Positron thermalization in Si and GaAs

Positron thermalization in Si and GaAs has been studied both by experiments and simulations. The decrease in the positron mean energy due to the interactions with longitudinal-acoustic phonons was calculated down to 4 K by solving numerically the Boltzmann equation for the positron momentum distribution. We find that the differences in the strength of the positron-phonon coupling can result in considerable variations in the thermalization time. At 10 K, the time needed by the positrons to reach twice the thermal energy is 25 ps in Si, and 80 ps in GaAs. We find experimental support for the calculated thermalization behavior by studying the temperature dependence of the positron trapping rate at negative vacancy-type defects in Si and GaAs. In Si, we observe that positron lifetime data depends strongly on the sample temperature at least down to 8 K, which supports the predicted fast thermalization. In GaAs, the trapping rate below 20 K is observed to increase considerably less than expected for positrons thermalized instantly after implantation. This demonstrates experimentally that the thermalization time in GaAs is indeed much longer than in Si. We show further that the calculated positron energy-loss rates can explain quantitatively the temperature dependence of the experimental trapping rate in GaAs down to 8 K.