Theoretical Study on Threshold Energy and Impact Ionization Coefficient forElectrons in Si(1-x)Ge(x)

摘要

Threshold energy and electron impact ionization coefficients (alpha) arecalculated for unstrained and strained Si(1-x)Ge(x) on (1OO) silicon substrate using nonparabolic and ellipsoidal band structure for conduction band and k.p method for valence band. The threshold energy in the unstrained Si(1-x)Ge(x) is smaller than that in pure silicon due to the reduced band-gap energy. The strain causes band degeneracy lifting for both the conduction band and valence band. It gives an additional band-gap narrowing which leads to a much smaller threshold energy. On the basis of these results, the electron impact ionization coefficient is estimated up to 30% germanium using a Monte Carlo simulation. The reduced threshold energy is found to be the most dominant factor in determining alpha in the strained Si(1-x)Ge(x). As a result, the strained Si(1-x)Ge(x) has much larger a than pure silicon while the unstrained Si(1-x)Ge(x) does not due to the effect of alloy scattering and the relatively small change of the threshold energy.