Impact ionisation in AlInP photodiodes

The aim of this work is to characterise and understand the impact ionisation characteristics of AlInP towards the exploitation as the avalanche medium in GaAs based avalanche photodiode (APD) for optical communication. Initial room temperature spectral response measurements on AlInP with avalanche multiplication region thickness, w, of 1.03 μm demonstrates a Gaussian like spectral response in the blue green region with a narrow full width half maximum of ~ 40 nm and peak responsivity of 0.12 AW-1 at 474 nm wavelength. These results are very close to the published responsivity data of 0.16 AW-1 at 480 nm wavelength and suggesting the suitability for underwater optical communication. Room temperature avalanche multiplication measurements were performed on a comprehensive set of p+-i-n+ (n+-i-p+) diodes with different w, ranging from 1.03 μm to p+-n+ junctions. The effective impact ionisation coefficients, i.e. αeff for electrons and βeff for holes (described in section 2.2.2), were deduced from diodes with fairly uniform electric fields using the local model and the results show that αeff > βeff, which in turn, suggest that electron is more readily ionising carrier type compared to holes in this semiconductor. This is further reinforced by excess noise measurements on w ~ 1.0 µm p+-i-n+ (n+-i-p+) diodes and calculation of electron and hole local impact ionisation coefficients using the recurrence equations. Temperature dependence of avalanche multiplication and breakdown voltage from 77 to 400 K were also investigated on AlInP diodes with w ranging from 1.03 μm to 0.04 µm. It is found that the αeff > βeff for the range of temperature investigated, suggesting that APD utilising AlInP as the avalanche multiplication medium should be designed for electron initiated impact ionisation event within this operating temperature range. However, αeff/βeff reduces to unity with increasing temperature mainly due to βeff is being less temperature dependent compared to αeff. The temperature coefficient of breakdown voltage in AlInP p+-i-n+ diodes with w of 1.03, 0.23 and 0.04 μm is ~ 12.90, 3.45 and 0.77 mVK-1 respectively, lower than the values reported for many other semiconductors in the literature for a given w, except for AlSb0.44As0.56. The potential of utilising AlInP as the avalanche multiplication medium with features such as weak temperature dependence avalanche multiplication and breakdown voltage as well as low excess noise due to electron initiated impact ionisation have been demonstrated.