W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an $11.3\mu \mathrm{m}$ cutoff and 34% external quantum efficiency (at $8.6\mu \mathrm{m}$ ) operating at $80\mathrm{K}$ . The new WSL design employs quaternary ${\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.49}{\mathrm{In}}_{0.11}\mathrm{Sb}$ barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of $p\text{-}i\text{-}n$ WSL photodiodes with background-doped $i$ -region thicknesses varying from $1\text{to}4\mu \mathrm{m}$ , the authors determine that the minority-carrier electron diffusion length is $3.5\mu \mathrm{m}$ . The structures were grown on semitransparent $n\text{-}\mathrm{Ga}\mathrm{Sb}$ substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections.