Glassy substrates TeSeSn thin films were thermally evaporated onto chemically cleaned glass. The as-deposited (as-prepared) and annealed thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical transmission. The optical absorption of the as-prepared and annealed TeSeSn thin films is studied in the wavelength range of 300 nm - 900 nm. The direct optical energy gap ( E g) increases from 1.989 to 2.143 eV with increasing the thickness of the as-prepared films from 100 to 200 nm. The annealed TeSeSn films showed a decrease in the optical energy gap with increasing the annealing temperature. The effect of heat treatment on the lattice dielectric constant ( ε L) and carrier concentration ( N) are also studied.
Chalcogenide glasses are used as photographic materials and have gained much importance recently. The shortcomings of pure glassy Se used for photographic drums are its short lifetime and low sensitivity [
The present work deals with some experimental observation on the effect of heat treatment on structure and optical properties of TeSeSn films. Also, we report the study of the dependence of the optical properties of the TeSeSn thin films on annealing temperature.
The bulk TeSeSn was prepared by the melt-quench technique. Materials (99.99% pure) Te, Se and Sn (from Aldrich, UK) were weighted (6 g total weight) according to their atomic percentage. The weighted elements were placed into a quartz glass ampoule and sealed under vacuum of 10 - 5 Torr. The sealed ampoule was heated in Heraus programmable tube furnace (type R 07115). The heating rate was approximately 4 K/m. The temperature was kept at 800˚C for 14 h and the ampoule was rocked during the melt process to ensure complete mixing and reaction. After that, the ampoule was quenched into ice-water mixture.
Thin films were prepared by thermal evaporation under vacuum of 10 - 5 Torr using the Edwards E-306 coating system. A constant evaporation rate (4 nm/sec) was used to deposit the films. The evaporation rates as well as the films thickness were controlled using a quartz crystal monitor (FTM5). TeSeSn films were annealed at different temperature (323 ≤ Tann ≤ 423 K) for one hour under gas Nitrogen. The morphology for as-deposited and annealed films were investigated using (SEM) type JEOL JSM-T200. The crystalline phases for as-prepared and annealed films were identified using a Philips diffractometer type 1710. The X-ray diffraction patterns were analysed using some software. Peakfit program was utilized to determine and identify the peaks of the patterns. These peaks were employed using Williamson-Hall method to estimate the crystallite size and lattice strain for the samples. Chekcell program was used to obtain and refine the cell parameters [
The morphology of the samples for as-prepared and after annealing temperature was examined using SEM. The samples were gold coated before SEM examination to study the surface morphology. The scanning micrograph specimens of as-prepared and annealed at 373 K for hour are shown in
In order to determine the crystalline phases that appeared in SEM the X-ray diffraction pattern of films was analyzed.
vary quite differently with respect to Bragg angle, θ:
where K is the crystallite shape constant (≈0.89), ε lattice strain and L crystallite size. First contribution varies as 1/cosθ and the other as tanθ. The first contribution of crystallite size was measured by the Scherrer method as given in Equation (1) [
If we multiply this equation by cosθ we get:
and comparing this to the standard equation for a straight line (m = slope; c = intercept). We see that by plotting βtotcosθ versus sinθ we obtain the strain component from the slope (4ε) and the size component from the intercept (Kλ/L). Such a plot is known as a Williamson-Hall plot [
The deduced L, δ and ε are listed in
The spectral distribution of transmittance and reflectance for as-prepared at different thickness TeSeSn films are shown in
Average of dislocation density δ × 1014 (lines/m2) for SeTe phase | Average of strain values (lin−2∙m−4) ×10−3 for SeTe phase | Average of crystal size L (nm) for SeTe phase | Kind of phase | (h k l) | d. (stand.) | d. (exp.) | TeSeSn |
---|---|---|---|---|---|---|---|
Se | (1 1 2) | 3.895 | 3.894 | As-prepared | |||
SeTe | (1 1 2) | 3.574 | 3.570 | ||||
40.43 | 13.43 | 15.728 | SeTe | (4 2 0) | 2.221 | 2.225 | |
SeTe | (0 5 2) | 1.738 | 1.738 | ||||
Te | (2 1 2) | 1.310 | 1.311 | ||||
Se | (1 1 2) | 3.895 | 3.873 | Ann. at 373 K | |||
SeTe | (1 1 2) | 3.574 | 3.553 | ||||
40.75 | 11.72 | 15.666 | SeTe | (4 2 0) | 2.221 | 2.225 | |
SeTe | (0 5 2) | 1.738 | 1.734 | ||||
Te | (2 1 2) | 1.310 | 1.311 | ||||
Se | (1 1 2) | 3.895 | 3.873 | Ann. at 423 K | |||
SeTe | (1 1 2) | 3.574 | 3.562 | ||||
62.52 | 11.36 | 12.647 | SeTe | (4 2 0) | 2.221 | 2.228 | |
Te | (2 1 2) | 1.310 | 1.311 |
Sample | a (in Å) | c (in ´10−2 nm) | Volume of unit cell (in ´10−3 nm3) |
---|---|---|---|
As-prepared | 44.58 | 59.03 | 101.602 |
Ann. at 373 K | 44.52 | 59.65 | 102.405 |
Ann. at 423 K | 44.53 | 59.49 | 102.140 |
where d denotes the film thickness, R and T are the reflection and transmission coefficients, respectively.
According to Tauc [
where B is a characteristic parameter (independent of photon energy) for respective transitions [
direct band gap transition in TeSeSn films. The values of optical energy gap, Eg, have been determined by extrapolating the linear portions of the respective curves to (αhν)2 = 0. In the exponential edge region, the absorption coefficient is governed by the relation [
where Ee is the band tail width and hν is the photon energy, therefore, plotting the dependence of (lnα) versus (hν) should give a straight line. The inverse of the slope gives the band tail width (Ee) of the localized states at the band gap as shown in
The increase of energy gap (Eg) and the decrease of band tail width (Ee) may be explained in terms of unsaturated bonds present in amorphous materials. It is known that unsaturated bonds are produced as a result of an insufficient number of atoms deposited on the amorphous films [
On the other hand, the values of refractive index (n) and extinction coefficient (k) have been calculated using the following relations [
where α is absorption coefficient and R is reflectance, The spectral dependence of refractive index (n) and extinction coefficient (k) on the wavelength for as-prepared at different thickness TeSeSn thin films are shown in
In order to deduce the optical properties as a function of the annealing temperature, an analysis of the transmittance and reflectance spectra was done. The absorption coefficient was plotted as (αhν)2 versus photon energy (hν) for TeSeSn films (100 nm thickness) at various annealing temperature ranges from (323 ≤ Tann. ≤ 423), see
It is observed from
Tann. (K) | Eg (eV) | Ee (eV) | Eo (eV) | Ed (eV) | εL | N/m*× 1057 (m−3/kg) |
---|---|---|---|---|---|---|
As-prepared | 1.989 | 0.121 | 3.671 | 23.266 | 24.778 | 11.820 |
323 | 1.942 | 0.130 | 3.417 | 28.527 | 27.347 | 15.007 |
373 | 1.876 | 0.136 | 3.340 | 35.021 | 31.480 | 17.959 |
423 | 1.809 | 0.141 | 3.077 | 29.056 | 33.368 | 22.654 |
phous materials, the width of the localized tail states near the mobility edges of the band gap depends on the degree of disorder and the density of defects present in the amorphous state. In particular, it is known that unsaturated bonds together with some saturated bonds, such as like dative bonds [
where Eo is the oscillator energy, Ed is the dispersion energy, which measures the average strength of the interband optical transition and E is the photon energy.
where ε1 is the real part of dielectric constant, εL is the lattice dielectric constant or (the high frequency dielectric constant), λ is the wavelength, N is the free charge carrier concentration, εo is the permittivity of free space (8.854 × 10−12 F/m), m* is the effective mass of the charge carrier and c is the velocity of light. The real part of dielectric constants ε1 = n2 was calculated at different values of λ. Then, the obtained values of ε1 are plotted as a function of λ2 as shown in
The high frequency dielectric constant εL and the ratio N/m* of the as-deposited and annealed films can be determined. The values of these two parameters with annealing temperature are given in
TeSeSn thin films were deposited onto glass substrates under a vacuum of 10−5 Torr using a vacuum evaporation technique. X-ray analyses showed that the average particle size decreased with the increase in annealing temperature. The optical absorption measurements indicate that the absorption mechanism is due to a direct forbidden transition. The optical energy gap (Eg) increases with the increase of the film thickness and decreases with the increase of the annealing temperature; the optical data may also be fitted to an exponential Urbach formula. The optical parameters, (Eg) and (Ee), are affected by both film thickness and annealing temperature; this confirms the effect of these two factors on the density of localised states. It was found that the dispersion of refractive index obeyed the single oscillator model. On the other hand, the high frequency dielectric constant (εL) and the ratio (N/m*) increase with increasing the annealing temperature.
The authors wish to thank the Ibb University for financial support. Assuit University (Department of Physics) has been thanked for availing the research equipment.
A. Elwhab B.Alwany,O. M.Samir,Mohammed A.Algradee,M. M.Hafith,M. A.Abdel-Rahim, (2015) Investigation of the Effect of Film Thickness and Heat Treatment on the Optical Properties of TeSeSn Thin Films. World Journal of Condensed Matter Physics,05,220-231. doi: 10.4236/wjcmp.2015.53023