Molecular Dynamics Simulation Of Silicon Crystal Melting And Growth

As a semiconductor material with good performance,single crystal silicon has a relatively perfect preparation technology,which is the basis of large-scale integrated circuits and solar photovoltaic cells.The main method of obtaining single crystal silicon material is to solidify the molten high-purity silicon material,and to control the crystal growth through the supercooling degree of the melt.Since the quality of the crystal directly affects the efficiency of the photovoltaic cell and chips,and the melting and growth behavior and microstructure of the crystal determine the various properties of the crystal.Therefore,the basic mechanism and laws of the melting and growth of single-crystal silicon are studied to continuously improve the quality of the crystals obtained,and it has a very important guiding role in the development of photovoltaic devices dominated by silicon.In this paper,molecular dynamics simulations are used to study the melting and growth process of silicon crystals,and discuss the differences caused by different melting models on the crystal melting phenomenon,and analyze the growth behavior of the crystals at different growth temperatures and different growth surfaces;at the same time,we use Jackson interface model,molecular dynamics and density functional theory to infer the roughness of the solid-liquid interface when silicon crystals grow along different crystal planes.The main results are as follows:1.When the silicon crystal undergoes the bulk melting process,the melting starting point will randomly appear inside the crystal,and the disordered region will diffuse rapidly with time,and the melting process will be completed in a short time.In the process of surface melting,the starting point of melting will appear on the crystal surface,and with time the melting part will gradually extend from the surface to the inside of the crystal until the melting is completed.In addition,the surface melting rate is significantly greater than the bulk melting rate,and by fitting the surface melting rate at different temperatures,it is calculated the thermodynamic melting point of the(100)face of the silicon crystal is below 2480 K.2.When the silicon crystal grows along the(100)plane,the crystal can undergo crystallization reaction in the temperature range of 1800 K to 2400 K,and the crystal growth rate increases first and then decreases as the temperature decreases(taking the melting pointtemperature as the starting point),and reaches the maximum value of 6.4 m/s at 2200 K temperature.In addition,at a temperature of 2200 K,the crystal growth rate first increased and then decreased over time.By analyzing the relationship between the diffusion coefficient,the degree of supercooling and the growth rate,it is found that the growth rate of crystal is affected by the crystallization driving force(thermodynamics)and atomic diffusion capacity(kinetics).Under the combined effect of supercooling and diffusion coefficient,the growth rate will increase first and the decrease.Finally,W-F model is used to predict the growth rate at different temperatures,and the calculation result have the same trend with the simulation results,but the temperature corresponding to the maximum rate is different,which shows that the atomic diffusion is the main factor leading the growth behavior of silicon crystal.3.After comparing the growth of silicon crystals along the(111)plane with the(100)plane,it is found that there is anisotropy in the growth of silicon crystals: at the same temperature,the growth rate of the(100)plane is always greater than that of the(111)plane,but the temperature corresponding to the maximum growth rate is the same.In addition,the kinetic coefficients of different crystal planes and the spacing between the crystal planes are analyzed,the results show that the silicon crystals do not meet the description of the crystal growth behavior of the BGJ model,and the surface spacing of the different crystal planes is not the main reason for the growth anisotropy of the silicon crystals.4.According to the calculation of Jackson interface theory,it is found that the Gibbs free energy reaches the minimum value when the(100)interface crystal phase atoms and the fluid atoms each occupy about 50% on the surface,while the(111)interface reaches the minimum value accounts for about 0% or 100% on the surface,indicating that when thermodynamics is in equilibrium,the(100)surface tends to be a rough interface,and the(111)surface tends to be a smooth interface.Molecular dynamics simulation shows that with the development of growth,the initial smooth solid-liquid interface will gradually change into a rough interface on the(100)surface,while the(111)plane will always maintain smooth interface growth,and will not change with the change of supercooling.DFT calculations have found that the adsorption energy of all growth positions on the(100)surface is close,which can achieve continuous growth,and there is a significant difference in the adsorption energy of the(111)plane,the growth atoms need to be adsorbed at the step to perform layered growth.This is the main reason that the growth rate of(100)surface is always higher than that of(111)surface.