Ion Implantation Doping of Semi-Insulating Polycrystalline Silicon

Abstract

ABSTRACTThe study of doping of silicon nano-grains in semi-insulating polycrystalline silicon is of paramount importance since intense room-temperature luminescence at 1.54 µm has been demonstrated in this silicon-based semiconductor when doped with erbium ions. We have investigated the formation of p- and n-type layers of semi-insulating polycrystalline silicon by implantation and diffusion of B, P, As, and Er. The room-temperature resistivity can be changed by more than six orders of magnitude for both p- and n-type doping. A dramatic decrease of resistivity is observed for dopant concentrations above a threshold level; this effect is explained by assuming that the free-carrier motion is limited by grain boundary barriers and the electrical conduction is due to thermionic emission and tunneling of the carriers through the barriers. The prevalence of one mechanism over the other depends upon temperature, oxygen concentration and doping. In the undoped material the barrier height is large (≈ 0.5 eV), but for dopant concentrations above the threshold, it decreases with the doping level. Correspondingly, the conductivity increases by many orders of magnitude. The determination of the threshold value allows the evaluation of donor and acceptor grain boundary trap densities. Diodes have been fabricated by implantation and diffusion of boron and erbium. The I-V characteristics of these diodes are interpreted on the basis of the material modeling.