Properties of the InGaAsSb Alloy and InGaAsSb-Based Photodiodes
Creators: name: Mamic, Katarina
Publisher: name: Lancaster University
Year: 2025

description: 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
 &lt; 0.15, with signs of thermodynamically-induced phase separation
 seen in samples with x &gt; 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.