Molecular Simulation Of Mechanism And Control Of Nucleation And Growth In Thin Film Deposition Process

The Nano Technology has been developed rapidly in recent years, which gota lot of applications in many fields. Further more, the research about NanoTechnology shows unlimited prospects in many areas, such as photoelectron,new energy and catalyst materials. Now, Nano Technology has already beenregarded as a kind of source technology and given more and more attention.In theresearch of nano materials, thin film material plays a decisive role due to theirvarious excellent properties, such as the excellent optical performance, corrosionresistance and wear-resisting performance. More over, various preparationmethods including Physical Vapor Deposition and Chemical Vapor Deposition.make thin film material more popular. So, more and more needs are needed toprepare the nano material and nano structure that meet special demand undercontrol.To realize controllable preparation of thin film materials and structures,researches about the deposition, diffusion, absorption, nucleation of atoms andthe thin film growth should be carried out. But the existing experiment methodsand techniques can’t deal with the molecular scale and the femtosecond timescale, so we use molecular simulation method to study the nano/micro scaleprocesses of nucleation and aggregation, the mechanism of the nucleation andaggregation. The theoretical basis for the controllable preparation of materialsand at the same time try to guide experiments to improve the experimentefficiency and to reduce the blindness of experiments. This study has two main parts. The first part is to research the process andmechanism of the nucleation of the atoms in gas phase, and the second parts is toresearch the factors that influence the growth mode and morphology of thin filmformed on the substrate. Then both these two parts are taken into account foranswer to the theory basis of the controllable preparation of thin film.The nucleation processes of Ag atoms are investigated by molecularsimulation. It is found that the aggregation between clusters dominate thenucleation and aggregation process during the formation of Ag nano particles.The classical nucleation theory as well as other existing research is based on thehypothesis that the process of nucleation is single atom combination one by one.However, our simulation result indicate that it is easier to form a dimer or trimeror other small clusters in the initial nucleation period, then, as time increases,more and more small clusters gathered to form bigger clusters, rather than aprocess of single atom aggregation one by one. The interaction between clusters,such as clusters aggregation and clusters decomposition dominates the nucleationprocess. At the same time, the variation of the clusters size distribution as thetime indicates the same process of nucleation.The simulation results also show that whether the nucleus can grow up ornot decided by its own structure. The classical nucleation theory states clearlythat the nucleus growth depends on if its size is larger than the critical radius, butour results show that in the processes of nucleation and aggregation, someclusters can form stable crystal structure (inactive cluster), these kind of clustersare not easy to grow up or aggregate again, however, the other amorphousclusters (active cluster) are easy to aggregate and grow up. Therefore, whether anucleus can grow up or not depends on its structure but not the size of the clusters,which is also different from the classical nucleation theory. In addition, activeclusters dominate the growth of nuclei, but stable clusters are only theparticipants.The research of the controllable factors of the morphology and growth modein film deposition process shows that the diffusion rate has a great influence onthin film growth mode and morphology. When the diffusion rate is low, the maingrowth mode is formation of island and have a rough surface. When the diffusionrate becomes higher, a smooth surface is formed through the layer by layer mode.The increase of temperature leads to the increase of surface diffusion butdoes not have an obvious influence on the final morphology. Therefore, diffusionrate has more influence on the growth mode and morphology than thetemperature does. The island structure will lower the film quality and then shouldbe avoided in the film preparation process. Therefore, it is very important tocontrol the temperature of deposition region and the diffusion rate of depositionatoms during the preparation of thin film under control.