Structure Of Point Defects Research Articles

First-principles calculations of iodine-related point defects in CsPbI3.

We present here first principles hybrid functional calculations of the atomic and electronic structure of several iodine-related point defects in CsPbI3, a material relevant for photovoltaic applications. We show that the presence of neutral interstitial I atoms or electron holes leads to the formation of di-halide dumbbells of I2- (analogous to the well-known situation in alkali halides). Their formation and one-electron energies in the band gap are determined. The formation energy of the Frenkel defect pair (I vacancies and neutral interstitial I atoms) is found to be ∼1 eV, and as such is smaller than the band gap. We conclude that both iodine dumbbells and iodine vacancies could be, in principle, easily produced by interband optical excitation.

Energetics and electronic structure of native point defects in antiferromagnetic CrN

We present a detailed analysis of the role of native point defects in the antiferromagnetic (AFM) phases of bulk chromium nitride (CrN). We perform first-principles calculations by using the local spin-density approximation, including local interaction effects $(\text{LSDA}+U)$, to study the two lowest energy AFM models expected to describe the low-temperature phase of the material. We study the formation energies, lattice deformations, and electronic and magnetic structure introduced by native point defects. We find that, as expected, nitrogen vacancies are the most likely defect present in the material at low temperatures. Nitrogen vacancies present different charged states in the cubic AFM model, exhibiting two transition energies, which could be measured by thermometry experiments and could help identify the AFM structure in a sample. These vacancies also result in partial spin polarization of the induced impurity band, which would have interesting consequences in experiments. Other point defects have also signature electronic and magnetic structure that could be identified in scanning probe experiments.

First-principles study of CN point defects on sidewall surface of [0 0 0 1]-oriented GaN nanowires

By using the first-principles, the electronic structure and optical properties of CN point defects on sidewall surface of [0 0 0 1]-oriented GaN nanowires were calculated. We found that CN point defects are more likely to be enriched on the surface of the GaN NWs. In addition, the closer the CN point defect to the sidewall of the GaN NWs, the greater the effects of CN point defect to the top of valence band. The CN point defects in different depths may lead to the yellow luminescence, and the red shift of PL spectral wavelength would occur when it tends to the surface. This may be one of the reasons for the broadening of the yellow luminescence.

Point defect structure of La-doped SrTiO3 ceramics with colossal permittivity

Sr1-xLaxTiO3 (SLTO) ceramics with colossal permittivity were fabricated by conventional solid-state reaction method. The point defects of pure STO and SLTO ceramics were analyzed by Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening (CDB). The charge compensation mechanisms and dielectric properties of ceramics were investigated. The results indicated that the intrinsic defects in pure STO ceramics were mainly VTi″″. The charge compensation mechanism of SLTO ceramics was predominantly formation of VSr″. With increasing La content, εr of SLTO ceramics increased up to ∼70000 at room temperature. The results of first-principle calculations indicated that the colossal permittivity came from a sharp polarization increase caused by dipole structure of defects. tanδ of SLTO ceramics showed obvious Debye relaxation at high temperatures and the relaxation showed a multiple relaxation times derived from different kinds of polarization mechanism. The main polarization mechanism of SLTO ceramics gradually changed from ion displacement polarization to defect dipole polarization influenced by the concentration of La dopants.

Effects of Point Defects on Properties of B2 NiAl: A First-principles Study

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal were analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B0 and Cauchy pressure parameters C12-C44 values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.

Backward-Diode Heterostructure Based on a Zinc-Oxide Nanoarray Formed by Pulsed Electrodeposition and a Cooper-Iodide Film Grown by the SILAR Method

A heterostructure promising for designing a backward diode is formed from a zinc-oxide nanorod array and a nanostructured copper-iodide film. The effect of modes of successive ionic layer adsorption and reaction (SILAR) deposition and the subsequent iodization of CuI films on smooth glass, mica, and fluorine-doped tin oxide (FTO) substrates and on the surface of electrodeposited nanostructured zinc-oxide arrays on the film structure and electrical and optical properties is investigated. A connection between the observed variations in the structure and properties of this material and intrinsic and iodination-induced point defects is established. It is found that the cause and condition for creating a backward-diode heterostructure based on a zinc-oxide nanoarray formed by pulsed electrodeposition and a copper-iodide film grown by the SILAR method is the formation of a p+-CuI degenerate semiconductor by the excessive iodination of layers of this nanostructured material through its developed surface. The n-ZnO/p+-CuI barrier heterostructure, which is fabricated for the first time, has the I–V characteristic of a backward diode, the curvature factor of which (γ = 12 V–1) confirms its high Q factor.

Defect Dynamics in 2-D MoS2 Probed by Using Machine Learning, Atomistic Simulations, and High-Resolution Microscopy

Structural defects govern various physical, chemical, and optoelectronic properties of two-dimensional transition-metal dichalcogenides (TMDs). A fundamental understanding of the spatial distribution and dynamics of defects in these low-dimensional systems is critical for advances in nanotechnology. However, such understanding has remained elusive primarily due to the inaccessibility of (a) necessary time scales via standard atomistic simulations and (b) required spatiotemporal resolution in experiments. Here, we take advantage of supervised machine learning, in situ high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations to overcome these limitations. We combine genetic algorithms (GA) with MD to investigate the extended structure of point defects, their dynamical evolution, and their role in inducing the phase transition between the semiconducting (2H) and metallic (1T) phase in monolayer MoS2. GA-based structural optimization is used to identify the long-range structure of randomly distributed point defects (sulfur vacancies) for various defect densities. Regardless of the density, we find that organization of sulfur vacancies into extended lines is the most energetically favorable. HRTEM validates these findings and suggests a phase transformation from the 2H-to-1T phase that is localized near these extended defects when exposed to high electron beam doses. MD simulations elucidate the molecular mechanism driving the onset of the 2H to 1T transformation and indicate that finite amounts of 1T phase can be retained by increasing the defect concentration and temperature. This work significantly advances the current understanding of defect structure/evolution and structural transitions in 2D TMDs, which is crucial for designing nanoscale devices with desired functionality.

(Invited) Characterization of Phases and Defects in Compound Semiconductors

Defects in semiconductors can be difficult to characterize, especially in complex semiconductors such as the chalcogenides. This talk presents several different and relatively unusual methods for their study along with examples of results. The talk will focus on semiconductors used for photovoltaic applications but the talk should illustrate methods that would be of use with any semiconductor material. Specifically the talk will address the use of admittance spectroscopy, nuclear magnetic resonance (NMR), scanning microwave impedance microscopy (SMIM), scanning tunneling spectroscopy (STS) and others to study defects in CdTe, CuInSe2 (CIGS), and Cu2ZnSnS4 (CZTS), and related compounds. Examples of results include the identification of two defects in the energy gap of CIGS by admittance spectroscopy, band edge fluctuations in CIGS by STS and their impact on the ability to obtain atomic resolution images, organized point defect structures in CZTS by NMR, and grain and grain boundary behaviors in CdTe by SMIM. Other examples will be included as time permits.

Atomic and Electronic Structures of Intrinsic Defects in Ta2O5: Ab Initio Simulation

The atomic and electronic structure of intrinsic point defects in orthorhombic tantalum oxide has been studied by numerical simulation within the density functional theory. It has been shown that all defects responsible for metal enrichment of Ta2O5 serve as electron and hole traps. Under conditions of strong oxygen depletion and at a metal–insulator interface, which are characteristic of resistive memory elements, interstitial tantalum atoms compete with an oxygen vacancy in the formation of a conducting filament. Interstitial oxygen atoms are not involved in charge transport. Tantalum substituting oxygen can be considered as a combination of the oxygen vacancy and interstitial tantalum. The analysis of the calculated thermal and optical energies of trap ionization shows that the oxygen vacancy is a key defect for charge transport in Ta2O5.

Clusters of Point Defects Near Dislocations as a Tool to Control CdZnTe Electrical Parameters by Ultrasound

We studied the temperature dependence (77–300 K) of the electron concentration and mobility using the Hall method under ultrasound (the acoustic Hall method) to determine the mechanisms by which ultrasound influences the electrical activity of near-dislocation clusters in n-type low-ohmic Cd1−xZnxTe single crystals (NCl ≈ 1024 m−3; x = 0; 0.04) with different dislocation density (0.4–5.1) × 1010 m−2. Changes in electrophysical parameters were found to occur as a function of temperature and ultrasound intensity. To evaluate the relative contribution of different charge carrier scattering mechanisms (lattice scattering, ionized impurity scattering, neutral impurity scattering, and dislocation scattering) and their change under ultrasound, a differential evolution method was used. This method made it possible to analyze experimental mobility μH(T) by its nonlinear approximation with characteristic temperature dependence for each mechanism. An increase in neutral impurity scattering and a decrease in ionized impurity and dislocation scattering components were observed under ultrasound. The character and the amount of these acoustically induced changes correlate with particular sample dislocation characteristics. It was concluded that the observed effects are related to the acoustically induced transformation of the point-defect structure, mainly in the near dislocation crystal regions.

Implications of linear correlation between hyperfine parameters in iron oxide nanoparticles

Controlled annealing induced evolution of local structure and magnetic properties on oleic acid coated and bare nanoparticles composed of iron oxy hydroxide and cubic iron oxides were studied using Mössbauer spectroscopy. Mutually opposite variation of quadrupole splitting and magnetic hyperfine field with respect to that of isomer shift has been observed in both the cases of bare and oleic acid coated nanoparticles subjected to annealing treatments. This manifests in an interesting linear correlation between quadrupole splitting and magnetic hyperfine field. Physics of the observed linear correlation between hyperfine parameters has been elucidated based on the direct proportionality of superexchange interaction on the hyperfine contact interaction and a strong dependence of the strength of superexchange interaction on the structural distortion and point defects mainly due to anionic vacancies. Thus the result of this study plausibly points to a strong coupling between local structure in terms of structural distortions and point defects with magnetic properties while the nanoparticles of iron oxides are subjected to coarsening and reduction.

Defect transitions in nematic liquid-crystal capillary bridges.

We use experiment and computational modeling to understand the defect structure and director configuration in a nematic liquid crystal capillary bridge confined between two parallel plates. We find that tuning of the aspect ratio of the bridge drives a transition between a ring defect and a point defect. This transition exhibits hysteresis, due to the metastability of the point-defect structure. In addition, we see that the shape of the capillary-bridge surface determines whether the defect is hyperbolic or radial, with waistlike bridges containing hyperbolic defects and barrel-like bridges containing radial defects.

KH2PO4 single crystals activated with the Ti4+ ions in the form of TiO2−x × nH2O nanoparticles: Structural peculiarities, point defects, and dielectric properties

Potassium dihydrogen phosphate KH2PO4 (KDP) single crystals activated with Ti4+ ions in the form of TiO2−x × nH2O nanoparticles in the η-phase, synthesized by the sulfate method using TiOSO4 × хH2SO4 × yH2O, have been first grown by the temperature lowering method and cut from pyramidal (P) and prismatic (Pr) growth sectors. The first-performed neutron powder diffraction investigation of P and Pr samples cut from the KDP:Ti4+ crystal allowed us to reveal vacancies in the K and H sites for both samples, their number being larger in the Pr structure compared to the P one. Taking into account the deficiency of the K and H sites, full occupation of the O site, presence of Ti4+ ions in the structure, and the electroneutrality condition, a partial substitution of (PO4)3− anion by the (SO4)2− one, larger for the Pr sample, was observed. The real compositions of P and Pr samples, correlated with the cation-anion internuclear distances, were refined. The dielectric permittivity of the Pr sample was significantly lower than that of the P one; it decreases with decreasing К–O, P–O, and O···H distances and increasing deficiency of the K and H sites.

Fine representation of dielectric properties by impedance spectroscopy

Dielectric spectroscopy is used as a quick, precise and nondestructive examination tool of macroscopical dielectric properties for dielectrics. However, if the information of the microstructure and the location of a certain dielectric relaxation can be learned also by dielectric spectroscopy, it will help to reveal point defect structure and macroscopic and microscopic mechanisms of conduction and dielectric relaxation. Fine representation of dielectric properties by impedance spectroscopy (IS) is analyzed carefully in this paper. It is found that IS is not a good tool to describe a uniform system because a pseudo relaxation peaks exists at low frequency limit corresponding to direct current (DC) conductivity, and two relaxation peaks appears simultaneously corresponding to only one relaxation process for a high loss system with tanδ>1. However it is very convenient to describe a multiple phase system with IS for that only one Cole-Cole arc with some distortion appears for every phase with several relaxations, therefore it is very easy to distinguish different phases from each other. Especially, the location of a true relaxation process can be deduced by IS without any uncertainly according to pseudo relaxation theory which is built in this paper. Furthermore, when dielectric properties are shown with specific impedance spectroscopy (SIS), the information of the microstructure can be obtained conveniently. Based on these theoretical results above, dielectric properties of CaCu3Ti4O12 (CCTO) ceramics with giant dielectric constant (GDC) are investigated. It is found that GDC comes from pseudo relaxation of grain.

Blocking germanium diffusion inside silicon dioxide using a co-implanted silicon barrier

We investigate the effect of co-implanting a silicon sublayer on the thermal diffusion of germanium ions implanted into SiO2 and the growth of Ge nanocrystals (Ge-ncs). High-resolution imaging obtained by transmission electron microscopy and energy dispersive spectroscopy measurements supported by Monte-Carlo calculations shows that the Si-enriched region acts as a diffusion barrier for Ge atoms. This barrier prevents Ge outgassing during thermal annealing at 1100 °C. Both the localization and the reduced size of Ge-ncs formed within the sample region co-implanted with Si are observed, as well as the nucleation of mixed Ge/Si nanocrystals containing structural point defects and stacking faults. Although it was found that the Si co-implantation affects the crystallinity of the formed Ge-ncs, this technique can be implemented to produce size-selective and depth-ordered nanostructured systems by controlling the spatial distribution of diffusing Ge. We illustrate this feature for Ge-ncs embedded within a single SiO2 monolayer, whose diameters were gradually increased from 1 nm to 5 nm over a depth of 100 nm.

Point defect structure of γ-NaxCoO2

Oxygen nonstoichiometry and electrical conductivity were measured on the system of γ-NaxCoO2, allegedly so far the best p-type thermoelectric oxide, against oxygen activity (aO2) across its widest ever range below aO2=1 for a fixed Na-content x=0.706 at different temperatures in the range of 773–973K, and at a fixed temperature 973K for x=0.664, 0.706 and 0.731, respectively. It has been deduced therefrom that as aO2 decreases, the majority disorder type shifts from [VNa′]≈p to either [VNa′]≈2[CoNa⋅⋅] or [VNa′]≈3[Coi⋯], however, exhibiting a positive deviation from the ideal defect behavior. The latter is attributed to the positive deviation of holes due to their degeneracy. By taking into appropriate account of the activity coefficient of holes in terms of the Fermi-Dirac integral of order 1/2, the nonstoichiometry and electrical conductivity have been precisely and consistently depicted to evaluate the defect-chemical parameters including the effective mass and mobility of holes and the redox equilibrium constant. The phase-stability limit of γ-NaxCoO2 is documented against temperature and Na-content.

Reverse manipulation of intrinsic point defects in ZnO-based varistor ceramics through Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase

Intrinsic point defect structure plays a crucial role in functional ceramics with a grain-grain boundary microstructure. In the present study, a novel method of reversely manipulating intrinsic point defects (oxygen vacancy, Vo and zinc interstitial, Zni) in ZnO-based varistor ceramics is proposed, which makes use of Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase. It is found that Zr-doping not only modifies the grain growth by the formation of secondary Zr-rich phase, but also influences the intrinsic point defect structure via the stabilized βIII-Bi2O3 phase, resulting in reduced Vo density but increased Zni density. The reverse manipulation is unambiguously demonstrated by broadband dielectric spectroscopy and further confirmed by the accelerated ageing experiment. The proposed intrinsic point defect dynamics unveil an important but usually neglected function of the dopant that has little solid solubility in ZnO grains, which opens up a promising way to tailor the material property.

(Invited) Challenges the UWBG Semiconductors AlGaN, Diamond, and Ga2O3 Must Master to Compete with SiC and GaN HPE Devices

Ultra-wide bandgap (UWBG) semiconductor devices have the potential to operate more efficiently than, and could enable applications that cannot be realized with, SiC or GaN devices because the larger bandgap, EG, provides a larger critical electric field, ξC. This enables the semiconductor to be doped more heavily for a given breakdown voltage, VB, so the internal resistance, RON, and therefore heat loss, will be smaller provided that the probability of premature breakdown has not been significantly enhanced, mobility of the carrier has not been substantially degraded, charge carriers from the dopant states can easily access a conducting band at room temperature, and dopant and electrically active impurity and point defect concentrations can be adequately controlled. If these problems can be addressed, then devices used for continuous power applications such as high power electronic inverters can be built to run more efficiently, or RF HEMTs with more output power can be fabricated. In addition, pulsed power devices such as those that could be used for electronic armor, that do not currently exist, could be built. GaN transistors should be able to operate more efficiently than those made from SiC primarily because its 2-dimensional electron gas, 2DEG, created at the AlGaN/GaN interface has a carrier concentration that is 10X larger and an electron mobility that is 50X larger than that of SiC/oxide inverted surface; the bulk mobility is also 20% larger. However, SiC can be controllably doped at ~5x1014 cm-3, whereas background Si, O, and C impurity densities often exceed 1016 cm-3 in GaN. Si and O are shallow donors, and it has been suggested that their sources are the NH3 reactant gas in CVD growth and the NH3 or N2 reactant gas in MBE growth. Given the higher growth temperature, the quartz reactor used in CVD growth is also thought to be a source. C is amphoteric, but for n-type material it is thought to occupy a N site where it is a deep acceptor. The reactant gases containing N are thought to be a source for all types of growth; the SiC platen is thought to be a source for all types of CVD, and the metal reactant in MOCVD growth is yet another source. Si forms a DX center, O forms a deep donor, and C forms a deep acceptor in AlN so it is unlikely that AlN will be an active semiconductor in HPE devices. However, the C background concentration will be a problem for AlGaN with enough Ga so that Si acts as a shallow donor, ~ 20%. Thus, AlGaN has the advantage that it can be “tuned” over a wide range of compositions for this Ga content and above. However, alloy scattering in the 2DEG and the bulk will have to be determined to discover if the increase in the allowed maximum carrier concentration enabled by the larger EG can be more than offset by the reduction in the mobility caused by the alloy scattering. Researchers will also have to deal with the problem of lattice mismatch between the substrate and device structure. The problem is made more challenging by the facts that for structures with low Al content in the bulk layer, will be in tension because the substrate of choice is GaN, and for the high Al content structures, the drift region will have an Al content at least 30% less than the AlN substrate. Diamond is an attractive material because both electrons and holes have a large mobility, its ξC is almost as large as it is for AlN, and it has a larger thermal conductivity. However, both the P donor and B acceptor are very deep. Also, someone will have to identify a dielectric whose band structure properly matches it to create a 2DEG with a reasonable mobility. An alternative approach could be to fabricate a bipolar junction transistor, which is made more palpable by the fact diamond has an indirect bandgap Also, methods will have to be developed to grow high quality, larger area structures. High quality, large area, relatively inexpensive wafers of β-Ga2O3 are already available, and Si or Sn form shallow donors. However, the facts that it cannot now be doped p-type, the electron mobility of 200 cm2/V·s is quite small, its thermal conductivity is ~100X less than the other UWBGs, and its complex rhombohedral structure could create complicated point defect structures and not be compatible with materials with simpler crystal structures. Currently, the best HPE device option appears to me a MESFET.

High-quality monolayer superconductor NbSe2 grown by chemical vapour deposition

The discovery of monolayer superconductors bears consequences for both fundamental physics and device applications. Currently, the growth of superconducting monolayers can only occur under ultrahigh vacuum and on specific lattice-matched or dangling bond-free substrates, to minimize environment- and substrate-induced disorders/defects. Such severe growth requirements limit the exploration of novel two-dimensional superconductivity and related nanodevices. Here we demonstrate the experimental realization of superconductivity in a chemical vapour deposition grown monolayer material—NbSe2. Atomic-resolution scanning transmission electron microscope imaging reveals the atomic structure of the intrinsic point defects and grain boundaries in monolayer NbSe2, and confirms the low defect concentration in our high-quality film, which is the key to two-dimensional superconductivity. By using monolayer chemical vapour deposited graphene as a protective capping layer, thickness-dependent superconducting properties are observed in as-grown NbSe2 with a transition temperature increasing from 1.0 K in monolayer to 4.56 K in 10-layer.

RETRACTED: First-Principles Study of Advanced Nuclear Materials: Defect Behavior and Fission Products in U-Si System

Uranium silicides are envisaged as potential nuclear materials for the next generation. U3Si is featured by the high actinide density and the better thermal conductivity relative to UO2. To properly and safely utilize nuclear materials, it is crucial to understand their chemical and physical properties. First-principles in theory is mostly used to analyze the point defect structures for uranium silicides nuclear fuels. The lattice parameters of U3Si and USi2 are calculated and the stability of different types of point defects are predicted by their formation energies. The results show that silicon vacancies are more prone to be produced than uranium vacancies in β-USi2 matrix. The most favorable sites of fission products are determined in this work as well. According to the current data, rare earth elements cerium and neodymium are found to be more stable than alkaline earth metals strontium and barium in a given nuclear matrix. It is also determined that in USi2 crystal lattice fission products tend to be stabilized in uranium substitution sites, while they are likely to form precipitates from the U3Si matrix. It is expected that this work may provide new insight into the mechanism for structural evolutions of silicide nuclear materials in a reactor as well as to provide valuable clues for fuel designers.