Surface structure and growth mechanisms of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is a disordered semiconductor with many technological applications. Basic research is still being done on many aspects of a-Si:H. We focus on those issues related to a-Si:H surfaces. The structure of a typical surface at an atomistic level still needs to be investigated. The mechanisms of the growth of a-Si:H are not well understood, nor have the energetics of possible surface reactions been calculated. Other areas of interest are the vibrational signature of surface H and the subject of surface roughness.; We describe the basic experimental properties of a-Si:H. We review the key computer simulations on bulk a-Si:H and a-Si:H surfaces. Next we discuss the local-orbital method and the ab initio quantum chemistry code used for our research. We outline the subsequent approximations used in these computational methods and discuss their accuracy.; We report on the a-Si:H surface models we have constructed. We describe the method for making the four surfaces we developed. We analyze them for reasonableness in terms of geometric and electronic structure. We also calculate the vibrational properties of the H in these surfaces and compare with experimental results.; We examine the question of surface roughness in a-Si:H. It is hypothesized that dangling bonds on the a-Si:H growing surface prefer to be in microscopic valleys. SiH₃ bonding at these sites fills in the valleys, thereby smoothing the surface. We create valleys of various sizes and then compare the relative energies of dangling bonds in the valleys with those on the flat surfaces.; We investigate two pathways for SiH₃ incorporation into growing a-Si:H. It has been thought that growth proceeds by H abstraction by atomic H or SiH₃. Recently, a mechanism of direct insertion of SiH₃ into a surface Si-Si bond has been proposed to explain new IR data. We determine the activation energies for key steps in these pathways, and discuss the validity of each in light of our results.