Electron drift mobility model for devices based on unstrained and coherently strained Si/sub 1-x/Ge/sub x/ grown on >001< silicon substrate

摘要

The electron drift mobility for unstrained and coherently strained Si/sub 1-x/Ge/sub x/ grown on a >001< silicon substrate is analytically obtained for Ge fractions less than 30%. The method is based on the following two assumptions: the conduction bands of the unstrained alloy are Si-like for Ge fraction less than 30%, and in the case of the coherently strained alloy, strain-induced energy shifts occur in the conduction band valleys. The shifts in energy yield two different mobility values: one corresponding to the growth plane with a value larger than the unstrained mobility, and the other parallel to the growth direction and correspondingly smaller in value. In comparison to silicon, the results show a degradation of both the unstrained mobilities for doping levels up to 10/sup 17/ cm/sup -3/. Beyond this doping level, the strained mobility component parallel to the growth direction becomes slightly larger than the mobility of silicon.