シリコンナノ結晶の凝集過程と構造安定性:分子動力学シミュレーション

Abstract

The aggregation process of silicon nanocrystals was simulated using the molecular dynamics method and the stability of the nanocrystals was examined as a function of temperature and pressure. The specimens were constructed as follows: ultrafine particles of silicon with 1.6 nmφ, each of which has a diamond structure, were oriented randomly and set on the face centered lattice sites. The nanocrystal system was surrounded by perfect rigid body walls and pressed by the walls at a constant pressure. The temperature was held constant by ad hoc scaling. The Tersoff potential was assumed as the interaction mechanism of the silicon atoms, and the Morse potential was used to calculate the force between the silicon atoms and the rigid walls. After the system reached the equilibrium state, the radial distribution functions were calculated for each sample and the stability region of the silicon nanocrystal was determined. The critical temperatures for the stability were about 750 K for 1.013×105 Pa and 300 K for 1.013×109 Pa. The time change of the fraction of the coordinate number was also calculated, and it was clarified that the fraction of 4-coordinated bonds, which represents the local tetragonal structure in the diamond lattice, decreased substantially whereas the fraction of 3-coordinated bonds increased at high temperatures, and that the decrease in the fraction of 4-coordinated bonds was suppressed at high pressure.