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Unformatted text preview: VOLUME 84, NUMBER 5 P H Y S I C A L R E V I E W L E T T E R S 31 J ANUARY 2000 Bonding Arrangements at the Si-SiO 2 and SiC-SiO 2 Interfaces and a Possible Origin of their Contrasting Properties Ryszard Buczko,* Stephen J. Pennycook, and Sokrates T. Pantelides Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 and Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (Received 15 June 1999) We report ab initio calculations designed to explore the relative energetics of different interface bond- ing structures. We find that, for Si (001), abrupt (no suboxide layer) interfaces generally have lower energy because of the surface geometry and the softness of the Si-O-Si angle. However, two energeti- cally degenerate phases are possible at the nominal interface layer, so that a mix of the two is the likely source of the observed suboxide and dangling bonds. In principle, these effects may be avoidable by low-temperature deposition. In contrast, the topology and geometry of SiC surfaces is not suitable for abrupt interfaces. PACS numbers: 68.35.Ct, 68.35.Bs, 81.65.Mq, 82.65.Dp A key component of metal-oxide-semiconductor field- effect transistors (MOSFETs) is the semiconductor-oxide interface. The predominance of Si in microelectronics is due primarily to the properties of the Si- SiO 2 interface, which can easily be made very abrupt (minimal suboxide layer) and smooth (minimal steps) with a minimal den- sity of point defects . As scaling laws are pushing the technology to ultrathin SiO 2 layers, understanding and control of the interface on the atomic scale remain open challenges. For power MOSFETs, a semiconductor with a wider band gap is preferred. SiC, whose native oxide is also SiO 2 , is one of the options, but efforts to develop SiC-based MOSFETs have been thwarted by poor-quality SiC- SiO 2 interfaces. The abruptness and smoothness of the Si- SiO 2 interface are puzzling because SiO 2 is amorphous. In addition, O has a high solubility and diffusion constant in Si. In con- trast, O is not known to be an abundant impurity in SiC, and a recent calculation finds that O has very low solubil- ity in SiC . Yet, SiC interfaces are generally found to be rough for MOSFETs. Photoemission  has provided information on bonding arrangements at the Si- SiO 2 interface while microscopy and theory led to several interface models [4–10]. Total-energy calculations have accounted for the observed low density of interface point defects  and have found that, during thermal oxidation, lateral growth is preferred . Yet, the mechanisms that control abruptness and smoothness remain an open issue that is critical for the ultimate control of ultrathin gate oxides....
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