A. Chroneos

Anisotropic oxygen diffusion in tetragonal La2NiO4+δ: molecular dynamics calculations

Molecular dynamics simulations, used in conjunction with a set of Born model potentials, have been employed to study oxygen transport in tetragonal La2NiO4+δ. We predict an interstitialcy mechanism with an activation energy of migration of 0.51 eV in the temperature range 800–1100 K. The simulations are consistent with the most recent experiments. The prevalence of oxygen diffusion in the a–b plane accounts for the anisotropy observed in measurements of diffusivity in tetragonal La2NiO4+δ.

Vacancy-mediated dopant diffusion activation enthalpies for germanium

Electronic structure calculations are used to predict the activation enthalpies of diffusion for a range of impurity atoms (aluminium, gallium, indium, silicon, tin, phosphorus, arsenic, and antimony) in germanium. Consistent with experimental studies, all the impurity atoms considered diffuse via their interaction with vacancies. Overall, the calculated diffusion activation enthalpies are in good agreement with the experimental results, with the exception of indium, where the most recent experimental study suggests a significantly higher activation enthalpy. Here, we predict that indium diffuses with an activation enthalpy of 2.79eV, essentially the same as the value determined by early radiotracer studies.

Diffusion of n-type dopants in germanium

Germanium is being actively considered by the semiconductor community as a mainstream material for nanoelectronic applications. Germanium has advantageous materials properties; however, its dopant-defect interactions are less understood as compared to the mainstream material, silicon. The understanding of self- and dopant diffusion is essential to form well defined doped regions. Although p-type dopants such as boron exhibit limited diffusion, n-type dopants such as phosphorous, arsenic, and antimony diffuse quickly via vacancy-mediated diffusion mechanisms. In the present review, we mainly focus on the impact of intrinsic defects on the diffusion mechanisms of donor atoms and point defect engineering strategies to restrain donor atom diffusion and to enhance their electrical activation.

Vacancy-arsenic clusters in germanium

Electronic structure calculations are used to investigate the structures and relative energies of defect clusters formed between arsenic atoms and lattice vacancies in germanium and, for comparison, in silicon. It is energetically favorable to form clusters containing up to four arsenic atoms tetrahedrally coordinated around a vacancy. Using mass action analysis, the relative concentrations of arsenic atoms in different vacancy-arsenic clusters, unbound arsenic atoms, and unbound vacancies are predicted. At low temperatures the four arsenic-vacancy cluster is dominant over unbound vacancies while at higher temperatures unbound vacancies prevail. In terms of concentration, no intermediate size of cluster is ever of significance.

Carbon, dopant, and vacancy interactions in germanium

Electronic structure calculations have been used to study the interaction of carbon with isolated substitutional dopants (boron, phosphorus, or arsenic), vacancies, and dopant-vacancy pairs in germanium. For comparison, equivalent defects were examined in silicon. The calculations employed a plane-wave basis set and pseudopotentials within the generalized gradient approximation of density functional theory. The results predict a range of different association preferences, with carbon being strongly bound in some cases and unbound in others. For example, in germanium, the carbon-vacancy cluster is weakly bound whereas in silicon it is more strongly bound. Conversely, dopant-carbon pairs are not stable in either germanium or silicon compared to their isolated components. If, however, they are formed during implantation, they will act as strong vacancy traps. Details of clusters comprised of a dopant, carbon, and vacancy are also discussed with respect to their formation by the association of a vacancy or cl...

Strain-induced changes to the electronic structure of germanium

Density functional theory calculations (DFT) are used to investigate the strain-induced changes to the electronic structure of biaxially strained (parallel to the (001), (110) and (111) planes) and uniaxially strained (along the [001], [110] and [111] directions) germanium (Ge). It is calculated that a moderate uniaxial strain parallel to the [111] direction can efficiently transform Ge to a direct bandgap material with a bandgap energy useful for technological applications.

Point defect engineering strategies to suppress A-center formation in silicon

We investigate the impact of tin doping on the formation of vacancy-oxygen pairs (VO or A-centers) and their conversion to VO2 clusters in electron-irradiated silicon. The experimental results are consistent with previous reports that Sn doping suppresses the formation of the A-center. We introduce a model to account for the observed differences under both Sn-poor and Sn-rich doping conditions. Using density functional theory calculations, we propose point defect engineering strategies to reduce the concentration of the deleterious A-centers in silicon. We predict that doping with lead, zirconium, or hafnium will lead to the suppression of the A-centers.

Molecular dynamics study of oxygen diffusion in Pr2NiO4+δ

Oxygen transport in tetragonal Pr(2)NiO(4+delta) has been investigated using molecular dynamics simulations in conjunction with a set of Born model potentials. Oxygen diffusion in Pr(2)NiO(4+delta) is highly anisotropic, occurring almost entirely via an interstitialcy mechanism in the a-b plane. The calculated oxygen diffusivity has a weak dependence upon the concentration of oxygen interstitials, in agreement with experimental observations. In the temperature range 800-1500 K, the activation energy for migration varied between 0.49 and 0.64 eV depending upon the degree of hyperstoichiometry. The present results are compared to previous work on oxygen self-diffusion in related K(2)NiF(4) structure materials.

Phase separation in oxygen deficient ΗοBa2Cu3O7-δ single crystals: effect of high pressure and twin boundaries

We investigate the influence of high hydrostatic pressure on the electrical resistance in the ab-plane in HoBa2Cu3O7– δ single crystals with oxygen deficiency. It is determined that the high-pressure-induced redistribution of labile oxygen enhances phase separation, which is accompanied by structural relaxation and ascending diffusion within the volume of the sample. This results in a significant displacement of the temperature intervals that correspond to metal-to-dielectric-type transitions. It is determined that the formation of the low-temperature phase can occur at the twin boundaries.

c-axis hopping conductivity in heavily Pr-doped YBCO single crystals

We investigated the temperature dependence of the longitudinal and transverse conductivity of Y1−zPrzBa2Cu3O7−δ single crystals with different praseodymium contents. The correlation between the experimental results and the predictions of various theoretical models was analyzed. It is observed that the increase of the praseodymium concentration in Y1−zPrzBa2Cu3O7−δ single crystals leads to increased localization effects and suppression of the superconducting state. Interestingly, for single crystals with large praseodymium content the anisotropy of the normal electrical resistivity ρc/ρab(T) is described by the universal ‘law of 1/2’ for thermally activated hopping conductivity. This is in contrast to the case for comparable crystals of YBa2Cu3O7−δ.