Improvements to the Sink Strength Theory Used in Multi-Scale Rate Equation Simulations of Defects in Solids,
Tommy Ahlgren and Kalle Heinola,
Materials 13 (2020) 2621.
The application of kinetic Rate Equations (kRE) have proven to be a versatile method
in simulating defect dynamics and temporal changes in the microstructure of materials.
The reliability and usefulness of the method, however, depends critically on the defect interaction parameters used.
We show that the main interaction parameter for the kRE method, the sink strength, intrinsically depends on the detrapping, or the dissociation process itself
The correct sink strength required for a detrapping defect, is considerably larger than the values commonly used
Theory is presented how to determine the appropriate sink strengths
Hydrogen isotope exchange in tungsten during annealing in hydrogen atmosphere,
T. Ahlgren, P. Jalkanen, K. Mizohata, V. Tuboltsev, J. Raisanen, K. Heinola and P. Tikkanen,
Nuclear Fusion 59 (2019) 026016.
The radiological safety of the future thermonuclear fusion devices depends critically
on the total tritium inventory in the plasma-facing components.
We show that tritium removal from tungsten can be enhanced by the isotope exchange mechanism by doing the baking in H2 atmosphere
The retained deuterium from 30 keV implantation drop almost to zero after 24h annealing at 250C in H2 atmosphere
Annealing in vacuum requires temperatures above 400C for close to zero retention
The effect of vacancy formation energy, the hydrogen (or impurity) binding, and the divacancy binding energy
on the total equilibrium vacancy concentration in metals is given
The divacancies give the major contribution to the total vacancy fraction at high H fractions and cannot be
neglected when studying superabundant vacancies
At high hydrogen (impurity) fractions, superabundant vacancy formation takes place regardless of the binding
energy between vacancies and hydrogen (impurity)
The reason of superabundant vacancy formation mainly in the fcc phase (compared to bcc phase) is explained
The results of this theory are compared and found to be in good agreement with experiments
of H induced vacancy formation in fcc metals Pd, Ni, Co, and Fe
The diffusion of monovacancies in W is studied using molecular dynamics simulation (MD) and
density functional theory (DFT)
The diffusion pre-exponential factor for monovacancy diffusion is found to be two to three orders
of magnitude higher than commonly used in computational studies,
resulting in an attempt frequency of the order 10^{15} Hz
Multiple nearest neighbour jumps of monovacancy are found to play an important role in the
contribution to the total diffusion coefficient,
especially at temperatures above 2/3 of T_{m},
resulting in an upward curvature of the Arrhenius diagram
The kinetic Rate Equation (kRE) computer program HIM (hydrogen in metals) is presented, which can take
into account most of the dynamic processes, including defect annihilation, during irradiation
The results show that H is mainly trapped in W monovacancies, and trapping in larger vacancy
clusters increase with increasing H implantation energy
The slow H desorption observed in experiments after irradiation, was found to be mainly due
to detrapping of the weakly bound sixth hydrogen from monovacancies
Impurity self-interstitial atom complexes could be the nucleation site for formation of large
interstitial type dislocation loops observed experimentally
Plastic deformation of single nanometer-sized crystals,
L. Sun, A. V. Krasheninnikov, T. Ahlgren, K. Nordlund and F. Banhart,
Phys. Rev. Lett. 101 (2008) 156101.
in situ electron microscopy observations of the plastic deformation of individual
nanometersized Au, Pt, W, and Mo crystals
The correlation with atomistic simulations shows that the observed slow plastic deformation
is due to dislocation activity
We see evidence that the vacancy concentration in a nanoscale system can be smaller than
in the bulk material, an effect which has not been studied experimentally before
A very fast Monte Carlo method using ion packets is introduced
The method presented is optimal for the low probability scattering
simulations of multiple and plural backscattering effects
Concentration of interstitial and substitutional nitrogen in GaN_{(x)}As_{(1-x)},
T. Ahlgren, E. Vainonen-Ahlgren, J. Likonen, W. Li and M. Pessa,
Appl. Phys. Lett. 80 (2002) 2314.
The interstitial to substitutional nitrogen atoms in Ga(In)NAs is determined by nuclear reaction
analysis (NRA) and ion channeling techniques
The fraction of N atoms occupying substitutional sites was observed to increase linearly with
increasing N amount, while the concentration of interstitial N was nearly constant
Annealing at 750 C decreases the concentration of interstitial N
Origin of improved luminescence efficiency after annealing of Ga(In)NAs materials grown by molecular-beam epitaxy,
Wei Li, Markus Pessa, Tommy Ahlgren and James Dekker,
Appl. Phys. Lett. 79 (2001) 1094.
Positron annihilation spectroscopy (PAS) and nuclear reaction analysis (NRA) in
conjunction with ion channeling measurements show that
a high density of N interstitials and Ga vacancies are present in molecular beam epitaxy grown Ga(In)NAs
These measured point defects are believed to be the reason for the low
luminescence efficiency of as-grown Ga(In)NAs materials
Annealing is shown reduce the Ga vacancies and N interstitials, which
improves the luminescence efficiency
Identification of vacancy charge states in diffusion of arsenic in germanium,
E. Vainonen-Ahlgren, T. Ahlgren, J. Likonen, S. Lehto, J. Keinonen, W. Li and J. Haapamaa,
Appl. Phys. Lett. 77 (2000) 690.
Concentration dependent As diffusion in p-type Ge is observed
The concentration dependence is explained by a Fermi-level dependent diffusion model,
where the As atoms diffuse through Ge vacancies with the charge state 2- and 0
No presence of singly negatively charged Ge vacancy was observed, indicating
that Ge vacancy is a negative U center
A diffusion model that takes into account silicon located on Ga and As sites,
Si_{Ga}^{+}-Si_{As}^{-} pairs and interstitial Si is presented
The charge state of the interstitial Si was found to be +1 in order to best fit the experiments
Concentration dependent and independent Si diffusion in ion implanted GaAs,
T. Ahlgren, J. Likonen, J. Slotte, J. Räisänen, M. Rajatora and J. Keinonen,
Phys. Rev. B, 56 (1997) 4597.
A very fast numerical method to solve concentration dependent diffusion equation with
solubility limit is presented
Two silicon diffusion mechanisms in GaAs observed
Concentration independent diffusion: interstitial Si diffusion
Concentration dependent diffusion via vacancies in the Ga and As sublattices
The solid solubility limit of Si in GaAs is determined