Electron mobility and lifetime in a-Si1-xGex:H

A reduction in the mobility due to additional localization at the conduction band edge in a-SiGe-H alloys has been measured in transient photoconductivity experiments. The slope of the conduction band tail g(E), which shows hybrid structure (g(E) tends to become flat to a low level close to the mobility edge followed by a steep exponential decrease) flattens as the increased Ge concentration is enlarged, reflecting the additional chemical disorder introduced by the Ge. The neutral dangling bond density remains nearly unchanged for cGe ≤ 7at%, but for higher concentrations of Ge an exponential rise of defects causes the carrier lifetime to fall. DC dark conductivity measurements of phosphorous doped samples confirm the increase in localization which affects the doping efficiency and the Fermi-level position. In addition this method demonstrates a temperature dependent shift of the mobility edge at the conduction band which turns out to be more pronounced in a-SiGe:H than in a-Si:H