Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2002"

Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap.

Abstract: (a) Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia (b) Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany (c) Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan (d) Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus (e) LfI, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany

Band Gap of Hexagonal InN and InGaN Alloys

Abstract: A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 10 18 and 4 × 10 19 cm -3 . It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich In x Ga 1-x N alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of In x Ga 1-x N alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In 2 O 3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

Ab initio Study on the Magnetism in ZnO‐, ZnS‐, ZnSe‐ and ZnTe‐Based Diluted Magnetic Semiconductors

Abstract: The magnetism and the electronic structure of II-VI compound-based diluted magnetic semiconductors (DMSs) are investigated based on ab initio calculations. The stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe- and ZnTe-based DMSs is investigated systematically and materials design for ferromagnetic DMSs is given. In all host materials, it is found that V- and Cr-doped systems are ferromagnetic and Mn-doped systems are spin-glass state. On the other hand, for Fe-, Co- and Ni-doped systems, the ferromagnetic state is stable in ZnO-based DMS, however, the spin-glasss state is stable in ZnS-, ZnSe- and ZnTe-based DMSs. The carrier-induced ferromagnetism in ZnO-based DMSs is also investigated and it is found that their magnetic states are controllable by changing the carrier density. Analysing the calculated density of states, the mechanism to stabilize the ferromagnetic state in the DMSs is discussed.

p‐Type ZnO Layer Formation by Excimer Laser Doping

Abstract: A p-type ZnO layer was fabricated by excimer laser doping technique using thermally oxidized ZnO films. Epitaxial ZnO(0001) films were formed by thermal oxidation of epitaxial ZnSe films on Si(111) substrate grown by remote plasma enhanced metal-organic chemical vapor deposition. The p-type ZnO was fabricated by excimer laser irradiation with Sb as a dopant source. A good Ohmic contact was obtained between Sb doped ZnO layer and gold metal electrodes. The Sb doped ZnO layer showed positive Hall coefficient, the resistivity was 8 × 10 -3 Ω cm with a hole mobility of 1.5 cm 2 /Vs and an acceptor concentration of 5 x 10 20 cm -3 as p-type, respectively.

Use of Luminescent CdSe–ZnS Nanocrystal Bioconjugates in Quantum Dot‐Based Nanosensors

Abstract: Biomolecules labeled with luminescent colloidal semiconductor quantum dots (QDs) have potential for use in numerous applications, including fluoro-immunoassays and biological imaging. QD labels exhibit size-tunable narrow-band luminescent emission and high resistance to photodegradation. They also exhibit efficient Forster energy transfer between neighboring QDs of different sizes and their emission is readily quenched by bound fluorescent dyes. In this paper, we describe preliminary results aimed at defining conditions for the design and preparation of nanoscale QD-bio-conjugate sensors based on fluorescence quenching. We envision building sensor assemblies that employ quantum dots linked with dye-labeled biological receptors that utilize donor-acceptor energy transfer between QDs and receptors for conducting recognition-based assays. In particular, we report the effects of varying the concentration of energy acceptors bound to nanocrystal surfaces under both soluble and solid phase conditions on quenching phenomena.

Growth Temperature Dependence of Indium Nitride Crystalline Quality Grown by RF‐MBE

Abstract: We have investigated the growth temperature dependence of InN crystalline quality grown by RF-MBE. It was confirmed that the crystalline quality improved by increasing the growth temperature within the dissociation limit of InN. We obtained FWHM as narrow as 3.7 cm -1 for E 2 (high frequency) phonon mode peak of Raman spectroscopy and 24 meV for the band-edge luminescence at 77 K. These values prove excellent quality of our samples. In this study, we obtained optical band-gap energy of around 0.8 eV at room temperature from the high-quality InN samples.

X-ray Photoelectron Spectra and Electronic Structure of Bi2S3 Crystals

Abstract: The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of the semiconductive Bi 2 S 3 single crystal, which shows anomalies in various physical properties and weak phase transitions without change of symmetry. The XPS were measured with monochromatized Al K a radiation in the energy range 0-1400 eV and the temperature range 160-460 K. The valence band is located 0.55-6.5 eV below the Fermi level and shows non-linear dependence on temperature. Experimental energies of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the Bi 2 S 3 crystal. The chemical shifts in the Bι 2 S 3 crystal for the Bi and S states are obtained. Results revealed the origin of a weak polarity of the crystals at room temperature, which results in anomalies in the physical properties, and shifts the Fermi level and all the electronic spectrum.

Surface Potential at as‐Grown GaN(0001) MBE Layers

Abstract: The Fermi level pinning at GaN and AlGaN surfaces was investigated in-situ by X-ray photoemission spectroscopy (XPS). The measurement revealed, that the pinning position is strongly affected by the Ga/N ratio during MBE growth. The effect of the surface potential on the electronic properties of two dimensional electron gases (2DEG) forming at AlGaN/GaN interfaces was studied by means of self-consistent Schrodinger-Poisson calculations.

ZnS-Based Photonic Crystals

Abstract: Recent studies of the application of II-VI materials to the fabrication of photonic crystals are reported. Modeling studies show the potential of ZnS photonic crystals to provide a new method of controlling the emission characteristics of this material system so as to enhance color, intensity and decay time. The same structures are also shown to possess giant refraction and dispersion properties that can be used to control (collect, focus and steer) light. The fabrication of these period structures is addressed. Two approaches are being considered: the fabrication of two-dimensional ZnS photonic crystals by conventional electron beam lithography and the formation of three-dimensional ZnS photonic crystals by cost-effective self-assembly methods. ZnS and related II-VI compounds are very attractive for applications in photonic crystal devices operating in the visible and near IR region due to their high indices of refraction and large bandgaps that make them highly transparent in the visible. We report recent studies on the self-assembly of nanoparticles and subsequent ZnS infiltration techniques that can be used to control the emission and out-coupling properties of phosphors embedded in a photonic crystal.

Structural Investigation of Copper Phthalocyanine Thin Films Using X‐Ray Diffraction, Raman Scattering and Optical Absorption Measurements

Abstract: Structural properties of copper phthalocyanine (CuPc) powder and its thin films coated on glass plate are investigated using X-ray diffraction, Raman scattering and optical absorption techniques. These experiments are carried out at room temperature on CuPc powder and as-coated, 320 °C and 520 °C annealed, CuPc thin films. From the X-ray diffraction (XRD), we find that the CuPc powder exists in monoclinic phase, as-coated film is in orthorhombic α-phase and the film annealed at 320 °C exists in monoclinic β-phase with different lattice parameters as compared to those of the powder phase. The XRD peaks in 520 °C annealed film do not show the presence of CuPc but indicate the presence of copper oxide (CuO) nanoclusters of approximately 15 nm size. The optical Raman phonon frequencies of CuPc powder, as-coated and 320 °C annealed CuPc films show a slight shift in the intramolecular high frequency modes. Whereas, their low frequency Raman spectra from intermolecular modes are characteristically different indicating different phases. The different characteristic behavior of the optical absorption spectra of the as-coated and 320 °C annealed CuPc films corroborate the structural changes in the annealed film.

Strategies for Controlling the Conductivity of Wide-Band-Gap Semiconductors

Abstract: Wide-band-gap semiconductors have numerous applications in electronic and optoelectronic devices. Progress is currently hampered by a lack of control over electrical conductivity: for instance, ZnO is typically n-type conductive, the cause of which has been widely debated. A first-principles investigation, based on density functional theory, shows that native defects are unlikely to be the cause of the unintentional n-type conductivity. An investigation of likely donor impurities reveals that hydrogen acts as a shallow donor. In contrast, hydrogen acts as an amphoteric impurity in most other semiconductors; calculations for hydrogen in ZnSe are included here to highlight this difference. Experimental results are discussed in light of these new insights.

Quantum Confined Atoms of Doped ZnO Nanocrystals

Abstract: The luminescent properties of ZnO nanocrystals doped with divalent manganese and europium ions show efficient broad-band emission at room temperature. These ZnO based quantum confined atoms potentially offer efficient nanophosphors that can be excited in the near UV range for applications in the solid state lighting, displays and optically pumped lasers.

Architectures for a Spin Quantum Computer Based on Endohedral Fullerenes

Abstract: We present a discussion of recent concepts for the construction of a spin quantum computer using endohedral fullerenes. The fullerene molecule is a static, room-temperature trap for atoms with slowly relaxing electron andnuclear spins. The fullerene "containers" can be used to arrange the spins in complex structures such as a linear chain, to form a spin quantum register. We discuss the probable properties of such registers and different strategies to use them in a quantum computer design, including gating and read-out methods.

Interfacial elasticity corrections to size-dependent strain-state of embedded quantum dots

Abstract: Introduction Quantum Dots (QDs) have recently been the focus of several experimental and theoretical researchers due to the promise of improved and new opto-electronic properties [1]. Frequently, embedded QD structures (e.g. InAs/GaAs system), to preserve coherency, must accommodate large lattice mismatch. The ensuing elastic relaxation and the hydrostatic strain state within the QD structure are well known to impact its opto-electronic properties [2, 3]. Several works, of varying sophistication (both analytical and numerical), have focused on the “accurate” calculation of the strain state in buried quantum dots [4–8]. Recently, two papers have caught the present authors attention. Pan and Young [4] indicated significant difference in strain calculation when anisotropic elastic behavior is assumed compared to simplified isotropic elasticity. Further, a recent article by Ellaway and Faux [8] indicated (via atomistic simulations) that the elastic properties of QD are staindependent and such a consideration on strain calculation can result in a significant correction (of 16%) to the hydrostatic strain (in their article, for a buried spherical InAs/GaAs QD). In this communication, we show that the so-far unconsidered interfacial elastic properties can also significantly alter the strain calculations; the exact correction sensitively being dependent upon the size of the QD structure and the interfacial elastic constants. We find that the correction resulting from interfacial elasticity is comparable to that due to either anisotropic or strain-dependency effects. Classical elasticity (on which most of the previous works are based) does not admit intrinsic size dependence in the elastic solutions of embedded inhomogeneities. For structures with sizes > 50 nm, typically, the surface-to-volume ratio is negligible and the deformation behavior is governed by classical bulk strain energy. Currently, no formulation exists which combines interface elasticity with bulk elasticity to analyze embedded inclusions. Eshelby’s [9] celebrated formalism, often used in QD literature, is based entirely on classical bulk elasticity. In this communication (using a variational approach) we derive a general expression for the correction in hydrostatic strain due to interfacial elasticity (for an embedded spherical quantum dot). Despite the lack of precise data, we are able to show (using InAs/GaAs as an example system) that inclusion of interfacial elasticity effects can result in minimum corrections between 1.8% and 12% in the typical size range of QD structures (2–20 nm).

Optical Properties of InxGa1—xN with Entire Alloy Composition on InN Buffer Layer Grown by RF‐MBE

Abstract: Optical characterization of In x Ga 1-x N layers was done by photoluminescence, cathodoluminescence and optical absorption measurements. PL emissions less than 1.9 eV were observed from In x Ga 1-x N films with x > 0.53 that showed no phase separation. A bowing parameter of 2.3 eV was obtained from the relationship between PL emission energy and alloy composition. Absorption edge dependence on In composition was similar to the luminescence peak dependence on In composition. These optical properties indicate that the bandgap energy of InN is below 1.0 eV.

Luminescent Quantum Dot-Adaptor Protein-Antibody Conjugates for Use in Fluoroimmunoassays

Abstract: A method for the preparation and characterization of bioinorganic conjugates made with highly luminescent semiconductor CdSe-ZnS core-shell quantum dots (QDs) and antibodies for use in fluoroimmunoassays is presented. The conjugation strategy employs two routes: 1. Use of an engineered molecular adaptor protein, attached to the QDs via electrostatic/hydrophobic self-assembly, to link the inorganic fluorophore with antibodies, and 2. use of avidin, also electrostatically self-assembled onto the nanocrystal surface, which allows QD conjugation to biotinylated antibodies via avidin-biotin binding scheme. With this approach, the average number of antibodies conjugated to a single QD can be varied. In addition, we have developed a simple purification strategy based on mixed composition conjugates of the molecular adaptor and a second inert two-domain fusion protein that allows the use of affinity chromatography. QD/adaptor-antibody conjugates were successfully employed in fluoroimmunoassays for the detection of small molecule analytes, 2,4,6-trinitrobenzene (TNB) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We also demonstrate the use of QD/avidin-antibody conjugates for fluoroimmunoassays using a model protein system.

The Photoionization Cross‐Section of Impurities in Quantum Dots

Abstract: In quantum dot structures, photoionization has been considered as an optical transition from the impurity ground state to the conduction subbands. Using a variational approach, we have calculated the photon energy dependence of the photoionization cross-section for a hydrogenic donor impurity in an infinite barrier GaAs quantum dot as a function of the sizes of the dot and the impurity position. The results we have obtained show that the photoionization cross-section is strongly affected by the quantum size effects and the position of the impurity and its overall shape seems to be a signature of the quantum dot system.

Anti‐Stokes Photoluminescence in II–VI Colloidal Nanocrystals

Abstract: For the first time highly efficient photon energy up-conversion (anti-Stokes photoluminescence) has been observed in colloids of CdTe nanocrystals (NCs) and CdSe NCs covered by ZnS shell. The maximum magnitude of the up-converted blue shift ΔE max UC = 335 meV was obtained for CdSe/ ZnS NCs under excitation by a low-power He-Ne laser. The maximum efficiency for energy conversion was estimated to be about η max = 2.7 x 10 -2 % at room temperature. The intensity of the anti-Stokes photoluminescence increases with temperature and shows a linear dependence on the excitation intensity. We suggest that subgap surface states are involved as intermediate states in the up-conversion process rather than nonlinear two-photon absorption or Auger processes.

Lattice dynamical properties of the rare earth aluminum garnets (RE3Al5O12)

Abstract: This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al-O and the dodecahedral RE-O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.

Percolation Approach to Hopping Transport in Organic Disordered Solids

Abstract: Percolation approach is used to study the dc hopping conductivity and thermopower in systems with a Gaussian density of localized states typical for disordered organic materials. It is shown that the theoretical methods developed earlier for the description of hopping transport in disordered inorganic solids, such as amorphous semiconductors, can also be successfully applied to description of hopping transport in organic disordered solids, such as conjugated or molecularly doped polymers. Calculations within the percolation approach give results in excellent agreement with those obtained by using a more transparent, though less rigorous approach based on the concept of the transport energy.

Electron Holography Studies of the Charge on Dislocations in GaN

Abstract: The measurement of charge on dislocations in GaN by electron holography is described. Recent results are presented showing that edge dislocations in n-doped GaN are highly negatively charged, whereas those in p-doped GaN are positively charged. It is shown that the results are consistent with a model which assumes Fermi level pinning at dislocation states about 2.5 V below the conduction band edge. The application of electron holography to screw dislocations, and the dependence of the observations on the dislocation core structure, are also discussed.

Zero‐Phonon Lines: The Effect of a Strong Softening of Elastic Springs in the Excited State

Abstract: The optical dephasing, caused by a quadratic interaction with acoustic phonons, is studied on the basis of the nonperturbative theory. The effect of a strong weakening of the elastic springs on the electronic transition is studied. It is found that, if the weakening brings the system close to the dynamical instability, then the usual ∝ T7 dependence of the homogeneous width of a zero-phonon line (ZPL) is replaced by the ∝ T3 dependence in a broad temperature interval. An equation is derived which describes the additional broadening of a ZPL caused by the dynamical instability of the final state. The model explains the temperature broadening and shift of the zero-phonon lines of some nitrogen-containing centers in diamond crystals.

Phase Diagram of Ising Nano-Particles with Cubic Structures

Abstract: The method of variational cumulant expansion (VCE) on the Ising model is applied to investigate phase transitions in magnetic alloy nano-particles, in which a random site distribution of component atoms is assumed. By introducing the effect exchange constant, it is shown numerically that nano-particles can be treated the same way as bulk materials. The Curie temperature and the Neel temperature of binary magnetic nano-particles with cubic lattices are calculated. It is found that both the Curie temperature and the Neel temperature decrease with decreasing size of the nano-particle. Phase diagrams of binary alloy nano-particles are investigated for variable doping concentration and the results are discussed. As the nano-particle size grows larger, the critical point approaches that of a bulk. On the other hand, the tri-critical point is independent of the nano-particle size and is always the same as that of the bulk.

Hopping Models and ac Universality

Abstract: Some general relations for hopping models are established. We proceed to discuss the universality of the ac conductivity which arises in the extreme disorder limit of the random barrier model. It is shown that the relevant dimension entering into the diffusion cluster approximation (DCA) is the harmonic (fracton) dimension of the diffusion cluster. The temperature scaling of the dimensionless frequency entering into the DCA is discussed. Finally, some open problems regarding ac universality are listed.

Simple Estimates of the Electron Transport Properties of Molecules

Abstract: We discuss how the complex bandstructure describes tunneling within a molecule, and how it can be used to understand single molecule conduction. Simple estimates of the conduction properties of some molecules can be obtained with minimal (or no) computational effort. Examples considered include tunneling current through SiO 2 , transistor action of biphenyl-dithiol, the effect of benzene ring rotation on tunneling through polyphenyl chains, and a designed molecule which -according to the form of its complex bandstructure - may function as a p-channel transistor.

Electron and Positron Energy Levels and Deformation Potentials in Group‐III Nitrides

Abstract: Electron and positron energy levels and deformation potentials are calculated in III-V bulk cubic nitrides using the pseudopotential formalism and the independent particle model. The calculated electron deformation potentials are found to be in reasonable agreement with the available data in the literature, whereas our obtained value for the positron deformation potential in BN disagrees with that calculated by other authors using ab initio methods. Furthermore, the positron affinity to different studied materials and heterostructures formed by these materials has been determined. This quantity is important to the understanding of positron trapping at interfaces and at precipitates. An attempt has been made to scale positron affinities and deformation potentials with the lattice constant. Such scaling is found to be not possible.

First‐Principles Study of Doping Limits of CdTe

Abstract: First-principles total energy and band structure calculations are performed to understand the factors that limit doping in CdTe. We calculated systematically the formation energies and transition energy levels of intrinsic and extrinsic defects and defect complexes. We find that n-type doping in CdTe is limited by the spontaneous formation of the intrinsic closed-shell cation vacancy V 2- Cd and/or DX centers. For p-type doping, it is limited by not having a dopant with both high solubility and shallow acceptor level.

Key Aspects of CdTe/CdS Solar Cells

Abstract: Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb 2 Te 3 in contacts to CdTe. Nominally identical solar cells fabricated using 99.999% pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher V oc , FF and efficiency, and larger grain size. Quantum efficiency and C-V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb-Te contacts to CdTe reveals that annealing in air at 400 °C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation.