InAs avalanche photodiode linear array for low photon applications

Avalanche photodiodes (APDs) are used in photon-starved applications for their ability to amplify low levels of incoming signals. A special class of APDs, known as electron APDs (e-APDs), can significantly improve the signal-to-noise ratio of an optical receiver system due to single carrier multiplication and the lack of hole feedback. InAs is one of such material with a band gap of 0.36 eV which makes InAs an attractive material of choice for low photon, infrared applications such as LiDAR and active imaging. Conventional mesa structure InAs APDs face many problems such as lateral side etching and the need for sidewall passivation. Planar topology InAs APDs previously developed using a high energy ion implantation scheme have shown performance comparable to the mesa structure APDs.

In this thesis, planar InAs APDs with a shallow implant depth of 100 nm are investigated. Long duration anneals have been employed to promote dopant diffusion to form the p type region. The dark current and responsivity of the planar APDs show improvement over as processed samples following a shallow surface etch, indicating the removal of surface damage induced by ion implantation.

Linear arrays of mesa and planar InAs APDs are fabricated and the array uniformity is evaluated to show that superior homogeneity of diode performance, from current-voltage and responsivity measurements, in the planar linear array. The challenges faced during semiconductor processing of mesa linear arrays are detailed and discussed to illustrate low device yield in mesa linear arrays.

Finally, the planar linear array is characterised in detail at the temperature range of 300 – 77 K. Good uniformity in dark current and gain characteristics are demonstrated at the temperatures measured. At 77 K, the signal-to-noise ratio increases with bias voltage (and gain) with a maximum of 62.8 dB at -22 V before the diode shot noise dominates. Current-voltage characteristics of the diode array before and after multiple thermal cycles show minimal degradation, highlighting the robustness of the fabricated planar array.