Longwave and bi-color type-II InAs/(In)GaSb superlattice infrared detectors
Infrared (IR) photodetectors are useful for a variety of military and civil applications such as target acquisition, medical diagnostics, pollution monitoring, to name just a few. Presently photonic IR detectors are based on interband transitions in low bandgap semiconductors such as mercury cadmium telluride (MCT) or InSb or in intersubband transitions in hetero-engineered structures such as quantum well or quantum dot infrared photodetectors (QWIPs or QDIPs). These detectors operate at low temperatures (77 K-200 K) in order to obtain high signal to noise ratio. The cooling requirement limits the lifetime, increases the weight and the total cost, as well as the power budget, of the whole infrared system. There is a concerted effort to develop photonic detectors operating at higher temperatures. In the past few years, interband transitions in type II InAs/GaSb strain layer superlattices (SL)have emerged as a competing technology among other IR systems. Although MCT and QWIP technologies are relatively more mature than the SL technology, the SL technology has potential to enhance performance in several key areas. One of the main advantages of this system lies in the fact that the effective band gap of the SL can be tailored over a wide range (3 μm < λc < 30 μm) by varying the thickness of two mid bandgap' constituent materials, namely GaSb and InAs. Tunneling currents in SL are reduced due to a larger electron effective mass. Large splitting between heavy-hole and light-hole valence subbands due to strain in the SLs contributes to the suppression of Auger recombination. Moreover, the band structure of the SL can be engineered to enhance carrier lifetimes and reduce noise at higher temperatures. SL based IR detectors have demonstrated high quantum efficiency, high temperature operation, and are suitable for incorporation in focal plane arrays (FPA) by tapping into the mature III-V based growth and fabrication processes. The recently proposed nBn heterostructure design has demonstrated a 100 K increase in background-limited infrared photodetection (BLIP) for InAs-based device, by decreasing Shockley-Read-Hall generation currents and by suppressing surface currents using specific processing. Third generation IR detectors have three main emphases, high operating temperature (HOT), multicolor capability, and large format arrays. This work concentrates on multicolor and HOT IR detectors based on nBn design. Contributions of this thesis include 1. Development of design and growth procedure for the long-wave (LW) SL detectors leading to an improved detector performance: 13 MLs of InAs and 7 MLs of GaSb with InSb strain compensating layer were designed and optimized for LW SL detectors. LWIR pin and nBn detectors were introduced and their optical and electrical properties were compared. LW nBn detectors show higher device performance in terms of lower dark current density and higher responsivity as compared to the LW pin detectors. The reduction in dark current in LW nBn detector is due to reduction of SRH centers as well as surface leakage currents. The increase in responsivity for LW nBn detectors is due to reduction non-radiative SRH recombination. 2. Design, growth and characterization of bi-color nBn detectors: Present day two color SL detectors require two contacts per pixel leading to a complicated processing scheme and expensive read out integrated circuits (ROICs). The nBn architecture was modified to realize a dual-band response by changing the polarity of applied bias using single contact processing. The spectral response shows a significant change in the LWIR to MWIR ratio within a very small bias range (\u223c100 mV ) making it compatible with commercially available ROICs. 3. Investigation of background carrier concentration in SLs: The electrical transport in SLs was investigated in order to improve the collection efficiency and understand SL devices performance operating at ambient temperature. For this purpose background carrier concentration of type-II InAs/GaSb SLs on GaAs substrates are studied. The hall measurements on mid-wave SLs revealed that the conduction in the MWIR SLs is dominated by holes at low temperatures (< 200 K) and by electrons at high temperatures (> 200 K) and is dominated by electrons at all temperatures for LWIR SLs possibly due to the thicker InAs (residually n-type) and thinner GaSb (residually p-type) layers. By studying the in-plane transport characteristics of LW SLs grown at different temperatures, it was shown that interface roughness scattering is the dominant scattering mechanism at higher temperatures (200 K- 300 K).'
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