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Theoretical study on threshold energy and impact ionization coefficient for electrons in Si1−xGex

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The American Institute of Physics
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Abstract

Threshold energy and electron impact ionization coefficients (α) are calculated for unstrained and strained Si1−xGex on {100} silicon substrate using nonparabolic and ellipsoidal band structure for conduction band and k⋅p method for valence band. The threshold energy in the unstrained Si1−xGex 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 α in the strained Si1−xGex. As a result, the strained Si1−xGex has much larger α than pure silicon while the unstrained Si1−xGex does not due to the effect of alloy scattering and the relatively small change of the threshold energy.

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