Shockley–Haynes Characterization of Minority-Carrier Drift Velocity, Diffusion Coefficient, and Lifetime in HgCdTe Avalanche Photodiodes
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A combined study of the avalanche gain characteristics of HgCdTe electron-avalanche photodiodes (e-APDs) and of the minority electron properties in the p-type absorber using Shockley–Haynes (SH) measurements is presented for various Cd compositions xCd. Ideal gain performance associated with a low excess noise factor F = 1.2 have been measured at T = 80 K down to cutoff wavelengths of λc = 2.9 μm. The observation of both a record high, exponentially increasing gain of M = 600 in short-wave e-APDs and a low excess noise factor proved that the exclusive electron multiplication is stable down to xCd = 0.4. Zero-flux measurements at 80 K confirmed that the dark current tends to decrease at constant gain as xCd increases. Measurements using a readout integrated circuit allowed us to establish a new record in sensitivity for APDs: Ieq_in = 2 aA, corresponding to 12 e/s at gain of M = 24 in an e-APD with λc = 2.9 μm. SH measurements enabled direct estimation of the electron diffusion coefficient, drift velocity, and lifetime in the p-type absorber of the e-APDs as a function of electric field at temperatures between 80 K and 200 K. Measurements at 80 K yielded lifetimes consistent with the values expected for the nominal doping of the samples. The low-field electron drift mobility, estimated from the drift velocity, was found to be a factor of 0.4 to 0.5 lower than the mobility in n-type material with the same composition. In mid-wave (MW) infrared samples with λc = 5.3 μm, the mobility was observed to be μep = 15 kcm2/Vs to 20 kcm2/Vs, being less than μen ≈ 40 kcm2/Vs to 50 kcm2/Vs. The reduction in mobility can, in part, be attributed to scattering by ionized acceptors and heavy holes. The diffusion mobility, estimated from the diffusion coefficient, was systematically higher than the drift mobility, indicating diffusion of hot electrons with a temperature higher than that of the lattice. The saturation velocity, vsat_ep = 2 × 106 cm/s to 6 × 106 cm/s, did not correlate with the Cd composition in the samples. The measured saturation velocities made it possible to estimate the timing jitter in p-type absorbers with a built-in electric field. Jitter below 100 ps was estimated for SW and MW APDs with absorbing layer thicknesses up to 4 μm.
KeywordsHgCdTe e-APDs response-time jitter Shockley–Haynes drift velocity mobility lifetime SW MW
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