Interatomic Potential Fitting Study Of Ag, Si And C Based On First Principles

In this thesis we establish a three-atom interaction potential of AgSiC system, which makes it possible to simulate the diffusion of Ag in SiC using molecular dynamics method.In this thesis, we use Tersoff potential function as the force field of AgSiC three-atom system. Combined with annealing simulation and least squares method, we use "force-matching" method to reproduce the reference configuration's mechanics, energy and other information to obtain the global optimum of potential function. And we use DFT-based ab initio calculations as the original data of various reference configurations, and in order to describe some of the dynamic process, we also adopt the first-principle molecular dynamics data. To ensure the reliability of our DFT calculations, the comparisons are made with the results of other DFT calculations.In the fitting process of Tersoff potential, we use stepwise approach to fit the interatomic potential gradually. Fitting results show that:the results of single-atom system (C and Si) potential is consistent well with the theoretical values.Among them,the lattice constant maximum error is 0.0031 A, maximum error of cohesive energy per atom is 0.004 eV, and bulk modulus error is about 5%;except the formation energy error of C vacancy is large (3.5 eV), other values are all consistent well with the theoretical values. The results of the Si-C interaction potential fitting show that except the elastic constant C44 deviation is larger, values including Si and C and vacancy formation energy and other elastic constants are all in line with the theoretical values. Especially the values of SiC crystal cohesive energy, lattice constant and bulk modulus are in line with the theoretical values; and they are respectively-6.433 eV,4.3778 A and 2.13 Mbar, and theoretical values are respectively -6.433 eV,4.3796 A and 2.12 Mbar, the corresponding errors do not exceed 0.5%.In addition, we compare values of our potential with experimental data and find that the calculated lattice constant is generally too high, and binding energy is lower. Compared with the results of the literature (Phys. Rev. B 39,5566(1989)),the accuracy of vacancy and interstitial formation energy in Si or C crystal in this paper are improved. Besides, the calculated values of Vsi(Si vacancy formation energy) and Vc (C vacancy formation energy) are more closer to theoretical value. Fitting results in Ag-C and Ag-Si atomic potential show that our potential's calculated cohesive energys of 16 kinds of Ag defect systems are all relatively accurate,and the error are about 0.6%.