| We have used cross-sectional scanning tunneling microscopy to examine MBE material quality in a series of GaSb/InAs/AlSb multiple quantum wells grown under conditions routinely employed for type-II quantum well and interband cascade lasers. The anion defects noted within the antimonide layers of these structures are established as arsenic for antimony (AsSb) substitutions following cross-sectional scanning tunneling microscopy experiments performed on MBE samples grown with intentional arsenic cross-incorporation. We have further identified the signatures and host lattice sites of surface (first layer, AsSb(1)) and subsurface (second and third layer, AsSb(2) and AsSb(3)) arsenic-for-antimony substitutional defects. These experiments also reveal that the frequency of arsenic cross incorporation in AlSb is roughly 2.5 times higher than that in GaSb for the same growth temperature and arsenic source conditions.; We have also investigated the influence of arsenic cross incorporation on the microstructure of the GaSb-on-InAs interfaces in these same structures. Cross-sectional scanning tunneling microscopy detects an excess of As Sb substitutions at the GaSb/InAs interface when compared to the amount expected from arsenic cross incorporation in the bulk; detailed analysis of the arsenide-on-antimonide interface roughness furthermore shows that these excess substitutions introduce a white-noise component that dominates the interface roughness spectrum at short wavelengths. We propose that this interfacial excess of AsSb substitutions is the remnant of an incomplete exchange between antimony from the vapor and arsenic at the GaSb-on-InAs heterojunction during the crossover from arsenide to antimonide growth.; We have also utilized cross sectional scanning tunneling microscopy to probe atomic-scale compositional grading in GaSb/AlSb/InAs quantum wells, as well as GaInSb/InAs superlattices. In particular we have studied how segregation, which is spatially inhomogeneous in the growth direction, may be experimentally distinguished from cross incorporation, which is spatially uniform. Layer-by-layer analysis of the SbAs(1) fraction as a function of distance from the arsenide-on-antimonide interface for InAs/GaSb, InAs/AlSb, and InAs/GaInSb heterojunctions indicates the presence of a "universal" antimony seed, as well as a "universal" segregation coefficient governing the subsequent propagation of this seed throughout the developing arsenide film. We believe the physical origin of this universality lies in the (1 x 3) surface reconstruction shared by GaInSb, GaSb, and AlSb, which is terminated by an antimony overlayer whose stochiometry quantitatively accounts for the common segregation seed. |
