Substitutional-interstitial silver diffusion and drift in bulk (cadmium,mercury) telluride: Results and mechanistic implications

The diffusion and electromigration of Ag in crystals of Cd_xHg_1-xTe is studied, as a function of original doping level and of the concentration of mercury. In materials with x = 0.55-0.8, Ag dopes p-type, when diffusing in at <125°C. This should be contrasted to what is found in n-CdTe, where in-diffusion of Ag at 200°C increases the net donor density, leaving the material n-type. Our results show that the higher is the mercury content or the hole concentration in Cd_xHg_1-xTe (x = 0.55-0.8), the faster Ag will diffuse in these materials. We explain our results, building on earlier suggestions made for Hg-rich materials, by assuming that silver diffuses by way of a substitutional-interstitial mechanism; i.e., it is present as two species with opposite charge, one of which dominates and is practically immobile, while the minority species diffuses rapidly. These forms equilibrate, at room temperature, within a few seconds, something that can be understood by postulating silver-mercury complex formation. If both forms of silver are bound to mercury, then this hypothesis explains the strong influence of mercury content on the diffusion behavior.