Medium- and long-wavelength InAs/InAsSb strained-layer superlattices for applications in infrared nBn detectors

Letka, Veronica and Marshall, Andrew (2019) Medium- and long-wavelength InAs/InAsSb strained-layer superlattices for applications in infrared nBn detectors. PhD thesis, Lancaster University.

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Abstract

InAs/InAs1−xSbx type-II strained-layer superlattices (SLS) have potential applications in infrared detection, owing to their narrow tunable bandgap and decreased Auger recombination currents. Furthermore, the so-called nBn device architecture promises to suppress the majority carrier dark current and the surface current thanks to its inbuilt potential barrier, as well as the Shockley-Read-Hall (SRH) process due to the structure’s lack of a depletion region. This thesis reports on the growth, modelling, and characterisation of InAs/InAs1−xSbx nBn devices in an effort to deliver a viable alternative to the HgCdTe-based technology. Sample growth proceeded with the use of molecular beam epitaxy (MBE) on both native GaSb substrates and lattice-mismatched GaAs equipped with GaSb and AlSb buffer layers. Excellent crystallinity of devices grown on GaSb is confirmed with the use of X-ray diffraction (XRD) and transmission electron miscroscopy (TEM) scans; the GaAs-grown samples, meanwhile, display broadened XRD peaks and dislocations largely originating at the buffer interfaces. Group V intensity data reveals a Sb segregation length of ∼ 0.95 nm. Temperature-dependent transmission spectra of undoped SLS samples with varying xSb are obtained using Fourier Transform Infrared (FTIR) spectroscopy, and used to derive their absorption coefficients α, which are shown to be comparable to other technologies operating in the same wavelength range. Direct fundamental optical bandgaps Eg are extracted, and incorporated into the Kronig-Penney model of the superlattice band structure. Further, several nBn device parameters are characterised. Dark current-voltage characteristics and activation energies show diffusion-limited behaviour at higher temperatures, especially in the medium-wavelength (MWIR) detectors, but voltage-dependent current mechanisms dominate at lower T and in some long-wavelength (LWIR) devices. Responsivity R of two MWIR and two LWIR devices is obtained at target operational temperatures of 160 and 77 K respectively; the peak value in the MWIR range is found to be 1.70±0.11 A/W, corresponding to an external quantum efficiency of ∼ 33%. When the dark current density data is combined with responsivity results, the resultant specific detectivity D∗ is found to be 5.3×1010 cmHz1/2W−1 for the MWIR samples at the operational bias and temperature of -0.1 V and 160 K, while one of the LWIR samples shows 1.1×1010 cmHz1/2W−1 at -0.2 V and 77 K.