III-V semiconductor quantum well and superlattice detectors

The paper reviews the development of IR detectors for the 8-12 mu m wavelength range based on GaAs/AlGaAs quantum well structures and InAs/(GaIn)Sb short-period superlattices (SPSLs) at the Fraunhofer-Institute LAF. Photoconductive GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are used for the fabrication of starring IR cameras for thermal imaging in the third atmospheric window. The long wavelength infrared (LWIR) camera, developed in cooperation with AEG Infrarot-Module (AIM), consists of a two-dimensional focal plane array (FPA) with 256 x 256 detector elements, flip-chip bonded to a read-out integrated circuit (ROIC). The technology for the fabrication of FPAs, electrical and optical properties of single detector elements in the two-dimensional arrangement and the properties of the LWIR camera system are reported. A noise equivalent temperature difference (NETD) below 10 mK has been measured at an operation temperature of T = 65 K with an integration time of 20 ms. More than 99.8% of all pixels are working and no cluster defects are observed. InAs/(GaIn)Sb SPSLs with a broken gap type-II band alignment are well suited for the fabrication of IR detectors covering the 3 - 12 mu m spectral range. Due to the lattice mismatch of the InAs/(GaIn)Sb SPSL with respect to GaSb, tight central of thickness and composition of the layers and a controlled formation of the chemical bonds across the interface in the SPSLs are used for strain compensation. Photodiodes with a cut-off wavelength lambda(c) = 8 mu m and a current responsivity R-lambda = 2 A/W exhibit a dynamic impedance of R(0)A- = 1 k Omega cm(2) at T = 77 K. This leads to a Johnson-noise limited detectivity in excess of D* = 1 x 10(12) cm(Hz)(1/2)/W for these type of detectors.