Assessing the potential of perfect screw dislocations in SiC for solid-state quantum technologies

Although point defects in solids are one of the most promising physical systems to build functioning qubits, it remains challenging to position them in a deterministic array and to integrate them into large networks. By means of advanced ab initio calculations we show that undissociated screw dislocations in cubic 3C-SiC, and their associated strain fields, could be used to create a deterministic pattern of relevant point defects. Specifically, we present a detailed analysis of the formation energies and electronic structure of the divacancy in 3C-SiC when located in the vicinity of this type of dislocations. Our results show that the divacancy is strongly attracted towards specific and equivalent sites inside the core of the screw dislocations, and would form a one-dimensional arrays along them. Furthermore, we show that the same strain that attracts the divacancy allows the modulation of the position of its electronic states and of its charge transition levels. In the case of the neutral divacancy, we find that these modulations result in the loss of its potential as a qubit. However, these same modulations could transform defects with no potential as qubits when located in bulk, into promising defects when located inside the core of the screw dislocations. Since dislocations are still mostly perceived as harmful defects, our findings represent a technological leap as they show that dislocations can be used as active building blocks in future defect-based quantum computers

First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers

Among the thin-film solar cells, the maximum efficiencies are achieved by devices that use Cu(In,Ga)Se2 as absorber. However, this fact should not mask that there is room for improvement, if we could mitigate the main sources of efficiency loss in this solar cell type, which are induced by lattice defects. Therefore, a more complete picture of the nature of defects in Cu(In,Ga)Se2-based solar cells would help to improve the growth process in such way that detrimental defects are avoided and the efficiency increased. In order to achieve this goal, first-principles calculations provide valuable insights that complement experimental studies and can also be used as predictive tools. These calculations have been and continue to be successfully used for the case of point and planar defects in Cu(In,Ga)Se2-based solar cells. However, a defect type that has been studied to a lesser extent are lattice dislocations. The aim of this thesis is to carry out a complete study of the structural and electronic properties of Frank partials and perfect dislocations in CuInSe2 and CuGaSe2 . Results from this study allow us to solve, at least partially, the puzzle of Cu(In,Ga)Se2-based solar cells which exhibit decent efficiencies and at the same time have a very high dislocation density. Specifically, in the case of Frank partials our results suggest that these cores prefer to be non-stoichiometric and, as a consequence, are expected to be highly detrimental. Therefore, this defect type should not be present in a fully grown and highly efficient device. Furthermore, we relate the beneficial effect of the Cu-rich stage of the three-stage co-evaporation process used to deposit the absorber in high-efficiency devices with the disappearance of these loops. In the case of stoichiometric perfect dislocations, our results show that their electrical activity is related to the presence of cation-cation or anion-anion "wrong" bonds in the cores. Moreover, we found that cation-rich α-cores are active in the Cu(In,Ga)Se2 semiconductor alloy, whereas the anion-rich β-cores are not. These results, along with the study of sodium segregation tendency into the electrically active cores, are put in perspective with respect to the experimental findings and structural models available in literature

Native defects in monolayer GaS and GaSe: Electrical properties and thermodynamic stability

Structural, electronic, and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab initio calculations. Based on their charge transition levels we assess the influence of the studied defects on the electrical properties of the monolayers. Specifically, we show that native defects do not behave as shallow dopants and their presence cannot account for the experimentally observed intrinsic doping. In addition, we predict that native defects are efficient compensation and recombination centers. Besides pointing out their detrimental nature, we also calculate the corresponding finite-temperature formation energies and provide a window of growth conditions able to reduce the concentration of all relevant native defects

Anisotropic solid–liquid interface kinetics in silicon: an atomistically informed phase-field model

We present an atomistically informed parametrization of a phase-field model for describing the anisotropic mobility of liquid–solid interfaces in silicon. The model is derived from a consistent set of atomistic data and thus allows to directly link molecular dynamics and phase field simulations. Expressions for the free energy density, the interfacial energy and the temperature and orientation dependent interface mobility are systematically fitted to data from molecular dynamics simulations based on the Stillinger–Weber interatomic potential. The temperature-dependent interface velocity follows a Vogel–Fulcher type behavior and allows to properly account for the dynamics in the undercooled melt

First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers

Among the thin-film solar cells, the maximum efficiencies are achieved by devices that use Cu(In,Ga)Se2 as absorber. However, this fact should not mask that there is room for improvement, if we could mitigate the main sources of efficiency loss in this solar cell type, which are induced by lattice defects. Therefore, a more complete picture of the nature of defects in Cu(In,Ga)Se2-based solar cells would help to improve the growth process in such way that detrimental defects are avoided and the efficiency increased. In order to achieve this goal, first-principles calculations provide valuable insights that complement experimental studies and can also be used as predictive tools. These calculations have been and continue to be successfully used for the case of point and planar defects in Cu(In,Ga)Se2-based solar cells. However, a defect type that has been studied to a lesser extent are lattice dislocations. The aim of this thesis is to carry out a complete study of the structural and electronic properties of Frank partials and perfect dislocations in CuInSe2 and CuGaSe2 . Results from this study allow us to solve, at least partially, the puzzle of Cu(In,Ga)Se2-based solar cells which exhibit decent efficiencies and at the same time have a very high dislocation density. Specifically, in the case of Frank partials our results suggest that these cores prefer to be non-stoichiometric and, as a consequence, are expected to be highly detrimental. Therefore, this defect type should not be present in a fully grown and highly efficient device. Furthermore, we relate the beneficial effect of the Cu-rich stage of the three-stage co-evaporation process used to deposit the absorber in high-efficiency devices with the disappearance of these loops. In the case of stoichiometric perfect dislocations, our results show that their electrical activity is related to the presence of cation-cation or anion-anion "wrong" bonds in the cores. Moreover, we found that cation-rich α-cores are active in the Cu(In,Ga)Se2 semiconductor alloy, whereas the anion-rich β-cores are not. These results, along with the study of sodium segregation tendency into the electrically active cores, are put in perspective with respect to the experimental findings and structural models available in literature

Atomic and electronic structure of perfect dislocations in the solar absorber materials CuInSe2 and CuGaSe2 studied by first-principles calculations

Structural and electronic properties of screw and 60◦-mixed glide and shuffle dislocations in the solar absorber materials CuInSe_2 and CuGaSe_2 are investigated by means of electronic structure calculations within density functional theory (DFT). Screw dislocations present distorted bonds but remain fully coordinated after structural relaxation. Relaxed 60◦-mixed dislocations, in contrast, exhibit dangling and “wrong,” cation-cation or anionanion bonds, which induce deep charge transition levels and are electrically active. Analysis of Bader charges and local density of states (LDOS) reveals that acceptor and donor levels are induced by α and β cores, respectively. Moreover, there is local charge accumulation in the surrounding of those cores which contain dangling or “wrong” bonds. Thus the apparently harmless nature of dislocations is not because they are electrically inactive, but can only be a result of passivation by segregating defects

Anisotropic solid-liquid interface kinetics in silicon: An atomistically informed phase-field model

We present an atomistically informed parametrization of a phase-field model for describing the anisotropic mobility of liquid-solid interfaces in silicon. The model is derived from a consistent set of atomistic data and thus allows to directly link molecular dynamics and phase field simulations. Expressions for the free energy density, the interfacial energy and the temperature and orientation dependent interface mobility are systematically fitted to data from molecular dynamics simulations based on the Stillinger-Weber interatomic potential. The temperature-dependent interface velocity follows a Vogel-Fulcher type behavior and allows to properly account for the dynamics in the undercooled melt

Secondary-Phase-Assisted Grain Boundary Migration in CuInSe2

Significant structural evolution occurs during the deposition of CuInSe2 solar materials when the Cu content increases. We use in situ heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu Se phase topotactic with the CuInSe2 grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu Se phase makes the grain boundaries highly mobil