Vertical transport study of InAs/GaSb type-II superlattice nBp MWIR detectors using electron beam-induced current measurements

To improve the performance of photodiodes based on narrow-bandgap InAs/GaSb type-II strained layer superlattices (T2SLs), knowledge of the vertical minority carrier transport is necessary. For this purpose, the key parameters influencing vertical minority-carrier electron transport in an nBp MWIR detector were studied: diffusion length, lifetime, mobility. The detectors were designed with p-type, 10/10 ML, InAs/GaSb T2SL absorbers, targeting a 50% cut-off wavelength of 5.0 µm at 80 K. The nBp structure is attractive because the junction field predominately drops across a relatively wide-gap InAs/AlSb SL barrier, which reduces the expected generation-recombination dark current. Measurements of the electron beam-induced current (EBIC), combined with minority carrier lifetime results from microwave reflectance measurements, enabled the determination of the minority carrier diffusion length (Le) and mobility in the growth direction as a function of temperature. The Le was extracted at each temperature by fitting the EBIC data to analytical expressions for carrier collection efficiency. The EBIC measurements were also repeated at different electron-beam energies to vary the distribution of minority carriers near the surface to gauge the surface recombination velocity. Microwave reflectance allowed for accurate measurement of the minority carrier lifetime over a large dynamic range of excess carrier concentrations, enabling a separation of recombination mechanisms. The lifetime and extracted diffusion length data were then used to estimate the diffusion coefficient and mobility versus temperature by applying the Einstein diffusion relationship.