Monte Carlo simulation of impact ionization rates in InAlAs-InGaAs square and graded barrier superlattice

The Monte Carlo method is used to analyze impact ionization rates for electrons and holes in a ⟨100⟩ crystal direction In_0.52Al_0.48As-In_0.53Ga_0.47 As square and graded barrier superlattice. The calculated impact ionization rate ratio α/β is enhanced to more than 10 in a wide barrier and narrow-well square barrier superlattice. This is because the hole ionization rate β is greatly reduced in the narrower well superlattice, while the electron ionization rate α is less sensitive to well and barrier layer thickness. These results are explained by a combination of the ionization dead space effect for the barrier layer and the electron ionization rate enhancement in the well layer due to large conduction band edge discontinuity. Furthermore, it is found that in a graded barrier superlattice, the impact ionization rate ratio α/β is smaller than that for a square barrier superlattice having the same barrier and well thickness. This is due to the occurrence of hole ionization in the narrow bandgap region in graded barriers. The band structure effects on hot carrier energy distribution, as well as impact ionization, are also discussed