Showing papers on "Band gap published in 1989"

Band lineups and deformation potentials in the model-solid theory.

Abstract: Semiconductor heterojunctions and superlattices have recently shown tremendous potential for device applications because of their flexibility for tailoring the electronic band structure. A theoretical model is presented to predict the band offsets at both lattice-matched and pseudomorphic strained-layer interfaces. The theory is based on the local-density-functional pseudopotential formalism and the ``model-solid approach'' of Van de Walle and Martin. This paper is intended as a self-contained description of the model, suitable for practical application. The results can be most simply expressed in terms of an ``absolute'' energy level for each semiconductor and deformation potentials that describe the effects of strain on the electronic bands. The model predicts reliable values for the experimentally observed lineups in a wide variety of test cases and can be used to explore which combinations of materials and configurations of the strains will lead to the desired electronic properties.

Calculation of the band gap for small CdS and ZnS crystallites

Abstract: The tight-binding approximation and the recursion method are used to study the size dependence of the band gap for small CdS and ZnS crystallites (20--2500 atoms). Because of the lack of accurate experimental data, a simple model of the crystal is considered; one which has no dangling bonds and a symmetrical shape. It is then possible to have a good evaluation of the band gap, even for the largest crystallites. The optical-absorption spectra exhibit an excitonic peak; we determine the peak position from a simple evaluation of the binding energy. The results are compared with the results of other calculations based upon the effective-mass approximation and some experimental data.

Biosynthesis of cadmium sulphide quantum semiconductor crystallites

Abstract: NANOMETRE-SCALE semiconductor quantum crystallites exhibit size-dependent and discrete excited electronic states which occur at energies higher than the band gap of the corresponding bulk solid1–4. These crystallites are too small to have continuous energy bands, even though a bulk crystal structure is present. The onset of such quantum properties sets a fundamental limit to device miniaturization in microelectronics5. Structures with either one, two or all three dimensions on the nanometer scale are of particular interest in solid state physics6. We report here our discovery of the biosynthesis of quantum crystallites in yeasts Candida glabrata and Schizosaccharomyces pombe, cultured in the presence of cad-mium salts. Short chelating peptides of general structure (γ-Glu-Cys)n-Gly control the nucleation and growth of CdS crystallites to peptide-capped intracellular particles of diameter 20 A. These quantum CdS crystallites are more monodisperse than CdS par-ticles synthesized chemically. X-ray data indicate that, at this small size, the CdS structure differs from that of bulk CdS and tends towards a six-coordinate rock-salt structure.

Photonic band structure: The face-centered-cubic case.

Abstract: We employ the concepts of band theory to describe the behavior of electromagnetic waves in three dimensionally periodic face-centered-cubic (fcc) dielectric structures. This can produce a ``photonic band gap'' in which optical modes, spontaneous emission, and zero-point fluctuations are all absent. In the course of a broad experimental survey, we have found that most fcc dielectric structures have ``semimetallic'' band structure. Nevertheless, we have identified one particular dielectric ``crystal'' which actually has a ``photonic band gap.'' This dielectric structure, consisting of 86% empty space, requires a refractive index contrast greater than 3 to 1, which happens to be readily obtainable in semiconductor materials.

Optical dispersion relations for GaP, GaAs, GaSb, InP, InAs, InSb, AlxGa1−xAs, and In1−xGaxAsyP1−y

Abstract: A method is described for calculation of the optical constants (the refractive index, extinction coefficient, and absorption coefficient) of some III‐V binaries (GaP, GaAs, GaSb, InP, InAs, and InSb), ternaries (AlxGa1−xAs), and quaternaries (In1−xGaxAsyP1−y) in the entire range of photon energies (0–6.0 eV). The imaginary part of the dielectric function [e2(ω)] is derived first from the joint density‐of‐states functions at energies of various critical points (CPs) in the Brillouin zone; then its real part [e1(ω)] is obtained analytically using the Kramers–Kronig relation. The indirect band‐gap transitions are also assumed to provide a gradually increasing e2 spectrum expressed by a power law of (ℏω−EIDg)2, where ℏω is the photon energy and EIDg is the indirect band‐gap energy. The optical dispersion relations are expressed in terms of these model dielectric functions. The present model reveals distinct structures in the optical constants at energies of the E0, E0+Δ0 [three‐dimensional (3‐D) M0 CP], E1, E...

Luminescence of erbium implanted in various semiconductors: IV, III-V and II-VI materials

Abstract: Luminescence of erbium implanted in various semiconductors such as Si, InP, GaAs, AlGaAs, GaInAsP, ZnTe and CdS is presented. The Er/sup 3+/ emission wavelength is the same for all these semiconductors with a bandgap energy greater than the intrashell transition energy of Er 4f electrons (0.805 eV). The Er/sup 3+/ emission intensity depends strongly on both the bandgap energy of the host semiconductor and the material temperature. To obtain an intense room temperature emission, a wide-gap semiconductor must be used.

Density-functional calculation of the parameters in the Anderson model: Application to Mn in CdTe

Abstract: We discuss methods for ab initio calculations of the parameters in the Anderson model. First, we present a very simple method for calculating the appropriate combination of hopping matrix elements needed in the impurity Anderson model. For a substitutional impurity, we show that to a good approximation it is sufficient to know the potential of the impurity atom and the local density of states of the unperturbed host. Calculations are performed for Mn substituting Cd in CdTe. As expected, the Mn 3d orbitals have a strong coupling to the Te 5p--derived valence band, but there is also a strong coupling to the conduction band. The dependence of the hopping matrix elements on the Mn configuration is studied. While there is a strong dependence on the Mn net charge, we find that the creation of, e.g., a core hole has a fairly small effect on the matrix elements, provided that the 3d occupancy is allowed to relax. Second, the Coulomb integrals between two Mn 3d orbitals and between a 3d orbital and a core orbital are calculated. The renormalization of these quantities due to the radial relaxation of the Mn 3d, 4s, and 4p orbitals, and due to charge-transfer effects, are analyzed in detail. Because of the nonmetallic character of CdTe, a change in the number of Mn 3d electrons is only partly screened by a charge transfer to the Mn 4s and 4p orbitals. Because of the moderate size of the band gap, this screening is, nevertheless, important. The radial relaxation of the Mn 3d, 4s, and 4p wave functions is also important. The relaxation of the neighboring atoms plays a rather small role. Results for the photoemission spectra are calculated including multiplet effects. The results are found to be in rather good agreement with experiment.

Growth of cubic phase gallium nitride by modified molecular‐beam epitaxy

Abstract: Gallium nitride is a compound semiconductor with a wide direct band gap (3.45 eV) and a large saturated electron drift velocity. Nearly all single‐crystal thin films grown to date have been wurtzite (hexagonal) structure. Cubic GaN has the potential for higher saturated electron drift velocity and somewhat lower band gap. These properties could increase its applicability for high‐frequency devices (such as impact ionization avalanche transit time diodes) as well as short‐wavelength light emitting diodes and semiconductor lasers. This paper reports the growth of cubic phase single‐crystal thin‐film GaN using a modified molecular‐beam epitaxy technique. A standard effusion cell was used for gallium, but to activate nitrogen gas prior to deposition, a microwave glow discharge was used. Auger electron spectroscopy showed a nominally stoichiometric GaN film. Transmission electron microscopy with selected area diffraction indicated the crystal structure to be zinc blende.

The electronic and electrochemical properties of poly(phenylene vinylenes) and poly(thienylene vinylenes): An experimental and theoretical study

Abstract: The electronic and electrochemical properties of poly(p‐phenylene vinylene), poly(thienylene vinylene), and their derivatives with electron donating moieties such as methyl, methoxy, and ethoxy are studied using the newly developed electrochemical potential spectroscopy (ECPS) and optical spectroscopy. It is shown that electrochemically derived band gaps agree well with band gap values obtained from optical measurements. Substitution with electron donating groups substantially lowers the ionization potentials and band gaps. A similar effect can be attributed to the incorporation of a vinylene linkage between rings of the polymer backbone. Our results imply that through a proper choice of substituents and backbone structure one can adjust the electrochemical potentials over a wide range as well as red shift the absorption edge of these polymers. In the case of the alkoxythienylene vinylenes the absorption edge is shifted through the visible range of the spectrum into the near infrared (NIR) yielding polyme...

Excitonic and nonlinear-optical properties of dielectric quantum-well structures

Abstract: Excitonic and nonlinear-optical properties of dielectric quantum-well (DQW) structures are investigated theoretically. A DQW is a quantum well sandwiched by barrier materials with a smaller dielectric constant and a larger band gap than the well material. The fundamental physics determining the excitonic properties in a DQW, i.e., exciton binding energy, exciton oscillator strength, and nonlinear-optical response, are clarified. The most important mechanisms for enhancing the excitonic properties are quantum-confinement effect, mass-confinement effect, and dielectric-confinement effect. Quantum confinement increases the spatial overlap between an electron and a hole as a result of the potential well confinement, and it enhances oscillator strength. Mass confinement is based on the penetration of the carrier wave function into barrier layers with a heavier effective mass than the well layer. It increases the exciton reduced mass and hence the exciton binding energy. Dielectric confinement arises from the reduction of the effective dielectric constant of the whole system due to the penetration of the electric field into the barrier medium having a smaller dielectric constant than the well and enhances the Coulomb interaction between the electron and hole. On the basis of these analyses, the general guiding principles are established for designing DQW structures with optimum excitonic properties. Various practical examples of DQW's are examined with respect to the lattice-constant matching, the difference in the dielectric constant, and the difference in the carrier effective masses. ZnSe is found to be one of the most promising candidates for the barrier material of the GaAs DQW.

Superconducting gap in bi-sr-ca-cu-o by high-resolution angle-resolved photoelectron spectroscopy.

Abstract: Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi2Sr2CaCu2O8. Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

Geometric and electronic structure of antimony on the GaAs(110) surface studied by scanning tunneling microscopy

Abstract: Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy. The Sb is observed to grow as a 1\ifmmode\times\else\texttimes\fi{}1 ordered monolayer. The positions of the Sb atoms in the surface unit cell are deduced from voltage-dependent imaging, and compared with previously proposed structural models. A best fit is found for a model in which the Sb atoms occupy positions similar to what would be the positions of Ga and As atoms at an unrelaxed GaAs(110) surface, in agreement with the results from low-energy electron-diffraction experiments. The surface-state density is measured with tunneling spectroscopy. Two filled-state peaks and one empty-state peak are observed in the spectra, as well as a band-gap region with a width nearly equal to the bulk band gap. These observations are compared with previous photoemission results and theoretical calculations.

Correlation of the optical gaps and Raman spectra of hydrogenated amorphous carbon films

Abstract: The Raman and infrared absorption spectra, optical gaps, and electron spin densities of amorphous carbon films deposited from hydrocarbon plasmas have been systematically studied as a function of deposition conditions and Raman probe wavelength. Although all other probes are consistent with a monotonic increase in intermediate‐range order with substrate bias voltage Vb, the optical gap decreases with increasing Vb (consistent with increasing graphitic domain size) only up to the onset of sputtering, where the gap sharply increases. We propose a simple structural model for a‐C:H which is consistent with these results and requires no sp3 bonding.

Band gaps and spin-orbit splitting of ordered and disordered AlxGa1-xAs and GaAsxSb1-x alloys.

Abstract: Spontaneous long-range ordering of the otherwise disordered isovalent semiconductor alloys A/sub x/B/sub 1-x/C has been recently observed in numerous III-V alloy systems exhibiting the CuAu-I, CuPt, and chalcopyrite structures. We present a theory for the ordering-induced changes in the Brillouin-zone-center electronic properties, with application to the Al/sub x/Ga/sub 1-//sub x/As and GaAs/sub x/Sb/sub 1-//sub x/ alloys. The dominant effect for these systems is shown to be level repulsion between different-symmetry states of the binary constituents which fold into equal-symmetry states in the ordered ternary structures. Strong variations in the band gaps, spin-orbit splittings, and charge densities among the three basic ordered structures reflect the different magnitudes of the symmetry-enforced coupling between the folded states. An extension of the model to the disordered alloys yields good agreement with the observed optical bowing parameters for the fundamental gaps; however, the positive (downward concave) bowing of the spin-orbit splitting observed in some common-cation semiconductor alloy remains an unexplained puzzle.

Band‐gap profiling for improving the efficiency of amorphous silicon alloy solar cells

Abstract: We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

Band‐gap narrowing in highly doped n‐ and p‐type GaAs studied by photoluminescence spectroscopy

Abstract: Band‐gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n‐ (Si) and p‐ (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line‐shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band‐gap narrowing as a function of concentration for both n‐and p‐doped GaAs is given.

Interband tunneling in polytype GaSb/AlSb/InAs heterostructures

Abstract: Polytype heterostructures of GaSb/AlSb/InAs show interband tunneling due to the 0.1 eV overlap of the InAs conduction band and the GaSb valence band. This broken‐gap configuration results in a novel mechanism for negative differential resistance that has potential applications in high‐speed devices. We have demonstrated for the first time interband tunneling in single‐barrier and double‐barrier polytype heterostructures. Single‐barrier structures show negative differential resistance due to the change in interband tunneling with applied bias. A peak‐to‐valley ratio of 2.7:1 at 77 K was observed in this case. Double‐barrier structures using an InAs quantum well exhibit resonant interband tunneling with a peak‐to‐valley current ratio of more than 60:1 at 77 K. This structure is promising for applications to three‐terminal devices because of the very wide quantum well that can be achieved.

Optical Studies of Hydrogen Above 200 Gigapascals: Evidence for Metallization by Band Overlap

Abstract: Direct optical observations of solid hydrogen to pressures in the 250-gigapascal range at 77 K are reported. Hydrogen samples appear nearly opaque at the maximum pressures. Measurements of absorption and Raman spectra provide evidence that electronic excitations in the visible region begin at about 200 gigapascals. The optical data are consistent with a band-overlap mechanism of metallization.

Distance of the image plane from metal surfaces.

Abstract: The data base of surface-state energies on the metals Cu, Ag, Au, Ni, Pd, and Pt is assembled and, with the aid of a simple model, is used to estimate the distance of the image plane and its trends from surface to surface and metal to metal. The model combines a nearly-free-electron representation of the crystal with a Jones-Jennings-Jepsen ansatz for the saturated image barrier. The projected bulk-band gaps are taken from published determinations. Constraints are placed on the surface barrier parameters by appeal to the results of self-consistent first-principles slab calculations. The general experimental trend observed is for the image-plane distance {ital z}{sub 0} to decrease in the sequence (111) to (001) to (110), in the same sense but not as rapidly as {ital z}{sub {ital J}}, the distance of the effective jellium edge. This trend is rationalized using a simple model of the tail of the surface charge density. Typical values for {ital z}{sub 0}{minus}z{sub J} fall in the range {minus}0.2 to +0.5 a.u., with the larger values occurring for the 3{ital d} metals Cu and Ni.

Linear- and nonlinear-optical properties of semiconductor clusters

Abstract: CdS and PbS clusters with sizes ranging from a few angstroms to 150 A can be synthesized in polymers. The dependence of the band gap on the cluster size deviates from the prediction of a simple particle-in-a-box model owing to the breakdown of the effective-mass approximation. Instead, the dependence can be described by a simple tight-binding cluster model. The optical nonlinearity, expressed as α2/α0, of 50-A CdS clusters in Nafion film is determined to be −6.1 × 10−7 cm2 W−1 at 480 nm. The nonlinearity originates from the bleaching of the excitonic absorption owing to the presence of trapped carriers on the cluster surfaces. By passivating the CdS surfaces with ammonia, we have shown that the nonlinearity can be controlled by surface chemistry. We have determined that the presence of one trapped electron–hole pair can bleach the excitonic absorption of the whole CdS cluster. This efficient bleaching can be understood by using a model that considers the shifting of the exciton resonance and weakening of its oscillator strength in the presence of a trapped electron or hole. We also discuss two new classes of material: superclusters in zeolites and surface-capped clusters. Both represent our first steps toward the systematic synthesis of clusters of controlled surfaces and sizes.

Optical evidence for the metallization of xenon at 132(5) GPa.

Abstract: Xe has been compressed in a diamond-anvil cell to approximately 200 GPa. The metallization of Xe by band-gap closure was investigated by obtaining optical data in both the metallic and insulating states. In the metallic state, the pressure dependence of the plasma frequency was determined from absorption data fitted with a free-electron model. In the insulating state, the pressure dependence of the band gap was determined from absorption data fitted with an indirect-band-gap model. The optical data indicate that the insulator-to-metal transition in Xe occurs at 132(5) GPa.

Effect of target‐substrate distance on the growth and properties of rf‐sputtered indium tin oxide films

Abstract: Indium tin oxide films have been deposited by rf sputtering an oxide target (90% In2O3+10% SnO2). Transparent conducting films have been obtained on room‐temperature substrates without any post‐deposition annealing. The effects of target‐substrate separation on the growth characteristics and film properties have been studied. The virtual‐source formation of the sputtered neutrals in the gap between the target and substrate has been observed. The effect of sputtering parameters on the position of the virtual source has been studied. It has been observed that the films deposited onto substrates kept below and above the virtual source have different properties. The films on substrates kept above the virtual source have (222) preferred orientations, with a direct optical band gap of 3.7 eV, an electron effective mass of 0.51 m, and a figure of merit ∼14×10−3 Ω−1. The films on substrates below the virtual source have preferred orientations along (440) and (400), with a band gap of 3.5 eV, an effective mass of ...

Quantum Wire Superlattices and Coupled Quantum Box Arrays: A Novel Method to Suppress Optical Phonon Scattering in Semiconductors

Abstract: Electronic structures in various coupled quantum box array systems including quantum-wire superlattices and planar superlattices are examined The optimal choice of coupling and periodicity in such systems results in miniband structures with medium width Eb and wide energy gap Eg and is found to provide a novel method to substantially reduce the optical phonon scattering of electrons in semiconductors Feasibility and significances of this approach are discussed

Polarized band-edge photoluminescence and ordering in Ga0.52In0.48P.

Abstract: We present the first experimental evidence for the spontaneous breaking of cubic symmetry in the band structure of films of ${\mathrm{Ga}}_{0.52}$${\mathrm{In}}_{0.48}$P grown by organometallic vapor-phase epitaxy on (100) GaAs substrates. We show how this effect is related to the spontaneous ordering of the alloy, and its correlation with the anomalous lowering of the band gap observed in these films.

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Abstract: Strained single quantum wells composed of GaAs/InGaAs/GaAs were grown by molecular beam epitaxy and characterized at room temperature by photoreflectance and at 6 and 77 K by photoluminescence spectroscopy. For the InGaAs/GaAs heterojunction, utilizing a band offset ratio of 85:15 (conduction band:valence band) for the intrinsic (nonstrained) interface and a contribution of the hydrostatic compression to the valence band movement corresponding to the pressure sensitivity of the spin orbit band, excellent agreement is found between calculated excitonic transition energies and those found by experiment at all temperatures studied. Our analysis indicates that material parameters and the combined strain components used to calculate band structure are not temperature dependent to our degree of sensitivity. An empirical equation, which differs slightly from that for bulk InGaAs crystals, describing the nonstrained band‐gap energy as a function of In fraction at 77 K is presented. The difference between band offset ratios for the intrinsic and strained heterojunction are found to be significant and the relative merits of each are discussed.

Metallicity and gap states in tunneling to Fe clusters in GaAs(110).

Abstract: We report the characteristics of tunneling to a GaAsO 10) substrate with distinct, nanometer-size Fe clusters, as a function of distance from and size of the clusters. We show that Fe clusters of volumes ∼ 150 A3, corresponding to ≃13 atoms, are observed to be nonmetallic with a gap at the Fermi level. Larger clusters with > 35 atoms begin to show metallic characteristics. We observe a continuum of cluster-induced gap states in tunneling to the GaAs substrate surrounding the metallic Fc clusters. The decay length of these states has a minimum decay of 3.4 A at midgap and diverges at the valence- and conduction-band edges.

Long‐wavelength infrared detectors based on strained InAs–Ga1−xInxSb type‐II superlattices

Abstract: We demonstrate that infrared detectors made from strained type‐II superlattices consisting of III–V semiconductors can have favorable properties for long‐wavelength (λc >10 μm) detection applications. We specifically consider InAs–Ga1−x Inx Sb strained‐layer superlattices with x≊0.4 . This is a type‐II superlattice where the conduction‐band minimum of InAs is lower in energy than the valence‐band maximum of Ga1−x Inx Sb. Electrons are localized in the InAs layers while holes are localized in the Ga1−x Inx Sb layers. Generally, in a type‐II superlattice such as InAs–GaSb, large absorption coefficients cannot be achieved at the small values of band gap necessary for long‐wavelength detection because the localization of electrons and holes in adjacent layers leads to small optical matrix elements. Therefore, small band gaps and large optical matrix elements appear to be two mutually exclusive requirements in the InAs–GaSb superlattice. Here, we show that it is possible to obtain large optical absorption coef...

Color-center-induced band-gap shift in yttria-stabilized zirconia

Abstract: An increase of the room-temperature band gap from 4.23 to 4.96 eV is observed in crystals of the superionic material yttria-stabilized cubic zirconia (YSZ) when the crystals are reduced either electrolytically or in a hydrogen atmosphere. The original absorption edge of 4.23 eV in unreduced YSZ can be accounted for by the excitation of an {ital F}{sub {ital A}} complex consisting of an Y{sup 3+} ion and an {ital F}{sup +} oxygen vacancy. We assume the ground state of this complex lies in the valence band, whereas its first excited state {ital F}{sub {ital A}}{sup *} formed by adding an additional electron lies in the gap 0.73 eV below the conduction band; the observed absorption is then due to optical excitation of this state from the valence band. Reduction of YSZ leads to the formation of doubly occupied oxygen vacancies, i.e., {ital F} centers, giving rise to a band of states in the gap. Arguments are put forth to show that as the {ital F}-center concentration increases, the mean energy of this band is raised by {ital F}-{ital F} interactions or by changes in the lattice relaxation; eventually, part of the band will lie above the {ital F}{sub {ital A}}{supmore » *} state, at which point the corresponding {ital F} centers will decay by losing an electron to one of the {ital F}{sub {ital A}}{sup *} states. This results in a shift of the optical absorption edge to the true band-gap energy, i.e., 4.96 eV, which is a true band-to-band transition.« less