Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

The study of the optical properties of the GaAs with point defects

GaAs is a common material for negative electron affinity photoelectric cathodes. It has a widely application in night vision. It is unavoidable to produce point defects in the process of the GaAs growth. In this article, the first principle method is used to calculate the formation energy and the optical properties of six types familiar point defects in GaAs. Three types of point defects (Ga vacancy defect, As antisite defect and As interstitial defect) are easier to be formed in these six types according to the formation energy calculation. In fact, crystal GaAs is grown in an As-rich environment. The Ga vacancy, As antisite and As interstitial defects are easier to be formed in reality. Hence the theoretical calculation results are in keeping with the experiments. In this article, the optical properties of these three types are compared with that of perfect GaAs. The electron structures are changed as a result of the point defects. Some of the defect levels enter into the band gap and the electron transitions become easier. It makes the optical spectra move to the low-energy region. Additionally, GaAs with Ga vacancy defect and that with As interstitial defect shows similar optical properties. Therefore both of the two types of defects can be considered as the same in the later experiments. The theoretical results in this article provide the basis for experimental study for photoelectric emission of the real GaAs material related to the optical properties and electronic structures.     

Comparison of vacancy and antisite defects in GaAs and InGaAs through hybrid functionals

The formation energies and charge transition levels of vacancy and antisite defects in GaAs and In(0.5)Ga(0.5)As are calculated through hybrid density functionals. In As-rich conditions, the As antisite is the most stable defect in both GaAs and InGaAs, except for n-type GaAs for which the Ga vacancy is favored. The Ga antisite shows the lowest formation energy in Ga-rich conditions. The As antisite provides a consistent interpretation of the defect densities measured at mid-gap for both GaAs/oxide and InGaAs/oxide interfaces.     

First-principle study of native defects in CuScO2 and CuYO2

This paper studies the electronic structure and native defects in transparent conducting oxides CuScO2 and CuYO2 using the first-principle calculations. Some typical native copper-related and oxygen-related defects, such as vacancy, interstitials, and antisites in their relevant charge state are considered. The results of calculation show that, CuMO2（M = Sc, Y） is impossible to show n-type conductivity ability. It finds that copper vacancy and oxygen interstitial have relatively low formation energy and they are the relevant defects in CuScO2 and CuYO2. Copper vacancy is the most efficient acceptor, and under O-rich condition oxygen antisite also becomes important acceptor and plays an important role in p-type conductivity.     

Defect stability in thorium monocarbide:An ab initio study

The elastic properties and point defects of thorium monocarbide(ThC) have been studied by means of density functional theory based on the projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study, including the vacancy defect, interstitial defect, antisite defect, schottky defect, and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect(0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for a carbon dimer diffusion in Th C.     

嵌入原子法研究Ni3Al中点缺陷以及Re择优占位和集团化

基于纯金属元素Ni，Al和Re的基本物理性质，建立了一个Ni-Al-Re三元体系的分析型嵌入原子多体势.结合分子动力学计算了Ni₃Al的平衡晶格常数、弹性模量、结合能、空位形成能以及反位置缺陷形成能，并分析了Ni₃Al中点缺陷的存在形式.计算结果表明，当成分偏离理想化学配比时出现反位置缺陷.同时研究了Re在Ni₃Al中的择优占位以及Re在Ni₃Al和Ni中的集团化行为.计算结果表明，Re在Ni₃Al中优先置换Al的位置，且发现当Re原子团的尺寸接近于11?时，Re原子团的长大趋势变弱.计算结果与实验以及其他的理论计算结果相符合. 关键词： 嵌入原子势 3Al')" href="#">Ni₃Al Re 占位 集团化     

Structural properties of oxygen on InN(0 0 0 1) surface

First-principles calculations are performed to study the various structures of oxygen (O) adsorbed on InN(0 0 0 1) surfaces. It is found that the formation energy of O on InN(0 0 0 1) decreases with decreasing oxygen coverage. Of all the adsorbate induced surface structures examined, the structure of InN(0 0 0 1)-(2 × 2) as caused by O adsorption at the H₃ sites with 0.25 monolayers coverage is most energetically favorable. Meanwhile, nitrogen (N) vacancy can form spontaneously. Oxygen atoms may also substitute N atoms, or accumulate at the voids inside InN film or simply stay on the surface during growth. The oxygen impurity then acts as a potential source for the n-type conductivity of InN as well as the large energy band gap measured.     

Ga sublattice defects in (Ga,Mn)As: thermodynamical and kinetic trends

We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga(1-x)MnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.     

Ab initio thermodynamic characteristics of the formation of oxygen vacancies,and boron,carbon, and nitrogen impurity centers in anatase

Using the ab initio projector augmented wave (PAW) method, calculations are performed for the electronic energy-band structure of titanium dioxide having the structure of anatase doped with boron, nitrogen, and carbon. Thermodynamic characteristics are determined for the formation of impurity centers, such as the preference energy for the interstitial position, the energy of impurity oxidation, and the energy of oxygen vacancy formation. It is shown that under the conditions of thermodynamic equilibrium the interstitial position of boron atoms is stable, whereas carbon atoms, depending on the oxygen pressure, can occupy both interstitial positions and substitutional positions of oxygen atoms, and nitrogen atoms replace oxygen atoms. It is shown that the presence of oxygen vacancies promotes the thermodynamic stability of carbon and nitrogen atoms. The obtained densities of electronic states correspond to ESR spectroscopy data, which indicates the presence of spin-polarized electrons in the states of the oxygen vacancy.     

锐钛矿型TiO2(101)面本征点缺陷的理论研究

采用平面波超软赝势方法计算了锐钛矿型TiO₂(101)面存在本征空位和间隙点缺陷的几何结构以及缺陷形成能.首先分析了点缺陷对表面结构的影响,发现不同类型缺陷导致缺陷周围原子有不同的位移趋势：O空位的产生导致空位周围的Ti原子向空位外移动,Ti₁和Ti₂空位的产生均使O₁自发地与周围的O原子团聚,O_i原子易被周围的氧原子吸附而成键,而Ti_i2缺陷几乎 关键词： 第一性原理 2')" href="#">TiO₂ 点缺陷 表面结构     

Energetics of native point defects in cubic silicon carbide

In this work we present a detailed investigation of native point defects energetics in cubic SiC, using state-of-the-art first principles computational method. We find that, the carbon vacancy is the dominant defect in p-type SiC, regardless the growth conditions. Silicon and carbon antisites are the most common defects in n-type material in Si-rich and C-rich conditions respectively. Interstitial defects and silicon vacancy are less favorite from the energetic point of view. The silicon vacancy tends to transform into a carbon vacancy-antisite complex and the carbon interstitial atom prefers to pair to a carbon antisite. The dumbbell structure is the lowest-energy configuration for the isolated carbon interstitial defect, and the tetrahedral interstitial silicon is a stable structure in p-type and intrinsic conditions, while in n-type material the dumbbell configuration is the stable one. Our results suggest that, in samples grown in Si-rich stoichiometric conditions, native defects are a source of n-doping and of compositional unbalance of nominally intrinsic SiC, in accord with experimental evidence.Received: 9 January 2004, Published online: 28 May 2004PACS: 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc. - 74.62.Dh Effects of crystal defects, doping and substitution     

Thermodynamic properties and structural stability of thorium dioxide

Using density functional theory (DFT) calculations, we have systematically investigated the thermodynamic properties and structural stabilities of thorium dioxide (ThO(2)). Based on the calculated phonon dispersion curves, we have calculated the thermal expansion coefficient, bulk modulus, and heat capacities at different temperatures for ThO(2) under the quasi-harmonic approximation. All the results are in good agreement with corresponding experiments proving the validity of our methods. Our theoretical studies can aid a clearer understanding of the thermodynamic behaviors of ThO(2) at different temperatures. In addition, we have also studied possible defect formations and diffusion behaviors of helium in ThO(2), to discuss its structural stability. It is found that in intrinsic ThO(2) without any Fermi energy shifts, the interstitial Th(i)(4+) defect rather than oxygen or thorium vacancies, interstitial oxygen, or any kinds of Frenkel pairs, is the most probable to form with an energy release of 1.74 eV. However, after upshifting the Fermi energy, the formation of the other defects also becomes possible. Regarding helium diffusion, we find that only through the thorium vacancy can it occur with the small energy barrier of 0.52 eV. Otherwise, helium atoms can hardly incorporate or diffuse in ThO(2). Our results indicate that upward shifting of the Fermi energy of ThO(2) should be prevented to avoid the formation of thorium vacancies so as to avert helium caused damage.     

Native defects and Pr impurities in orthorhombic CaTiO3 by first-principles calculations

Formation energies of native defects and Pr impurities in orthorhombic CaTiO₃ are explored using the first-principles calculations. The Ca vacancy (V_Ca), Ti vacancy (V_Ti) and Ca antisite (Ca_Ti) are found to be energetically preferable. The Ti antisite (Ti_Ca) and O vacancy (V_O) are not energetically favorable in the wide range of Fermi level. In Pr-doped CaTiO₃, Pr substituting for Ca (Pr_Ca) is likely to form under condition A in which CaTiO₃ is in equilibrium with CaO and O₂. Under condition B (TiO₂, CaTiO₃ and O₂ are in equilibrium), Pr_Ti defect is energetically preferable depending on the Fermi levels. Several native defects and the two sites of Pr impurities in CaTiO₃ are coincided with several different defects in Pr-doped CaTiO₃ reported in the literature. Based on the present calculations, we can elucidate that the Ca deficiency design of the traditional formula Ca_1−xV_Ca(x/2)Pr_xTiO₃ is not the best for efficient red photoluminescence, which is realized via the experimental measurements.     

Spontaneous N incorporation onto a Si(100) surface

Initial nitridation of the Si(100) surface is investigated using photoemission, ion-scattering and ab initio calculations. After dissociation of NO or NH3, nitrogen atoms are found to spontaneously form a stable, highly coordinated N[triple bond]Si(3) species even at room temperature. The majority of the N species is incorporated into the subsurface Si layers occupying an interstitial site, whose atomic structure and unique bonding mechanism is clarified through ab initio calculations. This unusual adsorption behavior elucidates the atomistic mechanism of initial silicon nitride formation on the surface and has important implication on the N-rich layer formation at the SiO(x)N(y)/Si interface.     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.     

Displacement field induced by a vacancy in nickel and some implications for the solubility of hydrogen

The displacement field induced by a vacancy in nickel and its possible implications for the hydrogen solubility in the interstitial sites is investigated up to 1100 K. The displacement field is extracted from our previous atomistic-scale density functional theory calculations and is compared with continuous isotropic models with and without image forces due to the distribution of the defects. This procedure put in forward the contribution of the elastic anisotropy and the vacancy concentration on the elastic displacement field. Then, we calculate the dilatation of the interstitial sites due to the presence of the defect and the local hydrostatic stress acting on them. This local hydrostatic stress modifies the hydrogen solubility in the interstitial sites. Elastic distortions contribute to reduce the local solubility inside the vacancy core and the opposite out of it. Finally, we extract the elastic contribution of the trapping energy of hydrogen in the displacement field of the vacancy. We find that the elastic energy is significant only for the closest interstitial sites of the defect core and limits the attractive electronic contribution to the trapping energy. In addition, we show that the elastic anisotropy and the vacancy concentration have moderate effects on the local solubility in the displacement field close to the defect core.     

Stability and magnetism of vacancy in NiO: A GGA+U study

Native vacancy defects in typical strongly correlated oxides NiO have been investigated using the GGA+U method. The defect formation energies under different conditions are determined and magnetisms induced by vacancies are studied. Our results indicate that the predominant defect is Ni vacancy under oxygen-rich condition and the most stable ionization state varies with different Fermi level. The Ni vacancy forms shallow acceptor, suggesting the native p-type conductivity originates from the cation vacancy. In addition, after introducing a Ni vacancy, a half-metallic antiferromagnet or half-metallic ferromagnet can form according to different ionization state of cation vacancy.     

First-principles calculations of F-, Cl-, and N-related defects of amorphous SiO_2 and their impacts on carrier trapping and proton release

The first-principles calculations based on density functional theory are performed to study F-,Cl-,and N-related defects of amorphous SiO2(a-SiO2) and their impacts on carrier trapping and proton release.The possible geometric configurations of the impurity-related defects,the formation energies,the hole or electron trapping of the neutral defects,and the mechanisms to suppress proton diffusion by doping N are investigated.It is demonstrated by the calculations that the impurity atoms can interact with the oxygen vacancies and result in impurity-related defects.The reactions can be utilized to saturate oxygen vacancies that will cause ionization damage to the semiconducting devices.Moreover,the calculated formation energy indicates that the F-or Cl-related oxygen vacancy defect is a deep hole trap,which can trap holes and prevent them from diffusing to the a-SiO2/Si interface.However,three N-related defects,namely N(2)o-H,N(2)o=O,and N(3)o-Vo,tend to act as shallow hole traps to facilitate hole transportation during device operation.The N(2)o and N(3)o configurations can be negatively charged as deep electron traps during the oxide charge buildup after ionization radiation.In addition,the nudged elastic band(NEB) calculations show that four N-related defects,namely N(2)o,N(2)o-H,N(2)o=O,and N(3)o are capable of capturing protons and preventing them from diffusing to and de-passivating the interface.This research reveals the fundamental properties of the F-,Cl-,and N-related defects in amorphous silica and the details of the reactions of the carrier trapping and proton release.The findings help to understand the microscopic mechanisms that alleviate ionization damage of semiconducting devices by doping a-SiO2.     

Shallow donor state due to nitrogen-hydrogen complex in diamond

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.     

Influence of hydrostatic pressure on the native point defects in wurtzite ZnO: Ab initio calculation

The formation energies and transition energy levels of native point defects in wurtzite ZnO under applied hydrostatic pressure are calculated using the first-principle band-structure methods. We find that the pressure coefficient of the (2+/0) level for oxygen vacancy is larger than that of the (2+/1+) level for zinc interstitial, which demonstrates that the donor level of oxygen vacancy is deeper than that of zinc interstitial, therefore the latter is the more probable electron resource in native n-type ZnO. And the significantly different pressure dependence of the transition levels between them can be used to determine the origin of the green luminescence center in ZnO. Zinc octahedral interstitial and oxygen tetrahedral interstitial configurations became the dominant defects under 5 GPa at their favorable growth conditions, respectively. The formation of defects under applied pressure is the result of fine interplay between internal strains, charges on the defects and applied external pressures.