Properties of the InGaAsSb Alloy and InGaAsSb-Based Photodiodes

Mamic, Katarina and Marshall, Andrew (2025) Properties of the InGaAsSb Alloy and InGaAsSb-Based Photodiodes. PhD thesis, Lancaster University.

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Abstract

The quaternary alloy InGaAsSb is a promising material system for applications in the short-wave infrared (SWIR) spectral region due to its flexible bandgap that can be tuned for wavelengths between 1.7 and 3 µm while remaining lattice matched to GaSb. Despite its increasing prominence in various infrared applications, there is a notable lack of reliable information available on some of the important material properties, particularly for alloys with low indium fractions. Therefore, a more comprehensive analysis of various material and device characteristics is needed to inform the design and development of optimal structures employing this material system. This thesis reports on the growth, fabrication, and characterisation of InxGa1−xAsySb1−y p-i-n photodiodes with 0 ≤ x ≤ 0.30 to investigate the potential of this alloy to provide an alternative to current SWIR technologies, across a more comprehensive bandgap range between 1.7 and 3 µm. The sample growth was carried out via molecular beam epitaxy (MBE) on n-type GaSb substrates. Good crystallinity was achieved in samples with x < 0.15, with signs of thermodynamically-induced phase separation seen in samples with x > 0.15. Both the absorption coefficient and external quantum efficiency (EQE) increased with increasing In fractions in the alloy, up to 8500 cm−1 at 2.2 µm and 70% at 1.55 µm measured in In0.26GaAsSb, respectively. Moreover, the EQE was largely independent of applied bias, suggesting a minority carrier diffusion length that is greater than 1 µm. The bandgap energies extracted from the absorption data exhibited the expected decrease from 0.722 eV measured in GaSb to 0.422 eV measured in In0.30GaAsSb. Electrical characterisation of InGaAsSb p-i-n photodiodes revealed a significant improvement in both dark current density, which was found to be limited by surface leakage, and zero-bias dynamic resistance-area (R0A) product with the introduction of In and As to the alloy. The best performance was seen in the In0.043GaAsSb photodiode which measured a dark current density and R0A product of 0.36 mA/cm2 at -100 mV and 194 Ωcm2 , respectively. Furthermore, the background carrier concentration extracted from capacitance-voltage (CV) measurements reduced significantly from 2.7 × 1015 cm−3 in GaSb to 6 × 1014 cm−3 in In0.30GaAsSb which points to a reduced native defect concentration. Dark current and R0A product gradually worsened towards even higher In fractions in the alloy as a result of the bandgap narrowing which increased Shockley-Read-Hall (SRH) generation-recombination. The photodetectors’ signal-to-noise performance was summarised with the calculation of specific detectivity which was found to be dominated by leakage and similarly peaked in the In0.043GaAsSb photodiode at 8.08 × 1010 Jones and -10 mV, before reducing towards even higher In fractions in the alloy. The performance of these photodiodes is comparable to commercially available extended InGaAs detectors.