Positron annihilation in silicon in thermal equilibrium at high temperature

Annihilation characteristics of positrons in Si in thermal equilibrium at high temperature were studied using a monoenergetic positron beam. The line-shape parameter of the Doppler broadening spectrum for the annihilation of positrons at the surface Ss and the diffusion coefficient of positrons D + were determined in the temperature range 298-1618 K. At 298-1373 K, because of the thermal desorption of positrons from the surface state and a resultant formation of positronium (Ps), the value of Ss increased with increasing temperature. However, it started to decrease above 1473 K. The decrease in the value of Ss was attributed to the suppression of the Ps formation due to the surface melting. With increasing temperature, the value of D + was decreased by the scattering of positrons with acoustic and optical phonons, and above 1073 K, it was almost constant; the lower limit of D + was determined as 0.48 cm2 s-1 . From the analysis of Doppler broadening spectra, the behaviour of positrons in bulk Si at high temperature are also discussed.