Atomistic Investigation On Structures And Behaviors Of Bulk Glasses And Interfaces In The Immiscible Tantalum-Copper System

The immiscible nature of the Ta-Cu system under equilibrium conditions makes Ta the ideal material to serve as diffusion barrier between Cu and Si/SiO₂ in semiconductor devices.Nonetheless,the amorphous structure,interfacial structure and interfacial behavior of Ta-Cu system as well as other systems with positive heat of formation remain unsufficiently investigated.Such investigations are however of great value in advancing the related fundamental theory as well as in promoting the fabrication of high performancenon equilibrium materials.This work therefore takes the immiscible Ta-Cu system as our major research object,and carries out molecular dynamics simulations based on an angular-dependent potential to examine the microstructural characteristics,thermodynamic characteristics and dynamics behaviors,crystalline-amorphous interfaces,solid-liquid interfaces,interfacial performance and behavior,with the intention to achieve an in-depth understanding of the above issues.The main results reached are:Molecular dynamics simulations on Ta-Cu amorphous alloys show that the microstructures and dynamics behaviors of this system have distinct features comparing to those with negative heat of formations.Microstructure evolutions of the supersaturated solid solutions suggest that they can transform into amorphous phases within a wide range of compositions.Besides,remarkable composition segregations,in other words,phase seperations,are observedin these amorphous alloys.This is further confirmed in the glassy alloy produced by rapid cooling of Cu₅₀Ta₅₀0 mets from high temperature,which exhibits a structure of dual-phase glass with marked phase separation phenomenon.However,it shows poor structural stability during high temperature annealing as crystallizationis observed.Interestingly though,the crystallizationis found to occur only in the Ta-rich amorphous phase,while isrestrained in the Cu-rich phase.Deformation after annealing could promotefurther crystallization in the Ta-rich phase;while the Cu-rich remains in the glassy state.These phenomena indicate that due to the envelope structure formed by high T_g Ta-rich phase,the dynamics of low T_g Cu-rich phase are confined,and finally restrains its cyrstallization.Simulations on the structural properties of interfaces between amorphous Cu₅₀Ta₅₀and crystalline Tareveal everal phenomena,namely,layering,crystallization,intermixing,and composition segregation in the Cu₅₀Ta₅₀ region adjacent to the interface.These interfacial behaviors are found to depend on the orientation of the underlying Ta substrate:the induced layering behavior is dominated by the interlayer distancesof the underlying Ta layers,while the degree of inter-diffusion is governed by the openness of theTa crystalline layers.In addition,composition segregations are observed in all interface models,corresponding to the immiscible nature of the Ta-Cu system.Furthermore,Voronoi polyhedra<0,5,2,6>and<0,4,4,6>are found to be abundant in the vicinity of the interfaces for all models,whose presence is believed to facilitate the structural transition between amorphous and bodycentered cubic structures.The structural and dynamical properties of solid-liquid interfaces formed between three low-index crystal layersof BCC Ta and liquid Cu were studied using molecular dynamics simulations.It is found that layering is induced in the liquid Cu adjacent to the solid Ta,while its behavior depends on the orientation of the underlying Ta substrate.The anisotropy in interfacial structure in turn results in the anisotropies of interfacial energy,interfacial width,and diffusivity of atoms adjacent to the interfaces.Specially,the diffusion coefficients of Cu depend not only on the orientation of the solid substrate,but also on their relative motion with respective to the interface:the in-plane component is greater than the out-of-plane one,which is also found to have a structural origin.