Quantum state-resolved studies of direct and precursor-mediated dissociative chemisorption of silane on Si(100)

Abstract

In this work, I present the results of my studies on the state-resolved reactivity of silane (SiH4) on the Si(100)-(2x1) surface. The results demonstrate a co-existence of both a direct and a precursor mediated mechanisms for the dissociative chemisorptions of SiH4 on Si(100)-(2x1). Above an incident kinetic energy of 34 kJ/mol, where SiH4 chemisorption occurs via the direct mechanism, I find that both translation and vibrational energy activate the chemisorptions. Vibrational energy is slightly less effective than translation energy in promoting the reaction since adding 51.6 kJ/mol of vibrational energy leads to the same reactivity increases as adding 47 kJ/mol of translational energy. Vibrational excitation leads to at most a factor of 4 increase in reactivity of SiH4 on Si(100)-(2x1). While translational activation was reported before [Jones et. al, Chem. Phys. Lett., 1994, 229, 401], this is the first time vibrational activation is observed for SiH4 chemisorption on Si(100)-(2x1). I also investigate mode-specific reactivity by comparing the reactivity of SiH4 excited to two different nearly isoenergetic vibrational states, |2000> and |1100>. The results show that the reactivity of SiH4 excited to the |2000> state is consistently higher than that of the |1100> state over the range of incident kinetic energy studied. This mode-specific reactivity of SiH4 on Si(100)-(2x1) is rationalized in terms of different vibrational amplitude of the |2000> and the |1100> states: the former contains two quanta of Si-H stretch in a single Si-H bond, whereas the latter contains one quantum of Si-H stretch in each of two Si-H bonds. Below 34 kJ/mol, where SiH4 chemisorption on Si(100)x(2x1) occurs via the precursor-mediated mechanism, the effect of internal vibrational energy on the reactivity is found for the first time. While increasing translational energy decreases SiH4 reactivity, vibrational energy increases its reactivity. After the incident SiH4 molecule is trapped in the weakly bound precursor state, its vibrational energy appears to be conserved long enough that it can still promotes the chemisorption reaction once the trapped SiH4 encounters a reactive site. The chemisorption of SiH4 is, however, also weakly activated with vibrational energy in the precursor-mediated mechanism, in which a reactivity enhancement of less than a factor of 2 is observed for vibrationally excited SiH4. The use of isotopic selective excitation in deposition experiments to control the isotopic abundance of Si is not successful due to the fact that SiH4 chemisorption on Si(100)-(2x1) is weakly activated with vibrational energy. With a reactivity enhancement of at most a factor of 4, the relative isotope abundance of 28Si is expected to increase by only 0.6% in the laser excitation experiment. Such a small change in the natural isotopic abundance could not be detected reliably in the limited analysis time by our SIMS setup. However, this experiment can be applied to other system, where vibration excitation is more efficient to promote the reaction. Furthermore, vibrational activation of precursor-mediated gas-surface reaction discovered in this work will open up a new possibility to control gas-surface reaction of physisorbed species on surfaces.