Cesar R.S. da Silva

Theoretical investigation of the pressure induced cubic-diamond-β-tin phase transition in the Si0.5Ge0.5

Abstract We theoretically studied the phase transition between the cubic-diamond and β-Sn structures for the Si 0.5 Ge 0.5 alloy. We performed first principles calculations based on the density-functional theory, using the local-density approximation, and the variable cell shape molecular dynamics approach. The alloy was modeled using an ordered zincblende type lattice, and we obtained a transition pressure of 8.35xa0GPa. We also calculated the enthalpy curves at zero as well as at the transition pressure. At zero pressure, the enthalpy difference between the two phases is 0.016xa0Ry/atom, and there is a barrier of approximately 0.026xa0Ry/atom. At the transition pressure, the enthalpy barrier is still 0.018xa0Ry/atom, a sign of the strong first order character of this phase transformation.

Crystalline silicon oxycarbide: Is there a native oxide for silicon carbide?

Using variable cell ab initio molecular dynamics, we have investigated hypothetical crystalline phases of silicon oxycarbide (Si1−xCxO2). We found that silicon oxide remains energetically stable with carbon incorporation, and the resulting oxycarbide material has a moderately large bulk modulus. Our results also indicated that there are at least two possible, and competing, crystalline phases for the Si2CO6. We discuss the possibility of those phases forming near the SiC/SiO2 interfaces.

Optimizing Metropolis Monte Carlo simulations of semiconductors

A new algorithm to accelerate Metropolis Monte Carlo simulations of semiconductors and other covalent materials is presented. The algorithm explores a usually missed side opportunity that appears when the Linked Cell and Verlet Neighbor List algorithms are combined. The central idea is quite general, and can be used to accelerate calculations other than Metropolis Monte Carlo with a variety of potentials. Tersoff potential is used throughout this study. Some more trivial optimizations, specific to the Tersoff potential, are also reviewed.

Structural and Electronic Properties of Si1-xCxO2

There is growing interest in understanding the properties of SiC-SiO2 interfaces, which can be formed by oxidation of silicon carbide surfaces. Here, we used variable cell shape ab initio molecular dynamics to investigate the structural and electronic properties of crystalline phases of silicon oxycarbide which could appear within such interfaces. We find that carbonoxygen single bonds may remain stable inside a silicon oxide matrix. For the Si2CO6 compound, there are at least two crystalline phases, both having large bulk modulii and wide bandgaps.

Ab initio studies of the Si1−xGex alloy and its intrinsic defects

Abstract We have performed systematical ab initio studies of the electronic and structural properties of the disordered phase of the Si 1− x Ge x alloy. For the defect free alloy, we find that the Si–Si bond length has a tendency to maintain its natural value ( x =0) all the way up to x >0.5, similarly to what is found experimentally. We also analyze the vacancy, and note, as expected for a structurally as well as chemically disordered solid, that there is no local symmetry around the vacancy site. Finally, we perform some studies of the alloy under pressure all the way up to 30 GPa, and find that the structure is highly distorted in some cases, indicating that an amorphous phase may be found in the pressure region of the diamond–β-tin phase transition.