HIGH-TEMPERATURE POSITRON DIFFUSION IN SI, GAAS, AND GE

Positron diffusion coefficients have been determined in Si, GaAs, and Ge in the temperature range 130-1000 K using the positron-beam technique. The diffusion coefficients at 300 K in Si, GaAs, and Ge are 2.3(2), 1.6(2), and 1-2 cm2/s, respectively. In Si, the diffusion coefficient has the temperature dependence T1/2 from 30 to 500 K [combined with the experiment of Makinen et al., Phys. Rev. B 43, 12 114 (1991)], consistent with positron scattering from longitudinal acoustic phonons. At T>500 K the diffusion coefficients are lower than extrapolated from the T-1/2 dependence indicating the onset of optical-phonon scattering. The first-order optical deformation-potential parameter is estimated to be 11 eV. Positron scattering from ionized impurities is observed in heavily doped n-type Si. In Ge, the temperature dependence of the diffusion coefficient is T-1/2 above 500 K. At room temperature the experiment yields an abnormally high diffusion length which is attributed to space-charge effects. In GaAs the diffusion coefficient is only weakly dependent on temperature from 300 to 800 K, which is interpreted to be due to positron scattering from both acoustic- and polar-optical-phonon modes. The agreement with theory is good using the theoretical deformation-potential parameters and the positron effective masses m* = 1.3m(e) - 1.6m(e) in Si, GaAs, and Ge.