Formation of Single Crystal Sulfur Supersaturated Silicon Based Junctions by Pulsed Laser Melting

Abstract

The authors demonstrate the formation of pn and nn+ junctions based on silicon supersaturated with sulfur (up to 0.46 at. %) using a combination of ion implantation and pulsed laser melting. Silicon wafers were implanted at 200 keV 32S+ to doses ranging from 1×1015 to 1×1016 ions/cm2 and subsequently melted and resolidified by using a homogenized excimer laser pulse. Above a threshold laser fluence of ~1.4 J/cm2, the process produces a single crystal supersaturated alloy, free of extended defects, with a sharp junction between the laser melted layer and the underlying substrate, located near the maximum penetration of the melt front. Hall effect measurements indicate that the laser melted layers are n doped with a free carrier density up to 8×1018/cm3 that decreases by one-third upon postirradiation furnace annealing at 550 °C. Dark current-voltage measurements performed on these structures show good rectifying behavior. The photovoltaic characteristics of the junctions were enhanced by postirradiation annealing at 550–800 °C. These effects are attributed to the evolution of a population of point defects that survive the laser treatment. The influence of ion implantation dose, laser fluence, and annealing temperature on the properties of the junctions is also presented and discussed.