Improving optical performance of W-structured type-II superlattices by adjusting As$_2$ and In flux

We are investigating ``W-structured'' type-II superlattices (W-SLs) for use in IR photodiodes. A typical W-SL period consists of a hole quantum well (QW) sandwiched by two electron QWs, which are in turn bounded by barrier layers (\emph{e.g.}, InAs/InGaSb/InAs/AlGaInSb), such that the bulk band-edges form a ``W'' pattern. The mini-band straddles the GaSb valence and InAs conduction bands, with a direct gap that can be varied from mid- to very long-wave IR. Photoluminescence (PL) has proven to be a sensitive indicator of photodiode performance, and here we present results correlating the PL intensity of W-SL's grown under varying conditions with their structure. X-ray diffraction is used to measure overall strain and periodicity, and cross-sectional STM (XSTM) is used to directly image the atomic-scale structure. We have discovered that roughness at InAs-on-InGaSb interfaces can be controlled by adjusting the As$_2$:In flux ratio and thereby reducing Sb cross-incorporation in the InAs. The smoother interfaces and reduced strain correlate with stronger PL. By independently adjusting the As$_2$ and In flux, W-SLs with low strain can be fabricated across a range of wavelengths with PL up to 20\% greater than we have previously achieved. We discuss our analysis using XSTM to correlate the PL spectra with the structure of the W-SLs as a function of As$_2$ and In flux.