Long-wavelength type-II InAs/GaSb superlattice infrared photodetectors

Type-II superlattices (T2SLs) have demonstrated great potential for long-wavelength infrared (LWIR) photodetectors but have yet to achieve their theoretical performance levels. In this research project, the capability at Cardiff University for simulation, fabrication, and characterisation of LWIR T2SLs is established and improved with the ultimate aim of improving the current performance levels of LWIR T2SL detectors. The strategies for improving performance, described herein, include improving the design of the superlattice (SL) period, optimisation of the fabrication process, and monolithic integration on GaAs substrates. 8 band k·p simulations are used for band structure modelling, which enables device design optimisation and informs analysis of experimental results. Molecular beam epitaxy (MBE) is used for growth of experimental T2SL reference wafers/samples, which employ novel SL period designs, GaAs substrates, or both. The material quality of these wafers is then investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Band structure information is obtained through photoluminescence (PL) measurements. Diodes were fabricated using a standard photolithography process which was later modified to incorporate experimental improvements. The diode performance of the T2SLs was then tested in a cryogenic probe station. Various strategies for improving device performance were tested and evaluated. By comparing different combinations of superlattice layer thicknesses, it was observed that the novel 12 ML InAs/4 ML GaSb superlattice structure was better suited to high-temperature operation than conventional designs. A comparison of the structural, optical, and device characteristics of LWIR T2SLs on GaSb and GaAs substrates was made to investigate the feasibility of LWIR T2SLs on a low-cost GaAs platform. It was observed that, despite using an interfacial misfit (IMF) array between the GaSb buffer and GaAs substrate, a material degradation due to the heteroepitaxial growth led to a corresponding degradation in optical and device performance. However, the reported work in this thesis is mere preliminary demonstration. There is a scope for improvement by involving different buffer layers, dislocation filters, and interfacial schemes of heteroepitaxy. The reasons for and nature of this material degradation were studied in detail using TEM. This thesis also reports intermediate results from the development of a dry etching process for T2SLs are reported. Scanning electron micrographs show this process significantly improves the sidewall verticality and fill factor compared to wet etching. I-V measurements indicate the performance of dry-etched devices is degraded by roughly two orders of magnitude at low temperatures compared to wet etched reference samples but is comparable at high temperatures