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The E centers (dopant-vacancy pairs) play a significant role in dopant deactivation in semiconductors. In order to gain insight into dopant-defect interactions during epitaxial growth of in situ phosphorus doped Ge, positron annihilation spectroscopy, which is sensitive to open-volume defects, was performed on Ge layers grown by chemical vapor deposition with different concentrations of phosphorus (∼ 1 × 10 18 – 1 × 10 20 cm − 3 ). Experimental results supported by first-principles calculations based on the two component density-functional theory gave evidence for the existence of mono-vacancies decorated by several phosphorus atoms as the dominant defect type in the epitaxial Ge. The concentration of vacancies increases with the amount of P-doping. The number of P atoms around the vacancy also increases, depending on the P concentration. The evolution of P n –V clusters in Ge contributes significantly to the dopant deactivation. [ABSTRACT FROM AUTHOR]

We present a computational scheme for orbital-free density functional theory (OFDFT) that simultaneously provides access to all-electron values and preserves the OFDFT linear scaling as a function of the system size. Using the projector augmented-wave method (PAW) in combination with real-space methods, we overcome some obstacles faced by other available implementation schemes. Specifically, the advantages of using the PAW method are twofold. First, PAW reproduces all-electron values offering freedom in adjusting the convergence parameters and the atomic setups allow tuning the numerical accuracy per element. Second, PAW can provide a solution to some of the convergence problems exhibited in other OFDFT implementations based on Kohn-Sham (KS) codes. Using PAW and real-space methods, our orbital-free results agree with the reference all-electron values with a mean absolute error of 10 meV and the number of iterations required by the self-consistent cycle is comparable to the KS method. The comparison of all-electron and pseudopotential bulk modulus and lattice constant reveal an enormous difference, demonstrating that in order to assess the performance of OFDFT functionals it is necessary to use implementations that obtain all-electron values. The proposed combination of methods is the most promising route currently available. We finally show that a parametrized kinetic energy functional can give lattice constants and bulk moduli comparable in accuracy to those obtained by the KS PBE method, exemplified with the case of diamond. [ABSTRACT FROM AUTHOR]

Tungsten is considered to be used in the future fusion reactors as plasma-facing material. In such extreme environments, defects are induced in materials that modify their macroscopic properties such as the mechanical ones. It is of paramount importance to be able to determine concentration and size of the vacancy defects, from the mono vacancy to the large cavities, to validate the models developed to predict the evolution of the microstructure of irradiated materials. Positrons are very useful non-destructive probes that can characterize vacancy-type defects in materials. We present a combined experimental and theoretical study on detecting and estimating the sizes of vacancy clusters that are invisible with electron microscopy in tungsten, using positron annihilation spectroscopy. We here model the positron annihilation in the tungsten lattice and in vacancy-type defects using state-of-the-art first principles methodology. The Doppler broadening spectra and positron lifetimes in tungsten are calculated with two-component density functional theory with local density approximation and weighted density approximation. Our calculations are in excellent agreement with our experimental results. We show that the sizes of vacancy clusters in tungsten can be well estimated by combining both positron lifetimes and Doppler broadening spectra. We also determine the limit of validity of the canonical calculation method, which here is shown to fail when the vacancy clusters grow beyond their nucleation stage. This work is a first step needed to better interpret the measured positron annihilation characteristics (Doppler and lifetime) in tungsten and then extract quantitative data on small vacancy defects required to improve the understanding of early-stage vacancy defect evolution in tungsten. The method used in this paper could be used to study other metallic materials. [ABSTRACT FROM AUTHOR]