Xie Quan

Growth Characteristics of Calcium Silicides Films from the Deposited Ca Films at the Different Sputtering Ar Pressure

Thin Ca films were deposited directly on Si(100) substrates at the different deposition Ar pressure using radio frequency (R.F.) magnetron sputtering system (MS) and subsequent annealed at 800 Celsius for 2.5 h or 1.5 h in a vacuum furnace, respectively. The double-structure Ca2Si films and Ca5Si3 film are grown directly and individually on Si(100) substrates by an interdiffusion process between the deposited Ca atoms and Si bulk crystals, respectively. Experimental results indicated that the selective grown of a single phase calcium silicide in multiple silicide phases in Ca-Si system depends on sputtering condition, annealing temperature and annealing time, especially sputtering condition is the principal factor for the selective grown because it is close contacted with the morphological character, the bombardment defect, the nucleation density of a calcium silicide in a calcium silicides system by bombardment intensity and so on. Besides, 800°C is the adaptive annealing temperature for a single calcium silicide film growth.

Evolution of microstructures during rapid crystallization of liquid GaAs

The technological importance of compound semiconductor GaAs are increasing because of their use in optoelectronic and microelectronic applications. Due to the high conversion efficiency and carrier mobility, GaAs can also be applied in solar cells and the recent study upon GaAs nanowires and their heterostructures has revealed that the conversion efficiency of GaAs nanowire array solar cells conversion is high up to 15.3%. Early the liquid and amorphous properties of GaAs were investigated by employing the first-principles calculations. The emergence of semi-empirical potential and the improvement of computer level have promoted the research and application of molecular dynamics (MD) simulation. MD simulation has now become one of the typical modeling methods at the molecular scale. The simulation is based on the known physical approximation of all particles in the system to solve the equation of motion, and obtain the atomic motion trajectory. Analytical potentials is very important in MD simulation as it is not feasible to solve the Hamiltonian by means of quantum-mechanical methods with huge computational complexity. Abell-Tersoff potential function is a short-ranged bond-order algorithm, which depends on bond lengths and bond angles and hence accesses information about the atomic structure. So it is suitable for simulating covalent bond species. Generally used for the IV elements and compounds like silicon, carbon, and others, but for the III-V compound semiconductor it is not very accurate due to the ionic bonds. Usually the modified tersoff potential, by the addition of Coulomb term, the modified exclusion potential and the truncation parameter, is used to simulate such semiconductor materials. Many studies on the bulk, surface and elastic properties of GaAs by means of MD method, are in good agreement with the experimental results. In this paper Karsten Albe’s Tersoff potential is adopted as it allows one to model a wide range of properties of GaAs compound structure. GaAs has two kinds of tetrahedral crystal structure, namely, Zinc-blende and Wurtzite, the former structure is more stable under normal conditions. But when reduced to a nanoscale scale, Wurtzite structure becomes stable. Different structures have distinct properties, similar to carbon and grapheme. But so far, there is no report on the evolution of the microstructure and the specific crystalline structure of GaAs during crystallization under rapid cooling. In this study, MD simulation was performed for liquid GaAs at the cooling rate 1×10 10 K/s. The pair distribution function, the total energy per atom, the bond angle distribution function, the dihedral angle distribution and visualization method were used to analyze the variations of microstructure during the solidification process. Results show that the onset temperature of crystallization of GaAs liquid is 1460 K. The random network is the essential structural feature of liquid. The rapidly cooled crystallization is Zinc-blende based polycrystalline structure, with the grain boundary in a eutectic twin structure is a layer of wurtzite structure. At temperature below 520 K, part of As atoms segregate into simple cubic structure As 8 .

Evolution mechanism of the topological structure during solid-liquid phase transition of InGaAs crystal

Melting refers to the phase transition from solid to liquid, which is a common physical phenomenon in nature. It is also a phase transformation process that the materials such as metals and semiconductors need to undergo in the process. It is closely related to the preparation and performance of materials. At present, it is difficult to trace the microstructure of the melting process, and computer simulation can get a lot of microstructure information of the melting process, which can well explain how the orderly arrangement of crystals becomes random liquid phase structure in the melting process. For the ternary semiconductor alloy In x Ga1 - x As, it has important application value in microelectronics and optoelectronic devices, because it can adjust its electrical parameters and optical band gap in a wide range composition. So the research of alloy structure has been paid more and more attentions for the ternary semiconductor. It is generally known that the macroscopic properties of the materials are mainly determined by their microstructures. However, most studies on the melting process are based on the dynamics, but the mechanism of the evolution of the microstructure during the melting process is lacking. Therefore, it is of great significance to study the evolution of microstructures during the melting process of InGaAs crystals in the development of novel optoelectronic materials and devices. At present, it is still difficult in the experiment to obtain the structural details of InGaAs system during the melting process. Molecular dynamics simulation is an efficient tool especially for such process. In this paper, the melting process of the ideal InGaAs crystal is simulated by using the molecular dynamics method, and the Tersoff potential function of the In x Ga1 - x As covalent bond system, which has been proved to be suitable for the simulation of complex structures. The structural evolution of the solid-liquid phase transition process was analyzed by using the radial distribution function, angular distribution function, coordination numbers and 3D visualization. From the results of our simulation, we find that the microstructures of the InGaAs phase change greatly during the solid-liquid phase transition, especially in the first-order phase transition. It shows significant variations in the average atomic energy and specific volume, the radial distribution function, angular distribution functions, coordination numbers and atomic cross-sections, local atomic distributions, and diamond structure analysis. In the melting process, covalent bonds the InGaAs atoms are broken, and the system changes from the four coordinated structure into the three coordinated structure. For the dominant three coordinated structure and a small amount of four coordinated structures, the three coordinated structure is connected with the three coordinated structure, and the three coordinated structure interpenetrated the four coordinated structure to form a disordered topological structure.

Selective growth of Ca 2 Si film or Ca 5 Si 3 film in Ca-Si system by R.F MS by annealing

Ca films were deposited directly on Si(100) substrates by Radio Frequency (R.F.) magnetron sputtering system (MS) and subsequent were pre-annealed at 600 °C for 2h in situ. Finally, the samples were annealed again at 750°C, 782°C, 795°C, 800°C and 850°C for 1h in a vacuum furnace by an interdiffusion process between the deposited particles and clusters and Si atoms, respectively. The structural and morphological features of the resultant films were tested by XRD, SEM, EDAX and FT-IR. The cubic phase Ca2Si film and the tetragonal phase Ca5Si3 film were grown directly and individually on Si(100) substrates, respectively. The experimental results indicate that the selective growth of a single phase Ca-silicide in Ca-Si system in the existence of multiple silicide phases depends on sputtering condition and annealing in twice. In addition, the electronic structure of stressed the cubic phase Ca2Si was calculated using the first-principle methods based on plane-wave pseudo-potential theory.