Electron and hole transport in a-Si1−xGex:H alloys

Abstract

Amorphous Si1−xGex:H films and p-i-n diodes were fabricated by decomposition of SiH4/GeH4 mixtures in a triode glow-discharge reactor. The photoconductivity under AM1 illumination in these alloys was constant over a range of band gaps between 1.8 (x=0) and 1.5 eV (x=0.25), while the solar cell conversion efficiency decreased at the same time from 8.6% to 4.3%. This can be explained by a reduction in the μτ product for holes with rising x in undoped samples as revealed by time-of-flight experiments. In contrast to μτ, the hole drift mobility μD,h remains constant. The opposite behavior is observed for electrons, whose drift mobility μD,e decreases as the mobility activation energy EA increases. The relation between EA and μD,e for variable x is suggestive of the Meyer–Neldel rule for the conductivity. In conjunction with space-charge-limited current and sub-band-gap absorption data we conclude that only the conduction-band tail is widened by the incorporation of Ge while the valence-band tail remains unaffected. The transport data for x>0 can no longer be explained by a purely exponential conduction-band tail. The rising density of midgap states shows an increasing capture cross section for holes and a decreasing one for electrons.