Characterisation of Low Noise 4H-SiC Avalanche Photodiodes

We report photomultiplication, M, and excess noise, F, measurements at 244nm and 325nm in two 4H-SiC separate absorption and multiplication region avalanche photodiodes (SAM-APDs). Sample A is a 4 x 4 array of 16 SAM-APDs. This structure possesses a relatively thin absorption layer resulting in more mixed injection, and consequently higher noise than sample B. The absorption layer of sample B does not deplete, so 244nm light results in >99% absorption outside the depletion region resulting in very low excess noise. Both structures exhibit very low dark currents and abrupt uniform breakdown at 194V and 624V for samples A and B respectively. Excess noise is treated using a local model [1]. The effective ratio of impact ionisation coefficients (k(eff)) is approximately 0.007, this indicates a significant reduction in the electron impact ionisation coefficient, alpha, compared to prior work [2-5]. We conclude that the value of alpha will require modification if thick silicon carbide structures are to fit the local model for multiplication and excess noise.