X-ray photoelectron spectroscopy study of cubic boron nitride single crystals grown under high pressure and high temperature

The defects, impurities and their bonding states of unintentionally doped cubic boron nitride (cBN) single crystals were investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that nitrogen vacancy (V_N) is the main native defect of the cBN crystals since the atomic ratio of B:N is always larger than 1 before Ar ion sputtering. After sputter cleaning, around 6 at% carbon, which probably comes from the growth chamber, remains in the samples as the main impurity. Carbon can substitute nitrogen lattice site and form the bonding states of CBN or CB, which can be verified by the XPS spectra of C1s, B1s and N1s. The C impurity (acceptor) and N vacancy (donor) can compose the donor-acceptor complex to affect the electrical and optical properties of cBN crystals.     

Influence of copper impurity and structure defects on the properties of compound CdSnAs2

A study was made of the mechanisms of formation and interaction of impurities and intrinsic defects in CdSnAs₂ crystals during diffusion doping of this compound with copper and during subsequent low-temperature (300°C) annealing. This was done by diffusing copper into samples with different initial defect structure under conditions of controlled arsenic vapor pressure and applying the method of quasichemical reactions to analyze the experimental data. It was found that the special features of the physical properties of n-type CdSnAs₂:Cu crystals — the low density and high mobility of electrons-were due to the high reactivity of copper resulting in binding of the original crystal defects to form neutral complexes. It was found that the anomalous annealing of CdSnAs₂: Cu samples, resulting in n-p conversion of the type of conduction, was associated with precipitation of a solid solution of an accidental donor impurity (most probably oxygen). The diffusion doping with copper was found to be one of the most effective methods for controlling the properties of CdSnAs₂, particularly in the preparation of high-resistivity crystals.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 5, pp, 102–108, May, 1980.     

Formation of defects in CdS and CdS:Cu single-crystals irradiated by E = 46 MeV protons

The results of an experimental study of the spectral dependence of the adsorption coefficient, reflection exciton spectra, luminescence, photoconductivity (in the wavelength range from 0.45 to 1 m), and dark conductivity of pure and copper-doped CdS single-crystals, irradiated by E=46 MeV protons, are presented. It is shown that irradiation produces characteristic defects both in the cadmium sublattice and in the sulfur sublattice, but for one and the same irradiation dosages the defect concentration in CdS:Cu is many times higher than the concentration of defects in the pure crystals. A significant difference is observed in the concentrations of defects in these samples only when they are irradiated by high-energy particles which strongly ionize the lattice of the crystals. It is proposed that there exists an ionization mechanism for the transfer of a significant part of the energy of the moving protons to Cu atoms, which form a large number of defects (more than 1 per copper atom).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 62–66, May, 1987.     

Periodic defect modes of one-dimensional crystals containing single-negative materials

The transmission properties of a one-dimensional (1D) photonic crystal containing two kinds of single-negative (permittivity or permeability negative) material with an inserted array of defects (RHM and LHM) were presented by the transfer matrix method (TMM). The dependence of the defect modes on the structure parameters was discussed by using the TMM. In contrast to the Bragg gaps, the properties (the central frequency and width of the gap) of zero effective phase (zero−φ_eff) and gap are invariant with a change in scale length and insensitive to the incident angles. The property of the periodic defect modes in the SNG host periodic structure was studied. It illustrates that the defect modes properties changed more by insert periodic defect than by single-media defect. The defect modes are sensitive to the parameters of the defect layers and the incident angle.     

“Defect” states of channeling electrons

We develop a theory of quantum states caused by defects of the crystal lattice during planar channeling of electrons. Points defects are considered. We obtain expressions for the wave function and the transverse energy of the defect states and make estimates for {110} planar channeling in silicon by electrons with energies of 1–10 MeV. Substitutional, interstitial, and vacancy impurity atoms are examined. We propose using properties of the electromagnetic radiation of electrons in defect states for diagnostics of single crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp.68–72, April, 1990.     

Ultraviolet-visible spectroscopic characterization of lanthanum beryllate crystals doped with Er,Nd, or Pr ions

Spectroscopic characterization of lanthanum beryllate La₂Be₂O₅ (BLO) single crystals doped with trivalent ions of Eu, Nd or Pr, was carried out in the ultraviolet-visible spectral range using synchrotron radiation spectroscopy in combination with conventional optical absorption and luminescence spectroscopy techniques. On the basis of the obtained data, the energy level diagram for these trivalent impurity ions in BLO host lattice was developed; the optical and electronic properties of the crystals were determined; the possibility of the 4f-4f, 4f-5d and charge transfer transitions was analyzed; spectroscopic properties of the lattice defects formed during the introduction of trivalent impurity ions in the BLO host lattice, were investigated. We found that the lattice defects are responsible for a wide-band photoluminescence (PL) in the energy region of 400–600 nm. The most efficient excitation of the defect photoluminescence in the energy gap of BLO occurs in broad PL excitation-bands at 270 and 240 nm. The PL intensity of defects depends on the type of impurity ion and increases in the sequence: Pr-Nd-Er.     

Collective low-energy excitations of two-level tunnelling defects in mixed crystals

The dynamics of strongly interacting two-level tunnelling defects in mixed crystals as KCl: Li is investigated in the framework of Mori's projection method. Applying a mode-coupling scheme on the memory functions and performing the average over the random defect configuration in a mean-field type approximation, the ensueing self-consistent equations can be solved analytically. At low concentration this solution coincides with the exact dynamics of the pair model, whereas for higher concentration the motional spectrum shows several novel features. In particular we find a strong relaxation peak and a characteristic two-peak structure for the density of states; there is an additional feature at energies far below the bare tunnelling amplitude which accounts well for several experiments indicating the existence of such low-energy states.     

First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO₃): (1) the oxygen-vacancy model , (2) the niobium-vacancy model , and (3) the lithium-vacancy model . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.     

Effects of topological defects in the semiconductor carbon nanotube intramolecular junctions

Several intramolecular junctions (IMJs) connecting two semiconductor single-wall carbon nanotubes (SWNTs) have been realized by using the layer-divided technique and introducing the pentagon-heptagon topological defects. The atomic structure of each IMJ is optimized with a combination of density-functional theory (DFT) and the universal force field (UFF) method, based upon which a -orbital tight-binding calculation is performed on its electronic properties. Obtained results indicate that different topological defects and their distributions on the interfaces of the IMJs have decisive effects on the electronic properties of the IMJs. The specific geometrical defects control the localized defect states chiefly, while the diameters of the SWNTs on both sides are also related to them. The influence on the experimental observation brought by the choice of the scanning line is also presented by comparing the scanning results performed on the defect side with those on the defect-free side. A new IMJ structure has been found, and it probably reflects the real atomic structures of the semiconductor-semiconductor (S-S) IMJ [Phys. Rev. Lett. 90, 216107 (2003)].Received: 29 November 2003, Published online: 9 April 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 73.20.At Surface states, band structure, electron density of states - 73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions     

Chemical diffusion in non-stoichiometric cuprous oxide

The kinetics and thermodynamics of point defect diffusion in non-stoichiometric cuprous oxide, Cu_2−yO, has been studied as a function of temperature (1173–1373 K) and oxygen pressure (10²–7×10⁴ Pa) using microthermogravimetric reequilibration technique. It has been shown that the defect diffusion coefficient of cation vacancies, constituting the predominant point defects in this oxide does not change with their concentration, clearly indicating that these defects do not interact and are randomly distributed in the crystal lattice. Consequently, cation vacancy diffusion coefficient, D_V, being the direct measure of defect mobility, can be described by the following empirical equation:

     

Generation of secondary electrons and radiation defects in matter during absorption of electrons with energy above 1 MeV

Using the model of sections with one-dimensional geometry, the output of electrons generated by electrons with energies from 1 to 10 MeV is considered as a function of the thickness and the atomic number of various absorbers. The depth distributions of thermalized electrons and radiation defect profiles are calculated for the model absorbers KCl and NaCl. Radiation defects due to irreversible displacement of lattice atoms upon elastic scattering of rapid electrons are considered. The microphotometry method is used to obtain depth distributions of enrichment centers in the ionic crystals KCl and NaCl for irradiation by electrons with energies E=1, 1.5, and 2 MeV for electron-beam incidence on the absorbers at angles of₀=0, 20, 45°. The integral density of the incident beam was varied from 1·10¹⁴ to 6·10¹⁵ cm^–2. It was found that in ionic crystals the enrichment center density is proportional to the density of thermalized electrons, if the concentration of radiation defects generated is less than 1% of the density of preradiation anion vacancies.Presented at the Third All-Union Conference on Radiation Physics and the Chemistry of Ionic Crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 105–111, April, 1977.     

Indium-defect complexes in silicon studied by perturbed angular correlation spectroscopy

The formation of molecule-like complexes, consisting of a defect and a radioactive¹¹¹In atom, is studied using the perturbed angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the¹¹¹In atom, its electric field gradient is measured after the decay of the radioactive¹¹¹In atom to¹¹¹Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.     

Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis

We report the effect of thermal annealing in air on the structural and optical properties of undoped and aluminium-doped (1%–4%) zinc oxide (AZO) thin films, grown by the spray pyrolysis technique on quartz substrates. Films were characterized by X-ray diffraction, low-temperature photoluminescence, electrical resistivity, and Raman spectroscopy after annealing at temperatures between 500 and 900 C. Annealing in air improves the long-range order crystalline quality of the bulk crystals, but promotes a number of point defects in the surface affecting both the resistivity and the photoluminescence.     

The differential geometry of elementary point and line defects in Bravais crystals

Since line defects (dislocations) and point defects (vacancies, self-interstitials, point stacking faults) in Bravais crystals can mutually convert, only theories which comprise these two sorts of defects can be closed in the sense of general field theory. Since the pioneering work of Kondo and of Bilby, Bullough, and Smith it is clear that differential geometry is the appropriate mathematical tool to formulate a field theory of defects in ordered structures. This is done here on the example of the Bravais crystal, where the above-mentioned defects are the only elementary point and line defects. It is shown that point defects can be described by a step-counting procedure which makes it possible to include also point stacking faults as elementary point defects. The results comprise two equations with the appropriate interpretation of the mathematical symbols. The point defects are step-counting defects and are essentially described by a metric tensorg, which supplements the torsion responsible for the dislocations. The proposed theory is meant to form a framework for defect phenomena, in a similar way that Maxwell's theory is a framework for the electromagnetic world.     

Application of PAC to study equilibrium point defects in intermetallic compounds

A new method has been developed to measure properties of equilibrium, or thermal, defects in intermetallics using the technique of perturbed angular correlations of gamma rays (PAC). After quenching, thermal defects are detected microscopically by distinctive nuclear hyperfine interaction signals produced when they localize next to probe atoms present in high dilution. Using a Schottky defect model and applying the law of mass action to defect equilibria, a linear Arrhenius temperature dependence is predicted for a normalized monovacancy site fraction. We have observed such linear dependences in PAC experiments on NiAl, CoAl and TiAl using the¹¹¹In probe. Features of the method are summarized, of which the most important are the abilities to discriminate between different defects and to determine absolute vacancy concentrations, formation enthalpies and entropies. Extremely large formation entropies have been obtained for NiAl, CoAl and TiAl, suggesting vacancy concentrations of 15% at the melting temperatures.     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

多原子极性晶体中表面激子的自陷条件

本文研究多原子极性晶体中表面激子的性质,采用微扰法导出表面激子的有效哈密顿量.在计及反冲效应中不同波矢的声子之间的相互作用时,讨论对电子、空穴间的相互作用势、表面激子的自陷能和自陷条件的影响.     

Investigations of As-antisite-related defects in GaAs

Optically Detected Magnetic Resonance (ODMR) is a powerful tool to study paramagnetic defects in semiconductors. As an example for the successful application of various methods based on ODMR, investigations of As antisite-related defects in GaAs are presented. Information about defect properties such as their microscopic structure, their metastability and thermal stability can be obtained by ODMR which was measured via the Magnetic Circular Dichroism of the optical Absorption (MCDA).Low temperature irradiation of GaAs (4.2 K) with 2 MeV electrons produces the Ga vacancy and some new As antisite-related defects. Annealing steps were investigated at 77 K and above. At 77 K the isolated As antisite defect is detected if semi-insulating (si) GaAs was irradiated. Between 200 K and 300 K the Ga vacancy decays. Its decay is correlated to the formation of an anti-structure pair. A second annealing step was found at about 520 K. There the anti-structure pair decays. EL2 is formed at that temperature if si material was irradiated, but this EL2 formation is not correlated with the anti-structure pair decay. We performed also magneto-optical measurements to investigate the metastability properties of the three As antisite-related defects. They all show metastability.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994