VACANCY-HYDROGEN INTERACTION IN H-IMPLANTED SI STUDIED BY POSITRON-ANNIHILATION

The density of vacancylike defects, produced in silicon by hydrogen im plantation at 15.5 keV and surviving to successive isochronal annealin gs, has been measured by means of a slow positron beam. The results sh ow that the number of defects acting as positron traps is a small frac tion of the Frenkel pairs produced by implantation. This number decrea ses, increases again, and eventually disappears after annealing at inc reasing temperatures. The mean depth of the positron traps in as-impla nted samples is smaller than the mean depth of vacancies predicted by computer simulations, but reaches, and in some cases surpasses, this l imit after annealing. A minimum in the number of the positron traps oc curs around 350-degrees-C when the number of displaced silicon atoms, produced by hydrogen agglomeration, is at maximum. Further annealing i ncreases the number of traps, until at high temperatures, above 700-de grees-C, all the traps disappear. This complicated behavior is interpr eted as the result of several concomitant effects: the formation of va cancylike defects during implantation, their partial annealing below 3 50-degrees-C, an initial passivation of the traps caused by hydrogen f ollowed by a reactivation stage, and the formation of thermally stable hydrogen complexes.