Showing papers in "Physica Status Solidi B-basic Solid State Physics in 1995"

Concentrations of Atomic Defects in B2FexAl1-x. An Ab-Initio Study

Abstract: The concentrations of various atomic defects are calculated within the framework of a grand canonical formalism for the ordered compound Fe x Al 1-x with B2 structure as functions of temperature and composition. The input parameters for the grand canonical formalism are determined by the ab-initio mixed-basis pseudopotential theory in local-density approximation. It is shown that Fe x Al 1-x is neither a compound with pure antisite disorder nor a compound with pure triple-defect disorder but that it exhibits a hybrid behaviour in which vacancies on the Fe sublattice and antisite atoms on both sublattices appear. The concentration of vacancies on the Al sublattice is negligibly small. The implications of this result for the modelling of diffusion mechanisms in Fe x Al 1-x are discussed.

InAs/GaAs quantum dots radiative recombination from zero‐dimensional states

Abstract: Nanometer-scale quantum dots are fabricated using the Stranski-Krastanov growth mode of InAs on GaAs(001) For an average coverage of four monolayers, 10 11 cm -2 pyramidal-shaped dots (12±1 nm base along , 5±1 nm high) are formed as observed in plane-view and cross-section transmission electron microscopy The quantum dots exhibit short-range order, aligning along rows in directions The three-dimensional confinement of the wave function results in ultrasharp luminescence lines (full width at half maximum <015 meV) from individual dots as revealed with highly spatially resolved cathodoluminescence Even at elevated temperatures extremely sharp lines are observed, proving the δ-like zero-dimensional electronic density of states

Langevin‐Type Charge Carrier Recombination in a Disordered Hopping System

Abstract: A Monte Carlo simulation study is performed to determine the ratio γ/μ of the rate coefficient for bimolecular charge recombination and the charge carrier mobility in a hopping system with built-in energetic disorder. The result indicates that γ/μ is independent of disorder but increases at large electric fields at which the charge carrier velocity saturates.

Self‐Diffusion of Ni in Single and Polycrystals of Ni3Al. A Study of SIMS and Radiotracer Analysis

Abstract: Diffusion of nickel in the intermetallic compound Ni 3 Al is measured in the temperature range from 1004 to 1422 K by applying two different techniques for the determination of the concentration profiles. The diffusion of the radioactive 63 Ni tracer in Ni 3 Al polycrystals (75.2 at% Ni) is analyzed in the temperature region from 1223 to 1422K by the conventional serial sectioning technique using a precision parallel grinding device and an improved technique of detecting the low-energy β-decays of 63 Ni with high efficiency by liquid scintillation counting. In the low-temperature region (1004 to 1259 K) the concentration profiles are determined by secondary ion mass spectrometry (SIMS) using the highly enriched stable 64 Ni tracer and Ni 3 Al single crystals (75.9 at% Ni). The SIMS technique is considered as particularly suitable for profile detection, owing to its potential of measuring also low diffusion coefficients with high accuracy. The temperature dependence of the diffusion coefficient D v Ni follows a perfect linear Arrhenius relation in the investigated temperature range for Ni 3 Al. The frequency factor D 0 = (3.59 -1.50 +2.59 x 10 -4 m 2 s -1 and the activation enthalpy Q = (303.0 ± 5.3) kJ mol -1 correspond to a vacancy mechanism. An enhanced diffusivity at lower temperatures, which was previously reported in the literature and ascribed to the presence of constitutional vacancies, is not observed. It is concluded that Ni self-diffusion in Ni 3 Al only occurs via thermal vacancies. This result is in agreement with recent positron annihilation studies by Badura and Schaefer where, independent of the composition, no constitutional vacancies are detected in the Ni 3 Al compound.

Luminescence Bands and their Proposed Origins In Highly Porous Silicon

Abstract: The striking photoluminescent properties of porous silicon nanostructures have received intense study since 1990. This literature review focuses on the large number of models proposed to account for efficient light emission. Two distinct luminescence bands in the visible spectral range have received most study. Strong evidence now exists that quantum confined carriers in crystalline Si can yield efficient red to green ('S band') luminescence. The blue ('F band') emission reported to date in oxidised porous Si is, however, unlikely to arise from quantum confinement effects.

Anisotropy of the ZnO single crystal reflectivity in the region of residual rays

Abstract: The consistent parameters of hexagonal zinc oxide single crystals are obtained in the frame of the one-oscillator model by analyzing the dispersion spectra of IR light reflection. The anisotropy coefficients of the electron effective masses and the plasmon damping Coefficients are measured. The calculated values of electron mobility and conductivity for zinc oxide are in good agreement with the reference data available. [Russian Text Ignored]

Highly-Ordered Organic Adsorbates: Commensurate Superstructures, OMBE, and 1D Nanostructures

Abstract: Big organic molecules deposited by vacuum sublimation on reactive surfaces form highly-ordered superstructures which are often commensurate to the substrate if covalent, nondissociative bonding occurs. The orientation of the molecules at the interface can be varied depending on reactive groups, substrate surface, and coadsorbates. Deposition of highly-ordered thin films, or even organic molecular beam epitaxy (OMBE),is difficult but appears possible under certain conditions. Organic nanostructures can be produced by self-organization of covalently-bound molecules and substrate steps on high-index surfaces. Experiments with different aromatic molecules like NDCA and PTCDA as well as with oligothiophenes (α4T and EC4T) on various substrates such as Ag(111), Ag(775), Ni(111), and Si(111) are reviewed using a variety of complementary surface sensitive techniques.

On the Dielectric Constants of AIBIIIC2VI Chalcopyrite Semiconductor Compounds

Abstract: Values ofthe static, e 0 , and high-frequency, e∞, dielectric constants of I-III-VI 2 chalcopyrite compounds are estimated by using an empirical model proposed by Nag for cubic semiconductors, which relates these constants to the average atomic number of its constituent atoms Z av . It is found, as in the cubic semiconductors, that the inverse of both these constants is linearly related to Z aV . It is also suggested that e 0 and e∞ thus obtained should be more reliable than those calculated from the energy gap Eg, due to the wide range of values of this parameter reported in some I-III-VI 2 chalcopyrite compounds.

Collective Optical Response of Molecular Aggregates

Abstract: Relative fluorescence quantum yield, fluorescence lifetime, optical dephasing, and absorption line shift data are presented for both excitonic transitions of the pseudoisocyanine (PIC) J-aggregate. The data indicate activation of the optical dephasing by exciton-acoustic phonon scattering at low temperatures. Above approximately 80 K the optical dephasing, line shift as well as the fluorescence lifetime lengthening call for activation by high frequency phonons. Moreover, resonance Raman data indicate the delocalization of low frequency vibrational modes in the aggregate.

The Dependence of the Debye Temperature on the Elastic Constants

Abstract: Sophisticated methods have been put forward in the literature to calculate Debye temperatures from the elastic constants C ik . As a simple alternative method for cubic crystals, a relation is often used that correlates Debye temperature and bulk modulus (c 11 + 2c 12 )/3 by a square-root law. It is shown that, with exception of the alkali metals, such a law is only poorly fulfilled for other cubic elements and compounds. If one takes, however, elastic moduli such as [c 44 (c 11 - c 12 ) (c 11 + c 12 + 2c 44 )] 1/3 or {c 44 [(c 11 - c 12 ) c 44 /2] 1/2 (c 11 - c 12 + c 44 )/3} 1/3 instead of the bulk modulus the square-root law is established for various cubic material classes. This procedure eventually allows to easily evaluate hitherto unknown Debye temperatures from the elastic constants with high precision.

Interband Optical Absorption and Plasma Effects in Half-Metallic XMnY Ferromagnets

Abstract: The frequency dependence of the real and imaginary parts of the complex dielectric constant ϵ(ω) in compounds XMnY (X = Ni, Pd, Pt, Cu; Y = Sb, Sn) having Clb structure are investigated in the spectral range ħω = 0.05 to 8 eV. The values of conduction electron plasma and relaxation frequencies ωp and γ, respectively, are determined. The evolution of interband conduction and electron energy-loss function -Im (e−1) for varying compositions is studied. In all the compounds, apart from the main absorption band located in the spectral range 1 to 7 eV, a low-energy (ħω = 0.3 to 0.7 eV) interband absorption is revealed. An estimation is made of the energy gap at point Γ of the Brillouin zone in the minority-spin subbands () for NiMnSb using optical data. [Russian Text Ignored].

Polaritons in multiple quantum wells

Abstract: A description of polariton effects on excitons in multiple quantum wells (MQWs), superlattices (SLs), and semiconductor microcavities is given, which is based on a nonlocal semiclassical theory in a transfer-matrix formulation. The radiative width in MQWs at k=0 increases first with the number of wells N, then goes to zero as N→∞, where stationary polariton states are recovered. A very similar behavior is found by treating the MQW with a local dielectric model. In the SL limit, the exciton-polariton dispersion at long wavelengths reduces to that of a light wave in a uniaxial material, described by an effective dielectric tensor. Comparing with the bulk, the polariton splitting is increased in SLs, and reduced in microcavities. Nevertheless, polariton effects in microcavities are easily observable, since spatial coherence along the growth direction is not required

Estimating Critical-Point Parameters of Modulated Reflectance Spectra

Abstract: Two new methods, based on the 3-point method and a Kramers-Kronig transformation, were recently published for estimating the energy, broadening, phase, and amplitude of critical points in modulated reflectance spectra which are describable by Aspnes' third derivative functional form. The use of the techniques is demonstrated by applying them to example experimental spectra, which are photoreflectance measurements of stress-induced splitting of the direct band gap E o transition in bulk GaAs. The results are compared and contrasted with those obtained from a least-squares fit of a theoretical model to the experimental spectra.

On the Theory of Reflectivity by an X-Ray Multilayer Mirror

Abstract: A general theory of reflectivity by a perfect X-ray multilayer mirror based on the usage of a recurrent relation is developed. The reflectivity is described by the well-known Fresnel formulae which take into account jumps of susceptibility at the boundary between the layers. The analytical solution in the frame of the most general dynamical approach is found and the condition of application of the kinematical approximation is discussed. Special attention is paid to the periodic multilayer structure containing two layers of different materials with more than one pair of these layers in the period.

Metal Oxide Surfaces: Electronic Structure and Molecular Adsorption

Abstract: The surface science of oxides has received considerably less attention than the study of metal surfaces. We review some of our work on structural characterization and electronic structure investigations on oxide surfaces which we prepare as thin oxide films on metallic substrates. This preparation method leads to certain experimental advantages concerning the application of electron spectroscopies on such systems. We discuss molecular adsorption on non-polar and polar surfaces of clean oxides. The influence of metal deposition onto the oxide surfaces and on their properties with respect to adsorption is described. Light induced processes with full quantum state detection of desorbing molecules are addressed. The possibility of studying dynamic processes in adsorbates on oxides via electron spin resonance spectroscopy is evaluated.

Impurity Binding Energy of a Cylindrical Quantum Wire in a Magnetic Field

Abstract: By using the strong-perturbation theory, suitable trial wave functions are chosen and the ground-state energy and the binding energy of the ground state of a hydrogenic donor in a quantum wire are calculated in the presence of a uniform magnetic field applied parallel to the wire axis. The quantum wire is assumed to be a cylinder of GaAs having an infinite confining potential. The ground-state energy and the binding energy increase as the strength of the magnetic field increases. Our results agree with those of previous calculations. The calculating method is novel and has general significance.

The ultrafast response of a scanning tunneling microscope

Abstract: The development of the ultrafast scanning tunneling microscope is reviewed. It is shown that the response of the tunneling gap of a scanning tunneling microscope to excitation by a subpicosecond electrical pulse has a capacitive component, the origin of which is quantum mechanical

Comparison of Surface-Enhanced Raman and Hyper-Raman Spectra of the Triphenylmethane Dyes Crystal Violet and Malachite Green

Abstract: Vibrational spectra of two dyes, malachite green and crystal violet are presented. Raman and hyper-Raman spectra are recorded as surface-enhanced (hyper-) Raman spectra on silver colloids. These data are compared to the respective IR spectra with respect to the point group of the molecules (C 2 and C 3 ). Theoretical calculations (PM3 semiempirical) and polarized Raman spectra are used to give an assignment of some vibrational bands. The advantage of SERS (surface-enhanced Raman scattering) and SEHRS (surface-enhanced hyper-Raman scattering) is discussed for the analysis of vibrational spectra

Two‐Photon Photoemission Spectroscopy of Electronic States at Metal Surfaces

Abstract: In two-photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon of the same pulse ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the position and the lifetime of both the initial and, in particular, the intermediate state. Most of the work in two-photon photoemission spectroscopy has been devoted to the investigation of image potential states (image states). They are bound states of electrons in front of metal surfaces in the potential well built by the image potential and the surface. The situation is analogous to the hydrogen atom, with a binding energy of the lowest state of 850 meV in first approximation. Up to four members of the Rydberg series are detected. Experimental results on the binding encrgy of image states in (111) and (100) surfaces of Ag, Au, Cu, and Pd agree with calculations by a one-dimensional scattering model. The results for Co, Ni, and Fe are not yet understood. The effective mass is close to unity except for Ag(111). Lifetimes between 5 and 170 fs arc measured. They can be explained by decay via electron-hole pair creation in the bulk. Image states on metal overlayers yield information on film growth and surface morphology. Dielectric overlayers are been investigated. Adsorbate states of atoms and molecules adsorbed on metal substrates can also be studied by two-photon photoemission spectroscopy. Results for Na, O, CO, and NO on Cu(111) are reported.

Polaronic Hamiltonian and Polar Optical Vibrations in Multilayer Structures

Abstract: A theory of polar optical vibrations in multilayer semiconductor structures is developed taking into account phonon dispersion. A method of diagonalization of the equations of motion for inertial polarization vectors in the finite basis is used, that is founded on a finite number of degrees of freedom of the system. The Hamiltonian of the electron- phonon interaction is deduced. The dispersion law and the spatial dependence of the obtained eigenmodes are in good agreement with experimental data and microscopic models. It is shown that the electron scattering rate in a magnetic field depends on the chosen model of polar optical vibrations.

Calculation of the Ground State Properties of Diamond and Cubic Boron Nitride

Abstract: Using the modified augmented plane wave method (MAPW), the ground state properties of cubic boron nitride and diamond are investigated within the framework of the local density approximation (LDA). As is common for LDA, the value of the band gap is underestimated by the one-particle excited states. Calculation of the total energy as a function of the lattice parameter yields equilibrium lattice constants within 1% of the experimental value. Furthermore, the hydrostatic bulk moduli obtained deviate from the experimental values by about 10% in the case of BN and 4% in the case of diamond. An estimate of the cohesion energy of both materials is obtained from the atomic limit within the MAPW scheme yielding improved values when compared to the difference between crystal and single atom LDA total energies. Mit Hilfe des MAPW-Verfahrens werden im Rahmen der lokalen Dichte-Naherung Grundzustands-eigenschaften der Halbleiter Bornitrid (Zinkblendestruktur) und Diamant bestimmt. Wie fur LDA ublich, wird der Wert der Bandlucke durch die Einteilchen-Anregungsenergien unterschatzt. Die Berechnung der Gesamtenergie als Funktion der Gitterkonstante liefert Ubereinstimmung mit dem experimentellen Wert der Gitterkonstante bis auf 1%. Der hydrostatische Kompressionsmodul weicht bei Bornitrid um 10% und bei Diamant um 4% vom Experiment ab. Eine Abschatzung der Kohasionsenergie der beiden Materialien wird aus den MAPW-Ergebnissen im Grenzfall a ∞ gewonnen; sie liefert gegenuber dem direkten Vergleich mit atomaren LDA-Resultaten verbesserte Werte.

Hole Transport in a Vapor Deposited Phenylenediamine Molecular Glass

Abstract: Hole mobilities have been measured in vapor deposited amorphous layers of N,N′-bis(1-naphthalenyl)-N,N′-diphenyl-4,4′-phenyldiamine (NPPDA) over a wide range of fields and temperature. The results are described within the framework of a formalism based on energetic disorder of the hopping sites. The variance of hopping site energies is 0.098 eV. The energy variance can be described by a model based on dipolar disorder. The model is based on the assumption that the total variance is comprised of a dipolar component and a van der Waals component. For NPPDA, the dipolar component is 0.080 eV and the van der Waals component is 0.067 eV.

Optical Properties of Undoped and V-Doped VA−VIA−VIIA Single Crystals

Abstract: SbSI: V, SbSeI: V, BiSI: V, and BiSeI: V single crystals are grown by the vertical Bridgman technique using ingots synthesized from mixtures of the elements (purity 99.9999%). The grown single crystals have an orthorhombic structure and an indirect energy band structure. The temperature dependence of the optical energy gap is well fitted by the Varshni equation. Impurity optical absorption peaks due to the doped vanadium appear and are assigned to the electron transitions between the energy levels of the V 2+ ion located at T d sites of the host lattice

Electron transport in vapor deposited molecular glasses: the role of group dipole moments

Abstract: Electron mobilities are measured in vapor deposited layers of N,N'-bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI). NTDI is a weakly polar compound with a molecular dipole moment of 0.3 D. The molecular moment is the vector sum of two group moments, each approximately 3.3 D. The results are described by a formalism based on disorder, due to Bassler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is the energy width of the hopping site manifold. For NTDI, the width is 0.093 eV. The width can be described by a model based on dipolar disorder. The model assumes that the width is comprised of a dipolar component and a van der Waals component. A comparison of the experimental results with predictions of the model leads to the conclusion that the dipolar component to the width is determined by group dipole moments of two carbonyl groups adjacent to the imide acceptor functionalities. The positional disorder parameter is 2.3 and attributed to packing constraints. The prefactor mobility is 1.0 x 10 -2 cm 2 /Vs.

Photoionization of Shallow Donor Impurities in Finite‐Barrier Quantum Wells

Abstract: The dependence of the photoionization cross-section on photon energy is calculated for shallow donors in finite-barrier GaAs/Ga 1−x Al x As quantum wells as well as the binding energy as a function of well width. The effect of a magnetic field is also considered

Blue‐Green ZnSe Laser Diodes for Optoelectronics. Present State at Würzburg University

Abstract: A review is given on the research activities at Wurzburg University in the field of blue-green emitting ZnSe laser diodes. Of great importance for both the structural and electrical properties is the possibility to grow a GaAs buffer layer on the GaAs substrate prior to the II–VI growth. Pulsed room temperature lasing with a duty cycle up to 17% is obtained using quaternary ZnMgSSe cladding layers, ternary ZnSSe waveguides, and a single CdZnSe quantum well. Important issues for long-term stability of such laser diodes like growth start, p-type doping, and ohmic contacts are discussed.

X‐Ray Diffraction Study of Thin Porous Silicon Layers

Abstract: Des experiences de diffraction de rayons X haute resolution sont effectuees sur des echantillons de silicium poreux (type p + , porosite 40%) de differentes epaisseurs (de 0,03 a 20 μm). Pour les couches les plus epaisses des pics de Bragg fins sont observes, avec des profils proches de ceux donnes par la theorie dynamique de la diffraction. Pour des epaisseurs plus petites (dans le domaine de 0,1 μm) le pic de Bragg de la couche poreuse n'est pas resolu, mais apparait comme un epaulement au pic du substrat. Des franges d'epaisseur sont observees, montrant ainsi que les surfaces de la couche poreuse sont bien definies.