Showing papers on "Band gap published in 1990"
TL;DR: In this paper, free standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography using electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers.
Abstract: Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under <1 mW unfocused (<0.1 W cm−2) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.
7,393 citations
TL;DR: In this paper, it was observed that the optical spectra of a nanometer-sized semiconductor crystallite are sensitive to size and the number of atoms in the crystallite.
Abstract: How can one understand the excited electronic states of a nanometer sized semiconductor crystallite, given that the crystallite structure is simply that of an excised fragment of the bulk lattice? This question is motivated by recent experiments on chemically synthesized "quantum crystallites," sometimes called "quantum dots," in which it is observed that the optical spectra are quite sensitive to size. For example, bulk crystalline CdSe is a semiconductor with an optical band gap at 690 nm, and continuous optical absorption at shorter wavelengths. However, 3540/~ diameter CdSe crystallites containing some 1500 atoms exhibit a series of discrete excited states with a lowest excited state at 530 nm (1-3). With increasing size, these states shift red and merge to form the optical absorption of the bulk crystal. Electron microscopy and Bragg X-ray scattering measurements show that these crystallites have the same structure and unit cell as the bulk semiconductor. Such changes have now been observed in the spectra of many different semiconductors. This phenomenon is a "quantum size effect" related to the development of the band structure with increasing crystallite size (4). Smaller crystallites behave like large molecules (e.g. polycyclic aromatic hydrocarbons) their spectroscopic and photophysical properties. They are true "clusters" that do not exhibit bulk semiconductor electronic properties. In this review
1,012 citations
TL;DR: In this paper, an inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature.
Abstract: An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.
600 citations
TL;DR: It is demonstrated that the formalism correctly predicts that MnO, FeO, CoO, NiO, and CuO are antiferromagnetic insulators, and that VO is a nonmagnetic metal.
Abstract: We present a method for performing fully self-consistent, ab initio, self-interaction\char21{}corrected, local-spin-density calculations. We demonstrate that the formalism correctly predicts that MnO, FeO, CoO, NiO, and CuO are antiferromagnetic insulators, and that VO is a nonmagnetic metal. The band gaps and moments are drastically improved compared with the local-spin-density approximation.
477 citations
TL;DR: In this paper, the mechanistic and kinetic details of the charge injection from excited CdS into a large bandgap semiconductor such as AgI and TiO{sub 2} have been investigated by coupling the two semiconductor systems in the colloidal form.
Abstract: The mechanistic and kinetic details of the charge injection from excited CdS into a large bandgap semiconductor such as AgI and TiO{sub 2} have been investigated by coupling the two semiconductor systems in the colloidal form. The interaction between the two colloids led to the quenching of CdS emission. The rate constants for the charge injection from excited CdS into the conduction band of AgI and TiO{sub 2} colloids were determined to be 2.2 {times} 10{sup 7} and >5 {times} 10{sup 10} s{sup {minus}1}, respectively. Transmission electron microscopic analysis indicated the possibility of several CdS colloidal particles interacting with a single particle of TiO{sub 2} and participating in the charge injection process. Primary photochemical events in the CdS-TiO{sub 2} system were investigated by picosecond laser flash photolysis. The charge injected into the TiO{sub 2} colloid and trapped at the Ti{sup 4+} site was characterized from its broad absorption in the region of 500-760 nm. The extended lifetime of these trapped charge carriers indicated an improved charge separation in the coupled semiconductor system.
417 citations
TL;DR: In this article, the authors explored the oxide-ion conductivity in oxygen-deficient perovskites with Brownmillerite and showed that Ba2In2O5 exhibits a low extrinsic O2-ionconductivity below 650°C, an intrinsic conductivity across an ordering energy gap △Hg that decreases with increasing temperature in the interval 650-C
325 citations
TL;DR: The superconducting-vortex-core spectra show a zero-bias peak which splits within a coherence length of the core and further away from the core these split peaks merge gradually with the gap edge and give a direct local measure of the superfluid velocity.
Abstract: The superconducting-vortex-core spectra show a zero-bias peak which splits within a coherence length of the core. Further away from the core these split peaks merge gradually with the gap edge and give a direct local measure of the superfluid velocity. The vortex-core states are imaged both at the Fermi energy and just below the gap to reveal two different sixfold star-shaped structures. The anisotropy and size of these images may be a consequence of the crystalline band structure with its charge-density-wave gap as well as the interaction of the neighboring vortices of the Abrikosov flux lattice.
309 citations
TL;DR: In this paper, the kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with photon counting luminescence decay measurements over wide temperature and energy ranges.
Abstract: The kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with picosecond photon counting luminescence decay measurements over wide temperature and energy ranges. The decay profiles were quantitatively examined with several models. The decays are composed of two distinct time regimes, each with very different temperature and emission energy dependence. The first (fast) regime is attributed to an unusually efficient thermal repopulation mechanism. The second (slow) component is well described by a distributed kinetic model. The kinetic behavior of wide (CdS) and narrow (Cd{sub 3}As{sub 2}) band gap materials was remarkably similar when composed of clusters in the quantum confined regime.
308 citations
TL;DR: Les calculs pour Si donnent des bandes interdites ameliorees ainsi qu'une contribution de l'echange a l'energie de liaison
Abstract: The exchange operator is divided into two parts, a Thomas-Fermi screened exchange operator and the remainder. The remainder and correlation are treated in the local-density approximation, while the screened exchange matrix elements are exactly evaluated. Calculations for Si result in much improved band gaps as well as an improved exchange contribution to the binding energy.
281 citations
TL;DR: In this article, the authors describe the continuum of metal-induced gap states (MIGS) which are derived from the virtual gap states of the complex semiconductor band structure, and the physical mechanism primarily determining barrier height is provided by the decay of the metal's electron wavefunctions into the semiconductor in the energy range between the top of the valence band and the Fermi level.
Abstract: Almost all metal-semiconductor or Schottky contacts exhibit rectifying behaviour which is caused by a depletion layer on the semiconductor side of the interface. The electronic properties of a Schottky diode are characterised by its barrier height which is the energy difference between the top of the valence band of the semiconductor and the Fermi level at the interface. The physical mechanism primarily determining barrier height is provided by the decay of the metal's electron wavefunctions into the semiconductor in the energy range between the top of the valence band and the Fermi level where the metal conduction band overlaps the semiconductor band gap. These wavefunction tails are described as the continuum of metal-induced gap states (MIGS) which are derived from the virtual gap states of the complex semiconductor band structure.
245 citations
TL;DR: In this paper, the surface reconstruction of a diamond C(100)-(1 × 1) surface was investigated and the presence of chemisorbed hydrogen on the surface was monitored by electron-stimulated desorption time-of-flight spectroscopy (ESD-TOF).
TL;DR: Based on the results, a model for the photocreation of defect states in leucoemeraldine base and emeraldine base is proposed and the central roles of phenyl-ring rotations and of massive polarons are discussed.
Abstract: We report the far-infrared through visible photoinduced absorption (PA) spectra of polyaniline in the emeraldine-base and leucoemeraldine-base forms. The direct-absorption spectrum of emeraldine base (EB) has a broad absorption centered at 2 eV (the ``exciton'' band) and an absorption band at \ensuremath{\sim}3.6 eV (the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band gap). The PA spectra of EB for pumping into the ``exciton'' band and across the band gap are nearly identical, indicating the same types of charged defect states are created upon photoexcitation. The direct absorption spectrum of leucoemeraldine base (LB) shows only the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band-gap absorption at \ensuremath{\sim}3.6 eV. For pumping into this absorption band, the PA spectrum of LB is very similar to that of EB, although important differences result from the lack of quinoid structures in this material. Based on our results, we propose a model for the photocreation of defect states in leucoemeraldine base and emeraldine base. The central roles of phenyl-ring rotations and of massive polarons are discussed.
TL;DR: In this article, a predictive model for band-gap narrowing has been applied to several III-V semiconductors, expressed as ΔEg =AN1/3+BN1/4+CN1/2 ; values for A, B, and C are predicted for these materials.
Abstract: A predictive model for band‐gap narrowing has been applied to several III‐V semiconductors. Band‐gap narrowing is expressed as ΔEg =AN1/3+BN1/4+CN1/2 ; values for A, B, and C are predicted for these materials. The commonly used N1/3 relation is shown to be valid for the p‐type materials considered, but not for n‐type materials.
TL;DR: La bande de valence experimentale est comparee a la densite d'etats theorique a partir des surfaces parfaites and defectueuses.
Abstract: The electronic properties of tin dioxide single-crystalline (110) surfaces have been studied in correlation with their structure by low-energy electron diffraction, angle-integrated and resonant photoemission using synchrotron radiation [ultraviolet photoemission spectroscopy (UPS)]. Energy distribution curves were measured from the Sn 4d core levels and from the valence band. The experimental valence band is compared with the theoretical density of states (DOS) from perfect and defective surfaces. UPS difference curves, normalized to the Sn 4d intensity, reflect mainly the increase in the oxygen partial DOS when the sample is annealed at increasing temperatures up to 1000 K after sputtering. Their comparison with simulated theoretical difference curves favors a bridging oxygen termination for annealing temperatures above 900 K. After argon-ion bombardment, band-gap defect states that are not predicted by the calculations are found at a maximum density 1.4 eV above the valence-band maximum (VBM). Various degrees of resonant enhancement occur throughout the valence band when the photon energy crosses the Sn 4d\ensuremath{\rightarrow}5p photoabsorption threshold, and these are used to establish the tin-derived character of the gap states, for which a tin 5s origin is proposed. Partial-yield spectra allow the localization of unoccupied Sn 5p states in the conduction band starting from 8 eV above the VBM with a maximum at 10 eV. The Sn 4d\ensuremath{\rightarrow}5p absorption threshold also shows possible core exciton formation for sputtered surfaces only.
TL;DR: In this paper, the authors identify the mechanism for band gap narrowing as band folding followed by repulsion between the folded states, coupled by the non-zinc-blende component of the superlattice potential.
Abstract: Either spontaneous or artificial ordering of semiconductor alloys into CuAu‐like, chalcopyrite, or CuPt‐like structures is predicted to be accompanied by a reduction in the direct band gaps relative to the average over the binaries. In this letter calculated results are presented for seven III‐V and II‐VI alloys. We identify the mechanism for this band‐gap narrowing as band folding followed by repulsion between the folded states. The latter is coupled by the non‐zinc‐blende component of the superlattice potential. The same physical mechanism (but to a different extent) is responsible for gap bowing in disordered alloys.
TL;DR: A simple model demonstrates that the oscillations arise from the formation of a miniband structure in the periodic crystal, including energy gaps and minibands which contain fifteen discrete states.
Abstract: We have studied the magnetotransport properties of an artificial one-dimensional crystal. The crystal consists of a sequence of fifteen quantum dots, defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure by means of a split-gate technique. At a fixed magnetic field of 2 T, two types of oscillations with different amplitude and period are observed in the conductance as a function of gate voltage. A simple model demonstrates that the oscillations arise from the formation of a miniband structure in the periodic crystal, including energy gaps and minibands which contain fifteen discrete states.
TL;DR: In this article, the chemical structures of thin films of amorphous carbon (C) and hydrogenated carbon (a-C: H) were determined using magnetron sputtering of a graphite target.
Abstract: Thin films of amorphous carbon (–C) and amorphous hydrogenated carbon (a–C: H) were prepared using magnetron sputtering of a graphite target. The chemical structures of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical band gap, and electrical resistivity were determined, and their relation to the film’s chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical band gap, mass density, and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp2-bonded clusters in the a–C films and changes in the termination of the graphitic clusters and sp3-bonded networks by hydrogen in the a–C: H films.
TL;DR: In this article, the systematics in the transition metal d-d Coulomb interactions and the anion to cation charge transfer energies were discussed, and the nature of the optical thresholds and their dependence on the cation and anion electronegativity were discussed.
TL;DR: In this article, the validity of some approaches currently used to determine the energy gap value (Eg) of semiconducting pyrite (FeS2) from optical and photoelectrochemical measurements was discussed.
TL;DR: The optimal volume- filling fraction of spheres for the creation of a gap in the photon density of states is found to vary substantially with the relative dielectric constant r, and this gap persists for r as small as 3.
Abstract: : Using both the plane-wave and the Korringa-Kohn-Rostoker method, the photon band structure is calculated within the scalar wave approximation for a fcc lattice of dielectric microspheres embedded within a uniform host medium. The plane-wave method is found to converge fairly rapidly. The optimal volume- filling fraction of spheres for the creation of a gap in the photon density of states is found to vary substantially with the relative dielectric constant r, and this gap persists for r as small as 3. Reprints. (rh)
TL;DR: In this paper, the optical transmission of LiNbO3 single crystals has been measured in the wavelength range 200-900 nm, for different degrees of reduction, to study the effect of reduction on the optical characteristics near the fundamental absorption edge.
Abstract: The optical transmission of LiNbO3 single crystals has been measured in the wavelength range 200–900 nm, for different degrees of reduction, to study the effect of reduction on the optical characteristics of LiNbO3 near the fundamental absorption edge. The optical transitions in LiNbO3 were found to be indirect and the band gap decreased with increasing degree of reduction. The band observed at 2.48 eV in the absorption spectrum in heavily reduced samples has been attributed to the formation of polarons, and the theoretical model of Reik and Heese [J. Chem. Solids 28, 581 (1967)] for small polarons is used to correlate the optical and electrical properties.
TL;DR: In this article, the optical and electrical properties of Ga2O3-doped ZnO films prepared by r.f. sputtering have been investigated as functions of preparation conditions and dopant concentration.
Book•
01 Jun 1990
TL;DR: Orbitals in atoms and ions optical spectra of metal ions electronegavity, polarisation and refractivity bonding in ionic, covalent and metallic materials the band gap and charge transfer properties of the oxide ion semiconductors glasses magnetic properties of solids as discussed by the authors
Abstract: Orbitals in atoms and ions optical spectra of metal ions electronegavity, polarisation and refractivity bonding in ionic, covalent and metallic materials the band gap and charge transfer properties of the oxide ion semiconductors glasses magnetic properties of solids.
TL;DR: In this paper, the first observation of different nitrogen pair complexes in GaAs was made by carefully tuning the pressure, making one after the other of the NNi pairs (1≤i≤10) appear in the band gap of GaAs and then become the major exciton recombination channel.
Abstract: We report on the first observation of different nitrogen pair complexes in GaAs. These complexes, which have been searched for since the ’60s, are studied under the application of hydrostatic pressure. By carefully tuning the pressure, we make one after the other of the NNi pairs (1≤i≤10) appear in the band gap of GaAs and then become the major exciton recombination channel. We compare our results for nitrogen states in GaAs with the classical case of NNi excitons in GaP.
TL;DR: In this article, a pseudopotential scheme, which incorporates compositional disorder as an effective potential, is proposed for calculation of the band structure of ternary compound semiconductors.
Abstract: A simple pseudopotential scheme, which incorporates compositional disorder as an effective potential, is proposed for calculation of the band structure of ternary compound semiconductors. It is shown that the present theory, which is free from any additional parameter, satisfactorily produces the band-gap bowings of ternary compounds when the lattice mismatch is small.
TL;DR: Plateaus in the electron-hole recombination energy and minima in the peak intensity at integer and fractional filling factors occur in the luminescence from ultrahigh-mobility GaAs single quantum wells.
Abstract: Plateaus in the electron-hole recombination energy and minima in the peak intensity at integer and fractional filling factors occur in the luminescence from ultrahigh-mobility GaAs single quantum wells. At Landau and spin gaps the regions of plateaus and intensity minima broaden as the temperature is reduced, in consort with the transport Hall resistance. A sharp intensity minimum and peak shift is seen at \ensuremath{
u}=2/3, while higher-field fractions are characterized by a splitting in the luminescence. The optical anomalies are directly related to the position of the Fermi energy in localized transport states.
TL;DR: In this article, anneal treatment was found to be optimal for removal of grown-in defect complexes without creating a significant density of misfit dislocations, and peak energies at 4.2 K varied from 620 to 990 meV for Ge fractions from 0.53 to 0.06.
Abstract: Intense photoluminescence (PL) from strained, epitaxial Si1−xGex alloys grown by molecular beam epitaxy is reported with measured internal quantum efficiencies up to 31% from random alloy layers, single buried strained layers, and multiple quantum wells. Samples deposited at ∼400 °C exhibited low PL intensity, whereas annealing at ∼600 °C enhanced the intensity by as much as two orders of magnitude. This anneal treatment was found to be optimal for removal of grown‐in defect complexes without creating a significant density of misfit dislocations. PL peak energies at 4.2 K varied from 620 to 990 meV for Ge fractions from 0.53 to 0.06, respectively. Efficient PL was due to exciton accumulation in the strained Si1−xGex layers of single and multiple quantum wells, where the band gap was locally reduced. Optical transitions associated with the PL occurred without phonon assistance.
TL;DR: In this article, the conduction and valence-band offsets for GaAs/Ga0.51In0.49P quantum wells were independently estimated by measuring the capacitance transient resulting from thermal emission of carriers from the respective wells.
Abstract: We have independently estimated the conduction‐ and valence‐band offsets ΔEc and ΔEv in GaAs/Ga0.51In0.49P quantum wells by measuring the capacitance transient resulting from thermal emission of carriers from the respective wells. The heterostructure samples were grown by low‐pressure metalorganic chemical vapor deposition. The band offsets are extrapolated from the emission activation energies with appropriate corrections. The estimated values of ΔEc and ΔEv are 0.198 and 0.285 eV, respectively.
TL;DR: In this paper, the electrical characteristics and switching behavior of amorphous ternary semiconducting CdGeAs 2 were studied under DC and AC conditions and the electrical band gap was determined to be 1.2 eV.
Abstract: The electrical characteristics and switching behavior of amorphous ternary semiconducting CdGeAs 2 were studied under DC and AC conditions. The samples tested were fabricated as as-quenched bulk compounds, roller splat-quenched ribbons, and Ar-sputtered thin films. Vapor deposited Ag, Au and Al behaved similarly as electrical contacts while the commercial Ag-paste electrodes revealed variable contact resistance. The threshold electric field of ribbon sample was ∼ 5 × 10 3 V/cm, smaller than ∼ 1.5 × 10 4 V/cm for the sputtered thin films. The electrical band gap of amorphous CdGeAs 2 was determined to be 1.2 eV (n-type). The “forming” process was believed to occur during the first switching cycle whereby highly conductive amorphous channels formed through the width of the samples. The effects of current on switching were investigated and shown not to be significant. The response time for switching decreased approximately exponentially with the applied voltage. The thermal assisted electronic model was postulated for the OFF-ON transition in these materials.
TL;DR: In this article, a simple model for the electron transfer from excited dyes to the conduction band of semiconductors is proposed, which does not contain the Franck-Condon term, but the state density of the semiconductor plays an important role.
Abstract: A simple model for the electron transfer (ET) from excited dyes to the conduction band of semiconductors is proposed. It is shown that the equation for the ET rate does not contain the Franck-Condon term, but the state density of the semiconductor plays an important role. The energy gap dependence of ET rates in the dye-sensitized semiconductors, which is quite different from those in molecular systems, is explained well by this model. The electron exchange energy is determined for Ru complexes and Rhodamine B on oxide semiconductors. Temperature dependence of the electron exchange energy term, caused by thermal motion of adsorbed dyes, is suggested as a new origin of activation energy of ET on the surface. The temperature dependence of the ET rates is explained well by this model.