Showing papers on "Band gap published in 1992"

New one-dimensional conductors: Graphitic microtubules.

Abstract: On the basis of realistic tight-binding band-structure calculations, we predict that carbon microtubules exhibit striking variations in electronic transport, from metallic to semiconducting with narrow and moderate band gaps, depending on the diameter of the tubule and on the degree of helical arrangement of the carbon hexagons. The origin of this drastic variation in the band structure is explained in terms of the two-dimensional band structure of graphite.

Are fullerene tubules metallic

Abstract: We have calculated the electronic structure of a fullerene tubule using a first-principles, self-consistent, all-electron Gaussian-orbital based local-density-functional approach. Extending these results to a model containing an electron-lattice interaction, we estimate that the mean-field transition temperature from a Peierls-distorted regime to a high-temperature metallic regime should be well below room temperature. Such fullerene tubules should have the advantages (compared to the other conjugated carbon systems) of a carrier density similar to that of metals and zero band gap at room temperature.

Electronic structure and optical properties of silicon crystallites: Application to porous silicon

Abstract: We have calculated the electronic structure of spherical silicon crystallites containing up to 2058 Si atoms. We predict a variation of the optical band gap with respect to the size of the crystallites in very good agreement with available experimental results. We also calculate the electron‐hole recombination time which is of the order of 10−4–10−6 s for crystallites with diameters of 2.0–3.0 nm. We conclude that small silicon crystallites can have interesting optical properties in the visible range. These results are applied to porous silicon for which we confirm that a possible origin of the luminescence is the quantum confinement.

First-principles calculations of the electronic properties of silicon quantum wires.

Abstract: We have performed first-principles pseudopotential calculations for H-terminated Si wires with thicknesses from 12 to 23 \AA{}, calculating the band gaps and optical matrix elements. Comparison with effective-mass theory shows that the latter is valid for wires wider than 23 \AA{}. We have used our data to analyze the luminescent properties of highly porous Si fabricated by electrochemical etching of Si wafers in HF-based solutions.

A new class of small band gap organic polymer conductors

Abstract: Polysquaraines and polycroconaines have been synthesized. They form a new class of polymers with a small band gap, down to 0.5 eV. The small gap arises from the regular alternation of strong donor and acceptor-like moieties in a conjugated backbone.

Epitaxial growth and characterization of zinc‐blende gallium nitride on (001) silicon

Abstract: GaN films have been epitaxially grown onto (001) Si by electron cyclotron resonance microwave‐plasma‐assisted molecular‐beam epitaxy, using a two‐step growth process, in which a GaN buffer is grown at relatively low temperatures and the rest of the film is grown at higher temperatures. This method of film growth was shown to lead to good single‐crystalline β‐GaN and to promote lateral growth resulting in smooth surface morphology. The full width at half‐maximum of the x‐ray rocking curve in the best case was found to be 60 min. Optical‐absorption measurements indicate that the band gap of β‐GaN is 3.2 eV and the index of the refraction below the absorption edge is 2.5. Conductivity measurements indicate that the films may have a carrier concentration below 1017 cm−3.

Modeling the optical dielectric function of semiconductors: Extension of the critical-point parabolic-band approximation.

Abstract: A model is proposed for the line shape of the optical dielectric function of zinc-blende semiconductors. For comparison with previously proposed models, this model is used primarily with spectroscopic ellipsometry data (but also transmission data below 1.5 eV) to obtain an analytic room-temperature dielectric function for GaAs. It is found to be more generally valid than the harmonic-oscillator model, the critical-point (CP) model, or the model of Adachi. It is applicable over the entire range of photon energies, below and above the lowest band gaps, incorporates the electronic band structure of the medium, and exactly satisfies the Kramers-Kronig transformation. It goes beyond the CP parabolic-band approximation in that it correctly takes into account the full analytic form of the electronic density of states and thus does not require the use of arbitrary cutoff energies. Also, it allows one to go beyond the usual approximation of Lorentzian broadening, which is known to be incorrect for elements and compounds above very low temperatures. For these reasons, it results in excellent quantitative agreement with experimental results for the dielectric function and for its derivatives with respect to photon energy, much better than that given by earlier models. Finally, the parameters of the model are physically significant and are easily determined as functions of composition for semiconductor alloys. Application of the model to the fitting of spectroscopic data on GaAs strongly suggests that spectroscopic ellipsometry does not measure the true bulk dielectric function. It also supports the conclusion that the line-shape broadening in GaAs at room temperature is more nearly Gaussian than Lorentzian.

Hydrogenic impurities in GaAs-(Ga,Al)As quantum dots.

Abstract: The ground-state energy and the binding energy of shallow hydrogenic impurities in spherical GaAs-(Ga,Al)As quantum dots have been calculated as functions of the radius of the dot. The binding energy has been calculated following a variational procedure within the effective-mass approximation. We have used a finite confining potential well with depth determined by the discontinuity of the band gap in the quantum dot and the cladding. Calculations were also performed for an infinite confining potential. For the infinite potential well we found that the impurity binding energy increases as the dot radius decreases whereas in the finite potential-well situation, the binding energy reaches a peak value as the dot radius decreases and then diminishes to a limiting value corresponding to the radius for which there are no bound states in the well. We found that the strong electronic confinement in these quantum dots reflects itself in the ground-state energy and in the impurity binding energies, which are higher than those found in GaAs-(Ga,Al)As quantum wells and quantum-well wires.

Optical properties of porous silicon: A first-principles study

Abstract: We show that first-principles electronic structure calculations of silicon wires with diameters up to \ensuremath{\sim}1.5 nm support the idea that quantum confinement and surface effects are responsible for the luminescence in porous silicon. Instead of the indirect gap of crystalline bulk silicon, the band structure of these wires exhibits a direct gap at k=0. The imaginary part of the dielectric function, polarized in the direction of the wire, shows a peak in the visible range. The dependence of this feature on wire size is analyzed and correlated to experimental luminescence spectra.

Photonic band gaps in two-dimensional square and hexagonal lattices.

Abstract: Two-dimensional square and hexagonal lattices exhibit photonic band gaps common to s- and p-polarized waves. These gaps occur from an overlap of the gaps between the first and second p bands and higher s bands. A dielectric structure with a hexagonal lattice of air holes requires a lower index contrast to generate a band gap and gives rise to larger gaps than a square lattice. Furthermore, square and hexagonal lattices of dielectric rods in air do not give rise to band gaps even when asymmetry is introduced to lift the degeneracies.

Graded band gap ohmic contact to p‐ZnSe

Abstract: We describe a low‐resistance quasi‐ohmic contact to p‐ZnSe which involves the injection of holes from heavily doped ZnTe into ZnSe via a Zn(Se,Te) pseudograded band gap region. The specific contact resistance is measured to be in the range of 2–8×10−3 Ω cm2. The graded heterostructure scheme is incorporated as an efficient injector of holes for laser diode and light emitting diode devices, demonstrating the usefulness of this new contact scheme at actual device current densities.

Empirical fit to band discontinuities and barrier heights in III–V alloy systems

Abstract: We present a figure summarizing the variation of conduction band discontinuity, valence band discontinuity, and gold Schottky barrier height for binary and ternary III–V semiconductors. This figure, which applies to unstrained material, makes use of the property of transitivity in band alignments, and the observed experimental correlation between barrier heights and band gap discontinuities, to consolidate a wide range of data. The figure should be very useful in the design of novel heterostructure electronic and optical devices.

Near‐band‐gap photoluminescence from pseudomorphic Si1−xGex single layers on silicon

Abstract: The systematic study of band‐edge luminescence in pseudomorphic Si/Si1−xGex/Si double‐heterostructure layers is reported for a wide composition range, 0.12

Theory of band tails in heavily doped semiconductors

Abstract: The random distribution of impurities in a semiconductor host lattice introduces potential fluctuations that allow energy levels within the forbidden energy gap. This statistical effect distorts the unperturbed density of states of the pure semiconductor, and, at high doping concentrations, substantial band tails appear. The changes in the density-of-states function are particularly important in determining the number of free carriers in a heavily doped semiconductor. Together with many-particle interactions, band tailing constitutes one of the most significant heavy-doping effects. Although the band-tailing phenomenon has been studied for many years, only a one-dimensional analytical model, which assumes a Gaussian whitenoise probability distribution of the potential fluctuation, exists. In this paper the different classes of theories that describe this band tailing of the density of states in heavily doped semiconductors are reviewed in detail.

Groundstate properties of a generalized VBS-model

Abstract: We study a generalized “q-deformed” VBS-model. This is a spin-1 quantum antiferromagnet with nearest neighbour interaction on a linear chain. The exact grounstate is determined in the form of a matrix-product of individual site-contributions. All relevant groundstate properties are calculated. The groundstate is unique, it has a finite gap to the excitations, and correlations decay exponentially. Thus the model has all the properties described by Haldane to be generic for certain quantum antiferromagnets with integral spin.

Luminescence quenching and the formation of the GaP1−xNx alloy in GaP with increasing nitrogen content

Abstract: A study of the luminescence properties of epitaxial GaP containing atomic N grown by molecular beam epitaxy using NH3 and PH3 as the column V sources was conducted. The 77 K photoluminescence spectra of the N‐doped epitaxial GaP showed a continuous redshift, from 5691 A (2.18 eV) to 6600 A (1.88 eV), resulted when the N concentration exceeded ∼5–7×1019 cm−3. This energy shift was found to be consistent with energy gap predictions using the dielectric theory of electronegativity for the GaP1−xNx system. The data also indicate that the emission intensity was maximum for N∼1×1020 cm−3, and then monotonically decreases with increasing N content. This is consistent with the formation of an indirect band‐gap semiconductor.

Influence of doping on the structural and optoelectronic properties of amorphous and microcrystalline silicon carbide

Abstract: Amorphous and microcrystalline silicon carbide, undoped and doped, has attracted a great attention for its optical and electrical properties. The introduction of dopant atoms in the network of amorphous films permits the control of electrical properties but it gives rise to a decreasing of the optical gap. Microcrystalline SiC:H films seem to provide films having a wide range of electrical conductivities without drastic change in the optical gap. This paper presents the results of a detailed study on the effects of boron and phosphorus doping on structural, optical, and electrical properties of a‐SiC:H and μc‐SiC:H films. An optical gap as high as 2.1 eV, together with a conductivity of 10−3 Ω−1 cm−1, are shown by doped μc‐SiC:H.

The electronic and atomic-structure of hydrogenated amorphous si-c alloys

Abstract: The electronic structure of amorphous Si—C (a-Si1-xCx) and hydrogenated amorphous (a-Si1-x Cx: H) alloys has been calculated as a function of composition, chemical ordering, C coordination and H configuration using the tight-binding method. The maximum in the optical bandgap observed in a-Si1-x Cx: H at around x = 0·6 is not due to chemical ordering but to a change in band edge character, from Si-Si bond states for x 0·6. The gap is controlled by the degree of clustering of sp2 sites for x > 0·6. Interpretation of the optical gap, photoemission and X-ray emission data suggests that a moderate degree of chemical ordering exists in a-Si1-xCx: H and is higher in a-Si1-xCx. Hydrogenation widens the gap over the entire composition range, in Si-rich alloys by a recession of the valence band and in C-rich alloys by reducing the cluster sizes. The position of defect states due to Si and C dangling bonds is calculated and combined into a band model for the alloys. Experimen...

Below-band-gap third-order optical nonlinearity of nanometer-size semiconductor crystallites.

Abstract: We find evidence that quantum confinement of electrons in small semiconductor particles causes the nonlinear optical properties in the transparency region to differ markedly from those of bulk semiconductors. The optical Stark effect makes the dominant contribution to the third-order refractive nonlinearity.

Intrinsic origin of visible light emission from silicon quantum wires: Electronic structure and geometrically restricted exciton.

Abstract: We theoretically investigate excitonic effects on the optical properties of silicon quantum wires, based on ab initio electronic structure calculations. The Si wires have a direct, allowed band gap in the visible energy range and exhibit a strong optical anisotropy. Taking into account the electron-hole Coulomb interaction, the geometrical restriction of excitons dramatically enhances the oscillator strength of the optical transitions. Comparisons with recent experimental results are also made.

Hydrogenated Si clusters: Band formation with increasing size.

Abstract: The electronic structures of various Si clusters of different sizes (with hydrogenated surfaces) are evaluated using a nearest-neighbor empirical tight-binding Hamiltonian which describes well the band structure and fundamental band gap of crystalline silicon. The largest cluster contains 3109 Si atoms and 852 H atoms, has a diameter of 49 \AA{}, and has both a normalized Si density of states and a band gap very close to those of crystalline Si.

Atomic and Electronic Structures of the 90° Partial Dislocation in Silicon.

Abstract: Two reconstructions of the 90\ifmmode^\circ\else\textdegree\fi{} partial dislocation core in silicon have been investigated using ab initio total-energy pseudopotential calculations. The asymmetric fourfold-coordinated configuration is shown to be stable and to be associated with only shallow states in the band gap. The symmetric quasi-fivefold-coordinated configuration is found to be metastable and to be associated with states that span the band gap. These results are reproduced with tight-binding Hamiltonians if the range of hopping integrals is restricted to include no more than four nearest neighbors.

The properties of niobium superconducting tunneling junctions as X-ray detectors

Abstract: The properties of superconducting tunnel junctions based on niobium are investigated. The limiting resolution of such junctions should be ⋍ 4 eV for 6 keV X-rays. Currently only between 2 to 25% of the theoretical charge is detected. The principal loss mechanisms, which not only reduce charge but seriously degrade resolution, are found to be phonon loss to the substrate, and recombination of the excess quasi-particle population in both films. The phonon loss is probably due to relaxation phonons from quasi-particles relaxing towards the bandgap. The quasi-particle self recombination is a direct result of the very large excursion from equilibrium produced during the X-ray photoabsorption process. Finally 6 keV X-rays have been detected directly in sapphire crystals by using the niobium junction only as a detector of beamed ballistic phonons. The use of a suitable crystal as the X-ray absorber and phonon source opens up interesting possibilities for position sensitive spectrometers based on high quality niobium junctions.

Ultrafast all‐optical switching in semiconductor nonlinear directional couplers at half the band gap

Abstract: Efficient ultrafast all‐optical switching in nonlinear directional couplers made of AlGaAs and AlGaAs/GaAs quantum wells near half the band gap is reported. The switching is limited by multiphoton absorption which is dominated by three‐photon absorption in this spectral range. The three‐photon absorption in the quantum well nonlinear directional coupler is stronger than that of bulk AlGaAs. Autocorrelations of the output pulses in the bar and cross states confirm pulse breakup through nonlinear coupling, and illustrate the effects of multiphoton absorption. All sets of experimental data are fitted well by a theoretical model.

Spin Gap and Antiferromagnetic Correlations in the Kondo Insulator CeNiSn

Abstract: Neutron scattering measurements show that the crossover (at T\ensuremath{\le}10 K) from metallic heavy-fermion to semiconducting behavior coincides with the formation of a gap in the magnetic excitation spectrum of CeNiSn. In contrast to the simple band picture of an insulator, the gap is well defined only at particular values of the momentum transfer Q. While substantial antiferromagnetic correlations in the a-c plane characterize the low-T state, the corresponding zero-frequency response function is Q independent.

New oxide phase with wide band gap and high electroconductivity, MgIn2O4

Abstract: It was demonstrated that the MgIn2O4 spinel is a very promising material as a transparent electronic conductor. By the measurements of diffuse reflectance spectra, the optical band gap of MgIn2O4 (∼3.4 eV) was found to be wider than that of ITO (indium tin oxide). Electrical conductivity of the sintered sample of MgIn2O4 at room temperature has reached almost 102 S cm−1 with no intentional doping. The conduction was found to be due to electrons introduced from oxygen vacancies.

Ultrathin SimGen strained layer superlattices-a step towards Si optoelectronics

Abstract: Ultrathin SimGen (m monolayers (ML) Si, n ML Ge) strained layer superlattices (SLS) have been grown by molecular beam epitaxy. The optical properties of these structures depend on the concept of band-structure engineering by Brillouin zone folding and strain adjustment of the SLS by a Si1-ybGeyb alloy buffer layer. The energies and the oscillator strengths of the bandgap and intersubband transitions have been studied theoretically for SimGen SLS with a variety of period lengths, particularly those of m+n=10. Various characterization tools such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, photoluminescence (PL) and photocapacitance measurements have been used to analyse growth quality, interface sharpness, morphology, strain distribution and optical properties of the superlattice experimentally. The PL data indicative of the quasidirect energy gap of the 10 ML strain-symmetrized SLS in the near-infrared spectral regime (h nu approximately 0.8 eV) are presented and discussed as well as complementary photocapacitance measurements on a p-n doped Si4Ge4 SLS diode. The fabrication of test mesa diodes from Si/Ge SLS structures is described. Finally, device applications offering the possibility of monolithic integration of superlattice devices with complex silicon-based electronic circuits are outlined.

Band Offsets in Semiconductor Heterojunctions

Abstract: Publisher Summary This chapter presents an overview of several theoretical approaches and experimental measurement techniques for determining band offset values and discuss the experimental and theoretical data reported for a number of specific heterojunction systems. It also evaluates the credibility and accuracy of the experimental measurements and provides a tabulation of reliable band offset values for as many heterojunctions as possible. Among the most important physical parameters for a given heterojunction system are the conduction- and valence-band offsets; indeed, the quality and even the feasibility of heterojunction device concepts often depend crucially on the values of these band offsets. The band offset is defined simply as the discontinuity in the band edge at the interface between two semiconductors. A number of current theories seem to yield band offset values in reasonable agreement with experiment, even though the physical ideas underlying these theories can be quite different. These ideas include electron affinities, Schottky barrier heights, bulk band structures on the same energy scale, and the definition of effective midgap energies corresponding to charge neutrality for each bulk constituent.