Rare Earths in Covalent Semiconductors: the Thulium-Gallium Arsenide System

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Abstract
Solutions containing radioactive 170Tm were applied to the surface of GaAs wafers that were heated to 800°, 900°, or 1000°C for times up to 100 h. The solubilities and diffusion coefficients were obtained by fitting the experimental data with complementary error functions. The resulting surface concentration (cm−3) was taken as the equilibrium saturation solubility and is represented below the eutectic temperature of 1150°C by 6.7×1025 exp (−2.3 eV/kT), which gives C max=4×1017 cm−3. The atom size, defined as the single‐bond radius, was found to be the dominant factor in limiting the solid‐solutionsolubility of the rare earths in GaAs.Diffusion was faster at lower temperatures, and the diffusion coefficient D (cm2/sec) was 2.3×10−16 exp (1.0 eV/kT). This inverse temperature dependence of the diffusion coefficient may be explained by the effective D derived from the concept of simultaneous diffusion and chemical reaction. The resulting diffusion coefficient in intrinsic material is D e i =D i C i i+ /C s 0 , where Di is the interstitial diffusion coefficient and C i i+ /C s 0 is the ratio of singly ionized interstitial concentration to neutral substitutional concentration. The inverse temperature dependence of D may be ascribed to the more rapid increase of C s 0 with temperature than that of D i C i i+ .