Studies On Novel InAs/GaAsSb Type-Ⅱ Superlattice Long Wavelength Infrared Photodetectors

InAs/GaSb type-Ⅱ superlattice（T2SL）materials have been considered as a superior candidate for the third generation of high-performance long wavelength infrared（LWIR）focal plane arrays（FPAs）due to their unique band structure and natural advantage in material growth.Especially in recent years,with the development of epitaxy and fabrication technologies,many large-format and high-performance T2SL-based FPAs have been demonstrated,which have aroused significant research interest.However,there are still some drawbacks for T2SL photodetectors,such as the short carrier lifetime and poor absorption coefficient,which have been the main obstacles to further improve the FPAs’performance.To solve these problems,many new material designs and device structures have emerged,opening a new era for T2SL photodetectors.InAs/GaAsSb T2SL,which can theoretically improve the epitaxy quality and also help to extend the cut-off wavelength,is such a novel material to make T2SL LWIR detection more promising.So far,the research on InAs/GaAsSb T2SL is still in infancy,thus there are little experimental evidences to prove its superiority.In this work,the optical and electrical properties of InAs/GaAsSb T2SL have been systematically studied by both experimental and theoretical means,which verifies the feasibility of InAs/GaAsSb T2SL in LWIR detection.The details are as follows:1.The influence of the doping type and thickness of the absorption region on the optical response for InAs/GaAsSb T2SL LWIR photodetectors were investigated.The optical performance of devices with P-doped absorbers show significant improvement over devices with intrinsic N-type absorbers,and the quantum efficiency（QE）increased from 25%to 40%due to the change of absorbers’doping type.Besides,the optical response of InAs/GaAsSb devices are always higher than these in GaSb-based InAs/GaSb devices.For P-type InAs/GaAsSb T2SL photodetectors,the peak QE increases with the absorber thicknesses,and reaches a maximum of 65%for sample-800,and a long minority diffusion length（⁷.28μm）is inferred by further optical response measurements at variable temperatures,which is really beneficial for elevating the QEs.2.The absorption coefficients of InAs/GaAsSb T2SL materials were calculated from the transmission spectra.The transmission spectra of epitaxy and corresponding substrate were measured by FTIR in two steps,then an absorption coefficient about1200 cm^-1 for InAs/GaAsSb T2SL（lower than that of GaSb-based material）was obtained under consideration of interface reflection.3.Based on the contradiction between the high response and low absorption coefficient of the InAs/GaAsSb T2SL devices,the internal response mechanism was further explored.The optical field of the device was simulated by FDTDs.By analyzing the simulations and experimental results,multiple absorption and the multi-layer thin-film interference due to the interface reflection contribute to a higher responsivity,when there is a low-refractive-index InAs buffer layer in the device structure.That is,although a low absorption coefficient was indicated in InAs-based materials,a high QE can be still achieved through reasonable device structure design.In addition,it has been confirmed by simulating the optical distribution in a back-side illumination configuration for FPAs that the buffer layer with a lower refractive index helps boost the total response,whether or not the InAs substrate is completely removed.4.The effects of device structure and fabrication process on the dark current properties of InAs/GaAsSb T2SL LWIR photodetectors were studied.It was found that the barriers are useless in suppressing the dark current by comparing the IV characteristics of devices with different device structures（PIN,PπN and PBπBN）,and the dark current in these detectors is always dominated by the surface leakage.Thus,a shallow-etch-mesa device structure and dielectric passivation were tried separately to solve the excess leakage,and significant improvements of dark current performances were achieved in both above treatments.5.The chlorine-based ICP etching technique for InAs/GaAsSb T2SL material was investigated.Etching mechanism for In-rich material in chlorine-based ICP etching was inferred by calculating the vapor pressures of the etching products,then proper etching parameters for InAs/GaAsSb T2SL were achieved under both direct contact and contact with vacuum grease.A wider process window and superior surface topography can be obtained by the etching technique using vacuum grease,and high-performance InAs/GaAsSb T2SL LWIR photodetectors were also achieved based on this technique.6.InAs/GaAsSb T2SL LWIR FPA was fabricated and evaluated.The world’s first InAs/GaAsSb T2SL LWIR FPA was achieved.The device has a format of 320?256with pixel pitch of 30μm.The cutoff wavelength for this detector is 8.1μm at 80 K.Under an aperture of F/2.0 and an integration time of 400?s,the NETD and uniformity are 20.7 mK and 4.6%,respectively.A high pixel operability of 99.23%is also achieved in this FPA.Based on this FPA,clear LWIR imaging is also performed.Realization of the high-performance FPA further confirms that the InAs/GaAsSb T2SL has great potential in LWIR detection.