Smoothing During Ion-Assisted Growth by Transient Ion Beam-Induced Defects
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PREVIOUS EXPERIMENTAL OBSERVATIONS This work was motivated by two previous experimental studies of low-energy ion bombardment. The first investigation monitored the evolution of the Ge(00 1) surface morphology during low-energy ion bombardment (200 eV Xe) and homoepitaxy using Reflection High Energy Electron Diffraction (RHEED)[4]. We demonstrated that a smoother surface can be obtained using a combination of ion bombardment and growth than can be obtained by ion bombardment or growth alone. We also showed that the smoothest steady-state surface morphology occurred when the ratio of net ion-induced defect production rate to deposition rate was approximately unity. As described earlier, we proposed several mechanisms for describing the enhancement of surface smoothness, however, the atomistic mechanism responsible for producing the surface smoothing enhancement was difficult to verify. Our attempt to better understand the mechanism of how lowenergy ion bombardment affects the dynamic surface morphology began with an investigation of low-energy ion bombardment on static Ge(001)-2x1 surfaces[7]. RHEED was used to quantify the number of surface vacancies and adatoms produced per incident ion at different substrate temperatures. For 200 eV Ar/Xe ion bombardment, the observed surface defect yield (number of surface vacancies and adatoms produced per incident ion) decreased abruptly by an order of magnitude as the substrate temperature was increased. We developed Monte Carlo simulations of defect production and diffusion and showed that surface recombination of point defects explained the observed temperature dependence[8]. All that remains after complete recombination of vacancies/adatoms at typical growth temperatures are the uncompensated sputter vacancies or sputter yield, one defect/ion. Although the observed defect yield may be on the order of one defect/ion at typical growth temperatures, an order of magnitude more defects transiently reside on the surface prior to recombining or annihilating. The following experimental observations are central to our current simulation study: simultaneous low-energy ion bombardment and growth produces a smoother surface morphology than growth alone and a large population of ion-induced defects exist temporarily on the surface at typical growth temperatures. MONTE CARLO SIMULATIONS To understand the enhancement in surface morphology in terms of the interaction of the transient ion-induced defects with growth atoms, we have developed simple kinetic Monte Carlo simulations of growth and surface defect diffusion to model the interaction between growth atoms and ion-induced defects. The Monte Carlo simulations used in this study have been described in detail previously[9]. We employed the following procedure. Each deposition event was represented by placing a single adatom at a random location on the surface, while each ion impingement event was represented as the production of multiple adatoms and monovacancies (typically ten defects per ion), with a sputter yield of one defect per io
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