Band gap tuning of InAs∕GaSb type-II superlattices for mid-infrared detection

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.