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The superlattice (SL) of a 40 period InAs∕GaSbSL structure were varied around the 20.5ÅInAs∕24ÅGaSb design in order to produce a device with an optimum mid-infrared photoresponse and a sharpest photoresponse cutoff. The samples for this study were grown by molecular beam epitaxy with precisely calibrated growth rates. Varying individual layer width around the nominal design, we were able to systematically change the photoresponse cutoff wavelength between 4.36 to 3.45um by decreasing the InAs width from 23.5 to 17.5Å, and between 4.55 to 4.03μm by increasing the GaSb width from 18 to 27Å. Therefore, the cutoff changes faster with decreasing InAs rather than increasing GaSb width. However, increasing GaSb width more effectively enhances the sharpness of photoresponse near band edge. The effect of design parameters on the photoresponse cutoff and other effects are explained by a nonperturbative, modified envelope function approximation (EFA) calculation that includes the interface coupling of heavy, light, and spin-orbit holes resulting from the in-plane asymmetry at InAs∕GaSb interfaces. Using the modified EFA model, the SL design at fixed period of 44.5Å was adjusted for the optimum performance.

Using a newly developed envelope function approximation model that includes interface effects, several InAs∕GaSb Type-II superlattices (SLs) were designed for the 4μm detection threshold. The present model predicts that a given threshold can be reached with a wide range of progressively thinner SL periods and these thinner designs hold a promise of higher mobilities and longer Auger lifetimes, thus higher detector operating temperatures. The proposed SL structures were grown by molecular-beam epitaxy with slow growth rates. As predicted, the band gaps of SLs determined by low-temperature photoluminescence remained constant around 330meV for the samples in the period range from 50.6to21.2Å.

We report the growth of a new closely lattice-matched II-VI heterostructure: ZnTe/CdSe (Δa/a∼0.3%). Epilayers of zinc blende CdSe grown on ZnTe buffer layers are shown to have much better quality than those grown earlier (with a 7% mismatch) on GaAs substrates. This permitted the first successful growth of high quality superlattices of ZnTe/CdSe. The superlattices were studied by x-ray diffraction, transmission electron microscopy, and optical techniques. Results of photoluminescence and optical transmission measurements show that ZnTe/CdSe superlattices have a very small valence-band offset.

The effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. Although the oscillator strengths are predicted to be larger for thinner period superlattices, the absorption coefficients are comparable because of the compensating effect of larger band widths. However, larger intervalence band separations for thinner-period samples should lead to longer minority electron Auger lifetimes and higher operating temperatures in p-type SLs. In addition, the hole masses for thinner-period samples are on the order the free-electron mass rather than being effectively infinite for the wider period samples. Therefore, holes should also contribute to photoresponse. A number of superlattices with periods ranging from 50.6 to 21.2 Å for the 4 μm detection threshold were grown by molecular beam epitaxy based on the model design. Low temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5. Overall, the present study points to the importance of interfaces as a tool in the design and growth of thin superlattices for mid-IR detectors for room temperature operation.