Molecular Dynamics Simulations On Mechanical Properties And Structural Evolutions Of Ⅲ-nitrides

Many research methods and theoretical models on the mechanical properties and internal evolution of materials have been proposed at the macro scale.However,the research at nano scale is still in the stage of understanding and development.As the main third-generation semiconductor materials,III-Nitrides nanomaterials have been widely used in many fields such as aerospace coating and electrical power systems,and the mechanical properties are the basis of their service.At the same time,mechanical regulation can also change the mechanical and electrical properties of materials.In order to investigate the mechanical properties and structural evolutions of III-Nitrides,molecular dynamics（MD）simulations are adopted in this paper.We find that B4-B1 phase transformation,amorphous and dislocation slip appear in single crystal AlN.Through anisotropy indentation analysis,two kinds of transition paths of B4-B1 phase transformation are found.Path I can be summarized as the realization of B4-B1 phase transformation by two parallel motions of Al and N atoms:an anti-parallel vertical movement of Al and N atoms along[0001]axis,followed by horizontal rearrangement of the two types of atoms.The path II is similar to the path I,but the two parallel motion sequence of Al and N atoms is opposite to the path I.In addition,the phase transition path I appears in[0001]and[（?）]crystal orientation indentation,while path II appears in[（?）]and[（?）]crystal orientation indentation.At the same time,dislocation slip also appears during the anisotropy indentation.In the initial deformation stage,the phase transition is the main deformation mechanism in[0001]and[（?）]crystal orientation indentation,while the main deformation is dislocation slip in[（?）]indentation,and the crystal direction has both slip and phase transformation in[（?）]indentation.During further indentation,phase transformation and dislocation slip will occur in all crystal directions.In addition,we proposed two mechanisms for the formation of dislocation loops of III-Nitrides.One is the“lasso”-like mechanism,similar to that observed in metals,and the other is“nested-loops”mechanism,which has never been reported in literature.The screw segments of two different shear loops,be adjacent to each other,can intersect with each other,resulting in a prismatic dislocation loop.The pile-up symmetry after unloading,associated with dislocation loops propagation,can be found,which is consistent with experimental results.More detailed analysis of dislocation patterns and directions indicate that plastic deformation in dominated by the shuffle-set dislocation along ₃¹á1120?,and the glide-set dislocations along ₃¹á1100?are asymmetric.We also studied the interface structures and mechanical properties of AlN/GaN multilayer films under compression and tension.We found that most of the interface dislocation lines after relaxation is the perfect dislocation of ₃¹á1120?,and the Burgers vector is perpendicular to dislocation lines which shows the dislocation line is edge dislocation.Moreover,a short number of unidentified edge dislocations exist in the interface.These dislocations form a dislocation network near hexagon,surrounded by coherent regions.Unlike metal multilayers and metal/ceramic multilayers,the compression and tension of AlN/GaN multilayer films have only one yield point,and it is found dislocations form from the interface dislocation networks in the soft layer and emit to the hard layer at once.The twice nucleation occurs almost at the same time,and the reason may be that dislocation nucleation induces a large stress and lattice distortion on the interface,which will quickly stimulate the structural deformation in the other layer,and leads to the almost simultaneous nucleation of the second layer and only one yield point appears.