Investigation of InAs/GaSb superlattice based nBn detectors and focal plane arrays
Infrared (IR) detectors are used for a variety of imaging applications, such as terrestrial surveillance, biomedical diagnostics, and target acquisition. Presently, the most common photon detectors for these applications are based on interband transitions in pn junctions made of mercury cadmium telluride (MCT) or InSb, and intersubband transistions in quantum well infrared detector(QWIP). However, lack of spatial uniformity over larger area and difficulties with the material growth plagues MCT detectors. The quantum efficiency of QWIPs is low and they cannot couple normal incidence photons due to absorption selection rules. The operation temperature of all these detectors is limited to < 200 K in order to obtain a high signal-to-noise ratio. The need for cooling increases the weight and cost of a infrared system. The InAs/Ga(In)Sb strained layer superlattice (SLS) material system proposed for the IR detection in 1987, has been considered in recent years as an interesting alternative to the present day IR detection technologies. InAs/GaSb SLS has a type II band alignment such that the valence band of the GaSb layer is higher than the conduction band of the InAs layer. The effective band gap is decided by the energy difference between the electron miniband and the first heavy hole state, which depends on the thickness of constituent layers, leading a wide range of cutoff wavelength (3-30μm). Also, the amplitude of the electron wavefunction is mostly confined to the InAs layer, whereas the amplitude of the hole wavefunction is mainly confined in the GaSb layer. These distributions of electrons and holes hinder the Auger recombination, leading to longer electron lifetime. However, the spatially indirect optical transitions lead to low absorption. The electron effective mass of the InAs/GaSb SLS (m*/mo \u2248 0.02-0.03) is larger than MCT (m*/mo \u2248 0.009) with the same bandgap (Eg \u2248 0.1 eV). Thus, tunneling current in SLS can be suppressed compared to MCT. These properties make the InAs/Ga(In)Sb SLS a promising material system for next generation IR sensors. This work is focused on the performance improvement of InAs/GaSb SLS detectors by implementation of the nBn design and a novel SU-8 passivation scheme, and realization of an nBn SLS FPA. This thesis covers three topics: 1) optical and electrical characteristics of single pixel nBn SLS devices operating in the MWIR and LWIR spectral regions; 2) development of a MWIR focal plane array (FPA) based on the InAs/GaSb SLS detector with the nBn design; 3) development of a new passivation method for mid wave infrared (MWIR) and long wave infrared (LWIR) p-i-n SLS detectors. As a result of this work, the following milestones were achieved: (1) first report of nBn MWIR FPA (Kim et al, APL. Vol. 92, 183502, 2008); (2) first report of use of SU-8 as a passivation material for MWIR and LWIR SLS single element detectors and FPAs (Kim et al, APL. Vol. 96, 033502, 2010); (3) development of an n-type ohmic contact for low-doped MWIR SLS material (Kim et al, Electronics Letters, Vol. 44, No 14, 2008).
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