Models of GaSb/InAs type-II infrared detectors at very long wavelengths: band offsets and interface bonds

摘要

We report a series of studies on GaSb/InAs superlattices, pseudomorphically strained to GaSb buffer layers. These heterostructures have recently been grown using molecular beam epitaxy for very long wavelength infrared photodetectors. We calculated the valence band alignment with the widely used model solid theory and evaluated the electronic band structure by employing an empirical pseudopotential (EP) scheme. The absorption coefficient was subsequently calculated at the far-infrared range of the spectrum, using density matrix theory. This approach predicted optical cut-off wavelengths, showing poor agreement with values obtained from absolute spectral responsivity measurements. Variation of the bond types at the interfaces (InSb- or GaAs-like, or a combination) led to significant changes in the cut-offs and absorption magnitude. However, no combination of interface bonds gave rise to results which were consistent with the experimental cut-offs. Addressing the band offset, we employed a recently published, interface-specific model as an alternative to the model solid theory-derived value. Including this model in our EP scheme, we obtained good agreement with experiment for a superlattice containing an InSb-like bond at each heterojunction, the configuration which had been fabricated in the photodetectors.