Frequency dependent electrical transport in bismuth-modified amorphous germanium sulfide semiconductors

Abstract

AC conductivity of Bi-modified amorphous germanium sulfide semiconductors Ge 20S 80− x Bi x from 100 Hz to 10 kHz and in the temperature range 180–445 K is reported. The ac conductivity is proportional to ω s . In Ge 20S 80 composition, the value of σ ac( ω) depends linearly on the frequency and is nearly independent of temperature. In samples containing Bi, σ ac( ω) increases with Bi content, especially at high temperature and low frequencies. The temperature dependence of σ ac( ω) and S in all compositions examined is interpreted by a correlated barrier hopping model (CBH). Analysis of the results reveals that electronic conduction in unmodified semiconductors ( x = 0, 4) takes place via bipolaron hopping. Addition of Bi in higher concentrations induces new types of defects which take part in a single polaron conduction process. Following Phillip's constraint theory and cluster model, the modified glass has microclusters of Bi 2S 3 with tetradymite structure, embedded in matrix of GeS 2 and (S) n . These microclusters may have n-type defects situated at excess S − atoms on the cluster surface; these defect may be taking part in the single polaron hopping process.