Abstract: ABSTRACT Interatomic potentials approximate the potential energy of atoms as a function of their coordinates. Their main application is the effective simulation of many-atom systems. Here, we review empirical interatomic potentials designed to reproduce elastic properties, defect energies, bond breaking, bond formation, and even redox reactions. We discuss popular two-body potentials, embedded-atom models for metals, bond-order potentials for covalently bonded systems, polarizable potentials including charge-transfer approaches for ionic systems and quantum-Drude oscillator models mimicking higher-order and many-body dispersion. Particular emphasis is laid on the question what constraints ensue from the functional form of a potential, e.g., in what way Cauchy relations for elastic tensor elements can be violated and what this entails for the ratio of defect and cohesive energies, or why the ratio of boiling to melting temperature tends to be large for potentials describing metals but small for short-ranged pair potentials. The review is meant to be pedagogical rather than encyclopedic. This is why we highlight potentials with functional forms sufficiently simple to remain amenable to analytical treatments. Our main objective is to provide a stimulus for how existing approaches can be advanced or meaningfully combined to extent the scope of simulations based on empirical potentials. Graphical abstract read less

Abstract: Crystal imperfections such as dislocations strongly influence the performance and thermal transport behavior of GaN-based devices. We show that the experimental data used to parametrize the effect of dislocations on the thermal conductivity can be explained using only the reported film thickness and point defect concentrations. The analysis highlights the boundary-scattering-governed reduction of thermal conductivity in GaN, which had been underestimated in earlier models. To quantify the influence of dislocations on the thermal transport in GaN, we adopt a Green's function approach based on accurate ab initio interatomic force constants. While calculations at the level of density functional theory are necessary for three-phonon and point defect scattering, we show that scattering due to dislocations can be satisfactorily approximated using semiempirical potentials. This makes the Green's function approach to dislocation scattering a quantitatively predictive, yet computationally practical, method for obtaining detailed phonon scattering rates. read less

Abstract: The present study examines the interaction of hydrogen and nitrogen plasmas with gallium in an effort to gain insights into the mechanisms behind the synergetic effect of plasma and a catalytic metal. Absorption/desorption experiments were performed, accompanied by theoretical-computational calculations. Experiments were carried out in a plasma-enhanced, Ga-packed, batch reactor and entailed monitoring the change in pressure at different temperatures. The results indicated a rapid adsorption/dissolution of the gas into the molten metal when gallium was exposed to plasma, even at a low temperature of 100 °C. The experimental observations, when hydrogen was used, indicate that gallium acts as a hydrogen sink in the presence of plasma. Similar results were obtained with Ga in the presence of nitrogen plasma. In addition, density functional theory calculations suggest a strong interaction between atomic hydrogen and molten gallium. This interaction is described as a high formation of Ga-H species on the surfa... read less

Abstract: Piezoelectricity has proved itself a promising mechanism for energy conversion and signal sensing by taking advantage of its ability to convert mechanical energy into electricity. Here, we demonstrate that the piezoelectricity in free-standing non-centrosymmetric nanowires can also be triggered directly by heat to produce electricity. The feasibility of the idea is first analyzed by the dynamic theory of crystal lattices and then confirmed by molecular dynamics simulations. The most salient point is that the heat-induced voltage drop across the cross section of the free-standing nanowires alternates periodically with the vibration of the nanowire. Moreover, the electric potential induced by heat here (as large as 0.34 V) is proved to be comparable with the previously reported potentials induced by mechanical energy, and the maximum value can be tuned by controlling the size of the nanowire and the applied heat. read less

Abstract: Using molecular dynamics (MD) simulations, the influence of the vacancy defects on the mechanical properties of gallium nitride (GaN) nanosheets is investigated. Two types of defective nanosheets are studied. In one of them, only one atom is removed at the vacancies and in the other, the number of removed atoms is not limited. It is shown that GaN nanosheets with multiple vacancies have larger in-plane elastic modulus than nanosheets with single vacancies. Besides, the ultimate stress and strain of GaN nanosheets are computed. Compared to perfect nanosheet, a significant decrease is observed in the ultimate stress of GaN nanosheet with only 2% defect. By plotting the fracture evolution of nanosheets under uni-directional tensile loading, three different patterns are observed. Moreover, by using bi-directional tensile tests on the nanosheets, the bulk moduli of perfect and defective GaN nanosheets are computed. read less

Abstract: Gallium Nitride (GaN) possesses superior electronic properties for RF power electronics that play critical roles in various wireless communication technologies and military applications [1]. Heat generated as a byproduct of operation in these devices, increases their operating temperature and degrades their performance and lifetime. While bulk GaN has a high thermal conductivity (k) approaching 250 W/m-K, [2, 3] GaN thin films and devices experience a much lower k due to the presence of additional phonon scattering mechanisms and departures from Fourier transport [4, 5]. We will review thermal transport in GaN based devices, broadly addressing the impact of heat source dimensions, film thicknesses, interfaces, and defects. read less

Abstract: In this paper a new interatomic potential based on the Kieffer force field and designed to perform molecular dynamics (MD) simulations of carbon deposition on silicon surfaces is implemented. This potential is a third-order reactive force field that includes a dynamic charge transfer and allows for the formation and breaking of bonds. The parameters for Si–C and C–C interactions are optimized using a genetic algorithm. The quality of the potential is tested on its ability to model silicon carbide and diamond physical properties as well as the formation energies of point defects. Furthermore, MD simulations of carbon deposition on reconstructed (100) silicon surfaces are carried out and compared to similar simulations using a Tersoff-like bond order potential. Simulations with both potentials produce similar results showing the ability to extend the use of the Kieffer potential to deposition studies. The investigation reveals the presence of a channelling effect when depositing the carbon at 45° incidence angle. This effect is due to channels running in directions symmetrically equivalent to the (110) direction. The channelling is observed to a lesser extent for carbon atoms with 30° and 60° incidence angles relative to the surface normal. On a pristine silicon surface, sticking coefficients were found to vary between 100 and 73%, depending on deposition conditions. read less

Abstract: Size-dependent lattice expansion of nanoparticles is observed for many ionic compounds, including metal oxides, while lattice contraction prevails for pure metals. However, the physical origin of this effect, which is of importance for the thermodynamic, chemical and electronic properties of nanoparticles, is discussed controversially. After a survey of the experimental literature, revealing a wide variety of materials with size-dependent lattice expansion, we show that the negative surface stress is the key reason for lattice expansion, while the excess of lattice sums or point defects of various charge states can be excluded as general explanations. Ab initio calculations of surface stresses for various surface structures of metal oxides confirm the model of a surface-induced lattice expansion. read less

Abstract: Hydrogen (H) behavior in ZnO based diluted magnetic semiconductors (DMSs) was investigated theoretically. It was found that H exhibits diverse electronic and structural behavior across a range of different DMSs, depending on the doped transition metal element. For instance in the extensively debated Co doped ZnO system (ZnO:Co), H dopants do not introduce significant carrier concentrations at room temperature thus carrier mediated magnetism is not attainable by H codoping. In this case, magnetism can be manipulated by other mechanisms. In contrast, in the ZnO:V system, H is positively charged for the entire bandgap region, meaning carrier mediated magnetism may be possible. read less

Abstract: Aluminum nitride (AlN) is a popular buffer layer and interlayer. The understanding of how AlN serves as a wetting and fracture‐mitigating layer relies on molecular pictures of the AlN layer and the interfaces. However, molecular dynamics (MD) simulation studies on AlN system, particularly on its wurtzite phase, have been limited. This is because most existing interatomic force fields of AlN target the less common zinc blende phase. Here, we report a new Tersoff‐based AlN force field for its wurtzite structure. This potential has been extensively tested in terms of lattice parameters, bulk modulus, cohesive energy, and heat capacity. In addition, thermal expansion coefficient (TEC) of wurtzite AlN, a key property to precisely model heterostructures, has been calculated using MD method. The value of 2.66 × 10−6 K−1 calculated at 300 K for TEC is in excellent agreement with the reported experimental value. read less

Abstract: We address the issue of accurate parametrization for the Abell-Tersoff empirical potential applied to tetrahedrally bonded semiconductor materials. Empirical potential methods for structural relaxation are widely used for group IV semiconductors while, with few notable exceptions, work on III-V materials has not been extensive. In the case of the Abell-Tersoff potential parametrizations exist only for III-As and III-N, and are designed to correctly predict only a limited number of cohesive and elastic properties. In this work we show how by fitting to a larger set of cohesive and elastic properties calculated from density functional theory, we are able to obtain parameters for III-As, III-N, III-P, and III-Sb zinc blende semiconductors, which can also correctly predict important nonlinear effects in the strain. read less

Abstract: Two-dimensional (2D) materials have attracted extensive research interest in various applications in recent years due to their superior thermal, electrical, and optical properties, making them preferable for potential electronic and optoelectronic applications. These 2D materials form Van der Waals interfaces with common substrate materials due to fabrication and/or device requirements. Since the generated heat during the operation of the devices cause degradation and reliability concerns, interface thermal boundary conductance (TBCs) and in-plane thermal conductivities of the interfaces should be well understood for proper thermal management. Herein, we investigate the TBC and in-plane thermal conductivities of the Van der Waals interfaces of 2D materials by approach to-equilibrium molecular dynamics (AEMD) and non-equilibrium molecular dynamics (NEMD) simulations. Our results show that the TBC is higher for the interfaces with stronger phonon DOS and lattice match. Also, the increasing number of 2D material layers increases the TBC of the interface. The results also showed that the thermal conductivity of the materials forming the interface could affect each other's in-plane thermal conductivity. Changes in thermal conductivities of individual in-plane thermal conductivities can be as high as 70%. Change in thermal conductivity depends on the difference in thermal conductivities of materials in contact and only visible in the vicinity of the interface. Thermal management strategies should pay attention to the trade-off between the changes in individual thermal conductivities and TBC of the interfaces. read less

Abstract: In this article, we study the deposition of AlGaN film on AlN template by molecular dynamics (MD) simulations. The effects of growth temperature and film thickness on the dislocation of deposited AlGaN film are simulated and studied. The atomic structure of deposited AlGaN film is also investigated. We find that the dislocations usually occur at the interface between AlN template and AlGaN film and then extend towards the growth direction. The dislocation density decreases with the increase of AlGaN film thickness, which indicates that increasing the thickness of deposited AlGaN film to a certain extent is beneficial to reducing dislocation. In addition, increasing the growth temperature can also effectively reduce the dislocation in deposited AlGaN film. Furthermore, the crystallinity of deposited AlGaN film could be improved by increasing the growth temperature. This is consistent with the dislocation discussion. The mobility of adatoms increases as the growth temperature increases. So it is easier for adatoms to find their ideal lattice points at higher temperature. Thus the dislocation and other defects can be effectively reduced and the crystal quality of deposited AlGaN film could be improved. GRAPHICAL ABSTRACT read less

Abstract: We outline a molecular mechanics model for the interaction of gallium and nitride ions ranging from small complexes to nanoparticles and bulk crystals. While the current GaN force fields allow the modelling of either bulk crystals or single ions dispersed in solution, our model covers both and hence paves the way to describing aggregate formation and crystal growth processes from molecular simulations. The key to this is the use of formal  +3 and  −3 charges on the gallium and nitride ions, whilst accounting for the charge transfer in GaN crystals by means of additional potential energy terms. The latter are fitted against experimental data of GaN in the wurtzite structure and benchmarked for the zinc-blende and rock-salt polymorphs. Comparison to quantum chemical references and experiment shows reasonable agreement of structures and formation energy of [GaN]n aggregates, elastic properties of the bulk crystal, the transition pressure of the wurtzite to rock-salt transformation and intrinsic point defects. Furthermore, we demonstrate force field transferability towards the modelling of GaN nanoparticles from simulated annealing runs. read less

Abstract: The analytical bond-order potential has been developed for simulating fission product (Ag, Pd, Ru, and I) behavior in SiC, especially for their diffusion. We have proposed adding experimentally available elastic constants and physical properties of the elements as well as important defect formation energies calculated from density functional theory simulation to the list of typical properties as the extensive fitting database. The results from molecular dynamics simulations are in a reasonable agreement with defect properties and energy barriers of their experimental/computational counterparts. The successful validation of the new potential has established a good reliability and transferability of the potentials, which enables the ability of simulation in extended scale. The kinetic behavior such as diffusion of different interstitials is then realized by applying the new interatomic potentials. The bulk diffusion is less likely to dominate the transport of the four fission products under pure thermal condition, when we refer to their extremely small values of the effective diffusion coefficients. The interstitial mechanism is hard for Pd, Ru, and I to access due to the high formation energy and high migration energy. However, it is found that the migration energy of silver is relatively low, which indicates Ag diffusion via an interstitial mechanism being feasible, especially under irradiation condition, where massive interstitials can be formed in high-temperature nuclear reactors. read less

Abstract: Laser-assisted atom probe tomography (APT) and high-resolution dark-field electron holography (HR-DFEH) were performed to investigate the composition of a polar [0001] GaN/AlxGa1 − xN/InyGa1 − yN light emitting diode. In particular, the III-site fraction of both AlxGa1 − xN and InyGa1 − yN alloys was studied adopting a comparative approach. HR-DFEH allows mapping the projected strain with a subnanometer spatial resolution which is used for the calculation of the two-dimensional alloy composition distribution. APT provides three-dimensional alloys composition distribution with a nanometer spatial resolution. However, here we reveal that important inaccuracies affect local composition measurements. A Ga-poor composition is obtained in high DC-electric field regions. Moreover, such inaccuracies may be locally enhanced where the [0001] pole intersects the surface of the analyzed specimen, leading to a lower fraction of Ga measured. III-site fractions closer to the nominal values were measured at low field conditions. Ga loss is thought to be due to preferential DC field induced evaporation of Ga ions between laser pulses. This is explained in terms of formation of a metallic layer on the tip surface during APT analysis, where weak Ga-Ga bonds are formed, promoting the loss of Ga at high field conditions.Laser-assisted atom probe tomography (APT) and high-resolution dark-field electron holography (HR-DFEH) were performed to investigate the composition of a polar [0001] GaN/AlxGa1 − xN/InyGa1 − yN light emitting diode. In particular, the III-site fraction of both AlxGa1 − xN and InyGa1 − yN alloys was studied adopting a comparative approach. HR-DFEH allows mapping the projected strain with a subnanometer spatial resolution which is used for the calculation of the two-dimensional alloy composition distribution. APT provides three-dimensional alloys composition distribution with a nanometer spatial resolution. However, here we reveal that important inaccuracies affect local composition measurements. A Ga-poor composition is obtained in high DC-electric field regions. Moreover, such inaccuracies may be locally enhanced where the [0001] pole intersects the surface of the analyzed specimen, leading to a lower fraction of Ga measured. III-site fractions closer to the nominal values were measured at low field con... read less

Abstract: Silicon is an important material and many empirical interatomic potentials have been developed for atomistic simulations of it. Among them, the Tersoff potential and its variants are the most popular ones. However, all the existing Tersoff-like potentials fail to reproduce the experimentally measured thermal conductivity of diamond silicon. Here we propose a modified Tersoff potential and develop an efficient open source code called GPUGA (graphics processing units genetic algorithm) based on the genetic algorithm and use it to fit the potential parameters against energy, virial and force data from quantum density functional theory calculations. This potential, which is implemented in the efficient open source GPUMD (graphics processing units molecular dynamics) code, gives significantly improved descriptions of the thermal conductivity and phonon dispersion of diamond silicon as compared to previous Tersoff potentials and at the same time well reproduces the elastic constants. Furthermore, we find that quantum effects on the thermal conductivity of diamond silicon at room temperature are non-negligible but small: using classical statistics underestimates the thermal conductivity by about 10% as compared to using quantum statistics. read less

Abstract: Thermal transport across solid interfaces is of great importance for applications like power electronics. In this work, we perform non-equilibrium molecular dynamics simulations to study the effect of light atoms on the thermal transport across SiC/GaN interfaces, where light atoms refer to substitutional or interstitial defect atoms lighter than those in the pristine lattice. Various light atom doping features, such as the light atom concentration, mass of the light atom, and skin depth of the doped region, have been investigated. It is found that substituting Ga atoms in the GaN lattice with lighter atoms (e.g. boron atoms) with 50% concentration near the interface can increase the thermal boundary conductance (TBC) by up to 50%. If light atoms are introduced interstitially, a similar increase in TBC is observed. Spectral analysis of interfacial heat transfer reveals that the enhanced TBC can be attributed to the stronger coupling of mid- and high-frequency phonons after introducing light atoms. We have also further included quantum correction, which reduces the amount of enhancement, but it still exists. These results may provide a route to improve TBC across solid interfaces as light atoms can be introduced during material growth. read less

Abstract: ABSTRACT In this work, we investigated the deposition of AlN film on GaN substrate by using molecular dynamics (MD) simulations. The effects of GaN substrate surface, growth temperature, and injected N: Al flux ratio on the growth of AlN film were simulated and studied. Consequently, the deposited AlN film on the (0001) Ga-terminated GaN surface achieves better surface morphology and crystallinity than that on the (000-1) N-terminated GaN surface due to the different diffusion ability of Al and N adatoms on two GaN surfaces. Furthermore, with the increase of growth temperature, the surface morphology and crystallinity of AlN film were improved owing to the enhanced mobility of adatoms. At the optimised injected N: Al flux ratio of 1, comparatively good surface morphology and crystallinity of deposited AlN films were realised. This method lays a foundation for the follow-up real-time study of defects and stress evolution of AlN on GaN and can be applied to film growth of other materials. GRAPHICAL ABSTRACT read less

Abstract: The molecular dynamics model of wurtzite crystal structure GaN in nano-grinding was established using the Tersoff multi-body potential. The complete structure defect-free GaN model and the defect-containing GaN model were set for comparison. The MD model used hemispherical diamond abrasive grains as the grinding tool, and the micro-regular ensemble (NVE) was used in the grinding process. Additionally, the GaN grinding simulation results under different loading conditions (changing grinding speed, depth) were analyzed to study the changes of internal crystal structure and the evolution of crystal defects. The results show that there is a transition from wurtzite to zinc-blende in GaN during the grinding. Moreover, defects and grinding force increase as the depth of grinding increases. The grinding speed has no obvious influence on the grinding force. When the grinding distance reaches 15 nm, the grinding force decreases slightly with the increase of the speed. read less

Abstract: In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case. read less

Abstract: Large-scale molecular dynamics (MD) simulations, along with bond-order interatomic potentials, have been employed to study defect production, clustering and their evolution within high energy displacement cascades in semiconductors. Based on the MD results, the damage density within a cascade core is evaluated, and used to describe a new energy partition function. In addition, we have further developed a model to determine the non-ionizing energy loss (NIEL) for semiconductors, which can be used to predict the displacement damage degradation induced by space radiation on electronic components. The atomic-level based NIEL model has been applied to GaAs and GaN. At low energies, the most surviving defects are single interstitials and vacancies, and only 20% of the interstitial population is contained in clusters in GaAs, but a direct-impact amorphization in GaAs occurs with a high degree of probability during the cascade lifetime for Ga PKAs (primary knock-on atoms) with energies higher than 2 keV. However, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume in GaN, and consequently, a great number of displaced atoms recombine significantly with vacancies at the same time, i.e., a pseudo-metallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplied Norgett, Robison and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. The calculated NIEL in GaN is often found smaller than that predicted by a model based on the simple Kinchin-Pease formula. The comparisons of defect creation, density and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs, and therefore, very suitable for use in high-power space-energy systems and space-probe applications. read less

Abstract: The growth of AlGaN has been extensively studied, but corresponding research related to the effect of AlN substrate surface has rarely been reported in literature. In this article, the effects of AlN substrate surface on deposition of AlGaN films were investigated by molecular dynamics (MD) simulations. (0001) Al-terminated and (0001¯) N-terminated AlN were considered as substrates. The quality of surface morphology and atomic scale structure of deposited AlGaN film are discussed in detail. The results show that the surface morphology and crystal quality of AlGaN film grown on (0001) Al-terminated AlN surface are better than for that grown on (0001¯) N-terminated AlN surface under various growing temperatures and Al/Ga injection ratios between Al and Ga. This can be attributed to the higher mobility of Al and Ga adatoms on the (0001) Al-terminated AlN surface. These findings can provide guidance for the preparation of high-quality AlGaN thin films on AlN substrate. read less

Abstract: We have employed large-scale molecular dynamics simulations to study defect production, clustering, and its evolution in GaN for energies of a primary knock-on atom ranging from 500 eV to 40 keV. In the presence of proton radiation, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume, but a great number of displaced atoms recombine significantly with vacancies at the same time, i.e., a pseudometallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplified Norgett, Robison, and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. Moreover, the damage density within a cascade core is evaluated and applied to construct a model to calculate an energy-partition function for studying the nonionizing energy loss (NIEL) in GaN. The calculated NIEL in GaN is often found to be smaller than that predicted by a model based on the simple Kinchin–Pease formula. The comparisons of defect creation, density, and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs at low temperatures. read less

Abstract: Thermal transport across solid interfaces plays important roles in many applications, especially in the thermal management of modern power electronics. In this study, we use non-equilibrium MD (NEMD) simulations to systematically study a model SiC/GaN interface, which is an important interface in GaN-based power electronics, mated by different intermediate layers (ILs) with the focus on how the atomic masses of the ILs influence the overall thermal conductance. To isolate the mass effect, the Tersoff potential with the same parameters is used to approximate the interatomic interactions between all atoms, with the only differences between materials being their atomic masses. The NEMD results show that the thermal boundary conductance (TBC) of IL-mated interfaces depends not only on the total primitive cell mass of the IL but also on the relative masses of the atoms within the unit cell. By analyzing the vibrational power spectra (VPS) of SiC, IL, and GaN, it is found that the optical phonons play important roles in thermal transport across the solid/solid interfaces. There is an optimal mass ratio of the atoms in the unit cell of the IL that can maximize the overlap of IL optical phonon VPS with those of SiC and GaN. Furthermore, the atomic masses of a number of III-V semiconductor compounds are studied for the ILs. It is shown that when only considering the mass effect, in the classical limit, AlN will be the best IL to enhance thermal transport across SiC/GaN interfaces with an improvement of as much as 27% over that of a pristine SiC/GaN interface. Despite the known limitation of the model (e.g., absence of strain and quantum effects), the results from this work may still provide some useful information for the design of ILs to improve thermal transport across solid/solid interfaces. read less

Abstract: The directional atomic layer etching (ALE) of GaN and AlGaN has been developed. The GaN ALE process consists of cyclic Cl2 plasma chemisorption and Ar ion removal. The etch per cycle (EPC) was 0.4 nm within the self-limiting regime, which is 50 to 100 V. The root-mean-square surface roughness RRMS was 0.6 nm, which was improved from an initial roughness of 0.8 nm. For AlGaN ALE, BCl3 was added to the chlorine step to obtain a smooth surface with RRMS of 0.3 nm and stoichiometry similar to the initial sample. The ultra smooth surface obtained by etching is promising for use in next-generation power devices. read less

Abstract: N2 gas is routinely used as a seeding species in fusion plasma to control the amount of power emitted from the plasma by radiation to the tungsten walls of an ITER-like divertor. Nitrogen atoms interact with the plasma-facing materials beryllium and tungsten, and form chemical bonds with the wall surfaces, as well as with plasma hydrogen isotopes, thus raising a special interest in W–N and N–H interactions in the fusion community. In this work we describe the development of an analytical interatomic potential for W–N interactions and benchmark the potential against DFT calculation results for N defects in tungsten. read less

Abstract: A systematic approach to the derivation of empirical interatomic potentials is developed for III–N semiconductors with the aid of ab initio calculations. The parameter values of empirical potential based on bond order potential are determined by reproducing the cohesive energy differences among 3-fold coordinated hexagonal, 4-fold coordinated zinc blende, wurtzite, and 6-fold coordinated rocksalt structures in BN, AlN, GaN, and InN. The bond order p is successfully introduced as a function of the coordination number Z in the form of p = a exp(−bZn) if Z ≤ 4 and p = (4/Z)α if Z ≥ 4 in empirical interatomic potential. Moreover, the energy difference between wurtzite and zinc blende structures can be successfully evaluated by considering interaction beyond the second-nearest neighbors as a function of ionicity. This approach is feasible for developing empirical interatomic potentials applicable to a system consisting of poorly coordinated atoms at surfaces and interfaces including nanostructures. read less

Abstract: The aim of the study described in this thesis is to obtain a profound understanding of transformations in NCs at the atomic level, by performing molecular simulations for such transformations, and by comparing the simulation results with available experimental high resolution transmission electron microscopy (HRTEM) data to validate the simulations and to reveal underlying physical mechanisms. These transformations include structural and morphological transitions and cation exchange processes in ionic nanocrystals (II-VI and IV-VI semiconductors). The main simulation method used is classical Molecular Dynamics (MD) simulation. First principles density functional theory (DFT) calculations were used to develop empirical force fields that are able to accurately reproduce phase transitions. Using these newly developed force fields, large scaled classical MD simulations were carried out and linked to HRTEM experiments. The partially charged rigid ion model (PCRIM) was chosen for the force fields. This PCRIM approach has a simple functional form with a few number of parameters and has a clear physical meaning for ionic crystals. To simulate cation exchange in colloidal NC systems at the NC/solution interface, we used a combination of all-atom force fields and a coarse-grained model. In Chapter 2, an ab-initio based force field for ZnO is developed within the framework of the PCRIM approach. The values of the partial charges were determined by Bader charge analysis of DFT calculations on various ZnO phases. Beside Coulombic interactions, only short-ranged pairwise interatomic interactions were included. An initial guess of the parameters of the short-ranged pair potentials were first obtained by the lattice inversion method. The parameters were further adjusted by an ab-initio potential surface fitting procedure. The new ZnO force field has a very simple functional form is able to accurately reproduce several important physical properties of ZnO materials. These physical properties include the lattice parameters and phase stability of several ZnO polymorphs, as well as the elastic constants, bulk moduli, phonon dispersion, and melting points of wurtzite ZnO. The transition pressure of the wurtzite-to-rocksalt transition calculated with the force field equals 12.3 GPa, in agreement with experimental measurements and DFT calculations. A wurtzite-to-honeycomb phase transition is predicted at an uniaxial pressure of 8.8 GPa. We found a rational and effective way to derive force fields with simple functional forms for accurate simulations of phase transitions in ionic crystals. In Chapter 3, we developed a transferable force field for CdS-CdSe-PbSPbSe solid systems. The selection of the force field and the fitting procedure are similar to that of the ZnO force field in Chapter 2. The challenges when developing this force field were to maintain the transferability of this force field for four materials (CdS, CdSe, PbS, and PbSe) and to describe their mixed phases. This was solved by assuming that different cations/anions have the same values of the partial charges, and that shortranged interatomic interactions between two cations/anions are the same in different materials. For the mixed phases, DFT calculations of the mixed phases were included in both the training and validation sets. This work is the first step for further simulation studies of these II-VI and IV-VI semiconductor NCs and heteronanocrystals (HNCs). In Chapter 4, a thermally induced morphological and structural transition of CdSe NCs was investigated using MD simulations. In MD simulations, a CdSe nanosphere with the ZB structure transforms into a tetrapodlike morphology at 800 K. In a CdSe tetrapod, four WZ legs attach to the {111} facets of a tetrahedral ZB core. This transformation is achieved by a layer-by-layer slip of the ZB-{111} bilayer. Simulations show that the slips are mediated by the formation of Cd vacancies on the surface of the NCs to overcome the potentially large energy barriers associated with slip. The morphology of the annealed NCs is found to be temperature and size-dependent. An octapod-like morphology is found in NCs with a relatively large NC size and in a certain range of the heating temperature. Surprisingly, nanoscale transformations of CdSe NCs have been directly observed in HRTEM in situ heating experiments. Our findings provide a simple method to modify the morphology of ionic NCs and can potentially be used in the synthesis of branched NCs. The cation exchange process of PbSe/CdSe HNCs has been investigated by HRTEM in situ heating experiments in combination with MD simulations and DFT calculations in Chapter 5. In the HRTEM experiments, we bserved that Cd atoms in PbSe/CdSe nanodumbbells (CdSe rods with one or two PbSe tip(s)) are replaced by Pb atoms. The exchange rate depends on the heating temperature and the amount of Pb atoms present in the system. Sometimes, fully converted PbSe nanodumbells can be observed. MD simulations were performed to investigate the mechanism of this cation exchange process. It was found that the the CdSe domains near the PbSe/CdSe interfaces have significant structural disorder. These findings are in line with the experimental observation that the exchange process proceeds in a layer-by-layer fashion along the WZ- direction. We concluded that cation exchange in PbSe/CdSe HNCs is mediated by the local structural disorder which enables the formation of vacancies and accelerated the motion of cations. In Chapter 6, a coarse-grained psuedoligand model was introduced to simulate cation exchange in PbS colloidal NCs taking into account the cation-solvent interactions. Modelling colloidal NC systems including interactions with the solvent has long been a challenge due to the large system size and long time scales. Here, we incorporated the effects of ligands and solvents into negatively charged large spherical coarse-grained psuedoligands. MD simulations combining coarse-grained and all-atom models can successfully reproduce the cation exchange process in PbS colloidal NCs. Simulations show that the exchange rate and system equilibrium can be controlled by the temperature and by changing ligands. The exchange process is directly related to vacancy formation and the high mobility of Cd ions at the PbS/CdS interface. Our simulations also predict that high-pressure conditions will be beneficial for achieving fast exchange at elevated temperatures. Our coarse-grained model can be easily extended to other systems for the computational investigation of transformations in nanostructures. read less

Abstract: The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten. read less

Abstract: Molecular beam epitaxial growth of GaN under Ga-rich conditions was simulated using a classical molecular dynamics method. We investigated nitrogen incorporation into the growth surface and the initial growth process using two kinds of simulation models: the Ga adlayer model and Ga droplet model. The simulation of the Ga adlayer model showed that the injected N atom diffused through the Ga adlayer and nucleation occurred in the solid/liquid interface. The simulation of the Ga droplet model showed that the injected N atom diffused on the bare GaN crystal surface and nucleation occurred at the edge of the Ga droplet. In the both simulations, scattering of injected N atoms on the surface of the Ga layer was often observed. Because Ga atoms in the Ga layer were intensively moving compared with that in the GaN crystal, injected N atoms were probably scattered by collisions with the Ga atoms in the Ga layer. read less

Abstract: Stainless steels found in real-world applications usually have some C content in the base Fe–Cr alloy, resulting in hard and dislocation-pinning carbides—Fe3C (cementite) and Cr23C6—being present in the finished steel product. The higher complexity of the steel microstructure has implications, for example, for the elastic properties and the evolution of defects such as Frenkel pairs and dislocations. This makes it necessary to re-evaluate the effects of basic radiation phenomena and not simply to rely on results obtained from purely metallic Fe–Cr alloys. In this report, an analytical interatomic potential parameterization in the Abell–Brenner–Tersoff form for the entire Fe–Cr–C system is presented to enable such calculations. The potential reproduces, for example, the lattice parameter(s), formation energies and elastic properties of the principal Fe and Cr carbides (Fe3C, Fe5C2, Fe7C3, Cr3C2, Cr7C3, Cr23C6), the Fe–Cr mixing energy curve, formation energies of simple C point defects in Fe and Cr, and the martensite lattice anisotropy, with fair to excellent agreement with empirical results. Tests of the predictive power of the potential show, for example, that Fe–Cr nanowires and bulk samples become elastically stiffer with increasing Cr and C concentrations. High-concentration nanowires also fracture at shorter relative elongations than wires made of pure Fe. Also, tests with Fe3C inclusions show that these act as obstacles for edge dislocations moving through otherwise pure Fe. read less

Abstract: The lattice thermal conductivity is known to depend on crystal quality, but the reduction in thermal conductivity due to specific defects is presently unclear. Molecular dynamics simulations were used to investigate the impact of microstructural defects on the thermal conductivity of gallium nitride. The conductivity of a finite crystal was reduced to (39 ± 4)% by a screw dislocation density of 2.0 × 1013 cm−2 and to (51 ± 4)% by an edge dislocation of similar density, illustrating that the type of dislocation is important for thermal conductivity. The effect of stacking faults on thermal conductivity was also investigated. read less

Abstract: The concept of eigenstrain is adopted to derive a general analytical framework to solve the elastic field for 3D anisotropic solids with general defects by considering the surface stress. The formulation shows the elastic constants and geometrical features of the surface play an important role in determining the elastic fields of the solid. As an application, the analytical close-form solutions to the stress fields of an infinite isotropic circular nanowire are obtained. The stress fields are compared with the classical solutions and those of complex variable method. The stress fields from this work demonstrate the impact from the surface stress when the size of the nanowire shrinks but becomes negligible in macroscopic scale. Compared with the power series solutions of complex variable method, the analytical solutions in this work provide a better platform and they are more flexible in various applications. More importantly, the proposed analytical framework profoundly improves the studies of general 3D anisotropic materials with surface effects. read less

Abstract: We investigated the fundamentals of the effect of C addition on Na flux GaN growth by first-principles calculation. We simulated C-added Na–Ga melts using molecular dynamics (MD) simulations to examine the local melt structure around a N atom. We also calculated C–N bond energy using constrained MD simulations. Results show that a N atom bonded to a C atom and there were no Ga atoms around the N atom because C–N bond energy was larger than Ga–N bond energy. This is the reason for the suppression of heterogeneous nucleation by C addition. It was also found that the C–N bond energy was affected by surrounding Ga atoms and that the C–N atomic distance increased with the Ga coordination number around the N atom. read less

Abstract: CdTe and Cd${}_{1\ensuremath{-}x}$Zn${}_{x}$Te are the leading semiconductor compounds for both photovoltaic and radiation detection applications. The performance of these materials is sensitive to the presence of atomic-scale defects in the structures. To enable accurate studies of these defects using modern atomistic simulation technologies, we have developed a high-fidelity analytical bond-order potential for the CdTe system. This potential incorporates primary ($\ensuremath{\sigma}$) and secondary ($\ensuremath{\pi}$) bonding and the valence dependence of the heteroatom interactions. The functional forms of the potential are directly derived from quantum-mechanical tight-binding theory under the condition that the first two and first four levels of the expanded Green's function for the $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-bond orders, respectively, are retained. The potential parameters are optimized using iteration cycles that include first-fitting properties of a variety of elemental and compound configurations (with coordination varying from 1 to 12) including small clusters, bulk lattices, defects, and surfaces, and then checking crystalline growth through vapor deposition simulations. It is demonstrated that this CdTe bond-order potential gives structural and property trends close to those seen in experiments and quantum-mechanical calculations and provides a good description of melting temperature, defect characteristics, and surface reconstructions of the CdTe compound. Most importantly, this potential captures the crystalline growth of the ground-state structures for Cd, Te, and CdTe phases in vapor deposition simulations. read less

Abstract: The unique properties of the group III-nitrides (Edgar et al., 1999; Jain et al., 2000; Kasap & Capper, 2006) make them the best semiconductor material for • optoelectronic devices emitting light in the visible and UV spectral ranges (Orton & Foxon, 1998), including sources for general illumination (Craford, 2005; Liu, 2009; Miyajima et al., 2001; Nakamura et al., 2000; Schubert & Kim, 2005; Schubert, 2006; Taguchi, 2003; Zukauskas et al., 2002), • photodetectors for these spectral ranges, including solar-blind UV detectors, • high power/high frequency electronic devices capable of operating at high temperatures and in harsh environment (Bennett et al., 2004; Morkoc, 1998; Pearton et al., 2000; Shur, 1998; Skierbiszewski, 2005; Xing et al., 2001). To fully exploit the potential of the group III-nitrides in optoelectronics and communication technology, two problems are to be solved: 1. difficulties of doping group III-nitrides, especially attaining the high-level p-doping in 1999 AlN was even called an "undopable" material (Fara et al., 1999) (p-doping is also a problem for other wide bandgap semiconductors oxides such as ZnO and chalcogenides such as ZnSe; ) and 2. the lack of large high crystalline quality native substrates with required electrical properties. read less

Abstract: CdTe and CdTe-based Cd(1-x)Zn(x)Te (CZT) alloys are important semiconductor compounds that are used in a variety of technologies including solar cells, radiation detectors, and medical imaging devices. Performance of such systems, however, is limited due to the propensity of nano- and micro-scale defects that form during crystal growth and manufacturing processes. Molecular dynamics simulations offer an effective approach to study the formation and interaction of atomic scale defects in these crystals, and provide insight on how to minimize their concentrations. The success of such a modeling effort relies on the accuracy and transferability of the underlying interatomic potential used in simulations. Such a potential must not only predict a correct trend of structures and energies of a variety of elemental and compound lattices, defects, and surfaces but also capture correct melting behavior and should be capable of simulating crystalline growth during vapor deposition as these processes sample a variety of local configurations. In this paper, we perform a detailed evaluation of the performance of two literature potentials for CdTe, one having the Stillinger-Weber form and the other possessing the Tersoff form. We examine simulations of structures and the corresponding energies of a variety of elemental and compound lattices, defects, and surfaces compared to those obtained from ab initio calculations and experiments. We also perform melting temperature calculations and vapor deposition simulations. Our calculations show that the Stillinger-Weber parameterization produces the correct lowest energy structure. This potential, however, is not sufficiently transferrable for defect studies. Origins of the problems of these potentials are discussed and insights leading to the development of a more transferrable potential suitable for molecular dynamics simulations of defects in CdTe crystals are provided. read less

Abstract: The authors simulate in three dimensions the molecular beam epitaxial growth of InxGa1−xN with classical molecular dynamics. Atomic interactions are simulated with Stillinger–Weber potentials. Both homoepitaxial and heteroepitaxial growths are studied. The effects of substrate temperature and indium concentration on quantum dot morphology, concentration profiles, and the thickness of wetting layers qualitatively agree with experimental findings. The authors’ simulations support earlier suggestions that quantum dot formation in the InGaN/GaN system is governed by a stress-driven phase separation mechanism. read less

Abstract: Past molecular dynamics studies of thermal transport have predominantly used Stillinger–Weber potentials. As materials continuously shrink, their properties increasingly depend on defect and surface effects. Unfortunately, Stillinger–Weber potentials are best used for diamond-cubic-like bulk crystals. They cannot represent the energies of many metastable phases, nor can they accurately predict the energetics of defective and surface regions. To study nanostructured materials, where these regions can dominate thermal transport, the accuracy of Tersoff potentials in representing these structures is more desirable. Based upon an analysis of thermal transport in a GaN system, we demonstrate that the cutoff function of the existing Tersoff potentials may lead to problems in determining the thermal conductivity. To remedy this issue, improved cutoff schemes are proposed and evaluated. read less

Abstract: In this study, the epitaxial growth of gallium nitride (GaN) on a GaN substrate is investigated by a molecular dynamics (MD) method. Furthermore, the difference between the surface diffusion of atoms of a strained substrate and an unstrained substrate is examined. From the results of this examination, it is found that the diffusion characteristic in the unstrained case is higher than that in the strained case. Therefore, in the unstrained case, GaN grows layer-by-layer. On the other hand, in the strained case, multiple layers of GaN grow simultaneously. Furthermore, it is also found that the wurtzite structure of GaN differs between the strained case and the unstrained case. read less

Abstract: In this work, an interatomic potential for the beryllium–tungsten system is derived. It is the final piece of a potential puzzle, now containing all possible interactions between the fusion reactor materials beryllium, tungsten and carbon as well as the plasma hydrogen isotopes. The potential is suitable for plasma–wall interaction simulations and can describe the intermetallic Be2W and Be12W phases. The interaction energy between a Be surface and a W atom, and vice versa, agrees qualitatively with ab initio calculations. The potential can also reasonably describe BexWy molecules with x, y = 1, 2, 3, 4. read less

Abstract: Statistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar+ ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence θi. As θi becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for θi=45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statis... read less

Abstract: Results from molecular dynamics simulations of continuous 50–200 eV Ar+ bombardment on wurtzite and zinc blende GaN surfaces are reported. A new analytical bond-order potential, originally developed for growth process studies, is used to investigate the low-energy physical sputtering of GaN compounds. Preferential sputtering of N atoms is initially observed up to 3.5×1015 ions/cm2 fluence, after which the layers reach steady state sputtering. The crystalline structure of the GaN sample does not have a major influence on the sputtering yield due to the rapid amorphization of the top surface after a few hundred impacts. Concentration depth profiles indicate a surface enrichment in gallium with a N/Ga concentration ratio equal to 0.59±0.1 for 100 eV bombardment, in agreement with published experimental studies. For the same conditions, Ga, N, and GaN species represent 25, 60, and 7% of the sputtered products. A significant fraction of those products leave the surface with kinetic energies sufficiently high t... read less

Abstract: Recent molecular dynamics simulation methods have enabled thermal conductivity of bulk materials to be estimated. In these simulations, periodic boundary conditions are used to extend the system dimensions to the thermodynamic limit. Such a strategy cannot be used for nanostructures with finite dimensions which are typically much larger than it is possible to simulate directly. To bridge the length scales between the simulated and the actual nanostructures, we perform large-scale molecular dynamics calculations of thermal conductivities at different system dimensions to examine a recently developed conductivity vs dimension scaling theory for both film and wire configurations. We demonstrate that by an appropriate application of the scaling law, reliable interpolations can be used to accurately predict thermal conductivity of films and wires as a function of film thickness or wire radius at realistic length scales from molecular dynamics simulations. We apply this method to predict thermal conductivities for GaN wurtzite nanostructures. read less

Abstract: A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t... read less

Abstract: We present an analytical interatomic potential for gallium nitride which is based on a new environment-dependent dynamic charge-transfer model. The model consists of a short-ranged bond-order potential that accounts for covalent/metallic interactions and an ionic Coulomb potential with effective point charges that are dynamically adjusted. In contrast to established models, these point charges are distance-dependent and vary with the number and type of nearest neighbour atoms. The basic concepts stem from the idea of bond charges. We assume pairwise symmetric charge transfer between atoms of different type forming a bond. Charge contributions of all bonds to an atomic site are weighted and added, yielding the effective charge per atom. Mulliken charges, as obtained from density-functional theory calculations within the local-density approximation, are used for adjusting the parameters and functional form of the potential. The short-range contributions are chosen as angular-dependent many-body bond-order potentials, which can be understood as an extension of a Finnis–Sinclair type potential. read less

Abstract: Analytical bond-order potentials for beryllium, beryllium carbide and beryllium hydride are presented. The reactive nature of the formalism makes the potentials suitable for simulations of non-equilibrium processes such as plasma–wall interactions in fusion reactors. The Be and Be–C potentials were fitted to ab initio calculations as well as to experimental data of several different atomic configurations and Be–H molecule and defect data were used in determining the Be–H parameter set. Among other tests, sputtering, melting and quenching simulations were performed in order to check the transferability of the potentials. The antifluorite Be2C structure is well described by the Be–C potential and the hydrocarbon interactions are modelled by the established Brenner potentials. read less

Abstract: An empirical bond‐order many body interatomic Tersoff potential is used for atomistic calculations of the multiple atomic configurations (5/7, 8 and 4) of the a‐edge threading dislocations in III(Al,Ga,In)‐N compound semiconductors. Structural‐ and energy‐related conclusions are drawn which are attributed to the complexity of the III–III metal type and N–N interactions (bondGa–Ga < bondAl–Al < bondIn–In) in connection with the difference of the lattice parameters (aAlN < aGaN < aInN) and the elastic constants. The 5/7‐atomic core configuration is calculated as the most energetically and structurally favourable in all the three compounds. Taking the 5/7‐atom model as a reference, the 8‐atom core model becomes the next favourable one when the lattice parameter increases (aInN) while the 4‐atom core model is the second energetically favourable when the lattice parameter decreases (aAlN). read less

Abstract: Atomistic simulations are used in combination with the two potential energy functions, namely, the Valence Force Field (VFF) model and the Tersoff model, to study the solution thermodynamics of In x Ga1−x As alloy. The simulation data, in the form of a T − x diagram, is contrasted with the results obtained by using the Ho and Stringfellow approach. It is observed that for the VFF model, the upper critical solution temperature obtained from simulation data is approximately 850 K, which is higher than the 729 K predicted by the Ho and Stringfellow treatment. The composition range for which the two-phase heterogeneous region exists is wider than that predicted by the Ho and Stringfellow approach. The Tersoff model predicts a complex miscibility diagram, where the 850 K temperature corresponds to the approximate ‘eutectic’ temperature. Further improvement of model predictions may be made possible by investigation of temperature and composition dependent interaction parameter in a modified regular solution theory, and investigation of non-random, non-ideal solution models in the Ho and Stringfellow treatment, development of temperature dependent VFF model parameters and adjustment of Tersoff model parameters to account for longer range interactions which exist at temperatures above 850 K. The miscibility diagram constructed using the Tersoff model simulation data can be used to provide information on the phase stability and equilibrium Indium content at any given temperature for the crystalline solid solution. read less

Abstract: Modified embedded-atom method (MEAM) interatomic potentials for the Ga–N and In–N binary and Ga–In–N ternary systems have been developed based on the previously developed potentials for Ga, In and N. The potentials can describe various physical properties (structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN and InN as well as those of constituent elements, in good agreement with experimental data or high-level calculations. The potential can also describe the structural behavior of Ga1−xInxN ternary nitrides reasonably well. The applicability of the potentials to atomistic investigations of atomic/nanoscale structural evolution in Ga1−xInxN multi-component nitrides during the deposition of constituent element atoms is discussed. read less

Abstract: Indium phosphide is investigated using molecular dynamics (MD) simulations and density-functional theory calculations. MD simulations use a proposed effective interaction potential for InP fitted to a selected experimental dataset of properties. The potential consists of two- and three-body terms that represent atomic-size effects, charge–charge, charge–dipole and dipole–dipole interactions as well as covalent bond bending and stretching. Predictions are made for the elastic constants as a function of density and temperature, the generalized stacking fault energy and the low-index surface energies. read less

Abstract: Molecular dynamics simulations have been performed using a Reax force field for C/H/Ni systems to study the structural changes of an Ni_(100) cluster adsorbed on a carbon platelet. Three different edges of a carbon platelet are considered. We find a complete restructuring of the initial structure of the Ni_(100) clusters adsorbed on the armchair and zigzag edges. Nonetheless, the mean Ni−Ni bond length hardly changes. Several preferential sites on each of the graphite edges are identified. Diffusion of the entire cluster is found both for adsorption on the basal plane and for binding to a hydrogen terminated graphite edge. read less

Abstract: III–N compound semiconductors are nowadays widely used in electronic device technology. Due to the complexity of their structures planar and linear defects may have various atomic configurations. Since in the wurtzite structure of AlN and InN the second‐neighbor distance is very close to the stable “metallic” Al–Al and In–In distances respectively, a III‐species environment approach based on a Tersoff empirical bond order interatomic potential is developed in which the cut‐off distance for Al–Al and In–In interactions is tuned. In particular, the work is focused on two issues: the development of an approach for the calculation of defected structures in III‐nitrides and the application of this method on a series of planar defects in wurtzite structure. Various structural and energy‐related conclusions are drawn that are attributed to the complexity of the III–III metal type and N–N interactions in connection with the difference of the lattice parameters and the elastic constants. Molecular dynamic simulations are led to the conclusion that structural transformations may also occur. The Austerman–Gehman and Holt models for the inversion domain boundary (IDB) on the (10$ \bar 1 $0) plane are higher in energy than the IDB* model of Northrup, Neugebauer, and Romano. The model of Blank et al. for the translation domain boundary (TDB) on the {1$ \bar 2 $10} plane is unstable with respect to Drum's model. The Austerman model for the IDB on the {1$ \bar 2 $10} plane is unstable with respect to the IDB* model appropriate for this plane. The Austerman {10$ \bar 1 $0} IDB model is recognized as a strong candidate, among the IDB atomic configurations. Moreover, models for IDBs on {10$ \bar 1 $0} planes in which the boundary plane intersects two bonds (type‐2 models) are less stable than models in which the boundary plane intersects one bond (type‐1 models), in all cases considered. It is confirmed that the III‐species environment approach describes the “wrong”‐bonded defect local configuration structures more realistically with respect to the standard approach. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Molecular dynamics simulations have been used to study the melting of GaN nanowires with triangular cross sections. The variation in potential energy as a function of the cross-sectional area of GaN nanowires, along with the atomic configuration, is used to monitor the phase transition. The thermal stability of GaN nanowires is strongly size dependent. The melting temperature of the GaN nanowires increases with increasing cross-sectional area to a saturation value. Melting of the nanowires is initiated at the surface edges formed by the triangular shape and then spreads across the nanowire surface. As temperature increases, the melting expands into the inner regions of the nanowires. read less

Abstract: Ion irradiation causes damage in semiconductor crystal structures and affects charge carrier dynamics. We have studied the damage production by high-energy (100keV–10MeV) H, He, Ne, and Ni ions in GaAs and GaAs90N10 using molecular dynamics computer simulations. We find that the heavier Ne and Ni ions produce a larger fraction of damage in large clusters than H and He. These large clusters are either in the form of amorphous zones or (after room-temperature aging or high-temperature annealing) in the form of vacancy and antisite clusters. The total damage production in GaAs and GaAs90N10 is found to be practically the same for all the ions. A clearly smaller fraction of the damage in GaAs90N10 compared to GaAs is in large clusters, however. Our results indicate that experimentally observed differences in charge carrier lifetimes between light and heavy ion irradiations, and before and after annealing, can be understood in terms of the large defect clusters. An increasing amount of damage in large clusters... read less

Abstract: An interatomic potential for zinc oxide and its elemental constituents is derived based on an analytical bond-order formalism. The model potential provides a good description of the bulk properties of various solid structures of zinc oxide including cohesive energies, lattice parameters, and elastic constants. For the pure elements zinc and oxygen the energetics and structural parameters of a variety of bulk phases and in the case of oxygen also molecular structures are reproduced. The dependence of thermal and point defect properties on the cutoff parameters is discussed. As exemplary applications the irradiation of bulk zinc oxide and the elastic response of individual nanorods are studied. read less

Abstract: A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W–C–H. The model combines Brenner’s hydrocarbon potential with parameter sets for W–W, W–C, and W–H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W–H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides. © 2005 American Institute of Physics. DOI: 10.1063/1.2149492 read less

Abstract: Exchange Monte Carlo calculations in the semi-grand canonical ensemble are used to determine the mixing properties of GaN and InN in both the wurtzite and zinc blende structures. Inter-atomic potentials are obtained via empirical fitting to the experimental, bulk properties of the end member materials. The difference in structure is reflected in the variation of the enthalpy of mixing with composition and phase diagrams for the hexagonal and cubic phases. The calculated consolute temperature is ≈1725 K, in line with previous calculations. The calculated phase diagrams for the two structures are markedly asymmetric with the maximum in the binodals lying markedly on the Ga rich side. Our results are compared with available experimental data. read less

Abstract: We present an analytical bond-order potential for silicon, carbon, and silicon carbide that has been optimized by a systematic fitting scheme. The functional form is adopted from a preceding work {\}Phys. Rev. B 65, 195124 (2002) and is built on three independently fitted potentials for Si-Si, C-C, and Si-C interaction. For elemental silicon and carbon, the potential perfectly reproduces elastic properties and agrees very well with first-principles results for high-pressure phases. The formation enthalpies of point defects are reasonably reproduced. In the case of silicon stuctural features of the melt agree nicely with data taken from literature. For silicon carbide the dimer as well as the solid phases B1, B2, and B3 were considered. Again, elastic properties are very well reproduced including internal relaxations under shear. Comparison with first-principles data on point defect formation enthalpies shows fair agreement. The successful validation of the potentials for configurations ranging from the molecular to the bulk regime indicates the transferability of the potential model and makes it a good choice for atomistic simulations that sample a large configuration space. read less

Abstract: Microscopic dislocation structures in wurtzite-type GaN crystal have been studied by the use of molecular dynamics simulation. Parameters for a two-body interatomic potential are determined by the Hartree-Fock ab initio method. A dislocation is generated by the coalescence of the facing planes of a semi-infinite trench structure in the crystal. Six types of trench structures with different depth-directions and extension-directions are examined. Temperatures of 1000 and 1500K are considered. A core structure with an eightfold ring has been confirmed for edge dislocations along the c axis, though there appear a few atoms that are shifted from the ring core structure. The ring core structure is consistent with reported theoretical expectations and experimental observations. A tenfold ring core structure is also observed for edge dislocations along the c axis. A screw dislocation is generated by an attracting force between gallium and nitrogen atoms across the trench space when the attracting force has a larg... read less

Abstract: A two-phase molecular dynamics simulation of coexisting solid and liquid has been carried out to investigate the melting point of wurtzite-type GaN crystals. The melting point is determined by examining the movement of the interface between the solid and liquid during the simulation. The potential is a two-body interatomic one composed of the long-range Coulomb interaction, the Gilbert-type short-range repulsion, the covalent bonding and covalent repulsion of the modified Morse type, and the van der Waals interaction. The melting point and the interface morphology depend on the crystallization direction. The melting point Tm(K) increases with pressure P(GPa), but there appears a discontinuity in the vicinity of 8–9GPa. This is due to the solid-electrolyte-like behavior of Ga atoms with a partial charge in the high-pressure region. The discontinuity has not yet been confirmed by experiment. The least-squares fitted result is Tm=2538+177P−4.62P2 at pressures lower than 8GPa and Tm=2825+210P−5P2 at pressures... read less

Abstract: This study examines an effect of pressure up to 50 GPa on the elastic and mechanical properties of wurtzite gallium nitride (w-GaN) by using classical potential within the Atomistic Tool Kit (ATK)-force field. The obtained results show that the elastic constants and other related parameters, such as Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, Pugh’s ratio, Zener anisotropy factor and Kleinman parameter increase monotonically with increase of pressure up to 32 GPa. Beyond this pressure, we observed a non-linear behavior with increase in pressure. This might be attributed to the phase transition in GaN in the pressure range of 33.4 44.6 GPa. The results obtained for zero pressure are consistent with both experimental data and the theoretical data shown in references. read less