The electronic band structures and optical properties of type-II superlattice (T2SL) photodetectors in the mid-infrared (IR) range are investigated. We formulate a rigorous band structure model using the 8-band k · p method to include the conduction and valence band mixing. After solving the 8 × 8 Hamiltonian and deriving explicitly the new momentum matrix elements in terms of envelope functions, optical transition rates are obtained through the Fermi's golden rule under various doping and injection conditions. Optical measurements on T2SL photodetectors are compared with our model and show good agreement. Our modeling results of quantum structures connect directly to the device-level design and simulation. The predicted doping effect is readily applicable to the optimization of photodetectors. We further include interfacial (IF) layers to study the significance of their effect. Optical properties of T2SLs are expected to have a large tunable range by controlling the thickness and material composition of the IF layers. Our model provides an efficient tool for the designs of novel photodetectors.

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We study midinfrared type-II InAs∕GaSb superlattice p-i-n photodiodes for high temperature operation. Representative samples exhibit a 3.9μm cutoff wavelength at 250K and detectivity of 4.9×1013, 1.0×1010, and 2.4×109cmHz1∕2∕W at 78, 240, and 300K, respectively. Longer-wavelength devices exhibit a 5.2μm cutoff wavelength at 240K, and detectivity of 1.3×1013 and 1.5×109cmHz1∕2∕W at 78 and 240K, respectively. The electron beam induced current technique is used to investigate the spatially varying carrier collection efficiency contribution to the quantum efficiency at different biases and temperatures. The residual doping in the i region is determined to be 6.0×1013cm−3 (n type) at 78K. The prospect of operating focal plane arrays based on the sample studied around 240K is quite promising.

We observed experimental evidences of surface channel current on the sidewall of InAs∕GaSb superlattice photodiodes. We investigated the surface channel current by measuring the current-voltage (I-V) characteristics of the diodes. The experimental data compare very well with our theoretical model before and after ammonium sulfide passivation. By using the passivation, we reduced the surface channel current by five times, which supports that the surface channel current is induced by surface carriers. We believe that the surface channel current results from the inversion layer of a p-type superlattice with surface Fermi levels pinned above the conduction-band minimum in InAs∕GaSb superlattices.