Interband Tunneling in a Type-II Broken-Gap Superlattice

Due to its unique features, like tunable gap and high absorption coefficient, type-II superlattices receive growing interest. Substantial progress has been made in the technology of these materials as well as in the processing of superlattice-based devices. On the theoretical side, whereas the methods for superlattice analysis are well developed, for the superlattice-based devices they are well behind. Usually, such devices are modeled with semiclassical methods, in which the superlattice is treated as a bulk material having effective parameters extracted from its analysis with full quantum methods. As there is little theoretical justification for such a substitution, attempts have been made to model a whole superlattice-based device on a fully quantum level. In this paper, such modeling is presented for a broken-gap type-II superlattice diode: the nonequilibrium Green's function method is applied to the two-band model of an $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ superlattice $p$-$i$-$n$ diode. The focus is paid on the band-to-band (BTB) tunneling with the aim of an assessment of equations used for its semiclassical description. The results of calculations presented in the paper demonstrate that, in the superlattice diode, the BTB tunneling occurs only for certain values of the in-plane momentum $k$, for which electronic and hole subbands cross. This is in contrast to the bulk materials, for which there is a range of such $k$ values. The simulations reveal much more differences. Accordingly, care must be taken when applying semiclassical models to describe the interband tunneling in superlattice devices.