International Journal of Soft Computing and Engineering (IJSCE)
ISSN: 2231-2307, Volume-3, Issue-1, March 2013
Structural & Optical Characterization of Gex Se80-x
Pb20 Thin Films Prepared by Thermal Evaporation
Technique
Pawan Kumar, Aravind Kumar, L.P. Purohit, Kapil Malik, Parvinder
Abstract- Vacuum evaporated films of GexSe80-xPb20 have been
characterized by using optical spectroscopy (especially
transmission and absorption spectra). The chalcogenide glass of
GexSe80-xPb20 has been prepared by melt quenching technique.
Thin films of GexSe80-xPb20 are deposited by vacuum thermal
evaporation technique on highly clean glass substrates and their
optical properties such as refractive index (n), extinction
coefficient (k), and energy band gap have been studied. The
transmission spectra in the spectral range 300-2000 nm has been
used to calculate the refractive index (n), extinction coefficient
(k) of the films have been studied. Optical spectroscopy of the
films has been done with the help of the Shimadzu U-3600 (UVVis-NIR) Spectrophotometer. The structural analysis is studied
through X-ray diffraction.
Keywords: Melt quenching, thermal evaporation, extinction
coefficient, structural analysis.
I.
INTRODUCTION:
Interest in the physical properties of amorphous
chalcogenide glassy materials, especially their potential
applications in areas of optoelectronics such as laser
technology and fiber optics, has gained attention since the
1950's and has coalesced within a field known academically
as "Physics of Non crystalline Solids". However, in contrast
to crystalline solids, for which the physical properties and
structures are essentially understood, their remain
considerable theoretical difficulties with amorphous solids,
and these have been amplified by a lack of precise,
organized experimental information. Chalcogenide glasses
are truly emerged as multipurpose materials and have been
used to fabricate technological devices such as IR detector,
electronic and optical switches and optical recording media.
Recently, great attentions have been given to chalcogenide
glasses mainly due to their wide range of applications in
solid state devices both in technological and scientific fields
[1].Elements of II–VI group are attracting a great deal of
attention because of their potential abilities in the wide
spectrum optoelectronic devices (Sharma et al 1979; Burger
and Roth 1984; Bassam et al 1988; Nasibov et al 1989;
Gupta et al 1995a; Deshmukh et al 1998).
Manuscript received on March, 2013.
Pawan Kumar, Department of Physics, Gurukula Kangri
Vishwavidyalaya, Haridwar-249404, India.
Aravind Kumar, Department of Physics, Kalindi College, Delhi
University, East Patel Nagar, Delhi, India.
L.P. Purohit, Department of Physics, Gurukula Kangri
Vishwavidyalaya, Haridwar-249404, India.
Kapil Malik, Department of Physics, Gurukula Kangri
Vishwavidyalaya, Haridwar-249404, India.
Parvinder, Department of Physics, Gurukula Kangri Vishwavidyalaya,
Haridwar-249404, India.
High absorption coefficient, high efficiency of radiative
recombination and nearly matching band gaps with the
visible region of the solar spectrum are the root causes of the
popularity of II–VI group semiconductors. As chalcogenide
glasses have poor thermo-mechanical properties, in order to
enlarge the domain of application, it is necessary to increase
their softening temperature and mechanical strength.
Researches [2-3] have been done in domain transformation
of chalcogenide. Often attempts have been made by
changing the composition [4] and introducing some extra
element [5] into binary alloy. Ternary materials provide a
possibility of tailoring their properties as per requirements
and hence project themselves as important semiconducting
materials for further advancements in the field of device
fabrication. Wide optical window (from 1-20 μm),
photoinduced reversible structure change and photoinduced
anisotropy of germanium based chalcogenides have shown
their potentiality in developing advance IR sensors for
biomedical process and gas detection [6], optical memories
[7] and opto-mechanical actuators [8].The widely used
envelope method has been developed for transmittance
measurements to evaluate the refractive index, extinction
coefficient,
and
absorption
coefficient.
Optical
characterization of thin films gives information about other
physical properties, e.g. band gap energy and band structure,
optically active defects etc., and therefore may be of
permanent interest for several different applications.
Considerable differences between optical constants of bulk
material and thin films or those of films prepared under
varying growth characteristics are often reported. Therefore
determination of optical constants for each individual film
by a non-destructive method is highly recommended.
Generally, the optical band gap (Eg) and absorption
coefficient α could be evaluated from transmittance or
absorbance spectra. Swanepoel [9] has improved this
method to determine more accurately the thickness (t),
absorption coefficient (α) etc. There are several reports on
this method [10–12]. In another conventional method, the
reflectance (R) and transmittance (T) spectra are used to
determine α. Since α is related to the extinction coefficient
k, which is defined as the imaginary part of the complex
refractive index, where n is the real part of refractive index,
an accurate determination of n and k is possible.
II.
EXPERIMENT
Glassy alloy of GexSe80-xPb20 (x = 0, 4, 6, 8) is prepared
by melt-quenching technique. Materials of 99.999% purity
are sealed in quartz ampoules (length 12 cm, internal
diameter 0.8 cm) with a vacuum of about 10 −6 torr. The
sealed ampoules are kept inside a furnace where the
temperature is raised slowly (5K/min) to 900 ◦C. The
413
Structural & Optical Characterization of Gex Se80-x Pb20 Thin Films Prepared by Thermal Evaporation
Technique
ampoules are rocked frequently for 12 h at the maximum
temperature to make the melt homogeneous. After rocking,
the alloys were quenched in ice water and the material was
separated from the quartz ampoules. Initially, a small
quantity of glassy alloy is kept in a molybdenum boat and
the chamber is evacuated to a vacuum of the order of 10 -6
torr. Thin films of GexSe80-xPb20 will be prepared by vacuum
evaporation technique. Well degassed corning glass plates,
having predeposited aluminium/silver/copper electrode for
electrical contact, will be used as a substrate for depositing
amorphous/crystalline films in the planer geometry and
different electrode gap. For deposition of film, highly
polished and thoroughly cleaned substrates are required.
First the substrates are cleaned using liquid detergent. Then
it is kept in dilute nitric acid. After this, they are cleaned
using distilled water and agitated ultrasonically in acetone.
The evaporant materials in the glassy form are kept in the
boat. The low tension (LT) supply for evaporation source is
obtained from a 230V input transformer by means of
parallel connections in the secondary side of the
transformer. Films will be prepared at a base pressure of 10 -5
torr by keeping the substrate at room temperature. Thickness
of thin films measure by using the single crystal thickness
monitor. Optical measurements can also be measure in thin
film by spectrophotometer. The optical spectrum was
recorded at room temperature for all samples using a UVVIS-IR (UV-3600) spectrophotometer (SHIMADZU, Japan)
in the wavelength range 300-2000nm.
x=0
x=2
x=6
x=8
55
50
at %
at %
at %
at %
45
Transmittance T %
40
35
30
25
20
15
10
5
0
200
400
600
800
1000
1200
1400
1600
1800
Wavelength  (nm)
Fig.1. Transmission spectra of GexSe80-xPb20
for different x.
x=0 at %
x=2 at %
x=6 at %
x=8 at %
6
5
Absorbance %
4
3
2
1
III.
OPTICAL CHARACTERIZATION
0
The optical constants (especially refractive index and
extinction coefficient) of these films have been determined
from transmission spectroscopy by using Manifcier’s
envelope method. The transmission and absorbance spectra
for GexSe80-xPb20 have been shown in Fig.1,2 within the
wavelength range 300-2000 nm. Transmission spectra of
vacuum evaporated GexSe80-xPb20 films were recorded at
room temperature with the help of Spectrophotometer
(Shimadzu U-3600) in the wavelength range 300-2000 nm.
The refractive index (n) has been determined from
transmission spectra by using the formula.
n = [N + (N2 + n0 2 n12)1/2]1/2
... (1)
Where n0 and n1 are the refractive index of air and
substrate, respectively. The number N is given by the
following equation
N = [(n02+ n12)/2] + 2n0n1 [(Tmax -Tmin)/ (Tmax+ Tmin)]
------ (2)
Where Tmax and Tmin are the upper extreme point and
lower extreme point at a particular wavelength, respectively.
The extinction coefficient (k) is given by an expression.
k = (-λ/4πt) lnP
... (3)
Where t is the thickness of the film and P is given by the
following equation:
P = C1/C2 [1-(Tmax/ Tmin)] / [1 + (Tmax / Tmin)] -- (4)
Where, C1 = (n + n0) (n + n1)
and C2 = (n-n0) (n1 - n)
Where n is the refractive index of the films at a particular
wavelength, n1 the refractive index of the substrates and n 0
is the refractive index of the air as given earlier.
200
400
600
800
1000
1200
1400
1600
1800
Wavelength  (nm)
Fig.2. Absorbance spectra of GexSe80-xPb20
for different x.
2.4
2.3
2.2
2.1
2
1.9
0
2
4
6
8
10
Fig.3.Refractive index (n) Vs Composition (x).
6
5
4
3
2
1
0
0
2
4
6
8
10
Fig.4. Absorption coefficient (α) Vs Composition (x).
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International Journal of Soft Computing and Engineering (IJSCE)
ISSN: 2231-2307, Volume-3, Issue-1, March 2013
It is observed that the refractive index decreases, as the
concentration increased. However, the extinction coefficient
increases with the increase of concentration. The optical
absorption and transmission measurements will be
performed on sample in the range 300-2000 nm. For
determination of optical band gap Eg, the Tauc’s plot
method will be used. The spectral dependence of the
absorption coefficient has the form αhν = B (hν - Eg) n/2
where hν is the photon energy, eg is the optical band gap
and n is a parameter depending on both the type of transition
(direct or indirect) and the profile of the electron density in
the valance and conduction band. The analysis of the nature
of glassy alloy thin film prepared by above mention
technique is done through study of X-ray diffraction pattern,
by which it is found that the films are of amorphous nature
as there is no significant peaks are obtained.
IV.
STRUCTURAL ANALYSIS
XRD patterns of the as-prepared amorphous alloy of
GexSe80-xPb20 (x = 0, 4, 6, 8,) thin films are presented in Fig.
5,6,7,8. There are no significant peaks observed for any of
the samples. It is clear that the alloys show an amorphous
nature.
Fig. 8 represents a curve between intensity & angle (2θ) of
Ge8Se72Pb20.
V.
VI.
Fig. 5 represents a curve between intensity
& angle (2θ) of Se80Pb20 .
CONCLUSION
Thin films of GexSe80-xPb20 (x = 0, 4, 6, 8) were deposited
using thermal evaporation method. These films were found
to be of amorphous nature as revealed from XRD result. The
optical band gap (Eg ) values were determined and are
found to decrease by increasing the concentration of Ge.
The absorption, extinction coefficients, and refractive index
were also obtained. The transmission spectra for the thin
films of GexSe80-xPb20 show strong absorption at wavelength
900nm and become highly transparent at wavelength above
800 nm. No significant changes are observed in intensity of
the spectra by increasing concentration of Ge. These results
might be useful for development of optical disks and other
semiconducting devices based on these films.
ACKNOWLEDGEMENT
The authors are thankful to the University Grant
Commission (U.G.C.), New Delhi for financial assistance
and also thankful to Department of Physics, G.K.V.
Haridwar for providing experimental facilities for this work.
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