STRUCTURAL AND MECHANICAL PROPERTIES OF AMORPHOUS SILICON: AB-INITIO AND CLASSICAL MOLECULAR DYNAMICS STUDY

Understanding the surface properties of amorphous silicon (a-Si) is extremely important for effective application of a-Si in its wide usage as a thin film material. Several theoretical works using ab-initio molecular dynamics schemes have been performed to investigate this subject. However, due to the limited time scale accessible to the ab-initio scheme, the resulting a-Si structure has not been subject to sufficient relaxation. One possible way to resolve this time scale problem is to employ classical molecular dynamics (CMD), because this method can handle the large time scale required to relax the amorphous structure. In this study, a combined method of a CMD calculation with the Tersoff empirical potential and an ab-initio calculation based on density functional theory has been performed. The main role of the CMD method is to sufficiently relax the amorphous structure. Subsequently, the results of the CMD calculations are used as starting points for ab-initio calculations. As a result, this combination method can provide quantitative evaluation of the surface energy and surface stress of well-relaxed amorphous silicon in addition to its structure. Using this combination method, a surface energy of 1.05±0.14 J/m2 and a surface stress of 1.5±1.2 N/m were obtained. This calculation also led to a new discovery of the microscopic characteristic of a-Si surface, which was not revealed through the use of an empirical potential. It was shown that there are two types of threefold coordinated atoms at the surface region; one with p3-like bonding and the other with sp2-like bonding. In addition, the investigation indicated that the microstructures of these defects were different from those of a threefold coordinated atom observed in the bulk structure.