Photoconductivity, optical and electrical properties of GaSe 0.95S 0.05 : Na mixed crystals

Abstract

To identify sodium-related and intrinsic structural defect levels in GaSe 0.95S 0.05 : Na mixed crystals; photoconductivity (PC), electrical conductivity (EC), current–voltage characteristics (CVC) and optical absorption spectrum (OA) were investigated. It was observed that these mixed crystals exhibit impurity PC bands at 1.81, 1.55 and 1.37 eV.These bands can be attributed to optical transitions from the localized acceptor levels located at energies of about 0.225, 0.48 and 0.67 eV to the indirect conduction band. Optical absorption spectrum at 300 K exhibits indirect band gap of 2.035 eV. Furthermore, it was found that photocurrent increases monotonically with increasing temperature from 300 K up to T cr=357 K and decreases for T> T cr. Moreover, the photocurrent–light intensity dependence in these crystals obeys the power law I ph∝ F γ with γ between 0.8 and 1. This indicates the continuous distribution of localized states in the band gap. It was also found that, temperature dependence of dark current in Ohmic and space-charge-limited current (SCLC) regions exhibits the same single thermal activation energy 0.151±0.004 eV. This reflects the presence of single set of discrete hole trapping level at E t=0.151±0.004 eV. The acceptor level at 0.225 eV from PC spectrum was also detected from temperature dependence of EC. These localized levels confirm the extrinsic nature of EC and PC in these mixed crystals. In the light of the experimental results; an energy band diagram can be suggested.