Showing papers on "Band gap published in 1978"

Band gap versus composition and demonstration of Vegard’s law for In1−xGaxAsyP1−y lattice matched to InP

Abstract: Measurements of lattice parameters and compositions for In1−xGaxAsyP1−y lattice matched to InP demonstrate the validity of Vegard’s law for this quaternary. The measured compositional dependence of the band gap shows a bowing parameter smaller than predicted from the previously measured band gaps of the four constituent ternaries.

Surface defects and the electronic structure of SrTi O 3 surfaces

Abstract: The electronic properties of various types of SrTi${\mathrm{O}}_{3}$ surfaces have been studied by using ultraviolet-photoemission and electron-energy-loss spectroscopy, low-energy electron diffraction and Auger spectroscopy. Vacuum-fractured surfaces exhibit weak photoemission in the region of the bulk band gap, which is probably due to residual surface defects. When surface defects are produced by Ar-ion bombardment, a much stronger band of surface states appears in the band-gap region. These states arise from the creation of ${\mathrm{Ti}}^{3+}$-O-vacancy complexes and are predominantly of $d$-electron character. Two surface defect phases are seen, one due to surface disorder and the other to changes in surface composition. The latter phase is stable under annealing to at least 1100 K. Exposure to ${\mathrm{O}}_{2}$, however, depopulates the band-gap surface states on both vacuum-fractured and ion-bombarded surfaces. Models for the detailed structure of the defect surface states are discussed.

Amorphous semiconductors equivalent to crystalline semiconductors produced by a glow discharge process

Abstract: A method of making an amorphous semiconductor film or the like having desirable photoconductive and/or other properties comprises depositing on a substrate a solid amorphous semiconductor film including at least one element, by glow discharge decomposition of a compound containing said at least one element and at least one alterant element in an atmosphere separately containing at least one different alterant element, wherein the plurality of different alterant elements comprise at least fluorine and are incorporated in said amorphous semiconductor film during the deposition thereof yielding an altered amorphous semiconductor material having reduced density of localized states in the energy gap thereof so that greatly increased diffusion lengths for solar cell application are obtained and modifiers or dopants can be effectively added to produce p-type or n-type amorphous semiconductor films so that the films function like similar crystalline semiconductors.

Electrical properties of polyacetylene with various cis‐trans compositions

Abstract: The electrical conductivity of crystalline polyacetylene films having various cis-trans compositions was measured. The resistivity and the energy gap of a 92,5% trans polymer were 1,01·104Ω·cm and 0,56 eV, respectively, whereas the values of a 20,0% trans polymer were 2,35·108Ω·cm and 0,93 eV, respectively. These differences are discussed in terms of the effective conjugation length estimated from visible spectra and ESR measurements, and the spacing between molecular planes obtained by x-ray diffraction measurements.

InAs-GaSb superlattice energy structure and its semiconductor-semimetal transition

Abstract: We performed a band calculation, based on the LCAO (linear combination of atomic orbitals) method, for the InAs-GaSb superlattice. Of possible semiconductor combinations the InAs-GaSb is of special interest because of indications that at a heterojunction interface the bottom of the conduction band of InAs lies below the top of the valence band of GaSb. Our results show that the InAs-GaSb superlattice is a semiconductor when the layers of the constituent materials are thin, and becomes a semimetal when the layer thicknesses are increased. The critical InAs thickness for this transition is approximately 115 \AA{}, over which the electrons from the valence band of GaSb "flood" the conduction band of InAs. For the thick-layer limit, we treat the problem in a Fermi-Thomas approximation.

The optical absorption edge of single‐crystal AlN prepared by a close‐spaced vapor process

Abstract: We have prepared several thicknesses of ∼1 cm by ∼1 cm epitaxial single crystals of AlN with a ’’high temperature (∼1800 °C) close‐spaced vapor transport’’ technique. These crystals have been grown both on (0112) and (0001) sapphire wafers. The optical absorption edge is measured at 300 K and for the first time at low temperature (5 K). This data is parametrized to include both reflection losses due to scattering off surface imperfections and contributions from impurity absorption bands. This parametrization technique is used to determine the band gap and the nature, direct or indirect, of the absorption edge. A value of 6.28 eV was found to be the best value of Eg(5 K) and the gap was found to be direct.

Band structure and superconductivity of Pd D x and Pd H x

Abstract: A study of the electronic band structure of the pailadium-hydrogen system was made using the augmented-plane-wave method. The calculations were performed self-consistently for two choices of the exchange parameters within the $X\ensuremath{\alpha}$ scheme. Spin-independent relativistic corrections were included explicitly. The results for the energy bands are compared with previous calculations. Our results are found to be in good agreement with photoemission measurements. The calculations of the superconducting properties of this system are studied as a function of hydrogen content, utilizing the theory of Gaspari and Gyorffy and neutron-scattering data. The results for the superconducting transition temperature ${T}_{c}$ are in very good agreement with the measured values. An analysis of these results attributes the observed inverse isotope effect to the effective increase of the Pd-H force constant over the Pd-D force constant, due to enhanced anharmonicity of the H motion, as originally proposed by Ganguly.

Electrical current in solids with position-dependent band structure

Abstract: Conduction in materials with a position-dependent band structure, such as graded band gap semiconductors and graded heterojunctions, is considered. A brief discussion of the meaning of spatially varying band structure, based on an extension of Wannier's theorem, is given. Using this extended theorem, it is shown that the conventional Boltzmann transport equation is inadequate for the situation under discussion; a generalized form of the Boltzmann transport equation is derived. From this the complete electron and hole currents are obtained. Besides drift, chemical, and thermal diffusion, they contain two extra terms. One of these involves the gradient of the electron affinity and of the band gap, while the other one depends on the gradient of the density of states. The results thus extend previous results by van Ruyven and Williams and by van Overstraeten et al. It is further shown that the results can be transformed to a form which resembles the conventional current equations, providing a modified position-dependent mobility and diffusivity are introduced. A discussion of the realm of applicability of these results is also presented.

Urbach's rule derived from thermal fluctuations in the band-gap energy

Abstract: The exponential absorption edge (known as Urbach's rule) observed in most materials is interpreted in terms of thermal fluctuations in the band-gap energy. The main contribution to the temperature shift of the band-gap energy is due to the temperature-dependent self-energies of the electrons and holes interacting with the phonons. Since the phonon number is fluctuating in thermal equilibrium, the band-gap energy is also fluctuating resulting in an exponential absorption tail below the average band-gap energy. These simple considerations are applied to derive Urbach's rule at high temperatures, while a simplified model with independent, noninteracting atoms is proposed to explain the behavior of Urbach's rule in the whole temperature range. The three parameters entering Urbach's rule are expressed in terms of parameters derived from the temperature shift of the band gap and from the exciton absorption. Comparison with experiments is performed for the II-VI compound ZnO. It is shown that a good agreement is found between the temperature shift of the exciton line observed experimentally and the temperature shift computed from the steepness parameter of Urbach's rule. The agreement with experimental values for the two other parameters is also satisfactory. It is shown that the band-gap shift (and absorption tail) in ZnO is caused by interaction with both acoustical and optical phonons. While the temperature-dependent polaron contribution can account for the optical-phonon contribution, the deformation-potential interaction with LA phonons is not sufficient to account for the acoustical-phonon contribution.

Energy bandgap and lattice constant contours of iii–v quaternary alloys

Abstract: Energy band gap and lattice constant contours are presented for the nine quaternary alloys formed from Al, Ga, In and P, As, Sb. The quaternary bandgaps were obtained using an interpolation formula proposed by Moonet al. The quater nary lattice constants were obtained by use of a linear interpolation technique using the binary lattice constants as boundary values.

Optical properties and electronic structure of UO2

Abstract: The near normal incidence reflectivity of UO2 single crystals has been measured in the photon energy range from 0.03 eV to 13 eV. From the reflectivity spectrum the complex dielectric function e (ω) =e1(ω)+ie2(ω) has been derived by means of the Kramers‐Kronig relation. In addition the absorption coefficient was determined from a direct transmission measurement on thin single crystal plates in the weakly absorbing spectral region below the absorption edge. An energy level scheme is proposed which allows a self‐consistent assignment of the structure in e2 with optical transitions between maxima in the density of states. The energy gap found at 2.1±0.1 eV is attributed to a 5f2→5f16deg transition. A crystal field splitting 10Dq=2.8 eV is derived for the 6d conduction states. Good agreement is obtained within this model with XPS measurements and a recent molecular cluster approximation. It disagrees with a previous interpretation of reflectivity data.

Effects of heat treatment on the optical and electrical properties of indium–tin oxide films

Abstract: The effect of heat treatment in various environments on the electrical and optical properties of indium–tin oxide (ITO) sputtered films has been investigated As the resistivity is decreased by heat treatment in H2 from 83×10−3 to 43×10−4 Ω cm, the optical band gap increases from 305 to 342 eV consistent with a Burstein shift and intrinsic band gap of 298 eV, and for resistivities less than 83×10−4 Ω cm there is a rapid decrease in transmission at longer wavelengths due to free‐carrier absorption and formation of dendritic precipitates Careful control over all sputtering variables is essential to obtain reproducible properties

Band-Gap Assignment in Sn O 2 by Two-Photon Spectroscopy

Abstract: We present for the first time two-photon measurements on rutile Sn${\mathrm{O}}_{2}$. The detailed polarization analysis yields ${{\ensuremath{\Gamma}}_{3}}^{+}$ symmetry for the direct forbidden $1S$ exciton and the upper valence band. Our results clear up the long-standing controversy concerning the symmetry of the upper valence band.

The Electron-Hole Liquid in Semiconductors: Theoretical Aspects

Abstract: Publisher Summary The chapter presents a discussion on the theoretical aspects of the electron-hole liquid in semiconductors. In an intrinsic semiconductor such as germanium or silicon, there are essentially no free carriers at very low temperatures. However, high densities of positive (hole) and negative (electron) charge carriers can be generated in these crystals by photoexcitation with a photon energy greater than the forbidden gap. In recent years, experiments in Ge and Si have revealed that optically pumped electrons and holes at high densities can undergo a phase transition at liquid helium temperatures into a metallic, liquid state. This condensate represents a state of matter particularly unique in nature because the electrons and holes, by reason of their small masses, are simultaneously in the quantum limit. This chapter presents a discussion on the collective behavior of electrons and holes at low temperatures and high densities. The chapter presents an historical introduction, mentions a few of the important early experiments performed, and presents a comparison of theory and experiment. Semiconductors can be divided into two classes. Those in which the conduction band minimum and valence band maximum occur at the same value of the wavevector are known as direct gap semiconductors, while those in which they are separated in k space are known as indirect gap semiconductors. The chapter discusses the latter.

Theory of electron-hole liquid in semiconductors

Abstract: The random-phase approximation is generalized to include the effects of band anisotropy, coupling between degenerate valence bands, coupling to optical phonons, and the Hubbard exchange correction. This method is used to calculate the ground-state energy and equilibrium density of the electron-hole liquid in Ge, Si, AgBr, and various II-VI and III-V compounds and the thallous halides and lead chalcogenides. The results are compared to the experiments where they are available. Agreement is excellent for Ge and Si and reasonable for the other materials, considering the large uncertainties in the values of the band masses, etc. Substantial discrepancies remain between theory and experiment, however, in the cases of GaAs and ZnO.

Carrier transport in oxygen‐rich polycrystalline‐silicon films

Abstract: A symmetrical‐Schottky‐barrier (SSB) band model is proposed for the carrier transport in oxygen‐rich polycrystalline‐silicon films. Semiquantitative analyses were made, comparing a one‐dimensional depletion approximation, with voltage and temperature‐dependent current measurements. The band gap is found to be similar to that of single‐crystalline Si, with electrons as the majority carriers. A net donor state density of 2.3×1019 cm−3, 0.12 eV below the conduction band edge, exists in the grains of the polycrystalline silicon, and these donors are completely ionized in the presence of large grain‐boundary surface states. This results in a grain‐boundary surface‐charge density of −2.3×1013q/cm2, setting up overlapped space‐charge regions which give rise to the SSB band structure with a maximum band bending of 0.44 eV. The Fermi level is found to be pinned at the midgap at the grain boundaries—suggesting the existence of an amorphous intergrain layer. At and above 373 K, the carrier transport is by thermionic...

Secondary electron emission from insulators

Abstract: The high yield of secondary electron emission from insulators due to electron bombardment may be the result of an increase of the depth of escape. The free-electron scattering theory is applied to the high energy of the primary beam, but it cannot be applied to the low energy of the secondary escaping beam because of the large energy gap of insulators. Then the plasmon loss with the valence electron is considered when the secondary electrons escape. Secondary electron emissions from insulators are calculated from the assumptions that the distribution of the secondary electrons due to both incident and back-scattered electrons within the target is isotropic and that it follows the absorption law of Lenard type. The universal yield-energy curve of secondary electron emission is derived.

(110) surface states of GaAs: Sensitivity of electronic structure to surface structure

Abstract: The (110) surface states of GaAs for three models of surface relaxation are examined by the tight-binding method. Although these relaxations, obtained from analyses of low-energy-electron-diffraction data, involve similar atomic displacements, they give rise to noticeable differences in the relative positions of some surface states when these states are identified by their orbital symmetries. All three relaxations are found to completely remove all surface states from the band gap and to give rise to new surface states. Our calculations indicate the presence of five to six filled and four empty surface states. Results for energies, local densities of state, and the orbital characters of these states are given and compared to experimental data.

Optical and electrical properties of CdGeAs2

Abstract: Large-grained ingots of CdGeAs2 have been grown from near-stoichiometric melts. Resistivity and Hall coefficient (RH) measurements were made on a large number of samples at 77 and 300 K and in some cases up to 450 K. Good fits to the log RH vs 1/T plots are obtained by using a model that assumes three kinds of electronic levels within the energy gap: donors, shallow acceptors, and acceptors with an ionization energy of 0. 30 eV. The deep acceptors are probably native defects, since their concentration varies by nearly four orders of magnitude from ingot to ingot with little change in impurity concentration. Between the intrinsic absorption edge at about 2. 5 Μm and the two-phonon absorption band at 18 Μm, the optical absorption increases with increasing deep acceptor concentration. By using oriented single-crystal samples ∼ 1 cm on a side, conversion efficiencies as high as 27% have been achieved for second-harmonic generation with single-mode pulses from a CO2 TEA laser.

High field electronic properties of SiO2

Abstract: The effects of very high electric fields on the transport of electrons and holes in SiO2 are discussed. At fields above 5 × 105 V/cm, electrons emit optical phonons, which is a very efficient energy loss mechanism. Holes on the other hand form small polarons in about 10−12 s, and their mobility becomes very low, but is unaffected by field up to 5 × 106 V/cm. The field dependent generation of electron-hole pairs is fit by application of the geminate recombination theory with a distribution of thermalization distances and excitation by X-rays and bandgap radiation is discussed. The first dependent bulk recombination coefficient is discussed in terms of high field mobility of the electrons. The impact ionization of electrons in SiO2 is discussed by comparing recent results for laser-induced breakdown in SiO2 with experiments on thin films involving photocurrents, space charge buildup and prebreakdown currents, and also theoretical predictions. Below 107 V/cm the laser experiments indicate higher impact ionization rates than the thin film experiments or theory.

The effect of elastic strain on energy band gap and lattice parameter in III‐V compounds

Abstract: The elastic and misfit strain in vapor‐grown InGaP/GaAs crystals was determined by measuring the lattice parameter of the InGaP before and after removal of the GaAs substrate. The energy‐band‐gap shift as a function of strain was measured in a similar manner using photoluminescence. Up to 70% of the misfit strain was found to be accommodated elastically. The critical resolved shear stress for dislocation motion was found to be ∼2×109 dyn/cm2. The rather low band‐gap shift with applied stress of ∼3×10−9 meV/dyn cm−2 was attributed to the Poisson effect. Photoluminescence was found to be a very accurate means to measure composition (and therefore lattice parameter), and empirical expressions were determined for the variation of photoluminescence wavelength with composition, lattice parameter, and energy band gap.

Composition dependence of the band gaps of In1−xGaxAs1−yPy quaternary solids lattice matched on InP substrates

Abstract: The In‐Ga‐As‐P quaternary phase diagram required for the growth of lattice‐matched In1−xGaxAs1−yPy layers on InP substrates has been determined experimentally at 650 °C. The liquidus isotherms were obtained by the seed‐dissolution technique. The solidus iostherms were determined by electron‐microprobe analysis performed on surfaces of quaternary epitaxial layers grown on Sn‐doped InP (111) B substrates from quaternary saturated melts. Lattice constants of layers were measured by an x‐ray‐diffraction technique. The liquid‐phase‐epitaxy growth conditions of lattice‐matched In1−xGaxAs1−yPy (0⩽x⩽0.47, 0⩽y⩽1.0) layers on InP were found from the results of the phase diagram and lattice constant measurements. Lattice‐matched layers with various band gaps (from 1.34 to 0.74 eV at room temperature) were grown by using these conditions. Band gaps of the layers were determined by photoluminescence measurements at 300 and 77 K. The band gap at each temperature was found to be linearly dependent on alloy‐composition p...

Secondary‐electron‐escape probabilities

Abstract: The probability B that a secondary electron internally excited in an insulator escapes upon reaching the surface is extracted from experiment and discussed theoretically for the first time. It is shown that B=2.5δme/E0m, where e is the electron‐hole‐pair creation energy and δm and E0m are the maximum secondary yield and the primary energy of maximum yield, respectively. It is also shown that e=2.8Eg, where Eg is the insulator band gap. Values of B are extracted from secondary‐electron‐emission data for many semiconductors. A simple theory relating B to χ/Eg, the ratio of the electron affinity and the band gap is developed and shown to define an upper bound for the experimental data. The free‐particle approximation adequately describes these hot‐ (energies ≳1 eV) electron phenomena.

Electrical properties of the gallium arsenide–insulator interface

Abstract: Extensive capacitance–voltage (C–V) measurements have been performed on three types of insulators on n‐type GaAs: pyrolytically deposited silicon nitride, sputtered silicon nitride, and the oxide produced by anodization. Evaporated aluminum gate contacts and alloyed Au–Ge ohmic contacts were applied and the C–V characteristics of these structures were measured from the quasistatic regime to 150 MHz. Although differences in detail are seen at intermediate frequencies, the data taken at the extreme frequency limits differ very little for the three types of specimens. The quasistatic and high‐frequency data are consistent with a model wherein the surface potential with zero applied gate bias is ∠−0.8 V. Application of electric fields of the order of ±106 V/cm at the semiconductor surface results in movement of the surface potential across approximately 1/3 of the band gap. An accumulation layer of surface electrons could not be produced on any of the devices tested.

The electron capture cross section and energy level of the gold acceptor center in silicon

Abstract: The electron capture cross section of the gold acceptor center has been measured by a direct technique which measures the electron‐trapping rate. The value was found to be 9±2×10−17 cm2 over the temperature range 77–280 K. Using this value together with thermal emission data, the energy level of the center was calculated from the detailed balance equation. Assuming a degeneracy factor of 4, the calculated temperature variation of the energy level (as measured from the conduction band) was found to be very similar to the temperature variation of band gap itself, indicating that the acceptor level may be fixed to the valence band edge with a separation of 0.632±0.004 eV.