We have used cross-sectional scanning tunneling microscopy (STM) to examine two important aspects of material and interface quality in the mixed-anion InAs/GaSb/AlSb system under growth conditions similar to those presently employed. for type-II quantum well and interband cascade lasers : (1) anion cross incorporation in the bulk (specifically, unintended substitution of As for Sb within the GaSb layers); and (2) the roughness spectrum at the common-anion AlSb-on-GaSb heterojunction. Two apparently different anion defects are noted in the GaSb layers, but the demonstration of a linear correlation between the bulk defect density observed with STM and the arsenic valve setting during antimonide layer growth clearly establishes background arsenic incorporation as the common origin for both of these defects. The roughness spectrum at the AlSb-on-GaSb interface displays a surprisingly short correlation length that is dominated by heterogeneity in the cation rather than anion sublattice. The relationship of these observations to fundamental materials-based performance limitations in type-II interband cascade lasers is briefly discussed.