# Effect of Ge Addition on the Optical Band Gap and Refractive Index of Thermally EvaporatedAs2Se3 Thin Films

Authors: Pankaj Sharma; S. C. Katyal
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

The present paper reports the effect of Ge addition on the optical band gap and refractive index ofAs2Se3 thin films. Thin films of As2Se3 and (As2Se3)90Ge10 were prepared by thermal evaporation technique at base pressure 10−4 Pa. Optical band gap and refractive index were calculated by analyzing the transmission spectrum in the spectral range 400–1500 nm. The optical band gap decreases while the refractive index increases with the addition of Ge to As2Se3. The decrease of optical band gap has been explained on the basis of density of states; and the increase in refractive index has been explained on the basis increase in disorder in the system.


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## Body

## 1. Introduction

Chalcogenide glasses based on
the chalcogen elements S, Se, and Te are attractive and widely investigated
materials as they possess high optical transparency in the IR region. They have
low phonon energy, high photosensitivity, easy fabrication and processing, and good chemical durability. So they are used
in ultrafast optical switches, frequency converters, optical amplifiers,
optical recording devices, integrated optics, infrared lasers, and infrared
transmitting optical fibres [1–3]. Chalcogenide semiconducting alloys have
found application not only due to their electrical and thermal properties, but
also due to their optical properties. Many amorphous semiconducting glasses, in
particular selenium, exhibit a unique property of reversible transformation. Its various device applications like rectifiers, photocells,
xerography, switching and memory, and so on have made it attractive, but pure
selenium has disadvantages like short lifetime and low sensitivity. Due to high
glass-forming ability of Se, it represents a good host matrix for the
investigation of chalcogenide glasses in the bulk and thin film forms [4–7].
Thus, the above problems can be overcome by alloying Se with some impurity
atoms (Bi, Te, Ge, Ga, As, etc.), which gives higher sensitivity, higher
crystallization temperature, and smaller ageing effects [8–10]. The As-Se-Ge system has been extensively studied [11–13] because of the fact that Ge,
As, and Se are the elements of same period in groups IV-VI, and it brings about the
covalent character of the interaction between their atoms. This results in a
broad glass formation region in As-Se-Ge system [14] among all investigated
three-component chalcogenide systems.The present work reports the
effect of Ge addition on the optical band gap and refractive index of arsenic
selenide thin films. The straightforward technique proposed by Swanepoel has
been used for analyzing the transmission spectra in the spectral range 400–1500 nm [15].


## 2. Experimental Procedure

Two compositions of (As2Se3)100-xGex(x=0,10) chalcogenide system were prepared using melt quench technique. The
materials (5 N pure) were sealed in evacuated (~10-4 Pa) quartz
ampoules each of 3 g batch, weighed according to their atomic weight
percentage.  The sealed ampoules were kept
inside a furnace where the temperature was increased up to 950°C in five
steps (200°C, 400°C, 600°C, 950°C)
gradually at a heating rate of 2-3°C/min.
The ampoules were frequently rocked for 24 hours at the highest temperature to
make the melt homogeneous. The quenching was done in ice cold water. Thin films
were prepared on cleaned glass substrates (microscopic glass slides). The
substrates were cleaned with soap solution, ultrasonically cleaned by
trichloroethylene, acetone followed by methyl alcohol. In last, the substrates
were washed by DI water and dried in oven at approximately 110°C.
Thin films of the alloys were prepared by thermal evaporation technique (Vacuum coating unit HINDVAC
12A4D Model) at room temperature and base pressure of
~10-4 Pa using a molybdenum boat. The thickness of the deposited
thin films has been measured by thickness monitor (DTM-101). The compositions
of evaporated samples have been measured by an electron microprobe analyzer
(JEOL 8600 MX) on different spots (size ~ 2 μm). For the composition analysis,
the constitutional elements (As, Se, and Ge) and the bulk original alloys, that
is, (As2Se3)100-xGex, are taken as
reference samples. The composition of 2×2 cm2 sample is
uniform within the measurement accuracy of about ±0.2%. Amorphous nature of
the bulk samples and thin films was checked by XRD technique. No prominent peak
is observed in the bulk as well as in thin films. The normal incidence
transmission spectra of (As2Se3)100-xGex thin films were obtained by a double beam ultraviolet-visible-near infrared
spectrophotometer [Hitachi-330], in the transmission range 400–1500 nm. The
spectrophotometer was set with a suitable slit width of 1 nm in the measured
spectral range. All optical measurements were performed at room temperature
(300 K).


## 3. Results and Discussion

Optical
transmission (T) is a very complex function and is strongly dependent on the
absorption coefficient (α). Figure 1 shows the variation of transmission (T) with
wavelength (λ) in As2Se3 and (As2Se3)90Ge10 thin films. According to Swanepoel’s method [15], the envelope of the
interference maxima and minima of transmission spectra can be used for deducing
optical parameters. The refractive index (n) of the thin films is
obtained by the envelope method by making use of the following expressions. In
the transparent region where the absorption coefficient α≈0, the refractive index (n) is given
by (1)n=[N+(N2−s2)1/2]1/2, where (2)N=2sTm−(s2+1)2 and Tm is the envelope
function of minimum transmittance, and s is the refractive index of
substrate. In the weak region, where absorption coefficient α⁢≠0,
the transmittance decreases due to the influence of α, the value of N in (1) is given by (3)N=2sTM−TmTMTm+(s2+1)2, and Tm is the envelope
function of maximum transmittance.Figure 1
Transmission spectrum for As2Se3 and (As2Se3)90Ge10 thin films.The extinction coefficient (k)
can be calculated using the relation k=(λ/4πd)ln⁡(1/x), where d is the thickness of the film
and x is the absorbance [15].  If n1 and n2 are the refractive indices of two adjacent
maxima or minima at wavelengths λ1 and λ2,
then the thickness of the film is given by d=λ1λ2/2(λ1n2−λ2n1).
The thickness of the thin films calculated using the above relation is given in
Table 1 and found to be within
±35 nm with thickness values measured while depositing the thin films using
thickness monitor.Table 1
Values of thickness (d),  n, k, and Egopt for As2Se3 and (As2Se3)90Ge10 thin films.
Compositiond (nm)at 800 nmEgopt (eV)nkAs2Se39342.500.00541.52(As2Se3)90Ge109002.640.00761.46The variation of refractive
index (n) and extinction coefficient (k) with wavelength is shown
in Figures  2 and 3, respectively. From
figures, it is clear that both refractive index and extinction coefficient decrease with the
increase of wavelength for the thin films under study. The decrease in the
value of refractive index with wavelength shows the normal dispersion behavior
of the material. On the part of Ge addition to As2Se3 thin
films, the refractive index has been found to have higher values. This increase
in the refractive index may be ascribed to increase of disorder in the
structure, change in stoichiometry and internal strain caused with the addition
of Ge. The values of n and k at 800 nm (as nearly at this
wavelength the spectrum region changes) are given in Table 1.Figure 2
Plot of refractive index versus wavelength for As2Se3 and (As2Se3)90Ge10 thin films.Figure 3
Plot of extinction coefficient versus wavelength for As2Se3 and (As2Se3)90Ge10 thin films.The optical band gap(Egopt) has been determined from absorption
coefficient data as a function of photon energy, according to the generally
accepted “nondirect transition” model for amorphous semiconductors [16],
proposed by Tauc [17] αhv=B(hv−Egopt)n, where hν is the photon energy, α is the absorption coefficient, Egopt the optical band gap, B is band tailing
parameter, and n=1/2 for direct band gap and n=2 for indirect
band gap. Figure 4 shows the variation of (αhv)1/2 with hν. Optical band gap can be determined by the extrapolation of best-fit
line between (αhv)1/2 and hν to intercept the hν axis (α=0)  for thin films. The
region of the transparency is limited by an intrinsic absorption at short and
long wavelengths, the position at short wavelength corresponds to λ=hc/Eg, where λ is wavelength, h is Planck’s constant, c is
light velocity, and Eg is energy width of the forbidden band [18]. So
best-fit line between (αhv)1/2 and hν to
intercept the hν axis (α=0) is taken only for short wavelengths,
that is, where absorption is strong. It is found that the optical band gap
decreases with the addition of Ge to As2Se3 thin films.
The variation in optical parameters with Ge incorporation is mainly due to
change in stoichiometry. The Ge additive in As2Se3 must
bring about a compositional change of host network of As-Se. In the fundamental
absorption region, the absorption is due to the transition from the top of
valence band to the bottom of the conduction band. Addition of Ge in As2Se3 thin film may cause an increase in the density of state-in-the-valence band.
The addition of Ge may also create localized states in the band gap [19]. This
will lead to a shift in the absorption edge towards lower photon energy, and,
consequently, the decrease in the optical energy gap can be explained by the
increased tailing [20] of the conduction band edge into the gap due to the
addition of germanium impurities.Figure 4
Plot of(αhv)1/2 versus hv for As2Se3 and (As2Se3)90Ge10 thin films.


## 4. Conclusion

The thermally evaporated thin films
of the As2Se3 as well as (As2Se3)90Ge10 were analyzed using their transmission spectra in the spectral range 400–1500 nm. The
optical parameters viz. n,k, and Egopt were calculated. Refractive index and
extinction coefficient were found to increase while optical band gap decreases
with Ge addition to As2Se3. The increase in the
refractive index with the addition of Ge has been explained on the basis of the
increase in disorder in the system. The decrease in optical band gap with Ge
addition has been explained on the basis of density of states.



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*Source: 826402-2008-03-12.xml*