The Effect of Hydrogen and Other Additives on the Electronic Properties of Amorphous Silicon

Abstract

This tutorial discussion is a personal view on three types of additives that are of interest to the study of the electronic properties of amorphous silicon (a-Si). These additives are: 1) dangling and other weak bonds, 2) hydrogen and other bond terminators, and 3) n- and p-type dopants. Although dangling bonds are not generally thought of as additives, conceptually they can be viewed as extrinsic to a perfect continuous random network and therefore, as something added to that as yet unobtainable ideal. Either accidentally or by design, hydrogen, oxygen and more recently fluorine, have been used as bond terminators to passivate the extrinsic effects of dangling bonds. Quantitative analysis of H in electronically interesting amorphous silicon, namely a-Si:H, gives a typical concentration of order 10–20 atomic percent, a level indicative of a role for H that involves more than defect passivation. Substitutional dopants that are widely used in crystal silicon (x-Si), e.g. P and B, are now commonly added to a-Si:H to make n-and p-type amorphous semiconductors. The doped materials are employed in diodes, transistors and other configurations of device potential. The assumption is that at least the electrically active P and B are substitutionally incorporated into the host Si network and therefore donate and accept electrons in ways analogous to shallow donors and acceptors in x-Si.