Influence of the incorporation of germanium on the tail states in amorphous alloys semiconductors

Abstract

Electrons and holes transport in hydrogenated amorphous silicon-germanium alloys, with fraction x of Ge between 0 and 24%, has been analyzed by means of a time of flight experiment. The shape of the valence band (VB) tail as well as the capture cross-section appear to be quite independent of x and of the deposition conditions for x = 0. These behaviours are in opposition with the enlargement of the conduction band (CB) tail and the increase of the apparent cross-section with Ge alloying. The last effect is interpreted as CB edge fluctuations. These results suggest that the CB tail is very sensitive to the increase of the disorder in the sample, as it is the case when increasing the Ge content, whereas the VB tail is not. We have also shown that the density of states beyond the VB tail, at say 0.45 eV above the VB edge, increases with x. A similar increase of the density of states below the CB tail, at say 0.4 eV below the CB edge, is observed in the same samples.