n-Type conduction in a-Ge₂₀Bi₄Se₇₆ thin films

Abstract

Amorphous thin 6lms of Ge₂₀Bi₄Se₇₆ exhibiting n-type conduction are reported for the first time. As compared to a-Ge₂₀Se₈₀ films, the modified films (a-Ge₂₀Bi₄Se₇₆) show an increase in room-temperature electrical conductivity by several orders of magnitude, a transition from p-to n-type conduction, a decrease in the optical gap from 1.9 to 1.5 eV and a drastic fall in the electrical activation energy from 0.85 to 0.17eV. The correlation of the electrical and optical data suggests the existence of localized states at the conduction band edge arising from the incorporation of bismuth. The decrease of the activation energy for electrical conduction is much larger than the decrease of one-half the optical gap. This indicates unpinning of the Fermi level with a shift towards the conduction band edge.