Date of Award
Doctor of Philosophy
Physics and Astronomy
Costas M. Soukoulis
Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) have been studied with molecular dynamics simulations. The structural, vibrational, and electronic properties of these materials have been studied with computer-generated structural models and compare well with experimental observations. The stability of a-Si and a-Si:H have been studied with the aim of understanding microscopic mechanisms underlying light-induced degradation in a-Si:H (the Staebler-Wronski effect);With a view to understanding thin film growth processes, a-Si films have been generated with molecular dynamics simulations by simulating the deposition of Si-clusters on a Si(111) substrate. The Biswas-Hamann interatomic Si potential has been employed. The a-Si films have 15-28% lower densities than the crystal, and voids that lead to intense small wave-vector scattering in the static structure factor. A remarkable result is that no floating bonds are found and all coordination defects comprise of dangling bonds;A new two- and three-body interatomic potential for Si-H interactions has been developed. The structural properties of a-Si:H networks are in good agreement with experimental measurements. The presence of H atoms reduces strain and disorder relative to networks without H;We have studied the stability of bulk a-Si models against local excitations. Local excitations lead to structural degradation in a-Si networks in which weak Si-Si bonds are broken and dangling bonds are produced, requiring a threshold of 0.8-1.0 eV. In contrast, c-Si is stable to local excitations as large as 8.0 eV;To investigate bond-breaking models of the Staebler-Wronski effect, we have examined structural changes against local excitations in a-Si:H models. The important result is that the monohydride-containing a-Si:H model, with no coordination defects or H-induced defects, is stable to bond-breaking excitations. The dihydride-containing a-Si:H model does exhibit higher energy dangling-bond states induced by local excitations, but these dangling-bonds can be easily annealed back to the initial configuration;We propose a two-level h-induced defect system as a mechanism for the Staebler-Wronski effect in a-Si:H. A bridge-bonded H interstitial defect is identified as the annealed state that can form a higher-energy metastable Si dangling-bond state in the light-soaked state. This two-level defect system can account for the saturation and metastability of the defects.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Kwon, Inhee, "Theoretical studies of amorphous silicon and hydrogenated amorphous silicon with molecular dynamics simulations " (1991). Retrospective Theses and Dissertations. 9653.