Quasi-2D analysis of the effect of passivant on the performance of long-wavelength infrared HgCdTe photodiodes

Abstract

The results of a quasi-two-dimensional model for calculating passivant-induced surface leakage currents due to band-to-band tunnelling in mercury cadmium telluride (MCT) photovoltaic (PV) diodes are presented. The object is to assess the effect of a fixed surface state charge density due to a passivant on the zero-bias resistance - area product for the technologically important case when surface state charges accumulate the MCT surface. Calculations are carried out to estimate the tolerable value of beyond which the of the MCT diode degrades. To the best of our knowledge, this is the first time that such a detailed calculation involving the acceptor concentration profile near the surface has been reported for long-wavelength IR (LWIR) MCT photodiodes. This calculation has been done numerically, and hence the depletion width (pinched near the surface), the electric field and the band-to-band tunnelling are calculated as a function of depth, layer by layer, from the passivant - semiconductor interface. The currents - diffusion, generation - recombination, band-to-band and trap-assisted tunnelling - have been calculated in each layer, and the zero-bias resistance - area product is determined for each mechanism. Hence, the resultant is calculated. The results are compared with the earlier step model of Bhan and Gopal (Semicond. Sci. Technol. 9 (1994) 289), which assumed a surface layer of constant concentration . The present model indicates that for photodiodes with a cut-off wavelength of and an acceptor concentration , operating at 77 K, a would degrade significantly. This value is insensitive to composition (in the LWIR). For an diode, the tolerable value of is found to depend on the concentration in the layer. Further, the tolerable value of for an diode is much higher than for an diode, for donor concentrations .