Craig, Adam Patrick and Marshall, Andrew (2016) Novel structures for lattice-mismatched infrared photodetectors. PhD thesis, Lancaster University.
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Abstract
Using the interfacial misfit (IMF) array growth mode, GaSb p-i-n diodes were grown on Si and GaAs lattice-mismatched substrates by molecular beam epitaxy (MBE) under optimised growth conditions. For the sample grown on Si, an AlSb nucleation layer was used to reduce the occurrence of twinning defects. In addition to the samples grown on mismatched substrates, an equivalent structure was further grown on a native GaSb substrate, for comparison. X-ray diffraction (XRD) was used to demonstrate that the layers were fully relaxed, and transmission electron microscopy (TEM) imaging showed arrays of 90° misfit dislocations with measured periodicities in agreement with atomistic modelling. However, after processing, device dark current densities of 0.9 Acm^-2 and 0.18 Acm^-2 were recorded for the sample grown on Si and the sample grown on GaAs, respectively, at -1.0 V and 300 K. These were compared to the sample grown on native GaSb, which had a dark current density of 0.01 Acm^-2 under the same conditions. Furthermore, TEM analysis revealed relatively high threading dislocation densities (TDDs) of ~10^8 cm^-2. It was proposed that not all the interfacial strain could be accommodated by the IMF arrays, since the array periods (9:8 for AlSb/Si and 13:14 for GaSb/GaAs) were not in exact agreement with ratio of the lattice constants (of AlSb to Si and GaSb to GaAs), i.e. a population of 60° misfit dislocations was still formed.It was therefore decided to investigate the use of nBn detector structures as lattice mismatched photodetectors. Using a design based on an InAsSb bulk-material absorber, a comparison was again drawn between two samples, one grown on mismatched GaAs and a second grown on native GaSb. This time, device dark current densities were found to be relatively similar when comparing the two samples (1.6×10^-5 Acm^-2 vs 3×10^-6 Acm^-2 at 200 K). D^* performance figures were also found to be within one order of magnitude (1.5×10^10 cmHz^1/2 W^-1 vs 9.8×10^10 cmHz^1/2 W^-1 at 200 K). Furthermore, diffusion limited performance was exhibited at all temperatures tested, so that the effects of Shockley Read Hall (SRH) generation were established to be absent (or at least much less significant). It was also found that absorption layer doping of around ~4×10^17 cm^-3 was necessary to ensure diffusion limited performance for the sample grown on GaAs and that, with this modification, diffusion limited performance was achieved even for a sample with a highly lattice-mismatched absorption layer (with higher Sb content and longer cut-off wavelength).While nBn detector structures offer very low dark currents, it will sometimes be necessary to have a detector which is sensitive to very weak signals. In telecoms applications, avalanche photodiode (APD) structures are often used as receivers for long-haul fibre optic systems. However, relatively few avalanche photodiode designs exist for wavelengths beyond 1.55 μm. Two novel separate-absorption-and-multiplication (SAM) APD structures were therefore demonstrated based on the IMF growth mode. In particular, by transitioning the lattice from 5.65 Å to 6.09 Å, it was possible to combine GaSb absorption layers with GaAs and (for improved noise performance) Al0.8Ga0.2As multiplication layers. Multiplication profiles were established using capacitance voltage modelling (together with ionisation coefficients from the literature) and excess noise measurements were then carried out. Through the presence of 1.55 μm photocurrent, it was confirmed that absorption took place in the GaSb regions, with transport to the p-n junction (in the multiplication region) taking place by diffusion. Through measurements showing 0.2<k_eff<0.4 and 0.1<k_eff<0.2 it was confirmed that multiplication of the photocurrent took place in the GaAs and Al0.8Ga0.2As layers. Extension of the designs for sensitivity at longer wavelengths would then be possible using other absor-ption layer materials which are lattice matched to GaSb. It should be noted that these include InGaAsSb (short-wave infrared) InAsSb (mid-wave infrared) and strained layer superlattices based on InAs/GaSb or InAs/InAsSb (long-wave infrared).