A STUDY OF THE GA-AS-ZN SYSTEM WITH APPLICATIONS TO THE DIFFUSION OF ZN IN GAAS

The purpose of this research was to study the influence of arsenic pressure on Zn diffusion in GaAs. In order to determine the arsenic vapor pressure during diffusion, the phase equilibria among Ga, As, and Zn were considered. The Ga-As-Zn ternary phase diagram was studied in detail for particular relevance to the diffusion problem. The phase rule was applied to each region of the diagram, and its application to the diffusion problem was discussed. Since no information on the vapor phase is contained in the diagram and since this information is important for determining the partial vapor pressures during diffusion, an optical absorption method was developed to measure the vapor pressure of the separate species in a closed ampoule. The arsenic and zinc vapor pressures along the liquidus lines in the Ga-As-Zn system at 900, 1000, and 1050 C were determined. The gallium pressures along the same liquidus lines were also determined from thermodynamic considerations. The activity coefficients of Ga, As, and Zn were calculated. Radiotracer Zn65 was diffused into a GaAs single crystal with excess arsenic sealed in an evacuated quartz ampoule at 900 and 1050 C. The variation of the diffusion profile with amount of added arsenic is quite different at these two temperatures. Based on the interstitial-substitutional model, the theoretical results of average conductivity, solubility, and diffusion coefficient for Zn in GaAs as a function of arsenic pressure were derived.