HOLE IMPACT IONIZATION ENHANCEMENT IN ALXGA1-XSB

The threshold energy and scattering rate for impact ionization in Al xGa1-xSb have been calculated for compositions x near the value where the spin-orbit-split gap Δ equals the band gap Eg. We have found that a minimum in the threshold energy for impact ionization near the Δ=Eg composition cannot explain the enhancement of the hole ionization coefficient, although it has been proposed as a possible mechanism. Rather, an increased scattering rate is responsible for this enhancement. It is caused by a minimum change of momentum in the hole ionization near the threshold and the mixing of an s-like state into the valence-like state induced by a composition disorder in this ternary compound. Our results indicate that the maximum hole ionization coefficient will occur at a composition x that lies between zero and the composition at which Δ=Eg.