Showing papers by "Vincent Ji published in 2007"

Ab initio modeling of CaTiO3 (110) polar surfaces

Abstract: The TiO, Ca, asymmetric A-type O-terminated and symmetric B-type O-terminated surfaces have been constructed for CaTiO3 110 surface. The cleavage and surface energies, surface grand potential, surface relaxation, and surface electronic structure have been calculated for these polar terminations by using ab initio plane waves ultrasoft pseudopotential method based on generalized gradient approximation. The results show that the favorable CaTiO3 110 and 001 surfaces are CaO-terminated 001 surface, A-type O-terminated 110 surface, and TiO2-terminated 001 surface successively, in view of the surface energy minimization. The Ca termination is stable in O- and Ca-rich environments, however, its complementary TiO termination is stable in O- and Ca-poor conditions. The A-type O termination shows a stability domain in moderate O and Ca environments. In the range of accessible values of O, only Ca and A-type O-terminated surfaces are likely to be observed. Moreover, a large surface rumpling s of 12.10% a is found for the TiO-terminated surface due to inward movement of Ti ion and outward movement of O ion. The surface band gaps for the relaxed TiO and A-type O terminations are larger than the bulk band gap; however, the values for Ca and B-type O terminations are smaller.

Representation surfaces of Young's modulus and Poisson's ratio for BCC transition metals

Abstract: The general expressions for the compliance s ijkl ′ , Young's modulus E(h k l) and Poisson's ratio υ(h k l) along an arbitrary direction [h k l] are given for cubic crystals. The representation surfaces, for which the length of the radius vector in the [h k l] direction equals to E(h k l) or υ(h k l), are constructed for seven BCC transition metals Cr, Fe, Mo, Nb, Ta, V and W. Neglecting W, which is isotropic, both representation surfaces of Young's modulus and Poisson's ratio are spherical surfaces. The remaining BCC transition metals may be grouped into two classes. In the first group (Cr, Mo, Nb and V) with negative values of sA, Young's modulus surface has eight depressed corners and six rounded protuberances at the centers of the faces. In the second group (Fe and Ta) with positive values of sA, on the contrary, Young's modulus surface has eight rounded protuberance corners and six depressions at the centers of the faces. The contrary conclusions are obtained for Poisson's ratio.

Anisotropy analysis of stresses and strain energies in diamond-cubic films

Abstract: A thin polycrystalline film bonded to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed. These stresses and the corresponding strain energies for grains having various crystallographic orientations ( h k l ) relative to the film surface have been calculated for diamond-cubic polycrystalline films diamond, Si and Ge. The results show that, only in (1 0 0)- and (1 1 1)-oriented grains, the stresses σ 1 and σ 2 in the plane of the film surface are equal and (1 0 0)-oriented grains have the lowest stress and strain energy and (1 1 1)-oriented grains the highest. So that, from stress and/or strain energy minimization, the (1 0 0) preferred orientation should be favorable in these films after annealing. The stresses σ 1 and σ 2 and strain energy densities w in other ( h k l )-oriented grains increase linearly with increasing angle between ( h k l ) and (1 0 0). This can be used to estimate the relative values of the stresses and strain energies from a deviation of a grain orientation from (1 0 0).

Dependence of the stresses on grain orientations in hexagonal films

Abstract: A thin polycrystalline film attached tightly to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed during device fabrication and in service. Calculations of these stresses in various ( h k l )-oriented grains relative to the film surface have been made for a polycrystalline film composed of the hexagonal metal Be, Cd, Co, Hf, Mg, Re, Ru, Sc, Ti, Y, Zr and Zn, respectively. For all these hexagonal films, the stresses σ 1 and σ 2 in plane of the film surface are equal only in (0 0 1)-oriented grains due to the highest six-fold rotation symmetry of the crystallographic Z -axis. Excepting σ 1 of Be, Ru, Zr, Zn and σ 2 of Cd, Zn, the maximum values of the film plane stresses σ 1 and σ 2 correspond to the (0 0 1)-oriented grains means that the significant reliability problems, such as, voiding, cracking, hillocking induced by the stresses may be taken place preferred in (0 0 1)-oriented grains.