Formation of void/Ga-precipitate pairs during Zn diffusion into GaAs: The competition of two thermodynamic driving forces
An experiment of diffusing Zn into GaAs has been conducted at 900°C using Zn metal as the source material in a quartz ampoule, with or without As being included. For cases without further including As in the ampoule, the Zn profile is box shaped and the Zn-diffused region contains dislocations and void/Ga-precipitate pairs, with the void to precipitate volume ratio being essentially constant throughout the Zn-diffused region. For cases including As in the ampoule, the Zn profile is of the kink-and-tail type with the Zn-diffused region containing dislocations and void/Ga-precipitate pairs. In the profile tail region, the Ga-precipitate to void volume ratio is substantial, while in the profile kink region of high Zn concentration near the surface only voids are left. The results are interpreted in accordance with Zn and diffusion-ambient-induced Ga-As-Zn ternary alloy system thermal equilibrium requirements in general, and the consequential solid phase composition variations in particular. Without As inclusion in the ampoule, the overall Ga-As material Ga concentration in the Zn-diffused region has exceeded the allowed limit of the GaAs crystal, with the crystal composition at a limit. With the inclusion of As in the ampoule, the GaAs crystal composition in the high Zn concentration region near the surface has reached an allowed As concentration limit, but in the profile tail region the overall Ga-As material Ga concentration has exceeded an allowed limit of the GaAs crystal. This is because the vapor phase Zn and As species constitute two conflicting thermodynamic driving forces for producing GaAs crystals with two opposite and extreme compositions: As for producing As-rich crystals, and Zn for producing Ga-rich materials. The common assumption that the inclusion of As in the diffusion ampoule will ensure the whole GaAs crystal essentially at an As-rich composition does not hold during high-concentration Zn indiffusion.
Jäger, W; Rucki, A; Urban, K; Hettwer, HG; Stolwijk, NA; Mehrer, H; Tan, TY
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