Maria Ganchenkova

Effect of sodium incorporation into CuInSe2 from first principles

The presence of small amounts of sodium has been shown to improve the electronic performance of Cu(In,Ga)Se2 (CIGS) solar cells, but the origins of this effect have not yet been fully resolved. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using density-functional-theory-based calculations. We find no direct way how the creation of Na-related point defects in bulk CIS would enhance p-type conductivity. Instead, we demonstrate that Na reduces copper mass transport due to the capture of copper vacancies by NaCu defects. This finding provides an explanation for experimental measurements where the presence of Na has been observed to decrease copper diffusion. The suggested mechanism can also impede VCu-related cluster formation and lead to measurable effects on defect distribution within the material.

Vacancies in CuInSe2: new insights from hybrid-functional calculations

We calculate the energetics of vacancies in CuInSe(2) using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level ε(0/2-). We find strong preference for V(Cu) and V(Se) over V(In) under practically all chemical conditions.

Mass transport in CuInSe2 from first principles

The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that...

Diffusion and clustering of substitutional Mn in (Ga,Mn)As

The Ga vacancy mediated microstructure evolution of (Ga,Mn)As during growth and postgrowth annealing is studied using a multiscale approach. The migration barriers for the Ga vacancies and substitutional Mn together with their interactions are calculated using first principles, and temporal evolution at temperatures 200–350°C is studied using lattice kinetic Monte Carlo simulations. We show that at the typical growth and annealing temperatures (i) Ga vacancies provide an efficient diffusion transport for Mn and (ii) in 10–20h the diffusion of Mn promotes the formation of clusters. Clustering reduces the Curie temperature, and explains its decrease during long-term annealing.

Current Status of Beryllium Materials for Fusion Blanket Applications

Abstract Beryllium is a promising functional material for several breeder system concepts to be tested within the experimental fusion reactor ITER and, later, implemented in the first commercial demonstration fusion power plant DEMO. For these applications its resistance to neutron irradiation and the detrimental effects of radiogenic gases (helium and tritium) is crucial for fusion reactor safety, subsequent waste management and material recycling. A reliable prediction of beryllium behavior under fusion irradiation conditions requires both dedicated experiments and advanced modeling. Characterization of the reference and alternative beryllium pebble grades was performed in terms of their microstructure and tritium release properties. The results are discussed with respect to their application in fusion blanket systems. The outcomes from the HIDOBE-01 post irradiation experiment (PIE) are discussed to highlight several interesting features manifested by beryllium irradiation at fusion relevant temperatures. Titanium beryllide is presently developed as a possible substitute for beryllium pebbles as it shows better oxidation resistance, higher melting temperature and tritium release efficiency. Pebbles consisting predominantly of Be12Ti phase were successfully fabricated at Rokkasho, Japan. Recent advances in modeling provide new insights on the production of point defects and the behavior of helium and hydrogen impurities in beryllium, improving understanding of the mechanisms of primary damage production, hydrogen’s effect on the size and the shape of gas bubbles, and tritium removal from the pebbles. The relevance of the experimental and modeling results on irradiated beryllium for the design of a fusion demonstration reactor is evaluated, and recommendations for future R&D programs are proposed.

Chapter Eleven – Mechanical Properties of Silicon Microstructures

Publisher Summary Mechanical properties of silicon microstructures and basic structural properties of crystalline silicon are discussed here. At ordinary pressure silicon crystallizes in a diamond structure with a basis of two atoms. All the theoretical calculations using force-field methods correctly describe bulk silicon in its diamond structure ground state, giving a value for the lattice parameter that is close to the value that is experimentally observed. Effects of pressure are explained here in detail. At high-elastic strains the harmonic approximation becomes insufficient to correctly describe the elastic energy. An alternative way to treat the nonlinearity effects is to include the higher than second-order terms in the formal expansion of the elastic energy in strains. Extended defects could be classified with respect to their dimensionality. The partial dislocations are always associated with stacking faults. Two types of dislocation in silicon are of special interest. Silicon has been a favorite material for theoretical and experimental investigations of dislocation nature and mobility. The second approach is the so-called cluster method, where a finite cluster surrounding the defect is constructed. Some examples of the proposed core reconstruction are presented. The dislocation segment where the dislocation line jumps over the Peierls barrier is called kink. Silicon belongs to the class of intrinsically brittle solids. The convenience and success of silicon material and micromachining technology have made silicon a natural choice for many MEMS applications, such as actuator, power generator, etc. The reliability of these applications is extremely important to ensure their effective performance.

Nanoporous carbon structures based on C[sub 20]

In this paper, we present computational results for C20 based solids. We propose structures that are shown to be energetically more favorable and stable than previously suggested structures. The so-called quasigraphite phase and base-centered-monoclinic type structures are found to be the energetically most favorable. The moleculardynamics stability of suggested structures was studied via constant-temperature and constant-pressure techniques and by examining phonon dispersion curves. All the predicted structures demonstrate high stability with respect to temperature and external load. By changing the geometry, the electronic properties can be varied from metallic to insulating.