The development of extremely low-noise InAs electron APDs for SWIR active or passive imaging

We report on the characterisation of impact ionisation in InAs and the development of practical InAs avalanche photodiodes (APDs) to exploit the properties identified. Avalanche multiplication measurements show that the hole ionisation coefficient is negligible in InAs. We have demonstrated that this results in extremely low excess multiplication noise, F<2, for electron initiated gain. Indeed the excess noise measured was comparable to the excellent results reported for HgCdTe APDs and notably lower than those reported for other III-V based APDs. It could be desirable to exploit this extremely low excess noise characteristic, now demonstrated in a III-V material, in a number of applications. In this work we consider in particular the exploitation in an InAs APD focal plane array, to achieve enhanced sensitivity imaging in the SWIR. The greatest barrier to such exploitation is the requirement to sufficiently suppress the reverse leakage current, typically present in InAs diodes under high reverse bias at room temperature. We report on initial developments in this respect, presenting leakage current results for the temperature ranges accessible by both thermoelectric and Stirling engine cooling. These demonstrate that InAs APDs have the potential to operate at higher temperatures than the sMWIR sensitive composition of HgCdTe currently used in emerging APD focal plane array applications. We also show that InAs APDs are capable of operating at bias voltages below 10V, supporting easy integration with fine pitch read out ICs, without band to band tunnelling contributing significantly to the leakage current, even at low operating temperatures.