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 te chnique. The experimental diffusion length values are strongly influenced b y positron trapping at vacancies and negative ions existing in the samples. After subtraction of the trapping effects the diffusion coefficient for fr ee 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 8 0 to 300 K the diffusion coefficient decreases strongly with increasing tem perature meaning that positron scattering from polar-optical phonons is swi tched on.