For photodetection up to 3.6 µm in low photon conditions, InAs electron avalanche photodiodes (e-APDs) present a low noise and high-speed alternative to other materials. Despite their excellent ionisation characteristics, the gain of InAs APDs is limited by tunnelling, the difficulty growing thick avalanche layers with low doping, and their detectivity is lowered by high dark currents. By better understanding the tunnelling
behaviour within InAs APDs and by developing InAs-based heterostructures using AlInAsSb, this work aims to overcome these challenges.
To investigate the tunnelling and gain in InAs e-APDs, a TCAD model using Sentaurus was developed. Verification against measured PIN and NIP devices showed that the model well simulated the drift-diffusion current at room temperature, band-to-band tunnelling, and avalanche gain of InAs devices. This was used later to find the gain when the tunnelling current reached an assigned current limit, finding that when the gain at this limit exponentially rose with the avalanche width provided that it was fully depleted and could improved by grading the P layer. Mesas with bevelled sidewalls reached this limit earlier due to hotspots, but could be mitigated when using thick NIP devices.
After this, two InAs lattice-matched random alloys of AlInAsSb were characterised to see their effectiveness as an absorption layer in an InAs heterostructure. Measuring a diffusion dominated current density of 0.0358 A/cm2, a responsivity of 1.39 A/W at 2 µm, a background doping of 6.5 × 1014 cm−3, and a lattice mismatch of 6.79 × 10−4 for the best wafer. By comparison, commercially available 2.6 µm extended InGaAs photodiodes have comparable responsivities of 1.27 A/W, but dark current densities nearly two orders of magnitude lower at 5.1 × 10−4 A/cm2. Overall then AlInAsSb was demonstrated as a viable absorber material on InAs. But better surface passivation and wafer growth is still needed for competitive devices due to the significant surface leakage and high dark currents observed in the measurements.
Finally, the total gain M and excess noise F of a novel InAs heterostructure with both an InAs and AlInAsSb avalanche layer was modelled using modified local model equations and an RPL model. If the individual gain and noise of the InAs layer were M1 and F1 respectively and M2 and F2 for the AlInAsSb layer it was realised that when InAs is the first avalanche stage that M = M1M2 and F = 2 + (F2 − 2)/M1. Leading to a low-noise, high-gain 2-stage APD concept that combines the low-noise behaviour of InAs with the higher potential gains of AlInAsSb, removing the need for thick, difficult-to-grow InAs avalanche
layers. Current InAs APDs have reached gains up to 330 while conventional GaSb lattice-matched AlInAsSb avalanche layers have shown k = 0.018 noise behaviour. TCAD simulations in this predict that 2-stage InAs / AlInAsSb APDs can achieve near k = 0 behaviour up to gains of 500.
