The Role of Preparation Conditions on the SIC(001) Surface Reconstructions: A First Principles Study
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ABSTRACT Using first principles molecular dynamics, we have studied the reconstructions and thermal properties of the (001) surfaces of cubic SiC. Our calculations show that C-terminated surfaces can have different reconstructions, depending on preparation conditions and thermal treatment, and that the Si-terminated surface geometry can be substantially affected by the presence of stress. Our findings allow us to interpret recent experiments about (001) SiC surfaces, in particular STM images. INTRODUCTION The notable, current interest in the characterization of SiC surfaces is motivated by the many technological applications of SiC films, which are usually prepared by chemical vapor deposition[l]. Moreover, SiC has emerged as a promising substrate for the growth of GaN films for applications in electronics[2]. We have addressed the issues of the effect of preparation conditions on the surface structure and the thermal stability of the (001) surfaces of cubic SiC, by performing a series of ab-initio molecular dynamics (MD) simulations[3, 4]. We have studied various reconstruction paths of the C- and Si-terminated surfaces, modeling different preparation conditions, and we identified experimental probes which could discriminate between the various reconstruction patterns. Here we focus on the finite temperature properties of the f-SiC (001) C-terminated surfaces, and on the influence of stress on the reconstructions of the Si-terminated surface. Our calculations were carried out within the local density functional approximation (LDA), using pseudopotentials and plane wave basis sets[3, 4]. C-TERMINATED SURFACES Experimentally, C-terminated surfaces of the cubic politype /-SiC are obtained either by ethylene decomposition on Si-terminated substrates[5, 6], or by Si sublimation[7, 8, 9] between 1200 and 1500 K. The two techniques lead to samples characterized by different diffraction data[6] and photoemission spectra[8, 10]. We devised two series of computations, to investigate the two preparation methods. The first series of calculations concerns Si sublimation experiments. Since Si removal is expected to yield an ideally C-terminated surface (C-SiC(001)), which then reconstructs, we studied the spontaneous reconstruction of an ideally terminated C-SiC(001) and the thermal stability of its reconstructed phases. Starting from the ideal geometry vibrating at room temperature, we performed microcanonical MD simulations. Depending on the initial conditions for ionic positions and velocities, the ideal geometry transforms either 59
Mat. Res. Soc. Symp. Proc. Vol. 492 © 1998 Materials Research Society
Figure 1: Side view of the ideal C-terminated surface (middle panel), which at room temperature reconstructs into either a staggered dimer (right panel) or a dimer row (left panel) geometry. Black and white spheres indicate C and Si atoms, respectively. into a c(2x2) staggered dimer (SD) pattern, or into a p(2x 1) dimer row (DR) geometry (see Fig. 1). At T = 0, the total energy of the DR is lower than that of the SD geometr
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