Impact of germanium on vacancy clustering in germanium-doped silicon

Recent density functional theory calculations by Chen et al. [J. Appl. Phys. 103, 123519 (2008)] revealed that vacancies (V) tend to accumulate around germanium (Ge) atoms in Ge-doped silicon (Si) to form GeVn clusters. In the present study, we employ similar electronic structure calculations to predict the binding energies of GeVn and Vn clusters containing up to four V. It is verified that V are strongly attracted to pre-existing GeVn clusters. Nevertheless, by comparing with the stability of Vn clusters, we predict that the Ge contribution to the binding energy of the GeVn clusters is limited. We use mass action analysis to quantify the relative concentrations of GeVn and Vn clusters over a wide temperature range: Vn clusters dominate in Ge-doped Si under realistic conditions

Resolving the structure of TiBe12_{12}

There has been considerable controversy regarding the structure of TiBe$_{12}$, which is variously reported as hexagonal and tetragonal. Lattice dynamics simulations based on density functional theory show the tetragonal phase space group $I4/mmm$ to be more stable over all temperatures, while the hexagonal phase exhibits an imaginary phonon mode, which, if followed, would lead to the cell adopting the tetragonal structure. We then report the predicted ground state elastic constants and temperature dependence of the bulk modulus and thermal expansion for the tetragonal phase.Comment: In press at Acta Crystallographica B. Supplementary material appende

Mechanisms of nonstoichiometry in HfN_1-<i>x</i>

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0 ≤ X ≤ 0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater

Defect interactions in Sn_1-<i>x</i>Ge_<i>x</i> random alloys

Sn1-xGex alloys are candidates for buffer layers to match the lattices of III-V or II-VI compounds with Si or Ge for microelectronic or optoelectronic applications. In the present work electronic structure calculations are used to study relative energies of clusters formed between Sn atoms and lattice vacancies in Ge that relate to alloys of low Sn content. We also establish that the special quasirandom structure approach correctly describes the random alloy nature of Sn1-xGex with higher Sn content. In particular, the calculated deviations of the lattice parameters from Vegard's Law are consistent with experimental results

<i>E</i> centers in ternary Si_1-<i>x-y</i>Ge_<i>x</i>Sn_<i>y</i> random alloys

Density functional theory calculations are used to study the association of arsenic (As) atoms to lattice vacancies and the formation of As-vacancy pairs, known as E centers, in the random Si0.375Ge0.5Sn0.125 alloy. The local environments are described by 32-atom special quasirandom structures that represent random Si1-x-yGexSny alloys. It is predicted that the nearest-neighbor environment will exert a strong influence on the stability of E centers in ternary Si0.375Ge0.5Sn0.125

Phase stability and the arsenic vacancy defect in In_xGa_1-xAs

The introduction of defects, such as vacancies, into InxGa1-xAs can have a dramatic impact on the physical and electronic properties of the material. Here we employ ab initio simulations of quasirandom supercells to investigate the structure of InxGa1-xAs and then examine the energy and volume changes associated with the introduction of an arsenic vacancy defect. We predict that both defect energies and volumes for intermediate compositions of InxGa1-xAs differ significantly from what would be expected by assuming a simple linear interpolation of the end member defect energies/volumes

Hydrogen solubility in zirconium intermetallic second phase particles

The enthalpies of solution of H in Zr binary intermetallic compounds formed with Cu, Cr, Fe, Mo, Ni, Nb, Sn and V were calculated by means of density functional theory simulations and compared to that of H in {\alpha}-Zr. It is predicted that all Zr-rich phases (formed with Cu, Fe, Ni and Sn), and those phases formed with Nb and V, offer lower energy, more stable sites for H than {\alpha}-Zr. Conversely, Mo and Cr containing phases do not provide preferential solution sites for H. In all cases the most stable site for H are those that offer the highest coordination fraction of Zr atoms. Often these are four Zr tetrahedra but not always. Implications with respect to H-trapping properties of commonly observed ternary phases such as Zr(Cr,Fe)2, Zr2(Fe,Ni) and Zr(Nb,Fe)2 are also discussed.Comment: manuscript accepted for publication in Journal of Nuclear Materials (2013

Effects of Gallium Doping in Garnet-Type Li7La3Zr2O12 Solid Electrolytes

Garnet-type Li7La3Zr2O12 (LLZrO) is a candidate solid electrolyte material that is now being intensively optimized for application in commercially competitive solid state Li+ ion batteries. In this study we investigate, by force-field-based simulations, the effects of Ga3+ doping in LLZrO. We confirm the stabilizing effect of Ga3+ on the cubic phase. We also determine that Ga3+ addition does not lead to any appreciable structural distortion. Li site connectivity is not significantly deteriorated by the Ga3+ addition (>90% connectivity retained up to x = 0.30 in Li7–3xGaxLa3Zr2O12). Interestingly, two compositional regions are predicted for bulk Li+ ion conductivity in the cubic phase: (i) a decreasing trend for 0 ≤ x ≤ 0.10 and (ii) a relatively flat trend for 0.10 < x ≤ 0.30. This conductivity behavior is explained by combining analyses using percolation theory, van Hove space time correlation, the radial distribution function, and trajectory density

Fluorine codoping in germanium to suppress donor diffusion and deactivation

Electronic structure calculations are used to investigate the stability of fluorine-vacancy (Fn)Vm) clusters in germanium (Ge). Using mass action analysis, it is predicted that the FnVm clusters can remediate the concentration of free V considerably. Importantly, we find that F and P codoping leads to a reduction in the concentration of donor-vacancy (DV) pairs. These pairs are responsible for the atomic transport and the formation of DnV clusters that lead to a deactivation of donor atoms. The predictions are technologically significant as they point toward an approach by which V-mediated donor diffusion and the formation of inactive D(n)V clusters can be suppressed. This would result in shallow and fully electrically active n-type doped regions in Ge-based electronic devices

Crew Motion and the Dynamic Environment of Spaceborne Experiments

Analytical study of crew motion on dynamic environment of orbiting laboratorie