Surface migration and volume diffusion in the AgGaSe2−Ag2Se system

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Abstract
AgGaSe2 crystals are useful for nonlinear optical applications after a post-growth heat-treatment in the presence of Ag2Se to eliminate an unavoidable precipitate phase that degrades their optical properties. To understand better the heat-treatment procedure, surface migration and volume diffusion were investigated in the Ag2Se–AgGaSe2 system using reactive diffusion couples which were analyzed by x-ray diffraction, optical microscopy, and electron probe microanalysis. Surface diffusivities of all mobile species were found to be much larger than volume diffusivities. Specific values determined were DsAg = 5.43 × 10−4 exp(-0.46 eV/kT) and DvAg = 2.40 × 10−7 exp(-0.84 eV/kT), where DsAg and DvAg are effective surface diffusivity and effective volume diffusivity of Ag, respectively. The corresponding diffusivities for Ga and Se were found to be almost the same, indicating that Ga and Se move together with Ag to maintain binary (Ag2Se and Ga2Se3) stoichiometry and electroneutrality. These findings are consistent with the pattern of annihilation of the second phase precipitates in the AgGaSe2 matrix during heat treatment.

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