Diffusion of Cadmium and Zinc in Gallium Arsenide

The diffusion of Cd and Zn in GaAs has been studied by using radioactive isotopes of these elements as tracers. The diffusion of Cd follows the correct solution to the diffusion equation and its temperature dependence is of the customary form, $D=D_0\exp (-\frac{E}{\mathrm{kT}})$ where $E$, the activation energy, is 2.43 ev and $D_0$ is 0.05 ${\mathrm{cm}}^2$/sec. The diffusion of Zn from the vapor cannot, however, be described in terms of a single diffusion constant. The penetration curves decrease much more sharply than they theoretically should. Hall measurements indicate that all the Zn is substitutional and that it forms an impurity conduction band merging with the valence band. When Zn diffuses from a thin electroplated layer of radio-zinc, then the penetration profiles do correspond to the proper solution to the diffusion equation. The diffusion constants so determined have the usual temperature dependence given by $D=D_0\exp (-\frac{E}{\mathrm{kT}})$, where $D_0$ is 15 ${\mathrm{cm}}^2$/sec and $E$ is the same as that found for Cd. From the work reported here and that of others, it is suggested that the diffusion of Cd and Zn in GaAs proceeds via vacancy migration within the gallium sublattice.