BEHAVIOR OF BORON AND NITROGEN IN A SURFACE-LAYER OF SILICON DURING SYNTHESIS OF BURIED LAYERS BY IMPLANTATION OF N+ IONS

The electrical properties of boron-doped surface layer of silicon upon implantation with large doses of 200-keV N+ ions have been studied. M easurements of the conductivity and of the Hall effect at various stag es of isochronous (t=10(3) s, T=700-1200-degrees-C) and isothermal (t= 10(2)-10(5) s, T=800-degrees-C) annealing revealed several important e ffects: 1) accumulation of boron atoms at the surface; 2) neutralizati on of boron even at dopant concentrations > 10(20) cm-3; 3) appearance of donor centers characterized by two peaks in their distribution, on e of which corresponds to boron and the other is localized at the dept h of the maximum elastic losses experienced by the N+ ions; 4) gradual change in the type of conduction of the surface layer with an increas e in the temperature or in the duration of annealing. The effects are attributed to precipitation of a supersaturated solution of N in Si du ring annealing, which is accompanied by the formation of silicon nitri de precipitates, recovery of the disturbed structure, and emission of mobile point defects. These defects are responsible for the accelerate d rising diffusion of boron toward the surface. The boron atoms, toget her with the radiation defects, become centers of precipitation of the nitride. In this process the boron atoms lose their acceptor properti es and the precipitates introduce an equivalent amount of donors. The change in the type of conduction of the surface layer during annealing occurs as a result of a gradual formation and dissolution of precipit ates, governed by their dimensions, and by the temperature and degree of supersaturation of silicon with nitrogen around the precipitation c enters.