Novel InAs/InAsSb Superlattice Designs and Device Structures for Higher Sensitivity Infrared Imaging

Seager, George and Marshall, Andrew (2025) Novel InAs/InAsSb Superlattice Designs and Device Structures for Higher Sensitivity Infrared Imaging. PhD thesis, Lancaster University.

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Abstract

The InAs/InAsSb Ga-free superlattice system has demonstrated substantial promise in recent years for both mid-wave and long-wave infrared imaging applications. Typically this takes advantage of the nBn barrier architecture to suppress dark currents. It is benefitted by the removal of Ga compared with the InAs/GaSb system as this increases the carrier lifetime. Despite these advantages this approach continues to have difficulty achieving the electro-optical performance comparable to that of the industry standard material HgCdTe. This thesis discusses several novel modifications to the superlattice unit cell with the aim of improving understanding of the parameters which impact absorption coefficient experimentally. This includes variations to the InAs/InAsSb superlattice period length, Sb content, the lattice constant to which it is lattice matched and the inclusion of thin GaAs insert layers into the InAs layer. A correlation between the near-bandgap (within 25 meV of) absorption coefficient and superlattice period length is found, with thinner periods resulting in higher absorption irrespective of other elements of superlattice design. A reduction in period from 13 nm to 6.5 nm results in an increase in near-bandgap absorption from ∼ 1600 cm−1 to ∼ 2400 cm−1. The GaAs insert layers are found to significantly shift the cut-off wavelength for a given InAsSb thickness and Sb content. The inclusion of 0.5 and 1.0 monolayers of GaAs resulting in shifts of 26 meV and 43 meV compared with a reference epilayer. Superlattice test devices following the nBn architecture were developed and characterised for three variations of conventional superlattice with Sb contents of 15%, 18% and ∼ 40% as well as a 19% Sb design lattice matched to the AlSb lattice constant. External quantum efficiencies at 4 µm and 140 K were observed for each of these with values of 30%, 45%, 40% and 20% respectively, however it should be noted that cut-off wavelengths are not equivalent between the designs. The dark current densities of the devices are 3 × 10−6A cm−2, 3 × 10−4A cm−2, 1.5 × 10−5A cm−2 and 1.5 × 10−4A cm−2 respectively under these same conditions and the operation bias of each design. The considerable variation in dark currents between the designs is a result of numerous factors the most significant of which are considered to be material quality and device cut-off wavelength. The specific detectivities of these devices were calculated with the highest magnitude exhibited by the 15% Sb design at 1.5 × 1012 cm Hz−1W−1. The other devices have reduced performance with a value of ∼ 2×1011 cm Hz−1W−1. Focal plane arrays were grown and processed utilising the 15% and 18% Sb designs on both GaAs and GaSb substrates. Mid-wave infrared (MWIR) images have been obtained with the 15% Sb design at a range of temperatures (140−180 K) and this has demonstrated a minimum noise equivalent temperature difference (NETD) of 20 mK at 140 K. A further method to increase the sensitivity and operating temperature of type-II superlattice (T2SL) nBn detectors for MWIR imaging applications was investigated. The device architecture was enhanced by integrating it with a resonant cavity in order to greatly thin the absorber and enhance quantum efficiency at a target wave length. Two test devices with resonances at 4.5 µm and 5.0 µm were grown and have demonstrated a peak responsivity over the 3.5 − 5.0 µm of 0.75 AW−1 in the latter case. The 4.5 µm device demonstrated a great reduction in dark current at 140 K of 8 ×10−7A cm−2 at operation bias. This is due to its reduced absorber thickness and relatively small bandgap of the absorber. The spectral shape of these devices is reversed compared to a broadband detector with a peak in absorption at longer wavelengths rather than at the shorter end of the MWIR window. The structure was simplified compared with a typical two DBR resonant cavity design by replacing the high reflectivity mirror with an evaporated metallic layer. Two metal contact designs were considered and compared, these being titanium gold and chromium gold. The chromium was found to have a higher quantum efficiency than the titanium design, which is in keeping with its known higher reflectivity in the MWIR.