The diffusion of mercury in cadmium telluride

The results of an investigation on the diffusion of mercury (Hg) into single crystal slices of cadmium telluride (CdTe), measured using a radiotracer sectioning technique, are reported in this paper. The diffusion anneals were carried out in the temperature range 23–460°C in evacuated silica capsules in which a saturated vapour pressure due to the metal was maintained throughout the diffusions. A combination of two sectioning techniques, anodic oxidation and dry mechanical lapping, was used to measure the diffusion profiles. All the profiles showed two diffusion components which were both Fickian, allowing a double complementary error function to be fitted to the data. When plotted on an Arrhenius graph, the results due to the fast diffusion component gave values of E = 0.789 eV and D 0 = 2.65 × 10 -1 cm 2 s -1, and the mechanism was probably due either to an interstitial, or to a defect mechanism associated with a residual impurity in the CdTe slice. The Arrhenius graph due to the slow diffusion showed two straight-line components with a change in slope occurring at 300°C. When a mathematical expression consisting of the sum of two Arrhenius functions was fitted to the data, the following values were obtained: E 1 = 1.572 eV, D 01 = 1.7 × 10 -2 cm 2 s -1 E 2 = 0.265 eV and D 02 = 2.84 × 10 -13 cm 2 s -1. This was interpreted in terms of two diffusion mechanisms occuring. The first was due to a defect mechanism associated with a residual impurity, or an interstitial in the CdTe, similar to that proposed for the fast diffusion, which operated over the whole temperature range in which measurements were made, and the second was due to a vacancy defect which became significant for T #62 300°C. Isoconcentration measurements did not show a similar behaviour. Arrhenius graphs of the surface concentration due to the Hg have been obtained for both diffusing components and each curve shows a sharp kink with a change of slope occurring at 300°C, again giving minimum values of C 0 s = 2.4 × 10 18 atoms cm -3 and C 0 f = 1.1 × 10 17 atoms cm -3 occurring at that temperature. This is probably associated with the peritectic that occurs in the Cd/ and Cd/CdTe phase diagrams. The addition of Cd to the diffusion source reduced both the diffusivity and the Hg concentration in the surface of the slice. The diffusion at 460°C appears to be pressure dependent.