title: First-principles theoretical analysis of silyl radical diffusion on silicon surfaces.
abstract: We report results from a detailed analysis of the fundamental radical precursor diffusion processes on silicon surfaces and discuss their implications for the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of SiH(3) radical migration on the hydrogen-terminated Si(001)-(2 x 1) surface with molecular-dynamics (MD) simulations of SiH(3) radical precursor migration on surfaces of a-Si:H films. Our DFT calculations yield activation energies for SiH(3) migration that range from 0.18 to 0.89 eV depending on the local electronic environment on the Si(001)-(2 x 1):H surface. In particular, when no substantial surface relaxation (Si-Si bond breaking or formation) accompanies the hopping of the SiH(3) radical the activation barriers are highest, whereas hopping between nearest-neighbor overcoordinated surface Si atoms results in the lowest radical diffusion barrier of 0.18 eV; this low barrier is consistent with the activation barrier for SiH(3) migration through overcoordinated sites on the a-Si:H surface. Specifically, the analysis of the MD simulations of SiH(3) radical migration on a-Si:H surfaces yields an effective diffusion barrier of 0.16 eV, allowing for the rapid migration of the SiH(3) radical prior to its incorporation in surface valleys; rapid migration and subsequent incorporation constitute the two-step mechanism responsible for the smoothness of plasma deposited a-Si:H thin films.
authors.affiliation: Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-3110, USA.
authors.forename: Tamas
authors.initials: T
authors.lastname: Bakos
authors.forename: Mayur S
authors.initials: MS
authors.lastname: Valipa
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Maroudas
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D019281:Dimerization; D006859:Hydrogen / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D012821:Silanes / Q000737:chemistry*; D017655:Silicon Compounds / Q000737:chemistry; D013499:Surface Properties
chemical_list: D012821:Silanes; D017655:Silicon Compounds; D006859:Hydrogen