The effects of ion implantation on the electrical properties of amorphous silicon

Abstract

In this paper we present the results of a systematic study of ion implantation doping in glow-discharge a-Si films using the substitutional impurities, P, As, Sb, Bi, B, Al, Ga, In and Tl and the interstitial impurities Na, K, Rb and Cs. Implantation doping provides the same range of control of the electrical properties as gas phase doping, but at a lower doping efficiency. The compensation of pre-doped specimens by ion implantation has been investigated and found to be feasible and predictable. As an example the characteristics of an implanted a-Si p-n junction are shown. Results with F, Cl, Br and I implantations indicate that the halogens do not act as interstitial acceptors. The effect of implantation damage on electrical properties and its dependence on implantation temperature have been investigated in some detail. Photoconductivity measurements provide a useful approach to the problem and it is concluded that the density of gap states increases with ion dose, even for hot implantations. A simple model is proposed for the interpretation of the results and the basic reasons for the low doping efficiencies are discussed. Finally, the implantation results are used to determine an approximate density of state distribution for a-Si which is compared with the field effect data.