Low-temperature positron diffusion in GaAs

Positron diffusion has been measured in undoped and Si doped (n = 2 x 10(18) cm(-3)) GaAs in the temperature range 20-290 K using the positron-beam technique. The experimental diffusion length values are strongly influenced by positron trapping at vacancies and negative ions existing in the samples. After subtraction of the trapping effects the diffusion coefficient for free positrons in the GaAs lattice is obtained. The diffusion coefficient is 14 +/- 2 cm(2) s(-1) at 20 K and 1.6 +/- 0.2 cm(2) s(-1) at 295 K. Below 80 K it follows the T-1/2 law due to scattering from acoustic phonons. From 80 to 300 K the diffusion coefficient decreases strongly with increasing temperature meaning that positron scattering from polar-optical phonons is switched on.