Study On Low-energy Deposition Of Noble Metals Surface By Molecular Dynamics Simulation

In the techniques with low-energy deposition,the micro-mechanism has a direct impact on the mode of thin film growth,surface morphology and microstructure,which will further determine the mechanical,electrical,magnetic and other physical properties of thin film material.The study on the micro-mechanism and process of thin film growth at atomic level will reveal the physical nature of thin film,and is of great significance in controlling growth conditions,optimizing the characters of thin films manufacture and developing novel thin film materials.In the thesis,Molecular dynamics simulation and embedded atom method was applied to study the process of low-energy deposition dynamics of noble metals surface.We simulated different projectile-target combinations of noble metal atoms(Cu,Ag,Au, Ni,Pd,and Pt) in the range of incident energy from 0.1 to 200 eV.It is found that the threshold energies for sputtering are different for the cases of M1＜M₂ and M₁≥M₂,where M₁ and M₂ are atomic mass of projectile and target atoms,respectively.The sputtering yields are found to have a linear dependence on the reduced incident energy,but the dependence behaviors are different for the both cases.The two new formulas are suggested to describe the energy dependences of the both cases by fitting the simulation results with the determined threshold energies.With the study on the energy dependences of sticking probabilities and traces of the projectiles and recoils,we propose two different mechanisms to describe the sputtering behavior of low-energy atoms near the threshold energy for the cases of M₁＜M₂ and M₁≥M₂,respectively.At low energies,the kinematics of the collisions is important and depending on the mass ratio M₁/M₂ there are two regimes.For M₁＜M₂ atoms may be scattered into large angles to sputter substrate atoms.For M₁＞M₂,however,this is difficult and the sputtering angle is smaller than the case of M₁＜M₂.Distribution of sputtered atoms is more uniform if the peak of sputtering-angle distribution is higher.Low-energy mechanism determines energy distribution of sputtered atoms.For M₁＜M₂,energy distribution tends to high energy regime with incident atoms directly resulting in sputtering;For M₁＞M₂,however,nergy distribution tends to low energy regime with recoil atom dominating sputtering.In the research of influence of low-energy atoms on surface morphology,we find that the implantation of projectiles in shallow layers below surface can be distinguished by subplantation(in the first and second layers) and implantation(deeper than the third layer). The transition from subplantation to implantation occurs at the incident energy of about 2E_th for the low-energy bombardment of M₁＜M₂.In the case of M₁＞M₂,a transition also occurs at the incident energy between E_th and 2E_th,however,the removal of atoms in the surface layers is in layer-by-layer,or layer-by-layer sputtering and substrate materials are destroyed can be divided by the trasition energy.The incident-energy dependence of defect yields is obviously different for subplantation and implantation of projectiles.Based on our MD simulations,we discuss the influence of low-energy bombardment on film growth and the guide to the search for optimum deposition parameters.For the study on heteroepitaxial growth,the mechanism of strain-dominated unsymmetrical growth behavior in mutual epitaxial processes for two different metals Cu and Au was discussed.By comparing the evolution behaviors of the Ag/Cu(111) and Au/Cu(111) systems,the atomistic mechanism that results in the formation of Moiréstructure in Ag/Cu(111) epitaxial layer is studied and is related to the physical properties of the components in the heteroepitaxial system.It is found that the adhesion between epilayer and substrate is one of important factors that affect the formation of Moiréstructure. Positive solution heat of the alloy enables the adhesion of heteroepitaxial system to be weak and is helpful forthe formation of Moiréstructure.The relaxing ability of atoms in epilayer is also important in the formation of Moiréstructure,which can be related to the bulk modulus of epilayer.The larger the bulk modulus of epilayer,the easier formation of Moiréstructure.On the other hand,the formation of Moiréstructure is related to the island size, which is caused by the boundary atoms confining the relaxation of inside atoms in the island.