Nonlinear stability of <i>E</i> centers in Si_1-<i>x</i>Ge_<i>x</i>: electronic structure calculations

Electronic structure calculations are used to investigate the binding energies of defect pairs composed of lattice vacancies and phosphorus or arsenic atoms (E centers) in silicon-germanium alloys. To describe the local environment surrounding the E center we have generated special quasirandom structures that represent random silicon-germanium alloys. It is predicted that the stability of E centers does not vary linearly with the composition of the silicon-germanium alloy. Interestingly, we predict that the nonlinear behavior does not depend on the donor atom of the E center but only on the host lattice. The impact on diffusion properties is discussed in view of recent experimental and theoretical results

Transmutation in 90SrF2: A density functional theory study of phase stability in ZrF2

The stability of multiple possible phases of ZrF2 is computed using density-functional theory. Motivated by radioactive samples of fluorite 90SrF2 stored at the Hanford site, we consider β− radioactive decay as the route by which the 90ZrF2 is generated. To find suitable structures for the ZrF2 compound two methodologies are used. The first follows imaginary phonon modes from the fluorite ZrF2 while the second employs random structure searching. Six possible ZrF2 phases are identified; however, none of the structures resemble the lone experimentally reported orthorhombic structure for ZrF2. Although we predict these phases to be less stable (~0.3 eV/f.u.) than a phase-decomposed mixture of β-ZrF4 and Zr metal, they still may be relevant due to the kinetics of formation via radioactive decay and raise questions as to the nature of the ZrF2 structure and the state of the samples at Hanford

Radiation tolerance of nanocrystalline ceramics: insights from Yttria Stabilized Zirconia.

Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr(+), 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces

Importance of dispersion in density functional calculations of cesium chloride and its related halides

The ionic compound cesium chloride adopts a cubic crystal structure bearing the same name. However, ab initio electronic structure calculations based on density functional theory methods using generalized gradient approximation functionals do not predict that cesium chloride adopts this phase. In this paper we apply semiempirical methods (density functional theory plus a pairwise dispersion correction) to account for missing van der Waals interactions within cesium chloride. The C6 and R0 dispersion parameters for cesium are established within Grimme's DFT+D2 formalism. Inclusion of the dispersion corrections is found not only to improve the quality of structures in comparison to experiment for all cesium halides, but also leads to the correct prediction of the ground-state phase under ambient conditions

Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3.

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in β-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10-4 Ω.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy measurements confirm this finding and the interpretation of the experimental results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time, it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices

Engineering the free vacancy and active donor concentrations in phosphorus and arsenic double donor-doped germanium

In germanium, donor atoms migrate or form larger immobile clusters via their interaction with lattice vacancies. By engineering the concentration of free vacancies, it is possible to control the diffusion of the donor atoms and the formation of those larger clusters that lead to the deactivation of a significant proportion of the donor atoms. Electronic structure calculations in conjunction with mass action analysis are used to predict the concentrations of free vacancies and deactivated donor atoms in germanium doped with different proportions of arsenic and phosphorous. We find, for example, that at low temperatures, the concentration of free vacancies is partially suppressed by increasing the proportion of arsenic doping, whereas at high temperatures (above 1000 K), the concentration of free vacancies is relatively constant irrespective of the donor species. It is predicted that the free vacancy and active donor concentrations vary linearly with the arsenic to phosphorous ratio across a wide range of temperatures