R. Zedric, “Defect-Enhanced Charge Multiplication in an Irradiated Avalanche Photodiode”, Ph.D. Dissertation, Nuclear Engineering, Texas A&M University, College Station, TX (2022).
Radiation effects on avalanche photodiodes (APDs) are important to understand when deploying these devices in satellites, high energy physics, military, and nuclear safeguards applications. Radiation can degrade or alter the performance of APDs and compromise their intended functions. Proton radiation combined with the ion beam induced charge (IBIC) technique was used to assess displacement damage effects on a commercial APD. Experiments showed that the bias applied during irradiation and the average proton flux had an outsized effect on the apparent levels of damage. An irradiation made at 500 V showed a drop in charge collection from pre-irradiation levels that was worse than an irradiation at 0 V with an order of magnitude higher fluence. Similarly, the loss in charge collection increased by an order of magnitude when the irradiation bias increased from 50 to 1500 V. Numerous irradiations also exhibited increased charge collection in the APD. This defect-enhanced charge collection (DECM) was observed over a subset of our experiments when the irradiation bias ranged from 170 to 1830 V and the irradiation flux ranged from 9.8×107 cm-2s-1 to 3.4×109 cm-2s-1. The charge collected during an IBIC pulse increased up to 186 +/- 24% after irradiation when compared with the pre-irradiation response. This type of response had not been reported in a Si APD before, although it had been demonstrated in other devices and materials. DECM was shown here to be a transient effect. When the response to an IBIC pulse was simulated using technology computer-aided design (TCAD) software, it was shown that DECM could be caused by introducing negative defects. The localized distortion of the internal electric field increased the rates of impact ionization and ultimately increased the total collected charge during a pulse. Further simulations showed that DECM during IBIC measurements does not necessarily correlate to an increase in photocurrent during optical stimulation of the APD. The results from this study are relevant to any device which uses high internal electric fields and is exposed to damaging levels of neutron or charged particle radiation.