Electronic Properties of Hydrogenated Amorphous Silicon--Germanium Alloys and Long-Range Potential Fluctuations

The charge transport properties and microstructure of hydrogenated amorphous silicon-germanium alloys (a-SiGe:H) prepared by the hot-wire chemical vapor deposition (HWCVD) process as a function of alloy composition have been investigated in detail by employing the photoconductive frequency mixing and small angle X-ray scattering techniques. Evidence for the presence of long-range potential fluctuations in a-SiGe:H alloys is revealed from the measurements of electric field dependence of the drift mobility. The effect of the long-range potential fluctuations is enhanced by the addition of Ge to the alloy system that results in the deterioration of the opto-electronic properties of a-SiGe:H alloys. Through the drift mobility field dependence, the depth and range of the potential fluctuations as a function of alloy composition are determined, and subsequently the charged defect density. It was found that at a composition of ∼ 10% Ge in Si, the photoresponse begins to decrease monotonically with increasing Ge content due to the decreases in the drift mobility and lifetime as a result of an increase in the concentration of charged defects, which lead to the long-range potential fluctuations whose depth increases, while the range decreases. The sharp changes in these parameters are demonstrated to be attributed to a concurrent abruptly increased structural heterogeneity due to the introduction of microvoids.