Characterisation of Low Noise InGaAs/AlAsSb Avalanche Photodiodes

This work aims at studying the excess noise characteristics of AlAsSb material and investigating the high speed properties of InGaAs/AlAsSb SAM APDs in optical communication. Current commercial InGaAs/InP APDs have been limited to low frequency operation, below 10 Gb/s, because scaling down of multiplication region thickness has reached its limit due to high tunnelling current. The new material AlAsSb has been shown to offer more promising performance in terms of negligible tunnelling current, excellent thermal stability and extremely low excess noise.

The fabrication of AlAsSb and subsequent passivation methods are presented. Since AlAsSb oxidizes easily when exposed to air, different etchants were tested. A selective etching method has been shown to provide the best result for homojunction AlAsSb and InGaAs/AlAsSb APDs. SU-8 and BCB passivated InGaAs/AlAsSb APDs have negligible degradation compared to unpassivated devices. However removal of BCB residue still needs to be optimised if it is to be used in high speed APD fabrication. Therefore in this work SU-8 has been identified as the dielectric material for passivation because of its simple process. Procedures for fabrication of high speed InGaAs/AlAsSb APD have been developed.

The excess noise and avalanche gain of two thin homojunction AlAsSb p-i-n structures were characterised under pure injection (using 442 nm laser) and mix light injection (using 542 and 633 nm lasers). The absorption coefficient of AlAsSb was estimated from the linear interpolation of absorption coefficients of AlAs and AlSb. Both the gain and excess noise in the two structures indicated that the electron ionisation coefficient in AlAsSb is slightly higher than hole ionisation coefficient. Very low excess noise with an effective ionisation coefficient ratio keff corresponding to 0.05 was observed in a 230 nm thick AlAsSb p-i-n structure.

An InGaAs/AlAsSb APD with a multiplication layer thickness of only 50 nm was studied to determine its temperature coefficient of breakdown Cbd, and compared to an InGaAs/InAlAs APD. Due to the relatively low doping in the charge sheet layers, the tunnelling current from the InGaAs absorption layer has been unavoidable. However using the linear extrapolation of 1/M to zero, very small Cbd of 8 mV/K was measured. This is lower than all the reported InGaAs/InAlAs and InGaAs/InP APDs.

The bandwidth of an InGaAs/AlAsSb APD was studied on different devices with diameters from 25 to 250 µm. Gain close to 100 was measured on the smallest device with 25 µm diameter. However it was found to be partly contributed by edge breakdown. The bandwidth measured was ~ 3.4 GHz, independent of gain, suggesting that it is not limited by the avalanche process. As the avalanche limited bandwidth decreasing was not observed, a potential high gain bandwidth product > 327 GHz is plausible. The bandwidths of all the devices are mainly limited to the RC effect as the contact resistance still needs to be improved. The similar amplification of ~ 25 dB, obtained at 10 GHz and at 1 GHz confirms the InGaAs/AlAsSb is useful for high speed application.