AlAs-in-AlSb digital alloy superlattice morphology versus growth temperature

We report a systematic study of how growth temperature affects the quality of AlAs-in-AlSb digital alloy superlattices grown by molecular beam epitaxy for barrier layers in type-II W-structure infrared lasers. Using cross-sectional scanning tunneling microscopy to characterize the atomic-scale structure of the material, we find substantial differences in the superlattice morphology for growth temperatures between 435 and 540 °C. At lower growth temperatures, the AlAs forms three-dimensional clusters, with continuous structures threading through multiple periods of the superlattice. With increasing temperature, the morphology of the digitally doped AlAs layers consistently improves, with nearly perfect delta doping observed at the highest temperatures studied. The changes in the superlattice structure can be attributed primarily to the known temperature dependence of the AlSb growth front morphology, with secondary effects associated with anion-exchange at the interfaces and the different surface reconstructions on the two growth surfaces.