Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.     

Correlation between energy positions of deep intrinsic point-defect levels and a limiting fermi level in irradiated III–V semiconductors

The energy levels of neutral anion (V_A) and cation (V_C) vacancies and antisite defects are calculated for the anion C_A and cation A_C sublattices of III–V semiconductors. An averaged energy level position for these defects is estimated to be E_av ^abs = 4.9 eV. The position coincides with the local charge electroneutrality level. It is shown that the case, where the total energies of formation of V_A, V_C and antisite C_A, A_C defects in the sublattices of binary semiconductors are similar, corresponds to the point-defect equilibrium condition and stabilization of the Fermi level in the proximity of the local charge electroneutrality level. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 17–22, May, 2007.     

Defect Formation on the Surface of ZnO Using Low-Energy Electrons

Defect formation on the surface of zinc oxide using low-energy electrons is studied by the total-current-spectroscopy method. The formation of cation and anion vacancies on the surface is shown. The energy threshold for the formation of anion vacancies (~20 eV) on the surface of ZnO is determined. It is shown that the negative potential of the surface decreases in the case of irradiation with electrons with energies above 120 eV. The experimentally observed kinetic characteristics of the charging of a ZnO crystal are attributed to modification of the crystal surface and the generation of radiation-induced defects, which are electron traps.     

Review of defect investigations by means of positron annihilation in II-VI compound semiconductors

1-x Cd_xTe after post-growth annealing. The experiments on defect generation and annihilation after low-temperature electron irradiation of II-VI compounds are also reviewed. The characteristic positron lifetimes are given for cation and anion vacancies. Received: 19 November 1997/Accepted: 20 November 1997     

Enrichment centers in sodium bromide crystals

The primary products of irradiation of alkali halide crystals are anion and cation Frenkel pairs [1–4]. Interaction of these products among themselves and with structural defects existing before irradiation leads to the appearance of various structural defects, the majority of which are optically active. The present study will treat enrichment centers which can be formed from the components of Frenkel cation pairs. Studies of this type are important in providing information on states in which radiation-created cation defects can exist stably in the lattice.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 119–121, August, 1979.     

Electroluminescence from Si/SiGe quantum cascade emitters

Intersubband electroluminescence results are presented from Si/SiGe quantum cascade emitters at 3.2 THz and at temperatures up to 150 K. The effect of adding doping into the active quantum wells was studied in addition to reduced barrier widths from previous measurements. While the current through the sample is increased by the addition of doping, the emitted power is reduced through additional free carrier absorption and Coulombic scattering. Free electron laser measurements confirm the intersubband transitions in the quantum wells of the cascade devices and produce non-radiative lifetimes of 20 ps between 4 and 150 K.     

Lifetimes of stark-shifted image states

The inelastic lifetimes of electrons in image-potential states at Cu(100) that are Stark shifted by the electrostatic tip-sample interaction in the scanning tunneling microscope are calculated using the many-body GW approximation. The results demonstrate that in typical tunneling conditions the image state lifetimes are significantly reduced from their field-free values. The Stark shift to higher energies increases the number of inelastic scattering channels that are available for decay, with field-induced changes in the image state wave function increasing the efficiency of the inelastic scattering through greater overlap with final state wave functions.     

N-丁基吡啶四氟硼酸盐/水二元体系的分子动力学模拟研究

利用分子动力学模拟方法研究了离子液体N-丁基吡啶四氟硼酸盐([BPy]BF₄)与Tip4p模型水分子的二元体系的微观结构.比较了各组分间的径向分布函数,结果表明随着离子液体比例的增加,水与阴离子、水与阳离子头部吡啶环、阳离子头部与阴离子上相关原子间的径向分布函数峰值都呈现递增的趋势,而阳离子上丁基链末端碳原子间的径向分布函数没有明显变化;空间分布函数则直观地反映出阴离子主要分布在阳离子的吡啶环周围,水分子在阴离子周围近似呈均等分布,且几率随离子液体比例增大而增加;另外还探讨了不同离子液体比例下的二元体系中氢键的数目和寿命,结果均呈现一定规律的变化.     

Cu-Zn disorder in Cu2ZnGeSe4: A complementary neutron diffraction and Raman spectroscopy study

The crystal structure of the quaternary compound semiconductor Cu₂ZnGeSe₄ (CZGSe) was investigated by the complementary use of neutron diffraction, and Raman spectroscopy. The powder sample, which resulting from wavelength dispersive X-ray spectroscopy (WDX) turned out to be single phase Cu-rich CZGSe, was synthesized by solid state reaction of the pure elements in an evacuated silica tube at 700 °C. Raman spectroscopy confirmed the homogeneity and phase purity of the sample, in addition, the kesterite type structure was suggested. Rietveld analysis of the neutron diffraction data confirmed that the compound crystallizes in the tetragonal kesterite type structure. The refined site occupancy factors were used to determine the average neutron scattering lengths of the cation sites, giving insights into cation distribution and finally point defect types. Thus it has been shown, that additional to the Cu_Zn-Zn_Cu anti-site defects in the lattice planes at z=¼ and ¾ (Cu-Zn disorder) also the off-stoichiometry type related point defects like Cu_i and Cu_Zn occur in Cu-rich CZGSe.     

Observation of vacancy defect migration in the cation sublattice of complex oxides by 18O tracer experiments

We report on 18O tracer diffusion experiments and model calculations for the study of cation vacancy migration in oxide crystals. The model takes advantage of the electrostatic coupling forces between anion and cation defects that allow the evolution of the cation vacancy profile to be observed by anion tracer experiments. Applied to SrTiO3, the ambipolar diffusion of strontium vacancies with H(A)=3.5 eV was found to be the dominant reequilibration mechanism of the cation sublattice. This result is in contrast to earlier studies proposing the formation of SrO intergrowth phases.     

Atomic hydrogen and muonium in alkali halides

Atomic hydrogen can be trapped at interstitial and substitutional cation and anion sites in alkali halides. The geometrical structure of these defects was established by electron nuclear double resonance (ENDOR). From the analysis of the ENDOR spectra also detailed information was obtained on the electronic structure. In this article the major experimental and theoretical results for atomic hydrogen in several alkali halides are briefly reviewed. Special emphasis is given to the isotope effects upon replacing hydrogen by deuterium. The nature of the dynamical hyperfine and superhyperfine interactions is discussed. Its magnitude to be expected for muonium is estimated. Recent results on muonium centres are discussed on the basis of the knowledge about the hydrogen centres.     

A.C. electrical conductivity of pure and doped potassium perchlorate

A.C. electrical conductivity of potassium perchlorate (KP) has been measured in the temperature range 25–325°C at frequencies ranging from 50–500 Hz using an automated technique. The results are interpreted in terms of a novel mechanism involving Schottky defects in the anion sublattice and Frenkel defects in the cation sublattice. The conductivity behavior of KP is compared with literature data on similar low-symmetry systems containing polyatomic ions.     

Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe2–xMnxO4 (x = 0.0–2.0) spinel nanocrystallites

The replacement of cations at the B-sites in the spinel ferrite ZnFe₂O₄ by Mn³⁺ ions brings in several interesting changes, the most striking among them being a transformation from the spinel cubic structure to a body-centered tetragonal one. Concomitantly, there are variations in the nanocrystallite sizes and also in the lattice parameters. These are examined through high-precision X-ray diffraction measurements and transmission electron microscopic analysis. A more interesting aspect is the success of positron annihilation spectroscopy comprising of the measurements of positron lifetime and coincidence Doppler broadening measurements in understanding the effects of cation replacement and the resultant generation of vacancy-type defects. There are definite changes in the positron lifetimes and intensities which show positron trapping in trivacancy-type defect clusters and the nanocrystallite surfaces. The presence of ortho-positronium atoms within the extended intercrystallite region is also identified, although in small concentrations. The cubic to tetragonal transformation is indicated through definite decrease in the values of the positron lifetimes. We also performed a model analysis to predict the expected effect of substitution on the positron lifetime in the bulk of the sample and the experimentally obtained positron lifetimes significantly differed, indirectly hinting at the possibility of a structural transformation. Finally, Mössbauer spectroscopic studies have indicated a ferromagnetic nature for one of the samples, i.e. the one with Mn³⁺ doping concentration x = 0.4, which incidentally had the lowest crystallite size ~10 nm.     

A multi‐component optimisation of experimental parameters for maximising SERS enhancements

We present a multi‐component optimisation strategy for a range of commonly used Ag colloids as surface‐enhanced Raman scattering (SERS) spectroscopy substrates. The concentration‐dependent interactions of a wide range of parameters, including colloid and aggregating agent species, were considered in order to achieve optimal SERS enhancements. Few papers in the literature discuss the optimisation of these parameters, never mind multi‐component optimisation, and suboptimal responses result in SERS not being utilised to its true capabilities. Significant differences in enhancement factor were observed as a consequence of colloid–aggregating agent interactions and these are shown to be analyte specific. Both the cation and anion of the aggregating agent were found to play an important role in the optimisation process. We show that through the optimisation of experimental conditions, the reliability and applicability of SERS can be significantly improved. Copyright © 2009 John Wiley & Sons, Ltd.     

Photoexcited hot phonons and phonon lifetimes in GaAs

A study of optical lifetimes as determined from surface conditions is made through Raman lineshape analysis and non-equilibrium phonon distributions induced by energetic photoelectrons. Results indicate that first-order Raman scattered linewidths are lifetime determined and that phonon lifetimes depend upon surface conditions. Our observations also give strong support to the mechanism and analysis proposed earlier for hot phonon generation through photoelectron relaxation.     

Lifetime measurements of levels in 26Si and the ratio of neutron to proton transition amplitudes for mirror nuclei

The mean lifetimes of the 1796, 2784, 3333 and 4138 keV levels of ²⁶Si have been measured using the Doppler shift attenuation method. The ratios of neutron to proton transition matrix elements determined from these data and additional data from mirror transitions in ²⁶Mg are compared with recent data from inelastic pion scattering and with shell model calculations. The isoscalar strenghts are compared with α-scattering data.     

Dynamics of low-energy electron attachment to formic acid

Low-energy electrons (<2 eV) can fragment gas phase formic acid (HCOOH) molecules through resonant dissociative attachment processes. Recent experiments have shown that the principal reaction products of such collisions are formate ions (HCOO-) and hydrogen atoms. Using first-principles electron scattering calculations, we have identified the responsible negative ion state as a transient pi* anion. Symmetry considerations dictate that the associated dissociation dynamics are intrinsically polyatomic: a second anion surface, connected to the first by a conical intersection, is involved in the dynamics and the transient anion must necessarily deform to nonplanar geometries before it can dissociate to the observed stable products.     

Theory of GaAsoxide interface states

The discrete GaAsoxide interface states observed by Lagowski et al. can be explained by defects in the GaAs surface: The deep levels are in agreement with our predictions for surface antisite defects, and the shallow levels (observed for thick oxides) agree with our predictions for surface oxygen impurities on anion and cation sites.     

Calculating a local oscillating quasiparticle’s lifetime at screened defects in solids

We report on a novel approach to calculate quasiparticle lifetimes of localized initial states, which decay into the continuum of underlying quasi-free quasiparticle states in the vicinity of point defects and steps in solids. By using this interpretation of the inelastic damping of wavefunctions, the lifetime becomes a local property. In particular, we consider electrons, which are injected by a scanning tunneling microscope tip into the surface state of a noble metal surface. We investigate numerically the configuration of a single scatterer, a chain of scatterers, and a triangular quantum corral. As compared to an exponential increase of the damping from prior theories, we find an oscillating damping together with a linear background of the resulting measurement signal. The different configurations show increased lifetimes with increasing dimensionality as their scattering phase space is decreased.     

Defect-induced increase in the phonon energy involved in the formation of Urbach tail in Cu-ternaries

From a combined analysis of the stoichiometric composition and Urbach tail in samples of CuInSe₂, CuInTe₂, and CuGaTe₂ of the I-III-VI₂ family of chalcopyrite semiconductors, it is found that the energy hν_p involved in the electron/exciton-phonon interaction is a linear function of a parameter Δz which is the sum of the deviations from ideal molecularity Δx and anion to cation ratio Δy. It gives evidence that in the copper ternaries hν_p is associated to the structural defects caused by cation-cation, cation-anion, and other intrinsic disorders. The high value of hν_p found in the studied samples, higher than the highest optical mode, is shown to come from the contribution of the additional phonon energy due to structural defects. This is in agreement with recently proposed models of the temperature dependence of the Urbach energy.