Diffusion and Solubility of Cu in CdS Single Crystals

Measurements have been carried out on the rate of diffusion of Cu in CdS single crystals. The temperatures covered ranged from 146 to 400°C, well below those covered by previous diffusion investigations. The method of measurement employed the compensation of high-conductivity $n-\mathrm{CdS}$ by the in-diffusion of Cu acceptors. The thickness of the compensated layer was calculated from capacitance measurements. Diffusion coefficients have been calculated explicitly for diffusion parallel to the $c$ axis from the compensation depth and the complementary error function. These values ranged from ∼${10}^{-14\mathrm{}}$ to ${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^2$/sec, with an activation energy of 0.96±0.05 eV and $D_0=2.1\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{cm}}^2$/sec. The solubility of Cu in CdS was determined by both colorimetric and spectrographic analysis for Cu content in diffusion-saturated CdS granules. The result found was $\left[\mathrm{Cu}\right]=6.6\mathrm{}\times {10}^{22}{e}^{-\frac{\left(0.505\mathrm{} \mathrm{eV}\right)}{\mathrm{kT}}}$. A marked anisotropy was found which favored diffusion of the compensated layers perpendicular to the $c$ axis by one to two orders of magnitude. Increasing the edge-dislocation density parallel to $c$ by bending a crystal around the $c$ axis significantly increased the diffusion rate in that direction. For this reason, the observed anisotropic behavior is ascribed to preferential orientation of dislocations perpendicular to the $c$ axis. However, the total Cd vacancy content of the dislocations, ∼${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$, could not act as the source for regeneration of Cd vacancies required for compensation. It is therefore tentatively concluded that Cu diffusion along the dislocations, together with thermal regeneration of Cd vacancies coming from voids (precipitated sulfur vacancies) in the bulk, is the primary mechanism of Cu compensation of CdS crystals.