Design and characterisation of InGaAs high speed photodiodes, InGaAs/InAlAs avalanche photodiodes and novel AlAsSb based avalanche photodiodes

Avalanche photodiodes (APDs) can provide higher sensitivity, when the noise is dominated by electronic noise, than conventional p-i-n photodiodes due to its internal gain achieved via the impact ionisation process. High speed and high sensitivity photodetectors operating at the wavelength of 1.55 m for optical communication have been intensely research due to the ever increasing internet traffic, particularly in the long-haul communication systems. In this dissertation high speed InGaAs p-i-n photodiodes, InGaAs/InAlAs separate absorption and multiplication (SAM) APDs are designed and characterised. The waveguide InGaAs photodiode exhibits a maximum -3 dB bandwidth of 26.5 GHz and external quantum efficiency of 38.4% giving a bandwidth-efficiency product of 10.2 GHz, which is higher than 7.14 GHz obtained from conventional vertically illuminated diodes fabricated from the same wafer. Building on the high speed InGaAs waveguide diodes, the InGaAs/InAlAs APDs were fabricated. We demonstrated low dark currents of ~50 nA at 0.9Vbd (Vbd is the breakdown voltage), low excess noise factor k  0.2 (k is the effective ratio of ionisation coefficients ratio in excess noise model) and wide bandwidth up to 40 GHz at low gains. Our APDs also achieve higher signal amplification than the best 40 Gb/s APD reported, confirming the suitability of our APDs for use in the 40 Gb/s optical communication systems. The signal enhancement of up to 24 dB was achieved at 35 GHz.

While the InGaAs/InAlAs APDs may be suitable for 40 Gb/s operation, the avalanche gain is limited due to their limited gain bandwidth products. Hence novel wide bandgap AlAsSb avalanche regions were characterised for next generation high speed SAM APDs. The temperature dependence of dark current and avalanche gain were investigated using AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Extremely low temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained in these AlAsSb diodes, which are significantly lower than all semiconductor materials, with similar avalanche region widths, in the literature. Band to band tunelling current was shown to be significantly lower than those in InP and InAlAs diodes with the same avalanche region widths. By utilising an extremely thin 40 nm AlAsSb as multiplication layer, low excess noise factor corresponding to effective k values of 0.1 to 0.15 in InGaAs/AlAsSb SAM APDs was demonstrated. This is lower than that from an InAlAs pin diode with a 100 nm avalanche region. Therefore the potential of using thin AlAsSb avalanche region for next generation high speed and high sensitivity photodetectors has been demonstrated.