) model (Tauc et
al., 1966; Davis and Mott, 1970) relating the absorption coefficient  ( ) to the photon energy
 :  ( )  B
   opt  m

is employed. In the above equation m 
1
3
and m  correspond
2
2
to direct allowed and forbidden transitions respectively. On the other hand, m  2 and m  3 for
indirect allowed and forbidden transitions respectively and B is a constant.
All possible transitions ( ( ) versus  ,
2
1
( ) 3
3
( ) 2
versus 
1
, ( ) 2
versus  and
versus  ) were examined by drawing graphs. The band gap,  opt , is determined by
extrapolating the absorption curve to zero in the energy axis. From all the possible curves,
3
careful inspection shows that only the graph of
1
( ) 2
versus  has a well-defined linear
region in the high absorption range. This is shown in Fig 2. The value of the observed band gap
of 95 nm thick film determined from Fig. 2 is found to be 0.6eV, which agreed well with the bulk
value (Kittle, 1986).
[Fig. 2]
Variation of dielectric constant of ZnSb films of thickness 70 nm and 95 nm in the wavelength
range 400-2500 nm is shown in Fig. 3. It is seen from the figure that for both the films the
dielectric constant varies with wavelength. For 70 nm thick film there is a maxima at 500 nm and
the value of the dielectric constant gradually decreases up to 900 nm. Then it increases again up
to 1500 nm and becomes approximately constant over the wavelength range 1500-2500 nm. For
95 nm thick film, the value of dielectric constant has maxima at 600 nm and then decreases
gradually to 1200 nm and again increase in the wavelength range 1200-2500 nm.
[Fig. 3]
Optical conductivity  1 of these films is determined from the absorption data by using Gravier
(Gravier et al., 1979) equation  1 
nk o c

per sec., where c is the velocity of light in m/sec.,  is
the wavelength in meter, k o is the extinction coefficient and n is the refractive index. The
variation of optical conductivity of these films is shown in Fig. 4.
[Fig 4]
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It is seen from the figure that the value of optical conductivity is very high for both the films in
the wavelength range of 400-500 nm and decreases rapidly in the 500-1000 nm range. Above
1000 nm the value of optical conductivity remains approximately constant.
4. Conclusion
The optical properties of thermally evaporated ZnSb films deposited on glass substrate at normal
angle of incidence held at room temperature have been presented. The optical band gap of these
films is found to be 0.6 eV, which is very close to the bulk value. The optical spectra of ZnSb
shows that the transmittance is approximately zero in the UV region. The analysis of optical data
shows that the value of dielectric constant of the material is low and the optical conductivity is
very high in the UV region indicating that the material absorbs large energy in this region. On
the other hand the energy absorption is low in the visible and near infrared region.
List of Figure caption:
Fig 1. Variation of absorbance with wavelength of two ZnSb samples.
1
Fig 2. A plot of (  ) 2 vs  curve for ZnSb sample.
Fig 3. Dielectric constant,  vs wavelength curves for two ZnSb samples.
Fig 4. Optical conductivity, 1 vs wavelength curves for two ZnSb samples.
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Illustrations
6
7
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