Positron thermalization in Si and GaAs

Abstract

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.

Annealing of electron-, proton-, and ion-produced vacancies in Si

“…In a recent work 47 it was suggested that the T −1/2 dependence at temperatures below 20 K would be modified due to the positrons not being fully thermalized, the consequence being that below ϳ20 K the trapping rate becomes nearly independent of temperature for negatively charged divacancies. The present data exhibit a similar effect, but in a significantly wider temperature range 8 -100 K, and at higher temperatures the trapping decreases much slower than T −n ͑n Ͼ 1.5͒, which is characteristic for negatively charged divacancies.…”

Review—Defect Identification with Positron Annihilation Spectroscopy in Narrow Band Gap Semiconductors

“…The techniques are based on the fact that the annihilating electron-positron pair has momentum, which will give rise to a Doppler broadening of the annihilation line. The positron thermalizes rapidly in solids, typically within a few ps at RT, 38 and therefore it has negligible momentum compared to the annihilating electron. Hence, the Doppler broadening of the annihilation line is a measure of the momentum distribution of the annihilating electrons in the direction of the detector.…”