The Conundrum of Relaxation Volumes in First-Principles Calculations of Charged Defects in UO2

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material.

Density-Functional Calculation of Carbon-Interstitial Energies in a 4H-SiC(0001)-SiO2 Interface

An atomistic model for an abrupt 4H-SiC(0001)-SiO2 interface is constructed. By means of density-functional calculations within the local-density approximation, the energy landscape of a carbon interstitial in this structure is investigated and estimates for the diffusion barriers within bulk SiC and the energy difference between interstitial positions in the bulk semiconductor and the amorphous oxide are extracted both for the neutral defect and two charge states. The electronic structure of the neutral defect in its energetically most favorable position is analyzed by means of a hybrid exchange-functional calculation.

CONDUCTIVITY OF GAPPED GRAPHENE WITHIN RANDOM PHASE APPROXIMATION

The gapped graphene is technologically more important as electronic devices made up of conventional semiconductors are based on the presence of a gap between the valence and conduction bands. In this paper, we report our analytical and numerical calculation of conductivity of gapped graphene on SiO 2 substrate and its variation with the governing parameters of carrier density, band gap, long range charge impurity scatterers, short range neutral defect scatterers, impurity distance from the graphene interface and potential strength of short range scattering centers. We find that the conductivity for unscreened potential decreases with increasing band gap but when screening is incorporated the trend is not only reversed but there is a sharp enhancement in its magnitude also. The sub linear behavior seen in conductivity in the case of gapless graphene at high carrier concentration due to enhanced scattering from the short range neutral defects is also observed for gapped graphene, and it is further observed that increasing the potential strength of the defect scatterers brings in the same behavior at relatively smaller carrier densities.

Experimental Evidence for an Electrically Neutral (N-Si)-Complex Formed during the Annealing Process of Si+-/N+-Co-Implanted 4H-SiC

Hall effect investigations taken on Si+-/N+-, C+-/N+- or Ne+-/N+-co-implanted 4H-SiC layers and deep level transient spectroscopy investigations taken on Si+-implanted 4H-SiC layers provide experimental evidence for an electrically neutral defect complex formed during the annealing process at temperatures between 1400°C and 1700°C. This defect complex consumes nitrogen donors and an intrinsic Si containing defect species (interstitial Si or Si-antisite) or Cvacancies. At our present knowledge, we favor an (NX-SiY)-complex.

Defects in CdTe-Based Photodetectors

The photoluminescence spectra, defect composition and optoelectronic properties of chlorine doped CdTe monocrystals, thick layers and thin films are investigated. It is supposed that the complex defect (VCd-2ClTe) is a neutral defect that causes high resistivity of Cl doped CdTe. This complex can dissociate into two charged defects (VCd-ClTe) and ClTe at 300 K. Acentre (VCd-ClTe) is a stable defect that is responsible for p-type conductivity of Cl doped CdTe. Depending on the C1 concentration high resistivity, high photoconductivity or high p-type conductivity can be formed in CdTe that is only chlorine doped.

Reduction of Neutral Dangling Bond Density by Light Soaking in Nanocrystalline Silicon

ABSTRACTThe effects of light exposure on neutral defect density at the surface of nanocrystalline Si are investigated by electron-spin resonance (ESR) experiments. A decrease of the neutral dangling bond density by light soaking was observed in this nanostructure. The reduction rate of ESR signal intensity becomes large with increasing light exposure intensity, and the reduction occurs from the excitation energy higher than 2 eV in vacuum. The reduction of the defect density can be explained in terms of the conversion of neutral states to charged states by carrier trapping.

Defect Distributions in Doped and Undoped A-SiGe:H Alloys

ABSTRACTIn this work, gap states in doped and undoped a-SiGe:H alloys are examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. Deconvolution methods, neglecting the condition of charge neutrality, can be used for a rough estimate of the defect density value but not for ob- taining detailed information on the distribution of gap states in undoped samples. Our numerical analysis uses adapted occupation statistics and takes into account the condition of charge neutrality. Good agreement between measured and simulated PDS and CPM spectra is obtained. For a certain composition, i.e. a certain bandgap, the investigation of doped films yields infor- mation on the density and the position of charged defect states in the bandgap. In addition, the density of neutral defect states can be derived from a comparison of CPM and PDS spectra. The results reveal the coexistence of charged and neutral defects. In doped as well as in undoped films, charged defect states dominate the defect density. In the investigated range of compo- sitions the defect distribution of a-SiGe:H is similar to those found in a-Si:H. The width of the defect distributions does not decrease with decreasing bandgap. No evidence for a different be- havior of Si- and Ge-related defect states can be found in sub-bandgap absorption spectra.