Ab initio study of intrinsic point defects in germanium sulfide

The chemical aspects of defects of relevance to inorganic crystalline solids are described, especially the close links with the composition of crystalline nonmetallic inorganic nonstoichiometric compounds. The discussion begins with a description of simple point defects in stoichiometric materials and the relationship between the population of these defects and the Gibbs energy of formation of the defects. This includes defects in monatomic structures and balanced populations of point defects, Frenkel and Schottky defects, in binary compounds. Superionic compounds that contain quasi‐liquid cation arrays are considered as extreme cases of these simpler defect populations. Nonstoichiometric compounds treated from the standpoint of small populations of point defect populations are considered, including color centers. Defect chemistry, the incorporation of point defects into chemical nomenclature, is described, including the Kroger–Vink notation and the construction of Brouwer diagrams. Grossly nonstoichiometric compounds, including those that can be considered to contain point defects or structurally organised defect clusters, such as the iron oxide wüstite, and the defect fluorite structures are illustrated. Extended defects portray a wider view of defect chemistry, in which planar faults and intergrowths are prominent structural features of many complex inorganic solids. To illustrate planar defects, examples are taken from crystallographic shear phases, twinned structures, intergrowth structures, and the niobium oxide block structures. Structures containing pentagonal columns linked by vertices provide an example of structures that contain columnar defects. Examples of inorganic solids that possess more than one defect type are taken from layered perovskite phases, tungsten bronzes, and high‐temperature ceramic superconductors. The final section is concerned with structures that were once described as containing defects but that have recently been found to be essentially defect free. These materials, in which defects are eliminated by the device of continuous structural modulation affecting one part of the structure, are widespread over many classes of inorganic solids. Examples drawn from oxides related to tantalum pentoxide, layer misfit phases, and anion‐excess fluorite‐related materials.

AlGaN nanowires are promising for deep ultraviolet optoelectronic applications, but their performance is significantly affected by defects, and the mechanism of defect regulation remains unclear. In this work, the effect of vacancy and interstitial defects on the stability, structural and electronic properties of AlGaN nanowires are systematically investigated via first-principles. Our results demonstrate that the N vacancy exhibits the lowest formation energy of 3.68 eV, indicating that it is the most thermodynamically favorable defect type in AlGaN nanowires. Besides, as the defect position migrates from the center to the surface layer of nanowires, the stability of N vacancy gradually decreases (formation energy increases from 3.68 eV to 4.56 eV), while that of Ga vacancy progressively enhances (formation energy decreases from 8.06 eV to 7.88 eV). In addition, the introduction of defects induces significant relaxation in the atomic structure, with bond length variations up to 4.35% ∼ 7.50% for Ga-Ga around N vacancy and 9.92% ∼ 13.95% for N-N around Ga vacancy. Despite the incorporation of point defects, AlGaN nanowires preserve the direct bandgap characteristic. Moreover, Al vacancy, Ga vacancy and N interstitial introduce shallow acceptor levels at 0.12 ∼ 0.37 eV above the valence band maximum, while N vacancy, Ga interstitial and Al interstitial induce deep donor levels at 0.42 ∼ 1.58 eV below the conduction band minimum. Furthermore, Mulliken charge calculations reveal that atomic charge alterations induced by defects mainly occur within the neighboring coordination shell, verifying the localized nature of defect effects.

Simulation of point defect diffusion in germanium

Lead halide perovskites (LHPs) have attracted considerable attention as promising materials for photovoltaic and optoelectronic applications. Intrinsic point defects play an important role in determining the performance of semiconductor devices. LHPs exhibit strong ionic character and unique electronic structure; thus, their defect properties are quite different from conventional covalent bond semiconductors. Understanding the defect science is crucial to the performance optimization of LHP-based devices. State-of-the-art first-principles calculation methods enable one to explore atomistic mechanisms of various defect-related processes, and tremendous efforts from theoretical simulations have provided invaluable insights to the defect physics and defect control of LHPs. In this review, we summarize recent progress, made with the help of theoretical modeling, on atomic-scale understanding about intrinsic point defects and related processes in LHPs. The fundamental properties of intrinsic point defects in LHPs are first introduced, including defect formation energy, charge transition level, and defect tolerance and its origin. A particular emphasis is given to the effects of band edge position on calculated defect properties. The impact of these defects on structural properties, carrier dynamics, and photoluminescence of LHPs is then presented. Advanced strategies to engineer the defects in LHPs are also reviewed, such as growth condition, defect passivation, and doping. Finally, we discuss open issues and outline directions toward a better understanding of defects of LHPs from a theoretical perspective. The goal of the review is to provide a comprehensive summary of atomic-scale understanding of intrinsic point defects in LHPs and to help further related research in the perovskite community.

Hexagonal boron nitride (hBN) is a wide gap 2D layered material with good insulating properties. Intrinsic point defects in hBN play an important role in its applications as a dielectric in 2D electronic devices. However, the electronic properties of these defects are still poorly understood. We have calculated the structure and properties of a wide range of intrinsic point defects in the bulk of hBN using hybrid density functional theory (DFT). These include vacancies and interstitial states of B and N as well as di- and tri-vacancies. For each isolated defect, multiple charge states are calculated, and for each charge state multiple spin states are investigated. Positions of defect charge transition levels in the band gap of hBN are calculated. In particular, we predict that B vacancies are likely to be negatively charged in contact with graphene and other metals. Calculations of the interaction between vacancies predict that divacancies in both B and N sublattices are strongly binding. Moreover, the interaction of single B and N vacancies in adjacent layers induces the creation of -N–N- and -B–B- molecular bridges, which greatly distort the local structure, leading to local bond weakening. These results provide further insight into the properties of defects which can be responsible for degradation of hBN based devices.

Using first-principles methods we have calculated electronic structures, optical properties, and hole conductivities of CuXO2 (X=Y, Sc, and Al). We show that the direct optical band gaps of CuYO2 and CuScO2 are approximately equal to their fundamental band gaps and the conduction bands of them are localized. The direct optical band gaps of CuXO2 (X=Y, Sc, and Al) are 3.3, 3.6, and 3.2 eV, respectively, which are consistent with experimental values of 3.5, 3.7, and 3.5 eV. We find that the hole mobility along long lattice c is higher than that along other directions through calculating effective masses of the three oxides. By analyzing band offset we find that CuScO2 has the highest valence band maximum (VBM) among CuXO2 (X=Y, Sc, and Al). In addition, the approximate transitivity of band offset suggests that CuScO2 has a higher VBM than CuGaO2 and CuInO2 [Phys. Rev. Lett. 88, 066405 (2002)]. We conclude that CuScO2 has a higher p-type doping ability in terms of the doping limit rule.

We have examined a range of point defects, Frenkel pairs, Schottky defects, and hydrogen-related defects in the ${\mathrm{PuO}}_{2}$ system (supercells of 96 and 768 atoms) using the onetep linear-scaling density functional theory code. Vacancy point defects related to oxygen are found to be more stable than those related to plutonium. The oxygen in the octahedral interstitial is higher in the formation energy than the plutonium in the same octahedral site, although the difference is less than 1 eV. We were also able to identify a stable peroxide species (1.57--2.67 eV) with a O-O distance of 1.46 \AA{}. Of the Frenkel defects we studied, we found that the oxygen is more stable than the plutonium, whereas the Schottky stability changes as a function of supercell size. Finally, we examined a number of likely hydrogen sites in the ${\mathrm{PuO}}_{2}$ lattice: octahedral interstitial, oxygen edge, hydroxyl, oxygen vacancy, and plutonium vacancy. We report hydrogen which exists as a hydride at oxygen and plutonium vacancies to be relatively high in energy (2.69--3.81 and 13.71--15.54 eV, respectively). The hydrogen was found to exist as a radical at the octahedral interstitial site (2.43--3.38 eV) and which is somewhat higher formation energy than other studies find. We find that the hydrogen at the oxygen edge (as a ${\mathrm{H}}^{+}$ cation) and at the oxygen cube corner (as a hydroxyl) are both lower in energy (1.14--1.40 and 1.17--1.56 eV, respectively) as opposed to hydrogen in the octahedral interstitial site but again higher than found by other studies. We discuss the data in the context of potential hydrogen transport pathways and how that might be modified by radiation damage.

Introduction One final category of extended defect remains, although it is not generally so described. This consists of undesired, non-uniform distributions of native point defects, impurities and alloy composition variations. These point defect maldistributions render the initial material properties non-uniform and interfere with the controlled introduction of the variations required for devices. Semiconductor processing starts with material that is uniformly sufficiently pure and perfect to exhibit the intrinsic properties of the semiconductor. Controlled concentrations of selected impurities are then introduced into chosen volumes to achieve the desired extrinsic properties. These include, for example, p - or n -type conductivity of the necessary value or luminescent emission of a certain wavelength and efficiency. For this, the impurity must occur as a uniform, random distribution of single, isolated impurity atoms of the desired element on substitutional sites in the required concentration. This chapter is concerned with crystal growth phenomena affecting point defect distributions and so materials uniformity and, therefore, capable of leading to failure to achieve successful device fabrication. Because of their importance and relative simplicity, point defects have been studied intensively throughout the history of semiconductor physics and chemistry. The properties of point defects are therefore well treated in many review articles (Queisser and Haller 1998) and books such as Stoneham (2000) as well as series of conferences. We shall, therefore, give only the necessary minimum background on a number of points required for the present purpose.

In ionic crystals, jogs on surfaces and dislocations act as sources and sinks for point defects. Establishment of equilibria with the vacancies, solute ions, and interstitials in the interior produces a net charge on the surface or dislocation, surrounded by a compensating space charge region. Within this space charge region, which typically has a thickness of the order of 10nm, there are very large electric fields (up to about 105 V/cm) and very large perturbations of point defect concentrations (by factors of several hundred). Thus, ion diffusivities can be dramatically perturbed. In addition, determination of the electric potential difference across the space charge region allows one to separate the formation enthalpies and entropies of Frenkel and Schottky defects into contributions from each component of the pair. Examples of such studies in the silver and alkali halides will be described.

Crystal defects play an important role in the material strength and its mechanical properties. The Zr-1%Nb alloy, because of its low cross-section for thermal-neutron capture, corrosion resistance in water and suitable mechanical properties, is widely used in nuclear reactors. This alloy has an HCP structure at low temperatures and for low concentrations of Nb impurity. In this work, using the first-principles density-functional theory calculations, as well as molecular dynamics calculations with interatomic potentials, we have investigated the properties of vacancy and self-interstitial point defects in pure zirconium. The formation energy and formation volume are calculated; the results show a good agreement with the experimental values. These quantities are calculated for the Zr-1%Nb alloy as well; the results do not show any significant differences with those of the pure Zr. In addition, the interaction between two vacancies is investigated; by the calculation of the binding energies for di-vacancy clusters in different configurations, it is shown that only those clusters are stable for which the vacancies are in the first neighbor positions. Finally, the displacement energy of a vacancy in the basal plane is calculated, showing a good agreement with experiment.

Chapter VI - Thermodynamics of Defects

In the fabrication and processing of silicene monolayers, structural defects are almost inevitable. Using ab initio calculations, we systemically investigated the structures, formation energies, migration behaviors and electronic/magnetic properties of typical point defects in silicene, including the Stone-Wales (SW) defect, single and double vacancies (SVs and DVs), and adatoms. We found that SW can be effectively recovered by thermal annealing. SVs have much higher mobility than DVs and two SVs are very likely to coalesce into one DV to lower the energy. Existence of SW and DVs may induce small gaps in silicene, while the SV defect may transform semimetallic silicene into metallic. Adatoms are unexpectedly stable and can affect the electronic properties of silicene dramatically. Especially, Si adatoms as self-dopants in silicene sheets can induce long-range spin polarization as well as a remarkable band gap, thus achieving an all-silicon magnetic semiconductor. The present theoretical results provide valuable insights into identification of these defects in experiments and understanding their effects on the physical properties of silicene.

General schema for [0 0 1] tilt grain boundaries in dense packing cubic crystals

Defects are inevitably present in materials and always can affect their properties. Here, first-principles calculations are performed to systematically study the stability, structural and electronic properties of ten kinds of point defects in semiconducting phosphorene, including the Stone-Wales (SW-1 and SW-2) defect, single (SV59 and SV5566) and double vacancy (DV585-1, DV585-2, DV555777-1, DV555777-2, DV555777-3 and DV4104) defects. We find that these defects are all much easily created in phosphorene with higher areal density compared with graphene and silicene. They are easy distinguish each other and correlate with their defective atomic structures with simulated scanning tunneling microscopy images at positive bias. The SW, DV585-1, DV555777 and DV4104 defects have little effect on phosphorene's electronic properties and defective phosphorene monolayers still show semiconducting with similar band gap values to perfect phosphorene. The SV59 and DV585-2 defects can introduce unoccupied localized states into phosphorene's fundamental band gap. Specifically, the SV59 and 5566 defects can induce hole doping in phosphorene, and only the stable SV59 defect can result in local magnetic moments in phosphorene different from all other defects.

We have investigated the molecular beam epitaxial growth and characterization of InN nanowires. Detailed optical and electrical transport studies confirm that nondoped InN nanowires can exhibit extremely low (< 10¹⁵ cm^-3) residual electron density. Furthermore, the near-surface Femi-level was measured to be 0.4 to 0.5 eV above the valence band maximum (VBM), suggesting the absence of Fermi-level pinning and surface electron accumulation. These features are fundamentally different from those of n-type degenerate InN nanowires or InN epilayers. The absence of surface electron accumulation was also observed in Mg-doped InN nanowires, where p-type conduction was directly measured via Mg-doped InN nanowire field-effect transistors. Furthermore, the near-surface Fermi-level can be tuned from 0.1 eV to 1 eV above the VBM, i.e., from p-type degenerate to n-type degenerate through controlled Mg and Si dopant incorporations, a first demonstration for any semiconducting nanowire structures.