Native and Irradiation-Induced Defects in SiO<sub>2</sub> Structures Studied by Positron Annihilation Techniques

Abstract

Positron annihilation studies on SiO2 structures show that a significant fraction of the positrons forms positronium (Ps). The fraction of Ps is obtained by lifetime measurements and 2D-ACAR experiments, as well as by decomposition of Doppler broadening (DB) spectra. The narrowest component is attributed to para-positronium (p-Ps). For all SiO2 structures studied the DB parameters S and W show similar values. Dramatic changes are observed when point defects are introduced by ion implantation. These defects are very efficient traps for positrons. Once positrons are trapped at the defects the Ps formation reduces to zero. In addition the S parameters drops by about 10%, while the W parameter increases by about 20%. Vacuum annealing experiments show that the original conditions are restored after treatments at about 800 K. For the first time we explain the transport of positrons in biased SiO2 in terms of Ps production and suppression.