Variable-area diode data analysis of surface and bulk effects in MWIR HgCdTe/CdTe/sapphire photodetectors

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Abstract
The authors investigate the separate dark current components which are dominant in the diffusion-limited regime in MWIR n/p HgCdTe/CdTe/sapphire photodetectors. Both mesa and planar configurations of variable-area diodes were fabricated and evaluated over the temperature range from 78 to 250 K. Simple analytical expressions are used to calculate the contributions of bulk, lateral and surface effects from the perimeter/area dependence of R0A and measurement of the minority carrier diffusion length. The analysis indicates that at 180 K the mesa diode results can be accounted for by bulk and lateral currents, but that the planar diodes are limited by surface currents. The 180 K median R0A for the mesa diodes ranges from 63 Omega cm2 for 500*500 mu m2 diode areas to 14 Omega cm2 for 30*30 mu m2 diodes at a cut-off wavelength of 4.64 mu m. Scanning laser microscope measurements determine the 180 K electron minority carrier diffusion length to be 17-18 mu m.

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