An Integrated Molecular Dynamics and Monte Carlo Approach to Study Epitaxial Deposition of Silicon
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An Integrated Molecular Dynamics and Monte Carlo Approach to Study Epitaxial Deposition of Silicon Sweta Somasi1, Bamin Khomami1 and Ronald Lovett2 1 Department of Chemical Engineering, Washington University, St. Louis, MO 63130, U.S.A. 2 Department of Chemistry, Washington University, St. Louis, MO 63130, U.S.A. ABSTRACT The length and time scales of an atomistic simulation are often too small for any direct comparison with experimental observations. In order to study the coverage of pits (COPs) found on the Si (100) surface by epitaxial deposition, we first calculate rate of individual steps using molecular dynamics and then define a sequence of Monte-Carlo steps to study the effect of various factors on effective coverage of COPs. INTRODUCTION One of the important uses of epitaxy deposition on silicon (100) surface is to cover voids or crystal- originated pits (COPs) that are detrimental to device performance. These voids are formed by vacancy-conglomeration and get exposed when the silicon crystal is cut to make wafers. Surface diffusion of deposited silicon atoms to lattice positions is the crucial step for ensuring the growth of single crystal and for successful coverage of COPs. A complete description of epitaxial process would require molecular details of the motion of silicon atoms on the surface as well as a description of the growth of the crystal. Due to the extremely different time scales of the two processes, an integrated Molecular Dynamics and Monte Carlo approach has been used to study the silicon epitaxy process. The key steps in the epitaxial growth of high quality single crystals are the adsorption, diffusion and desorption of various species on the crystal surface. The rates and mechanisms of surface diffusion and deposition of silicon adatoms on a plane Si (100) surface and a surface with single-height steps were determined using molecular dynamics simulations at 1000 K. The experimental studies of surface diffusion of silicon over Si(100) are able to provide only an average diffusion rate and details of molecular motion cannot be predicted[1]. Theoretical studies, though numerous, have in general predicted diffusion rates based on 0K potential energy surface and have not accounted for entropic or temperature effects[2]. The omission of temperature is an important drawback as the epitaxial process if carried out at high temperatures to enhance surface diffusion rates. Diffusion on Si(100) is slow enough that it remains a rare process in an atomistic simulation and the rates cannot be predicted at finite temperatures. We have circumvented the problem of long simulation times by developing a classical-density functional inspired MD simulation scheme that allows us to calculate the free energy surfaces of various processes. The rate is then determined by using simple transition state theory from the Molecular dynamics determined activation barrier and the pre-exponential. In order to simulate the growing of Si (100) crystal, a Monte Carlo scheme was used. The rates of the essential steps in the gr
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