Mukul C. Debnath

Extended-shortwave infrared unipolar barrier detectors

The e-SWIR wavelength band is a performance gap for infrared detectors. At both shorter and longer wavelengths, high performance detector technologies exist: SWIR InGaAs detectors (1.7 micron cutoff), and MWIR (3-5 micron) detectors such as InAs-based and GaSb-based Unipolar Barriers, MCT, and InSb. This work discusses development of high performance e-SWIR detectors with cutoff wavelengths in the 2.7 - 2.8 micron range. Two approaches for e-SWIR detector absorber materials were evaluated, lengthening the wavelength response of the SWIR InGaAs technology and shortening the wavelength response of MWIR GaSb-based technology. The InGaAs e- SWIR approach employs mismatched InGaAs absorber layers on InP substrates, using graded AlInAs buffer layers. The GaSb-based approach uses lattice-matched InGaAsSb absorber layers on GaSb substrates. Additionally, two device architectures were examined, pn-based photodiodes and unipolar barrier photodiodes. For both of the absorber materials, the unipolar barrier device architecture was found to be superior. The unipolar barrier device architecture enables both types of device to be free of effects of surface leakage currents and generation-recombination dark currents. InGaAsSb-based devices show excellent performance, with diffusion-limited dark current within a factor of 2-4 of the HgCdTe standard, Rule 07. They achieve background-limited (BLIP) performance at T=210K, which is accessible by thermo-electric coolers. As expected, defects associated with latticemismatch increase dark currents of the InP-based approach. The dark currents of the mismatched unipolar barrier photodiodes are 30x larger than those of the lattice-matched GaSb approach, however despite the defects, the devices still exhibit diffusion-limited operation, and achieve BLIP operation at T=190K Further improvements in the InP-based approach are expected with refinements in the epitaxial structures. Both types of detector approaches are excellent alternatives to conventional e-SWIR detectors.

III-V semiconductor extended short-wave infrared detectors

The extended-shortwave infrared wavelength range, encompassing wavelengths from 2.2 to 3 μm, is significantly underdeveloped when compared to the shortwave and midwave infrared bands. Achieving high performance detectors in the extended-shortwave range is desirable; however, it is unclear whether to approach the wavelength range via the detector structures and materials common to the shortwave regime or those common to the midwave regime. Both approaches are studied here. Electrical and optical characteristics of conventional photodiodes and nBn architecture detectors with 2.8 μm cutoff wavelengths are analyzed for detectors with both lattice-mismatched InGaAs and lattice-matched InGaAsSb absorbing regions. Regardless of the absorber material, the nBn detectors show nearly 3 orders of magnitude improvements in performance over the conventional photodiode architecture, and the lattice-matched InGaAsSb nBn exhibits a further reduction in the dark current by more than an order of magnitude when compared to t...