Enhanced diffusion and interdiffusion in HgCdTe from fermi-level effects

Abstract

Special Issue Paper 657 FERMI LEVEL ENHANCED INTERDIFFUSIONThe fabrication of all HgCdTe-based infrared detectors involves the formation of one or more heterointerfaces. With the increasing interest in developing detectors in the SWIR as well as the MWIR and LWIR bands, and the desire for multispectral sensitivity, the number and complexity of hetero-interfaces is increasing. Because the performance of these detectors depends critically on the precise placement of the hetero-interfaces, any interdiffusion at these interfaces must be understood and controlled. Although much work has been done in this area, the mechaExcessive 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 Fermilevel effect on diffusion with two examples: compositional interdiffusion in multicolor detectors and diffusion of indium in MWIR and SWIR detectors.

Planar p-on-n HgCdTe FPAs by Arsenic Ion Implantation

“…In MWIR HgCdTe, the band gap is approximately 0.37 eV at 420°C, which gives an intrinsic carrier concentration of about 2 9 10 17 cm À3 . The fact that these two values are relatively close together is coherent with the results previously reported by Robinson et al 9 that areas p-type doped at concentrations greater than that of the intrinsic carrier (n i ) may display enhanced diffusion. Hg and Cd can diffuse either by moving onto interstitial sites or by exchanging place with vacancies on the metal sublattice.…”

Point defects and diffusion in cadmium telluride

“…The diffusion velocity reaches a minimum for a medium vacancy density and increases both in the case of high and low vacancy density. The former behavior occurs due to the enhancement of the vacancy mechanism and was demonstrated in HgCdTe [88]. The latter occurs due to the kick out of impurity atoms into interstitial positions and is outlined in Fig.…”

“…Impurity effects on the creation and migration of Ga vacancies in GaAs were demonstrated by Lee et al [87]. The modelling of enhanced diffusion and inter-diffusion taking into account the Fermi-level effect was proposed with real parameters in HgCdTe by Robinson et al [88].…”

“…Simultaneously, the potential variations are assumed to be weak and the space charge is neglected (quasi-neutral approximation). Though both given assumptions need to be verified in special cases, they do not disrupt the credibility of most calculations and allow significant simplification of the mathematical treatment [88].…”

Equilibrium properties of indium and iodine in LWIR HgCdTe

“…2 we compare the calculated defect concentrations at 500°C and P Hg = 3 atm as a function of indium concentration with experimental results of Vydyanath. 11 The presence of the V Hg -In Hg pair in indium doped material is supported by perturbed angular correlation measurements reported by Hughes et al 12 The calculations predict that even at the higher indium concentrations, the indium atoms incorporate as isolated species and exclusively on the cation sublattice.…”