Effect of thermal annealing on some electrical properties and optical band gap of vacuum evaporated Se65Ga30In5 thin films

Abstract

Electrical properties and optical band gap of amorphous Se65Ga30In5 thin films, which were thermally evaporated onto chemically cleaned glass substrates, have been studied before and after thermal annealing at temperatures above the glass transition temperature and below the crystallization temperature. The I–V characteristics, which were recorded in the temperature range (200–300 K), were obtained at different voltages and exhibit an ohmic and non-ohmic behavior at low (0–5 V) and high (5–18 V) voltages, respectively, for annealed and as-prepared films. Analysis of the experimental data in the high voltage range confirms the presence of space charge limited conduction (SCLC) for annealed and as-prepared films. The dependence of DC conductivity on temperature in the low voltage region shows two types of conduction channels: The first is in the range 270–300 K and the other at the lower temperature range (200–270 K). The conduction in the first region is due to thermally activated process, while in the other is due variable range hopping (VRH) of charge carriers in the band tails of the localized states. After annealing, the conductivity has been found to increase but the activation energy decreases. This is attributed to rupturing of Se–In weak bonds and formation of Se–Ga strong bonds. This process changes the concentration of defects in the films which in turn decreases the density of states N(EF) as predicted by Mott's VRH model. Analysis of the absorption coefficient of annealed and as-prepared films, in the wavelength range 300–700 nm, reveals the presence of parabolic densities of states at the edges of both valence and conduction bands in the studied films. The optical band gap (Eg) was obtained through the use of Tauc's relation and is found to decrease with annealing temperature.