立方半导体中双空位态的电子结构(Ⅱ)——能级和简单的物理模型

利用文献[1]中给出的基本方程和Vogl等人的紧束缚哈密顿量,计算了十多种立方半导体的理想双空位在禁带中引入的能级,并用了一个简单的“空位分子”模型来讨论双空位态的物理问题。 关键词：     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

Divacancy contribution to impurity diffusion in noble metals

A model calculation on the contribution to electropositive impurity diffusion by divacancy processes in f.c.c. noble metals has been carried out. Four different types of divacancy configuration were considered. It is shown that ΔQ_2v = Qⁱ_2v ? Q^o_2v, the difference in the activation energy between impurity diffusi and its thermal average, ΔQ_2v^av, are generally more negative than ΔQ_tv^s as calculated by LeClaire. $\text{D}{}^{\mathrm{i}}{}_{\mathrm{2v}}\text{D}{}^{\mathrm{i}}{}_{\mathrm{1v}}$, the ratio of the impurity diffusion coefficient by the divacancy process to that by the monovacancy process, is found to be in the range of 8.2–16.7% for copper and 27.3–36.7% for silver at 1000°K, both noticeably larger than the corresponding ratio for the self-diffusion coefficients. In addition, the present calculation has shown appreciable improvement in edging closer to the experimental data than the previous calculations.     

Annealing of divacancy-related infrared absorption bands in boron-doped silicon

Abstract

This work is devoted to an “old” defect and its annealing characteristics, namely the divacancy centre in silicon. Infrared spectroscopy is applied to study the annealing kinetics of divacancy-related absorption peaks in electron-irradiated Czochralski-grown samples doped with boron. In all specimens the kinetics show an exponential decay (first-order process) and a model taking into account both diffusion and dissociation as annihilation mechanisms gives excellent quantitative agreement with the experimental results. Activation energy values extracted for diffusion and dissociation are 1.28 eV and 1.71 eV, respectively. In highly boron doped samples (0.1 °Cm), a strong dependence of the absorption bands on the Fermi-level position is observed during annealing below ～ 200°C and concurrently, a change of the divacancy charge state from neutral to singly positive takes place.     

First principle study of the behavior of helium in plutonium dioxide

The incorporation and solution of helium in plutonium dioxide have been investigated based on density functional theory. The GGA and GGA + U approximations were used with the projector-augmented-wave method. Several defects that are likely to accommodate the incorporation of helium in PuO₂, such as oxygen vacancy, plutonium vacancy, divacancy and Schotty defects were considered in this work. With GGA approach, the lowest incorporation energy corresponds to neutral trivacancy, followed by divacancy and plutonium vacancy, while the GGA + U scheme gave us that oxygen vacancy is the most favorable incorporation site for He. Both SP-GGA and SP-GGA + U methods obtained a same conclusion that the most favorable solution site for He is oxygen vacancy, interstitial site and plutonium vacancy for under-, perfect and over-stoichiometry, respectively. Additionally, the concentrations of the point defects and the solution energy of He for the different incorporation sites as a function of the stoichiometry were also obtained based on the point-defect model.     

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V₂) with the definite charge states, V ₂ ⁰ , V ₂ ^–1 or V ₂ ^–2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V ₂ ^– using a specimen containing oriented divacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Electronic energy structure of vacancy and divacancy in SiO2

We investigated electronic energy structure of vacancy and divacancy in SiO₂, and found that oxygen vacancy and divacancy give rise to bound-states near the edge of the conduction band, whereas localized states related to the silicon vacancy occur in the valence band. Our results demonstrate that the doubly occupied oxygen vacancy state yields electrons to silicon in Si-SiO₂ junction and serves as a fixed oxide charge.     

Absorption measurements in neutron irradiated silicon

Silicon samples have been neutron irradiated at 76 °K with fluences sufficient to allow measurement of the 1.7 μ divacancy band at 76 °K. The growth of the divacancy concentration and the recovery of the edge absorption were studied as a function of annealing temperature between 76 °K and 550 °K. Immediately after irradiation the divacancy concentration is about 25 per cent of its maximum value which is attained at 330 °K, the temperature at which the divacancies begin to anneal out. Increases in the 1.7 μ band intensity and recovery of the edge absorption can also be achieved at 76 °K by illuminating the sample with intense sub-bandgap light or white light. The experimental results suggest a neutron-induced cluster model in which the cluster is a vacancy-rich region whose annealing characteristics are controlled by the liberation and motion of vacancies. The injection effects can be explained by analogy to the charge state dependent mobility of the Si vacancy.     

Magnetic and binding properties of metal-divacancy complexes at MgO (0 0 1) surface: DFT calculations

We have analyzed the magnetic and binding properties of Ni, Cr, Mo, and Pt metals deposited on the defect free and defect containing surfaces of MgO by means of density functional theory calculations and embedded cluster model. Clusters of moderate sizes with no border anions, to avoid artificial polarization effects, were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. Spin quenching occurs for Cr and Mo complexes at the defect free (terrace) surface, and Cr, Mo, and Pt complexes at the defect containing “pit” divacancy surface. The binding energies of the metals are significantly enhanced on the cationic vacancy end of the divacancy. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. The encountered variations in magnetic properties of free metals and of metal complexes are correlated with the energy gaps of the frontier orbitals. Spin contamination affect the adsorbate-substrate distances, Mulliken charges, Mulliken spin densities, natural charge, natural orbital population, and provide rationalization for the reported magnetic and binding properties. The electrostatic potential energy curves provide clearer understanding of the nature of magnetic and binding interactions. The magnetic and binding properties of a single metal atom adsorbed on a particular surface result from a competition between Hund's rule for the adsorbed metal, and the formation of a chemical bond at the interface.     

Comparative study of the empirical interatomic potentials and density-functional simulations of divacancy and hexavacancy in silicon

The comparison between three classical potentials and density functional theory (DFT) is performed mainly for divacancy and hexavacancy in Si crystal. According to their performances on the formation energies and structural properties (distortion magnitude, relaxation volume and symmetry), the limitations and validities of classical potentials are discussed. It is found that the outward relaxation directions of EDIP and Tersoff (T3) are contrary to the DFT and Stillinger-Weber (SW) directions (inward), which restrict their application in the structural property calculations. Except for the divacancy symmetries, the results of SW are in agreement with the DFT results. Thus, it can be concluded that SW should be the best potential to describe V₂ and V₆.     

Piezospectroscopic study (78°K) of radiation- produced absorption bands in Si†

Abstract

A detailed study has been made on two sharp sub-bands, 3.45 μ (2900 cm^?1) and 3.61 μ (2765 cm^?1) of the 3.3 μ broad defect absorption band in irradiated Si. Our results indicate that the 3.45 μ and 3.61 μ bands are observed in all types of Si (n-, p-type, and intrinsic) subjected to fast neutron reactor irradiation. Spectra were measured at 78 °K under the effect of a variable uniaxial compressive stress with polarized light. A relatively large dichroism is observed for all three stress directions ?100?, ?110?, and ?111?. We also confirm that both sharp bands exhibit splitting and energy shifts under the action of stress. The model of the divacancy defect given by Watkins and Corbett⁽⁵⁾ has been utilized to explain the data. The dichroism can be accounted for by the effects of Jahn-Teller alignment which arises from the strong bonding of nearest neighbor atoms of the divacancy. The splittings of the bands are small indicating that the Jahn-Teller distortion is little altered in the ground and excited states. The splittings appear to be most sensitive to stress along the vacancy-vacancy axis and a tentative model for the transitions is given.     

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.     

Modeling complex diffusion mechanisms in L1 2 -structured compounds

We report on a procedure developed to create stochastic models of hyperfine interactions for complex diffusion mechanisms and demonstrate its application to simulate perturbed angular correlation spectra for the divacancy and 6-jump cycle diffusion mechanisms in L1₂-structured compounds.     

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (V_Si-V_C)⁰, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.     

First principles molecular dynamics study of nitrogen vacancy complexes in boronitrene

We present the results of first principles molecular dynamics simulations of nitrogen vacancy complexes in monolayer hexagonal boron nitride. The threshold for local structure reconstruction is found to be sensitive to the presence of a substitutional carbon impurity. We show that activated nitrogen dynamics triggers the annihilation of defects in the layer through formation of Stone-Wales-type structures. The lowest energy state of nitrogen vacancy complexes is negatively charged and spin polarized. Using the divacancy complex, we show that their formation induces spontaneous magnetic moments, which is tunable by electron or hole injection. The Fermi level s-resonant defect state is identified as a unique signature of the ground state of the divacancy complex. Due to their ability to enhance structural cohesion, only the divacancy and the nitrogen vacancy carbon-antisite complexes are able to suppress the Fermi level resonant defect state to open a gap between the conduction and valence bands.     

Microscopic origin of collective exponentially small resistance states

The formation of ‘zero’ (exponentially small) resistance states (ESRS) in high mobility two-dimensional electron systems (2DES) in a static magnetic field B and subjected to strong microwave (MW) radiation has attracted great theoretical interest. These states appear to be associated with a new kind of energy gap Δ. Here I show that the energy gap Δ is explained by a microscopic quantum model that involves the Prime Number Theorem, hitherto reserved for only mathematical contexts. The model also contains the zeroes of the zeta function, and explains the physical origin of the Riemann hypothesis.     

Vacancy-assisted diffusion in silicon: a three-temperature-regime model

In this Letter we report kinetic lattice Monte Carlo simulations of vacancy-assisted diffusion in silicon. We show that the observed temperature dependence for vacancy migration energy is explained by the existence of three diffusion regimes for divacancies. This characteristic has been rationalized with an analytical model. In the intermediate temperature regime the divacancy dissociation plays a key role and an effective migration energy E{v}{m} approximately 2 eV is predicted, computed from either full ab initio values or mixed with experimental ones. The exact position of this temperature regime strongly depends on vacancy concentration. Previous contradictory experimental results are revisited using this viewpoint.     

Proton emission in α-induced reactions at 43 MeVnucleon

Inclusive proton spectra and proton-proton correlations have been measured from α + ⁵⁸Ni and α + ¹⁹⁷Au reactions at energies E_α = 100 MeV and 172 MeV. The inclusive spectra are compared to results of a simple model assuming local equilibration of the energy transferred to the target nucleons. This model describes the energy and angular dependence sufficiently well at angles ? 40°. Due to finite number effects, it also explains the occurrence of differences between in-plane to out-of-plane pp correlations. In addition, the measured coincidences indicate contributions from quasifree pp scattering.     

六角密堆结构钇的解析型键序作用势

为了构建拟合势需要的数据库,采用密度泛函理论方法计算了六角密堆结构钇(hcp-Y)的晶格参数、弹性常数、内聚能、结构能差以及相关的点、面缺陷性质. 基于解析型键序作用势,构建了hcp-Y的多体作用势模型. hcp-Y势模型是通过拟合Y的晶格参数、弹性常数、体弹模量、内聚能、空位形成能和不同相之间的结构能差而构建.分析发现,所得到的势模型能够很好地描述hcp-Y的自填隙原子形成能、空位形成能、双空位键能以及其它体性质,同时,构建的势模型用来研究Y的热动力学性质的相关结果也比较理想.     

Cosmological model with dynamical cancellation of vacuum energy and dark energy

We propose a model with a compensating scalar field whose back reaction to the cosmological curvature cancels possible vacuum energy density down to the terms of the order of the time-dependent critical energy density. Thus, the model simultaneously solves the mystery of the compensation of vacuum energy with an accuracy of 120 orders of magnitude and explains the existence of the observed dark energy. At an early stage, the suggested cosmological model might experience exponential expansion without an additional inflaton field. However, the solution found is unstable with respect to small perturbations. The stability can be ensured by introducing nonanalytical terms depending upon the absolute value of the curvature scalar R. Unfortunately, stable solutions do not describe realistic cosmology at the matter-dominated stage.