The role of annealing temperature on the optical energy gap and Urbach energy of Se:2%Sb thin films

: The optical energy gap(E opt ) and the width of the tails of localized states in the band gap (ΔE) for Se:2%Sb thin films prepared by thermal co-evaporation method as a function of annealing temperature are studied in the photon energy range ( 1 to 5.4)eV.Se2%Sb film was found to be indirect transition with energy gap of (1.973,2.077, 2.096, 2.17) eV at annealing temperature (295,370,445,520)K respectively. The E opt and ΔE of Se:2%Sb films as a function of annealing temperature showed an increase in E opt and a decrease in ΔE with increasing the annealing temperature. This behavior may be related to structural defects and dangling bonds.


Introduction:
Optical properties of thin films depend mainly on their volume and surface structures [1][2].Film structure is affected by its thickness, conditions of preparation, films material,substrate bulk material and its treatment after preparation [3][4][5].In recent years,the optical memory effects in amorphous semiconducting films have been investigated and utilized for various device applications [6].The optical absorption coefficient for many amorphous and glassy materials is found to obey the relation: αhυ=ß(hυ-Eopt) r ………………(1) where υ is the frequency of the incident radiation, β is a constant which depends on density of state of conduction and valence bands, r is a constant depend on the nature of the transition and Eopt is the optical energy gap.
The relation was first derived by Tauc and Colleagues [1] who assumed that the electron density of states at band edges in regions of localized states is a parabolic function of energy .Davis and Mott[7] obtained the same relation.The width of the tails of localized states at the band edges can be estimated using the Urbach relation [8]: where αo is a constant and ΔΕ is a measure of the extent of the band tailing in the band gap of the material and determined from the reciprocal of the slope of lnα against photon energy.This report will give results of a systematic study of the optical properties of Se:2%Sb amorphous thin films at different annealing temperature (295,370,445&520)K.

Experiment:
The purity of the materials are (99.999%pure), were prepared at room temperature by thermal co-evaporation technique from two molybdenum boats in a vacuum at a pressure of about(5x10 -6 )Torr using Edwards 306 coating unit.The thickness of films was (~300±5)nm measured using Tolonsky methods.
The glassy nature of the samples were investigated using X-Ray diffraction.Spectral characteristics in the wavelength rang (200-1100)nm were measured using UV-visible recording spectrophotometer(UV-160 Schematize).The absorption coefficient(α) calculated from the relation [9]: *Department of Physics ,College of Science, University of Baghdad , Jadirya-Baghdad-IRAQ  3) where A is the absorbance and t is the thickness of the films.The optical band gap calculated from the intercept of (αhυ) 1/2 data plotted as function of photon energy.

Results and discussions:
The films samples unannealed and annealed at various temperature were amorphous which are conformed by X-ray diffraction as shown in fig.The increasing in annealing temperature changes the density of localized state to a lower values as well as the localized state near the edges.Band gap at high temperature may be related to a decrease the structure defects such as dangling bonds, voids and decrease the disorder of the atomic bonds.The band tailing is a function of structural defects, therefor it decrease with increasing the annealing temperature as shown in Fig. (5).

Conclusion:
The optical transmission of Se:2%Sb films with thicknesses of 300 nm have been measured in order to drive data on the absorption edge and band tailing .They found to be a indirect energy gap.The Eopt for Se:2%Sb films showed an increase from a value of 1.978eV at room temperature to 2.17eV at 520K.While ΔΕ showed to be decreases with increasing the annealing temperature.These results may be related to a decrease in voids and dangling bonds.12. Nang,T.T.,M.
(1).Fig.(2) shows the plots of absorption coefficient (α) versus photon energy (hυ) at different annealing temperature.As evident from Fig.(2), α varies exponentially with hυ in the measured range of α.The absorption edge at room temperature is in a good agreement with the result on Se:2%Sb glass reported by Al-Ani et.al.[10,11].Fig.(3)show plots (αhυ) 1/2 against photon energy of Se:2%Sb films deposit at room temperature and annealed to (370,445,520)K.Fig.(4) shows the plot of ln α against photon energy of Se:2%Sb films deposited at room temperature.The reciprocal of the slope of curve give the value of corresponding ΔΕ (0.45, 0.38, 0.29, 0.23) eV at annealing temperature (295,370,445.520)Krespectively The extrapolated value of the indirect energy gap were (1.973,2.077,2.096,2.17)eVat annealing temperature of (295,370,445,520)K respectively as shown in Fig.(5) which is shows the variation of Eopt and ΔE with T. The value of energy gap at room temperature is in agreement with Nang et.al.[12] but it´s disagree with Choudhuriet.al.[13]  which they found that the optical energy gap decrease with heat treatment.