Optical Study of Copper Diffusion in Cadmium Sulfide Single Crystals

Abstract

The diffusion of copper in cadmium sulfide single crystals has been studied by optical transmission in the temperature range 500°—700°C. Diffusion was made parallel to the c axis. The intensity of the monochromatic light beam (λ=5230–20 Å) transmitted through a small section of the sample was monitored along the diffusion profile. The relationship between the light transmission T and the penetration distance x has been derived and applied to the geometry employed in the experiments. From the slope of the plot lnln (1/T) vs x2, the diffusion coefficient has been determined and the activation energy for the copper diffusion along the c axis has been calculated (ΔE=13.4 kcal/mole). Light absorption by the doped crystals is primarily due to the copper acceptor centers and defects which are generated by the copper diffusion via the self-compensation mechanism. Scattering of light by copper precipitate aggregates has also been considered. The results of the several diffusion studies may be explained assuming that copper diffuses interstitially in the cadmium sulfide lattice and there is an equilibrium between interstitial and substitutional copper during the diffusion. The solubility of substitutional copper which occupies cadmium sites must be greater than the solubility of copper interstitials. The results of the optical studies of copper diffusion compare favorably with diffusion studies using radiotracer techniques. Since it is a nondestructive method, under certain conditions it should be more useful than other types of diffusion analysis.