PHYSICAL REVIEW B 84, 205314 (2011) Ab initio study of the early stages of gas-phase water oxidation of the Si(100) (2 × 1):H surface R. Lelis-Sousa * and M. J. Caldas † Instituto de F´ ısica, Universidade de S ˜ ao Paulo, Rua do Mat˜ ao, Travessa R 187, CEP 05508-900 S˜ ao Paulo, Brazil (Received 28 June 2011; revised manuscript received 25 October 2011; published 16 November 2011) We investigate different reaction mechanisms for the dissociation of a H 2 O molecule on the Si(100)(2 × 1):H surface, through ab initio calculations within density functional theory, comparing results using local density and generalized gradient approximations for the exchange-correlation potential. The reaction pathways were obtained with the “climbing image–nudged elastic band” procedure. In all cases we present complete analysis of the transition barriers and binding energies. Our results indicate that the oxidation route suggested by earlier works, which entails full chemisorption of the water molecule, is not favorable, and we propose two alternative routes with simultaneous release of one H 2 molecule. DOI: 10.1103/PhysRevB.84.205314 PACS number(s): 68.43.Bc, 68.47.Fg, 82.20.Wt, 82.65.+r I. INTRODUCTION Oxidation of silicon surfaces has been a frequent object of study since the beginning of device physics and continues to receive careful attention, 1–3 especially due to the advances in nanoscience and molecular electronics. 4–6 Oxidation can come by exposure to molecular O 2 gas, to ambient air, or to water, and in the last two cases it is important to understand the initial reaction with water molecules. We are interested here in water oxidation of the monohydrogenated Si(100)(2 × 1):H, of relevance to hybrid organic-on-Si interfaces. 6,7 The reaction of water with Si(100) surfaces has long been investigated, from the experimental and theoretical points of view, 8–32 and despite these efforts, there are still several open questions. For clean Si(100) surfaces it is agreed from the experimental results that, after interaction with water, almost 100% of the surface consists of very stable HSi– SiOH species 11–25,33,34 and the dimer-row pattern is maintained. In this case, theoretical studies find that the reaction proceeds without barrier. 17,33,35 As for hydrogen passivated Si(100):H surfaces, even though there are fewer experimental or theoretical studies of wet oxidation, analyses of infrared (IR) spectra confirm that they are more resistant to oxidative damage. 18–20,36,37 Monohydrogenated Si(100)(2 × 1):H reconstruction is very stable 38 and allows for the growth of samples with a high degree of homogeneity, which makes it an interesting substrate for applications in hybrid devices; 5–7 on the other hand, oxygen is known to be very important for determining surface reactivity (in fact some techniques rely specifically on using suboxides 4 ), and since many approaches to the assembling of organic layers are based on wet-chemistry techniques, a thorough investigation of the early stages of oxidation by water of this particular surface is, therefore, very useful and motivates this work. From a joint experimental and theoretical study 18–20 of the Si(100):H surface, based on IR spectroscopy and ab initio cluster-based simulations for vibrational modes, it has been proposed that, in the initial stages of oxidation, insertion of the water molecule occurs with dissociation as a silanol (SiOH) group and dimer-bond breaking (HSi-SiH + H 2 O → HSiH + HSiOH). It is well established that increasing the water flux on a Si(100):H surface results in the disappearance of the IR signal for the pure surface dimer HSi-SiH and gives rise to vibrational peaks related to oxidized structures; however, the silanol vibrational modes at 821 and 2081 cm −1 , present for oxidation of the clean surface, were not identified in the experimental data. At advanced stages of wet oxidation, the IR signal is dominated by peaks located at 962–979 and 2143 cm −1 that, based on the calculations for the vibrational frequencies, were attributed to the presence of neighbor dihydrogenated surface groups, both clean HSiH and with a back-bond oxygen HSi(O)H (here we label this structure C BB ). These results suggest that the back bond is the main site for oxygen chemisorption. 18–20,26 Still according to this experimental interpretation, 18–20,26 oxidation of the back bond would follow from a HSiH + HSiOH precursor (we label this complete structure C Sil ). An intriguing point is that at early stages of wet oxidation (data for a water exposure of 100 L), 18–20 the IR spectrum consists mainly of vibrational peaks in the ranges 803 to ≈870, ≈897 to 910, and 2120 cm −1 20 , and the frequencies for oxidized HSiH units (close to 962–979 cm −1 ) are not identified. Indeed, the theoretical simulations 14,18–20 suggest instead oxidized dimer species HSi-O-SiH or HSi-Si(O)H, including multiply oxidized groups such as HSi-O-Si(O)H. To our knowledge, issues related specifically to the inter- action between a water molecule and HSi-SiH groups have not yet been discussed in the literature. We present here a theoretical investigation of energy barriers, transition states, and reaction pathways related to oxygen insertion in the Si(100)(2 × 1):H first subsurface region, at early stages of wet oxidation. We show that the C Sil species will be formed on the surface, but the energy barrier for conversion into C BB is very high, implying that silanol is not an effective route for oxygen insertion in subsurface sites. We propose two efficient routes for initial oxidation without dimer-bond breaking, one regarding oxidation of the back-bond site and another for on-dimer oxidation, which have favorable dissociation routes; both occur with simultaneous release of a H 2 molecule, producing HSi-O-SiH + H 2 (C OD ) and HSi-Si(O)H + H 2 (C BB+H2 ) species. Again, to our knowledge, these structures have never been previously considered. An interesting result of our simulations is that silanol is actually a precursor not of the back-bond configuration, but of the on-dimer oxidation. II. METHODOLOGY The unit cell of the ideal Si(100)(2 × 1):H surface consists of two Si atoms per layer, with the surface dimers saturated 205314-1 1098-0121/2011/84(20)/205314(8) ©2011 American Physical Society