Enhanced diffusion and interdiffusion in HgCdTe from fermi-level effects

Excessive dopant or compositional mixing (interdiffusion) during the processing of HgCdTe photodiodes can lead to significant reductions in device performance. With the advent of multi-color and wider bandgap detectors, processes developed for single color LWIR and MWIR devices may not be transferable to the more complex structures. An important factor to account for in processing multicolor and wider gap HgCdTe is the effect of the Fermi level on point defect (PD) concentrations. In general, the density of PDs that have donor states in the band gap will be boosted in the presence of acceptors through the energy gained by the donor state electrons dropping into the vacant acceptor states. The density of PDs that have acceptor states in the band gap will be boosted in the presence of donors through a similar compensation mechanism. This Fermi-level effect is increasingly more important as the band gap is widened. Since almost all diffusion is mediated by either native and/or dopant point defects, and the intrinsic carrier concentration is relatively low at typical processing temperatures, significant broadening of composition and dopant profiles can occur in moderately and heavily doped HgCdTe. In this paper, we illustrate the Fermi-level effect on diffusion with two examples: compositional interdiffusion in multicolor detectors and diffusion of indium in MWIR and SWIR detectors.