Study of strain balance in long wavelength infrared InAs/GaSb superlattice materials

The epitaxial growth parameters optimized for mid wavelength infrared (MWIR) InAs/GaSb superlattice (SL) growth are not necessarily the best parameters for very long wavelength infrared (VLWIR) SL growth. While the cutoff wavelength of the SL structure can be easily extended from a MWIR to a VLWIR spectral range by increasing InAs layer thickness with a fixed GaSb layer thickness, the structural and optical properties of SLs are changing as well, and these changes are not necessarily beneficial to the material quality of VLWIR SLs. For instance, tensile strain in the SL rapidly increases as InAs layer thickness increases. This impacts the interface growth processes used to strain balance the average lattice constant of the SL to match the GaSb substrate, the interface engineering in a VLWIR SL is very different than that in a MWIR SL. Using a baseline SL design of 16 monolayers (MLs) InAs/7 MLs GaSb, a systematic study of controlling the Sb/As background pressure and shutter sequence during interface formation was performed in order to minimize tensile strain in the VLWIR SLs. The effect of various shutter sequences on the SL morphological quality and their impact on optical spectral response is reported. By inserting 0.5 MLs of InSb-like interfaces, using a migration-enhance-epitaxy technique, in the baseline SL design, while maintaining a total SL period of 23 MLs, we achieved a high structural quality, strain balanced LWIR SL with a photoresponse onset at 15 μm.