Showing papers on "Band gap published in 1986"

Electron correlation in semiconductors and insulators: Band gaps and quasiparticle energies.

Abstract: We present a first-principles theory of the quasiparticle energies in semiconductors and insulators described in terms of the electron self-energy operator. The full dielectric matrix is used to evaluate the self-energy operator in the GW approximation: the first term in an expansion of the self-energy operator in terms of the dynamically screened Coulomb interaction (W) and the dressed Green's function (G). Quasiparticle energies are calculated for the homopolar materials diamond, Si, and Ge as well as for the ionic compound LiCl. The results are in excellent agreement with available experimental data. In particular, the indirect band gap is calculated as 5.5, 1.29, and 0.75 eV as compared with experimental gaps of 5.48, 1.17, and 0.744 eV for diamond, Si, and Ge, respectively. The Ge results include relativistic effects. The calculated direct gap for LiCl is within 5% of experiment. Viewed as a correction to the density-functional eigenvalues calculated with the local-density approximation, the present results show a correction dominated by a large jump at the gap. It is found that because of the charge inhomogeneity, the full dielectric screening matrix must be included, i.e., local-field effects are essential. The dynamical effects are also found to be crucial. The required dielectric matrices are obtained within the density-functional approach for the static case and extended to finite frequency with use of a generalized plasmon-pole model based on sum rules. The model reproduces the \ensuremath{\omega} and ${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}1}$ moments of the exact many-body response function. The qualitative features of the electron self-energy operator are discussed. Using the static Coulomb-hole--screened-exchange approximation for illustration, the role of local fields in the self-energy operator are explained. The role of dynamical renormalization is illustrated. The same qualitative features are observed in both the homopolar and ionic materials.

Evaporated Sn‐doped In2O3 films: Basic optical properties and applications to energy‐efficient windows

Abstract: We review work on In2O3:Sn films prepared by reactive e‐beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C‐type rare‐earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50‐μm range, by X‐band microwave reflectance, and by dc electrical measurements. Hall‐effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon‐induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corre...

Density functional theory and the band gap problem

Abstract: How can the fundamental band gap of an insulator be predicted? As a difference of ground-state energies, the fundamental gap seems to fall within the reach of density functional theory, yet the predicted gaps from band structure calculations within the local density approximation (LDA) are about 40% too small. It is argued here that even the exact Kohn-Sham potential veff(r), which generates the exact density in a self-consistent-field calculation, generates a band structure which underestimates the gap. Within the context of the band gap problem, several recent developments in the density-functional theory of many-electron systems are reviewed: (1) The Langreth-Mehl approximation to the Kohn-Sham exchange-correlation energy and potential, based upon the Langreth-Perdew wavevector analysis of the density gradient expansion. This functional leads to more accurate ground-state energies and densities than those of the LDA with little change in the calculated band structures of solids. (2) The derivative discontinuity of the exchange-correlation energy, which is responsible for substantial underestimation of the fundamental gap by even the exact Kohn-Sham potential. (3) The self-interaction correction, which yields accurate gaps in insulators only by virtue of its orbital-dependent potential. (4) The density response function of the uniform electron gas, which suggests that the LDA gives a good estimate of the exact Kohn-Sham potential for a semiconductor with a weak periodic potential. In short, several very different (but admittedly approximate) numerical calculations suggest that most of the error in the LDA fundamental gap would persist in the gap of the exact Kohn-Sham band structure. This error would persist in any attempt to calculate the gap from LDA total energy differences for clusters of increasing size.

Optical band gap of indium nitride

Abstract: Room‐temperature optical absorption data in the 1.5–2.5‐eV range are reported for indium nitride thin films prepared by reactive radio‐frequency sputtering. The fundamental absorption edge in high‐purity material is located at 1.89±0.01 eV and corresponds to a direct transition at k=0, in agreement with band‐structure calculations. A significant Moss‐Burstein shift is noted for carrier concentrations in excess of 1019 cm−3 and obeys the empirical relationship EG =1.89+2.1×10−8 n1/3 eV.

Electronic Structure of the Si(111)2 × 1 Surface by Scanning-Tunneling Microscopy

Abstract: The tunneling current is measured as a function of voltage, lateral position, and vertical separation between a tungsten probe tip and a Si(111)2 \ifmmode\times\else\texttimes\fi{} 1 surface. A rich spectrum is obtained in the ratio of differential to total conductivity, revealing the structure of the surface-state bands. The magnitude of the parallel wave vector for certain surface states is determined from the decay length of the tunneling current. Real-space images of the surface states reveal a phase reversal between those states on either side of the surface-state band gap.

Correction to "Reduction of lasing threshold current density by the lowering of valence band effective mass"

Abstract: In present day semiconductor lasers, there is a serious asymmetry between the very light conduction band mass and the very heavy valence band mass. Under laser threshold conditions, the hole occupation remains classical even while the electrons are degenerate. This results in a significant penalty in terms of threshold current density, carrier injection level, and excess Auger and other nonradiative recombination. We propose a combination of strain and quantum confinement to reduce the valence band effective mass and to lessen the laser threshold requirements.

Localization of electronic wave functions due to local topology.

Abstract: We present a simple two-dimensional hopping model for independent electrons which has strictly localized states in addition to the extended states. These localized states can exist either in a band gap or within the continuum. The localized states persist if the lattice periodicity is destroyed. Finally, the effect is shown to hold for a much more general class of systems.

Band alignments of coherently strained GexSi1−x/Si heterostructures on 〈001〉 GeySi1−y substrates

Abstract: The self‐consistent ab initio pseudopotential results of C. G. Van de Walle and R. M. Martin [J. Vac. Sci. Technol. B 3, 1256 (1985)] have been combined with a phenomenological deformation potential theory to estimate the band gap and band offsets for coherently strained multilayers of GexSi1−x/Si for growth on 〈001〉 GeySi1−y substrates. It is found that ΔEc is negligible and the narrower GexSi1−x gap falls within the wider Si gap (type I band alignment) if the Si in the multilayers is cubic, whereas ΔEc can be appreciable and the GexSi1−x conduction‐band edge tends to be higher in energy than the Si conduction‐band edge(type II band alignment) if both the Si and the GexSi1−x are strained. In particular, the present results resolve the seeming paradox which arose from interpretations of modulation doping experiments using heterojunctions grown either on Si〈001〉 substrates or on an unstrained alloy buffer layer.

Energy band alignment in GaAs:(Al,Ga)As heterostructures: The dependence on alloy composition

Abstract: The band alignment in GaAs:(Al,Ga)As heterostructures has been investigated over the full range of alloy composition. The valence‐band discontinuity ΔEv is determined by measuring the activation energy for thermionic emission of holes from p‐GaAs over an undoped, square (Al,Ga)As barrier. The use of p‐type structures to measure ΔEv circumvents a number of complications involved in the measurement of ΔEc. The parameters required for analysis are determined by different measurements on the same structures and the analysis is performed so that the activation energy, extrapolated to zero bias, yields ΔEv directly. It is found that ΔEv is a linear function of the aluminum mole fraction xAl: ΔEv ≂0.55xAl (eV) (0≤xAl≤1). The validity of these data is supported by measurements of ΔEc in the direct band‐gap regime, where complementary values of ΔEv and ΔEc add up to the expected band‐gap difference. This relationship provides a simple description of the full band alignment in this heterosystem and should prove val...

Optical Stark effect on excitons in GaAs quantum wells.

Abstract: We describe the first reported experimental observation of an extremely fast shift of the n = 1 exciton transition energy in GaAs quantum-well heterostructures. The shift is produced by optical pumping below the band gap and is not associated with a carrier or exciton population. We interpret the shift in terms of an optical Stark effect. We present a model for the Stark effect on the ground-state exciton in quantum wells and find good agreement between the predictions of the model and our experimental results.

Towards organic polymers with very small intrinsic band gaps. I. Electronic structure of polyisothianaphthene and derivatives

Abstract: We present valence effective Hamiltonian (VEH) band‐structure calculations on polyisothianaphthene and some of its simple derivatives. Our goal is to rationalize the lowering of the band gap by about 1 eV which is experimentally observed in polyisothianaphthene (Eg=∼1 eV) relative to the parent polymer, polythiophene (Eg=∼2 eV). We show that the smaller band gap found in polyisothianaphthene can be understood on the basis of a relationship which can be established between the band gap energy and the importance of the effective quinoid contributions to the electronic structure. Our theoretical results also indicate that simple dimethyl, dimethoxy, or dicyano substitutions are not expected to affect the size of the band gap in any significant way.

Electronic and structural properties of BN and BP.

Abstract: We present a pseudopotential study within the local-density formalism of the structural and electronic properties of zinc-blende BN and BP. The ground-state properties of these systems such as bulk moduli, lattice constants, cohesive energies, and frequencies of the TO phonon mode are in good agreement with experimental results. The valence charge density of BP shows two local maxima along the bond, which is similar to the case of diamond. In contrast, the charge density of BN is similar to that of a typical III-V compound semiconductor. The resulting band structures have some important features which are in disagreement with previously published work. Like most III-V compound semiconductors, the fundamental gap in BP decreases with decreasing volume. The corresponding gap in BN, however, increases with decreasing volume as was also found in diamond.

Electronic Properties of the Interface between Si and TiO2 Deposited at Very Low Temperatures

Abstract: Uniform TiO2 thin films with a large er (up to 86) were prepared at low temperatures (200-400°C) by CVD. The films deposited at 200°C were amorphous and those at high temperatures were polycrystalline structures of anatase. The electronic properties of a TiO2/Si interface were analyzed in detail using metal-insulator-semiconductor structures. The minimum interface state density in the bandgap was as low as 2×1011 cm-2 eV-1, showing the usefulness of the TiO2 films for the gate insulators of MIS diodes. An anomalous behavior of photo-induced current observed for the first time is also presented.

Tight‐binding theory of heterojunction band lineups and interface dipoles

Abstract: A tight-binding theory of semiconductor heterojunction band lineups is presented. Interface dipoles are shown to play a crucial role in determining lineups, so that lineups obtained by using the vacuum level as a reference (e.g., the electron affinity rule) are not reliable. Instead, the self-consistent lineup can be obtained approximately by aligning the average sp 3 hybrid energies in the respective semiconductors. Numerical results are provided and compared with experiment, and the approximations and accuracy in this approach are discussed. The application of these ideas to Schottky barriers is also considered.

Doping superlattices ("n-i-p-i crystals")

Abstract: Semiconductors with doing superlattices exhibit a number of unique features by which they differ from uniform bulk crystals as well as from semiconductors with a compositional superlattice. The properties which make them particularly appealing as a new kind of semiconductor are tunability of carrier concentration, bandgap, two-dimensional subband structure, and recombination lifetimes, in combination with an enormous flexibility in tailoring. Moreover, the choice of host materials is not restricted by interface- or lattice-matching problems. The possibility of varying conductivity, absorption coefficient, optical gain, and luminescence spectra by light or external electrical potentials implies new concepts for photodetectors, tunable light sources, optical amplifiers, and modulators. The long recombination lifetimes result in large low-power nonlinearities of the optical absorption coefficient and the refractive index. These properties offer applications for saturable absorbers and bistable optical devices. In this paper the basic concept of doping superlattices and experiments which have provided its verification will be reviewed first. New physical phenomena and possible device applications will then be discussed. Finally, we will report some recent results of extensions of the concept to "hetero n-i-p-i's," obtained by periodic modulation of composition superimposed on the periodic n-i-p-i doping profile.

A class of narrow-band-gap semiconducting polymers

Abstract: Scientific interest in electrically conducting polymers and conjugated polymers in general has been widespread among workers in polymer science, chemistry, condensed matter physics, materials science and related fields since the discovery of doped conductive poly acetylene1,2 Many doped conducting organic polymers with conductivity spanning the range from insulator to near-metallic ∼10−15–103 ohm−1cm−1) are now known1–13 Of prime importance and fundamental interest in the continuing experimental and theoretical search for new conducting, and perhaps superconducting, polymers is the achievement of small or vanishing values for the semiconductor band gap (Eg), which governs the intrinsic electronic, optical and magnetic properties of materials Existence of a finite Eg in conjugated polymers is thought to originate principally from bond-length alternation, which is related to the Peierls instability theorem for one-dimensional metals114–17 Here I describe a novel class of conjugated polymers, containing alternating aromatic and quinonoid segments, whose members exhibit intrinsic band gaps as low as 075 eV, the smallest known value of Eg for an organic polymer

Real-space observation of π -bonded chains and surface disorder on Si(111)2×1

Abstract: Scanning tunneling microscopy is used to confirm the \ensuremath{\pi}-bonded chain structure of the Si(111)2\ifmmode\times\else\texttimes\fi{}1 surface. Both the amplitude and voltage dependence of the vertical corrugation exclude the buckling model for the structure of this surface. Spectroscopic measurements of the tunneling current versus voltage identify a band gap for the 2\ifmmode\times\else\texttimes\fi{}1 surface states. Spatial images of disorder-related states are obtained by tunneling at energies inside of this band gap.

Enhanced photoredox chemistry in quantized semiconductor colloids

Abstract: This, in turn, leads to enhanced redox potentials for photoexcited electrons and holes in very small semiconductor particles In this Letter, we present both photochemical and photophysical evidence for large quantization effects in small-sized HgSe, PbSe, and CdSe colloids To a first approximation, the energy of the quantized levels is inversely proportional to the effective mass and the square of the particle diameter In our study, HgSe and PbSe colloids were used because they have very small electron effective masses (-005) Since the band gaps of HgSe and PbSe are both 03 eV, colloids consisting of large particle sizes (>500 A) are black and opaque; colloids with small particle sizes exhibit different colors depending on the size CdSe, with an effective mass of 01 and a band gap of 17 eV, was studied for COz reduction since it has a greater stability against photocorrosion compared to HgSe and PbSe Colloids with extremely small particle sizes were made by controlled precipitation of metal selenide in water or acetonitrile in the presence of stabilizers, such as (NaP03)6, SO2, Nafion, styrene/maleic anhydride copolymer, and polyethylene glycol All these colloids show large blue shifts in their absorption edge and in their emission spectra All the stabilizers were checked for their resistance against reduction processes For this purpose, T1° colloids were used as the reducing agent; its standard redox potential is -19 V vs NHE We found that only Si02 and Nafion are very stable in the presence of TlO, and therefore only these stabilizers were used to study photoredox chemistry SiO, colloids9 with a particle diameter of 50 A were used for the preparation of selenide colloids in water The colloids were prepared in water-alcohol mixtures at -20 OC, and the sols were then filtered through membrane filters with a pore size of 100 A For HgSe, 20 mL of solution containing 2 X M HgC12, 6 X M CH3COONH4 (buffer) were M SOz, and 2 X

Optical spectra and energy band structure of layer-type AIVBVI compounds

Abstract: The electroreflectance and absorption spectra as well as the hydrostatic pressure dependence of the fundamental absorption edge of GeS, GeSe, SnS, and SnSe crystals are investigated in polarized light. The lowest direct energy gaps are found to be 1V1—1V1 (E | | a) and 1Λ4—1Λ4 (E || b) for Ge and Sn monochalcogenides, respectively. The pressure coefficient of the energy gap 1Λ4—1Λ4 dE/AP = = — (1.3 ± 0.1) × 10−4 eV/MPa of SnSe and SnS is almost twice larger than that of the gap 1V1—1V1 in GeSe, GeS, and SnSe. On the basis of these investigations, different absorption mechanisms responsible for the fundamental absorption edge are discussed. [Russian Text Ignored].

Theoretical studies on polyaniline

Abstract: Theoretical predictions for various neutral and charged forms of polyaniline are presented and discussed. The MNDO semiempirical method is used to predict geometries which serve as input for valence effective Hamiltonian (VEH) calculations of the electronic band structure. The VEH calculations provide predictions for the band gaps, ionization potentials, electron affinities, and redox potentials for the various forms of polyaniline. Where possible, comparison is made to experiment with generally favorable results. The discussion emphasizes the electrochemistry of polyaniline and the structural evolution of the polymer during electrochemical oxidation and reduction.

Preparation of highly photosensitive hydrogenated amorphous Si‐C alloys from a glow‐discharge plasma

Abstract: The preparation conditions for the deposition of hydrogenated amorphous silicon‐carbon alloys (a‐SiC:H) from a glow‐discharge plasma have been systematically scanned by changing the starting‐gas materials as well as the starting gas to hydrogen dilution ratio. A highly photosensitive alloy showing a photoconductivity to dark conductivity ratio of 107 at a band gap of 2.0 eV was prepared under optimized conditions.

Excitonic transitions and exciton damping processes in InGaAs/InP

Abstract: Detailed absorption measurements and the analysis of the absorption spectra of In1−xGaxAs lattice matched to InP are reported. The lattice matching parameter Δa/a covered a range from +4×10−3 to −1×10−3. From the absorption data of material with small matching parameter we obtain the value of the interband matrix element ( P2=20.7 eV), the excitonic Rydberg (Ex =2.5 meV), and damping constant (Γ0=5.1 meV) in the temperature range from 1.5 to 340 K. From the temperature dependent band‐gap shrinkage and exciton damping constant Γ, information on the carrier‐phonon interaction is obtained. The effect of the biaxial stress in the epitaxial layers caused by the mismatch with the substrate is demonstrated by absorption spectra which directly reveal the valence band splitting due to stress. Absorption measurements on samples with and without substrate indicate that the strained expitaxial layers do not relax completely if the substrate is etched away. The remaining strain field is probably caused by misfit dislo...

Pressure dependence of GaAs/AlxGa1−xAs quantum‐well bound states: The determination of valence‐band offsets

Abstract: We report experiment and theory on the pressure dependence of quantum‐well bound states formed in the GaAs/AlxGa1−xAs heterostructure system. Using MQW’s and SL’s of various barrier compositions x, we trace in photoluminescence (8 K), and in full‐scale pseudopotential calculation, the pressure‐induced evolution of the lowest spatially confined states within the wells. With increasing pressure Γ‐confined states follow the shift to higher energies of the direct GaAs band gap. At critical pressures a crossing occurs between these Γ bound states and the barrier indirect X states. Here, Γ intensities plunge and new emission tracking the X edges appears. Confirmed in wave function calculation, these new transitions occur across the heterointerface, between X‐confined electrons within the AlxGa1−xAs and Γ‐confined holes within the GaAs. Arising from valence‐offset‐induced staggered band alignment, critical pressures for observation of these states decrease with increasing Al mole fraction. We thus obtain, with p...

Measurement of electron lifetime, electron mobility and band-gap narrowing in heavily doped p-type silicon

Abstract: The parameters that control the transport of minority carriers in heavily doped Si:B have been measured by a combination of steady state electrical and transient optical techniques. Electron diffusion length and electron lifetime measurements have been conducted on doped-as-grown wafers to extract the minority carrier electron mobility as a function of acceptor doping density. Effective band-gap narrowing in p+epitaxial layers has been characterized using bipolar test structures. Significant findings: 1) the electron mobility is about 2.5 times larger in heavily doped p-type Si than in n-type. 2) bandgap narrowing exceeds 120 meV at N _{A} = 2 \times 10^{20} cm-3.3) minority electron lifetime in processed p+Si is not well modeled over a large doping range by an "Auger" coefficient.

Hole trapping and breakdown in thin SiO 2

Abstract: The field dependence of the hole generation rate, also known as the impact ionization coefficient α, in thin SiO 2 ( \alpha = \alpha_{0}e^{-H/E} where H = 78 MV/cm for electric fields ranging from 7 to 14 MV/cm, which covers the field range from the onset of significant Fowler-Nordheim current to instant breakdown. The similar field dependences of α and charge-to-breakdown supports the model that hole generation and trapping leads to oxide wearout. Because of the fact that positive charge generation is observed for oxide voltage well below the SiO 2 bandgap, we propose that the generated holes arise from transition between band tails in the amorphous SiO 2 . It is also observed that α decreases rapidly when the applied oxide voltage is very low; thus α is a function of both oxide field and voltage in general. This suggests that ultra-thin oxide with low operating voltages might be a good candidate for high endurance E2PROM devices at very low oxide field.