Molecular mechanisms for disilane chemisorption on Si(100)-(2 x 1)

Adsorption Behaviors of Chlorosilanes, HCl, and H2 on the Si(100) Surface: A First-Principles Study

The hydrochlorination process is a necessary technological step for the production of polycrystalline silicon using the Siemens method. In this work, the adsorption behaviors of silicon tetrachloride (SiCl₄), silicon dichloride (SiCl₂), dichlorosilane (SiH₂Cl₂), trichlorosilane (SiHCl₃), HCl, and H₂ on the Si(100) surface were investigated by first-principles calculations. The different adsorption sites and adsorption orientations were taken into account. The adsorption energy, charge transfer, and electronic properties of different adsorption systems were systematically analyzed. The results show that all of the molecules undergo dissociative chemisorption at appropriate adsorption sites, and SiHCl₃ has the largest adsorption strength. The analysis of charge transfer indicates that all of the adsorbed molecules behave as electron acceptors. Furthermore, strong interactions can be found between gas molecules and the Si(100) surface as proved by the analysis of electronic properties. In addition, SiCl₂ can be formed by the dissociation of SiCl₄, SiH₂Cl₂, and SiHCl₃. The transformation process from SiCl₄ to SiCl₂ is exothermic without any energy barrier. While SiH₂Cl₂ and SiHCl₃ can be spontaneously dissociated into SiHCl₂, SiHCl₂ should overcome about 110 kJ/mol energy barrier to form SiCl₂. Our works can provide theoretical guidance for hydrochlorination of SiCl₄ in the experimental method.

Revisiting the vibrational spectra of silicon hydrides on Si(100)-(2x1) surface: What is on the surface when disilane dissociates?

Even though the decomposition of disilane on silicon surfaces has been extensively studied, the molecular mechanism for its decomposition has not been fully resolved. The general view motivated partly by spectroscopic data is that decomposition occurs through silicon-silicon bond dissociation although there is evidence from kinetics that silicon-hydrogen bond dissociation is important, and perhaps even dominant. Thus, we reexamine the assignment of the experimental vibrational peaks observed in disilane and silane adsorption in order to assess the evidence for the silicon hydride species that are formed during decomposition. We calculate the vibrational density of states for a number of silicon hydride species on the Si(100)-(2x1) surface using Car-Parrinello molecular dynamics. We obtain the calculated vibrational frequency in the adiabatic limit by extrapolating to zero orbital mass, calibrating our method using the well-established monohydride peak. The calculated vibrational frequencies of the monohydride are in good agreement experimental data. Our results show that the spectroscopic data for silicon hydrides does not preclude the occurrence of Si(2)H(5) on the surface thus providing evidence for silicon-hydrogen bond dissociation during disilane adsorption. Specifically, we find that an experimentally observed vibrational peak at 2150 cm(-1) that has generally been attributed to the trihydride SiH(3) is more likely to be due to Si(2)H(5). Our results also clear up the assignment of two peaks for monohydride species adsorbed at the edge of a growing terrace, and a peak for the dihydride species adsorbed in the interdimer configuration.

Disilane chemisorption on Si(x)Ge(1-x)(100)-(2 x 1): molecular mechanisms and implications for film growth rates

At low temperatures, hydrogen desorption is known to be the rate-limiting process in silicon germanium film growth via chemical vapor deposition. Since surface germanium lowers the hydrogen desorption barrier, Si(x)Ge((1-x)) film growth rate increases with the surface germanium fraction. At high temperatures, however, the molecular mechanisms determining the epitaxial growth rate are not well established despite much experimental work. We investigate these mechanisms in the context of disilane adsorption because disilane is an important precursor used in film growth. In particular, we want to understand the molecular steps that lead, in the high temperature regime, to a decrease in growth rate as the surface germanium increases. In addition, there is a need to consider the issue of whether disilane adsorbs via silicon-silicon bond dissociation or via silicon-hydrogen bond dissociation. It is usually assumed that disilane adsorption occurs via silicon-silicon bond dissociation, but in recent work we provided theoretical evidence that silicon-hydrogen bond dissociation is more important. In order to address these issues, we calculate the chemisorption barriers for disilane on silicon germanium using first-principles density functional theory methods. We use the calculated barriers to estimate film growth rates that are then critically compared to the experimental data. This enables us to establish a connection between the dependence of the film growth rate on the surface germanium content and the kinetics of the initial adsorption step. We show that the generally accepted mechanism where disilane chemisorbs via silicon-silicon bond dissociation is not consistent with the data for film growth kinetics. Silicon-hydrogen bond dissociation paths have to be included in order to give good agreement with the experimental data for high temperature film growth rate.