53 research outputs found
Étude numérique des mécanismes d'autodiffusion dans les semiconducteurs
Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal
Thermally-activated charge reversibility of gallium vacancies in GaAs
The dominant charge state for the Ga vacancy in GaAs has been the subject of
a long debate, with experiments proposing 1, 2 or 3 as the best
answer. We revisit this problem using {\it ab initio} calculations to compute
the effects of temperature on the Gibbs free energy of formation, and we find
that the thermal dependence of the Fermi level and of the ionization levels
lead to a reversal of the preferred charge state as the temperature increases.
Calculating the concentrations of gallium vacancies based on these results, we
reproduce two conflicting experimental measurements, showing that these can be
understood from a single set of coherent LDA results when thermal effects are
included.Comment: 4 pages, 4 figure
The role of screened exact exchange in accurately describing properties of transition metal oxides: Modeling defects in LaAlO3
The properties of many intrinsic defects in the wide band gap semiconductor
LaAlO3 are studied using the screened hybrid functional of Heyd, Scuseria, and
Ernzerhof (HSE). As in pristine structures, exact exchange included in the
screened hybrid functional alleviates the band gap underestimation problem,
which is common to semilocal functionals; this allows accurate prediction of
defect properties. We propose correction-free defect energy levels for bulk
LaAlO3 computed using HSE that might serve as guide in the interpretation of
photoluminescence experiments
Self-vacancies in Gallium Arsenide: an ab initio calculation
We report here a reexamination of the static properties of vacancies in GaAs
by means of first-principles density-functional calculations using localized
basis sets. Our calculated formation energies yields results that are in good
agreement with recent experimental and {\it ab-initio} calculation and provide
a complete description of the relaxation geometry and energetic for various
charge state of vacancies from both sublattices. Gallium vacancies are stable
in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state
for intrinsic and n-type GaAs, confirming results from positron annihilation.
Interestingly, Arsenic vacancies show two successive negative-U transitions
making only +1, -1 and -3 charge states stable, while the intermediate defects
are metastable. The second transition (-/-3) brings a resonant bond relaxation
for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page
Pressure effect on diffusion of carbon at the 85.91◦ 100 symmetric tilt grain boundary of α-iron
The diffusion mechanism of carbon in iron plays a vital role in carburization processes, steel fabrication, and metal dusting corrosion. In this paper, using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm with on-the-fly catalog building that allows to obtain diffusion properties over large time scales taking full account of chemical and elastic effects coupled with an EAM potential, we investigate the effect of pressure on the diffusion properties of carbon in
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91
∘
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100
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symmetric tilt grain boundaries (GB) of
α
-iron up to a pressure of 12 kbar at a single temperature of 600 K. We find that, while the effect of pressure can strongly modify the C stability and diffusivity in the GB in ways that depend closely on the local environment and the nature of the deformation, isotropic and uniaxial pressure can lead to opposite and nonmonotonous effects regarding segregation energy and activation barriers. These observations are relevant to understanding of the evolution of heterogeneous materials, where variations of local pressure can alter the carbon diffusion across the material
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