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Kinetics of the Initial Oxidation of the (0001) 6H-SiC 3 × 3 Reconstructed Surface Jia Mei Soon,*,†,‡,§ Ngai Ling Ma,‡,| Kian Ping Loh,† and Osami Sakata§ Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, 117543 Singapore, Institute of High Performance Computing, 1 Science Park Road, The Capricorn, Singapore Science Park II, 117528 Singapore, and Japan Synchrotron Radiation Research Institute, Spring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan ReceiVed: March 13, 2008; ReVised Manuscript ReceiVed: August 14, 2008
Using first-principle calculations, the mechanism of oxygen insertion into the (0001) 6H-silicon carbide 3 × 3 reconstructed surface is investigated. The stable chemisorbed oxygen states, transition states, and activation energies adjoining each oxidation step are identified. Dissociative oxidation occurs barrierlessly at the surface dangling bond, while the lower layers are repulsive toward oxygen. Oxidation of the surface dangling bond will lower the energy barriers for subsequent lower layer oxidation, after which lower layer oxidation is possible. 1. Introduction Silicon carbide (SiC) is a promising material for applications in high-power, high-temperature, and high-frequency electronic devices.1,2 SiC offers several advantages over silicon. The breakdown electric field strength of SiC is more than 1 order of magnitude higher. Consequently, a device made of SiC can be ∼10 times thinner than a device made of Si at similar operating voltages. SiC devices can withstand and dissipate heat faster due to its thermal conductivity and high thermal stability. Hence, cooling installations can be reduced and the total system can be made much smaller compared to Si-based devices. The success of the Si technology is in part due to the excellent properties of SiO2 as an insulating gate and the low density of defects at the Si/SiO2 interface. Apart from Si, SiC is the only semiconductor which forms SiO2 as its native oxide. However, in the presence of oxycarbide species which form at the oxide/ SiC interface during oxidation, SiC exhibits worse electrical passivation characteristics when compared to Si. For this reason, studies of the oxidation mechanism and chemical composition of thermally grown oxides on SiC have attracted a lot of attention. An experimental study of the oxidation process has shown silicon oxycarbide species as the first oxidation products.3 On the basis of core level photoemission spectroscopy, it has been proposed by Amy et al. that initial oxygen insertion takes place below the surface close to the first carbon atomic layer, i.e., the coplanar layers, leaving the Si adatom surface dangling bond unaffected.4-7 Several authors have disagreed with this conclusion. Investigations performed on the (0001) SiC 3 × 3 surface using reflection high-energy electron diffraction (RHEED) dynamic rocking beam analysis by Xie et al.8 and scanning tunneling microscopy (STM) by Chen et al.9 suggest that O2 molecules undergo dissociative adsorption on the Si adatom at room temperature by reaction with the surface dangling bonds, leaving the coplanar layers unaffected. Using slab model calculations, * To whom correspondence should be addressed. E-mail: jamie@ spring8.or.jp. † National University of Singapore. ‡ Institute of High Performance Computing. § Japan Synchrotron Radiation Research Institute. | Current address: Department of Computational Chemistry, Novartis Institute for Tropical Diseases Pte Ltd., 10 Biopolis Road, No. 05-01 Chromos, Singapore 138670.
Xie and Chen suggested that, while both adatom and coplanar initial oxidation are energetically feasible, adatom oxidation is theoretically more stabilizing than coplanar oxidation, hence the preferred adatom oxidation observed experimentally. At higher oxygen doses, they calculated that adatom oxidation continues to be more stabilizing than coplanar oxidation. Indeed, accurate structural determination using synchrotron-source surface X-ray diffraction by Voegeli et al.10 showed that even at high oxygen doses of 200 and 10 000 L, only adatom site oxidation was observed. Until now, this discrepancy in the conclusions cannot be bridged because the kinetics of the oxidation is not known. The proposition that the Si adatom is the first point of oxygen attack has been presented by Xie et al.,8 Chen et al.,9 and Voegeli et al.,10 but comparative verification (with the coplanar site) and the oxidation mechanism has not been studied prior to the present work. Our contribution in this work is two-fold: First, we evaluate the initial site of oxygen attack by comparative verification between the adatom and coplanar site. Second, we elucidate the kinetic pathway of the oxidation process by calculating the transition-state structures leading to the oxidized product. Our results show that the approach of O2 toward the adatom site is the only spontaneous reaction pathway for initial oxidation. Oxidation of the adatom site is barrierless because the process is assisted by the large energy gain from chemisorption. Upon oxidation of the adatom, the coplanar layers experience lowered repulsion to the approach by oxygen. 2. Calculation Methodology The theoretical calculations are based on a 6H-SiC 3 × 3 reconstructed surface model with four Si atomic planes lying on a C plane. This model has been established to be most energetically viable experimentally and theoretically.11,12 A 115atom (inclusive of the terminating H) cluster model of the 6H-SiC 3 × 3 unit cell is used for all energy minimization calculations as illustrated in Figure 1. With the exception of the adatom dangling bond, all other dangling bonds on the clusters are terminated with hydrogen atoms. All atoms in the cluster model are allowed to relax fully using the two-layer ONIOM method13,14 in the Gaussian 03 suite,15 except for in the situation where the single-point energy calculation method is used. In the ONIOM QM/MM hybrid method, computational
10.1021/jp802306e CCC: $40.75 2008 American Chemical Society Published on Web 10/07/2008
Oxidation of the (0001) 6H-SiC 3 × 3 Surface
J. Phys. Chem. C, Vol. 112, No. 43, 2008 16865
Figure 1. Adatom, coplanar 1, and coplanar 2 sites of oxygen attack on the 6H-SiC 3 × 3 reconstructed surface. The seven-membered ring on coplanar site 1 and five-membered ring on coplanar site 2 are highlighted using black. The boundaries used for high- and low-level ONIOM calculation are also defined. H atoms are omitted from the model for clarity. Only the atoms on the high-level ONIOM are shown using the ball and stick scheme. The Si and C atoms in the low-level ONIOM layer are represented using the wire-frame model. Si (blue) and C (gray) are indicated on the wireframe.
effort is concentrated on the important parts of the cluster (“high level”), which may participate in the reaction, while the nonparticipating parts of the model (“low level”) are treated with a modest level of theory. In this work, atoms in the high level are calculated using the B3LYP hybrid functional in which the exchange functional is derived from Becke’s three-parameter functional (B3)16 and combined with the Hartree-Fock-type exchange. The B3LYP hybrid functional seems to be a suitable choice for Si-C systems.17,18 The correlation functional used is provided by Lee-Yang-Parr (LYP).19 The electronic structure is expanded using polarization basis functions of 6-31G(d) developed by George Petersson and co-workers.20,21 Atoms in the low level are treated with semiempirical AM1 parametrizations. The ONIOM method allows for working with much larger models without incurring large computational expenses. This method has shown good correlation with both experimental and periodic DFT results.22-24 All optimizations in this work are performed at 0 K. Optimization on solid-state systems at 0 K generally give reliable results for the energetics and kinetics occurring at room temperature. The missing entropy terms usually require only minor corrections. The frequency calculations are performed at 298.15 K and 1.0 atm. Oxidation of the 6H-SiC 3 × 3 surface is expected to start at the Si adatom. On the 6H-SiC 3 × 3 surface, the first four layers are Si8 and the carbon layer starts from the fifth layer. The carbon layer is considerably far away from the surface
where the oxidation reaction is expected to occur, so it is a doubt whether it plays a role in the initial oxidation of the SiC 3 × 3 surface. However, in a recent paper, Vines et al. reported that the carbon centers are not simple spectators in the molecular25 and atomic26 oxidation of the (001) surface of earlytransition-metal carbides (TMCs). In their work, they considered the non-Si-enriched 1 × 1 surface in which the carbon layer is in coexistence with a metal layer, which is slightly different from the surface considered in this work. However, this gives sufficient reason to check the role of the carbon center with respect to the initial oxidation when a model for transition-state calculations is being chosen. Clearly, the C centers are too deep down the layers to be physically bonding with the oxygen at the initial oxidation. The bond angles ∠Si-Si-C and ∠Si-C-Si directly below and around the adatom site before and after oxidation at the (1) “ad-ins” (IV) and (2) “ins-ins” (V) positions are checked, and a difference of
