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Showing papers on "Band gap published in 1968"


Journal ArticleDOI
TL;DR: In this paper, the optical constants of V${\mathrm{O}}_{2} have been determined between 0.25 and 5 eV both below and above the semiconductor-metal transition temperature.
Abstract: The optical constants of V${\mathrm{O}}_{2}$ have been determined between 0.25 and 5 eV both below and above the semiconductor-metal transition temperature ${T}_{t}=340\ifmmode^\circ\else\textdegree\fi{}$K. Reflectivity and transmission spectra have been measured on both single crystals and than films. The reflectivity spectra of the bulk crystals were measured with E \ensuremath{\perp} ($c$ axis) in the tetragonal phase [or \ensuremath{\perp} ($a$ axis) in the monoclinic phase], and with E parallel to these axes. While there are some differences in magnitude between the dielectric constants obtained from thin-film and single-crystal measurements, the structural features are in good agreement. Below ${T}_{t}$ there are four prominent absorption peaks centered near photon energies of 0.85, 1.3, 2.8, and 3.6 eV. Above ${T}_{t}$, metallic free-carrier absorption is observed below 2.0 eV, but the same two absorption peaks near 3 and 4 eV are present. The energy location and polarization dependence of these two higher energy peaks can be related to similar absorption peaks in rutile, and are interpreted using the rutile band structure. The results are consistent with a picture in which filled bands arising primarily from oxygen $2p$ orbitals are separated by approximately 2.5 eV from partially filled bands arising primarily from vanadium $3d$ orbitals. Transitions from the filled $2p$ bands are responsible for the high-energy peaks in the optical absorption in both the high- and low-temperature phases. In the high-temperature metallic phase, there is evidence that there is overlap among the $3d$ bands such that at least two bands are partially occupied by the extra $d$ electron per vanadium ion. In the low-temperature semiconductor phase, a band gap of approximately 0.6 eV opens up within the $3d$ bands, separating two filled bands from higher-lying empty bands. The two absorption peaks at 0.85 and 1.3 eV are due to transitions from these two filled bands.

509 citations


Journal ArticleDOI
TL;DR: In this paper, the authors discuss the direct band gap case and the case where a band gap is zero by symmetry, as in gray tin, and discuss the case when the band gap of a semiconductor is zero.
Abstract: An indirect band-gap semiconductor may be converted to a semimetal, or vice versa, by application of pressure. At low temperature, an excitonic phase or some other anomaly must occur in the neighborhood of the transition pressure. We also discuss the direct-band-gap case and the case where a band gap is zero by symmetry, as in gray tin.

341 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the Hall coefficient of β-FeSi2 doped with cobalt (n-type) or aluminium (p-type), and found that the activation energy of the mobility is 0.06 eV, the density of states N = 1.2 × 1022 cm−3.
Abstract: Electrical conductivity, thermoelectric power and Hall coefficient of β-FeSi2 doped with cobalt (n-type) or aluminium (p-type) are measured between 100 and 1200 °K. The conductivity of n-FeSi2 follows an exponential dependence on temperature. The temperature dependence of the thermoelectric power cannot be interpreted on the basis of conduction in a band. With the assumption that conduction in n-FeSi2 is caused by small polarons, the mobility at room temperature is found to be μn = 0.26 cm2/Vs. The activation energy of the mobility is 0.06 eV, the density of states N = 1.2 × 1022 cm−3. The electrical properties of p-FeSi2 can be interpreted using the band model with a hole mobility μp ≈ 2 cm2/Vs, which varies as T−1/2 in the region of extrinsic conduction. From intrinsic conduction a band gap of 0.9 to 1.0 eV is deduced. The disappearance of the thermoelectric power at high temperature is related to the semiconductor-to-metal transition at 1200 °K.

225 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that boron-nitride films of up to 6000Aa thick have been deposited on a variety of substrates at 600 °-1000 °C by a reaction between diborane and ammonia in hydrogen or inert carrier gas.
Abstract: Clear, vitreous films of boron nitride up to 6000Aa thick have been deposited on a variety of substrates at 600 °–1000 °C by a reaction between diborane and ammonia in hydrogen or inert carrier gas. Deposition rate may be readily adjusted to 50–1000 Aa/min. Most samples were made at either 600° or 800°, with some attendant variation in film properties. The 600° material contains some residual B‐H bonding. The film is essentially amorphous to electron diffraction. The refractive index is 1.7–1.8, the 1 MHz dielectric constant ~ 3 1/2, the dielectric strength , and the The band gap is 3.8 ev and the phonon temperature in the neighborhood of 2000°K. For semiconductor junction protection boron nitride has no advantage over silicon nitride. 600° deposition directly on Si has produced surface charges as low as , but there are room‐temperature drifts, and high‐field conduction also. BN deposited at 800° on Si is electrically similar to silicon nitride. Etching of BN film also presents the same problems as does silicon nitride. BN is not as good a barrier against sodium ion permeation. Attack by atmospheric moisture over a long period has varied from insignificant to extensive conversion to orthoboric acid.BN film on Si dopes the substrate with boron at temperatures above 900 °C in inert ambient. Uniform junction depths are produced. D‐C conductivity in 500–4000Aa films has been studied from room temperature to 270 °C. With fields ≥ 106 v/cm BN film shows stable, nonohmic conductivity which is independent of polarity. The 25 °C d‐c conduction is describable over at least seven decades of current by , , where , . The 600°‐deposited BN is the more conductive and can carry indefinitely. is linear, and the slope of the curve is in good agreement with the theoretical value for a Frenkel‐Poole conduction mechanism. Possible use of BN as a thin film varistor is discussed.

225 citations



Journal ArticleDOI
TL;DR: In this article, a theoretical model for dye-sensitization of photocurrents at electrodes of n-type zinc oxide and p-type perylene was presented and experimental data was presented.

195 citations


Book ChapterDOI
TL;DR: In this paper, a discussion of the models that have been presented to explain the nonconductivity of the ground state of transition metal oxides is presented, and the experimental results on a number of specific materials are reviewed.
Abstract: Publisher Summary The transition metals are interesting because, in addition to the s and p electrons found on each atom of ordinary metals, their atoms each possess from one to nine d electrons When these transition metal atoms overlap to form a solid, the d orbitals spread to form a relatively narrow d band This d band is partially full, and is primarily responsible for the magnetic properties of the materials The next higher s band is quite wide, and overlaps the entire d band Thus, there is always a partially full s band that results in metallic conductivity This chapter discusses the oxides of the transition metals The main part of this chapter is a discussion of the models that have been presented to explain the nonconductivity of the ground state It also reviews in detail, the experimental results on a number of specific materials, and then considers how successful the models are in explaining the actual observations

191 citations


Journal ArticleDOI
TL;DR: In this article, a two-valence-band model for room temperature electrical and thermal properties of p-type SnTe for the carrier concentration range 4 × 1019-2 × 1021 cm−3 have been quantitatively explained using the Cohen's dispersion law for energy surfaces with rotation plus reflection symmetry.
Abstract: Room-temperature electrical and thermal properties of p-type SnTe for the carrier concentration range 4 × 1019-2 × 1021 cm−3 have been quantitatively explained using a two-valence-band model in which the principal light mass band is always highly non-parabolic and the heavy mass band is parabolic. Band parameters of the model were determined. The theory was based on Cohen's dispersion law for energy surfaces with rotation plus reflection symmetry. The close similarity in band structure between SnTe and PbTe was established. It appeared that the 300°K band parameters of SnTe were very similar to the 0°K band parameters of PbTe, given by Rogers, but the energy separation between valence band edges was much greater in SnTe. Intravalley acoustic-mode lattice scattering was always dominant at room temperature.

159 citations


Journal ArticleDOI
Raphael Tsu1, W. E. Howard1, Leo Esaki1
TL;DR: In this article, the authors describe the variation with carrier concentration of the susceptibility mass and the absorption edge, in a range of hole concentrations of 1.5 and 0.34 eV for GeTe and SnTe, respectively.
Abstract: We describe first our measurements of the variation with carrier concentration of the susceptibility mass and the absorption edge, in a range of hole concentrations of 1\ifmmode\times\else\texttimes\fi{}${10}^{20}$ to 2\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The susceptibility mass for holes varies from 0.075 to $0.51{m}_{0}$ for the lowest and highest carrier concentrations, respectively. These properties, and similar published data for SnTe, together with the Fermi energies and energy gaps obtained from tunneling experiments, have been interpreted using a band structure obtained by applying the k\ifmmode\cdot\else\textperiodcentered\fi{}p perturbation approach at the $L$ point of a face-centered cubic zone. We have assumed a coupling scheme such that there is strong transverse coupling across the energy gap while the longitudinal coupling is between the principal conduction band and the second valence band. The energy separations for the second valence bands required in both materials are 0.5 and 0.34 eV for GeTe and SnTe, respectively. As a prerequisite to a meaningful interpretation of the measured quantities versus carrier concentration, we have calculated the ratio of the actual carrier concentration to the Hall concentration $\frac{1}{{R}_{0}e}$. We have made numerous computer calculations and have obtained a set of band parameters for these materials based on the best fit for a wide variety of experimental results.

151 citations


Journal ArticleDOI
TL;DR: The contribution of the valence electrons to the nonlinear optical susceptibilities may be estimated on the basis of simple tetrahedral bonding orbitals as mentioned in this paper, which contributes to the third-order nonlinearity, which describes scattering processes between four light waves.
Abstract: The contribution of the valence electrons to the nonlinear optical susceptibilities may be estimated on the basis of simple tetrahedral bonding orbitals. The coefficient for second-harmonic generation in III-V compounds is in satisfactory agreement with the theoretical estimate. The same model contributes to the third-order nonlinearity, which describes scattering processes between four light waves. Again reasonable agreement is obtained with data for combination frequency generation in Si and Ge. The much larger effects in $n$-type InAs and InSb are due to conduction electrons. Their contribution has been calculated exactly under the assumption of Kane's theory for the band structure. The contribution of the valence electrons is, however, not negligible.

133 citations


Journal ArticleDOI
TL;DR: In this paper, a model for the electronic structure of copper-containing sulfospinels is deduced from magnetic and electrical properties, which indicate that Cu is monovalent in these compounds.
Abstract: CuCr1+xRh1−xSe4 with −1 < x ≦ 1 and CuCr1+xTi1−xS4 with 0 < x ≦ 1 show p‐type metallic conduction and are ferromagnetic, while CuCr1+xTi1−xS4 with x<0 show n‐type metallic conduction and no magnetic ordering at 4°K. From the magnetic and electrical properties, which indicate that Cu is monovalent in these compounds, a model for the electronic structure of copper‐containing sulfospinels is deduced. Ferromagnetism and p‐type conduction are attributed to holes in a broad valence band, n‐type conduction to electrons in a broad conduction band. Susceptibility measurements on CuCo2S4, CuTi2S4, CuCoTiS4, CuRh2S4, and CuRh2Se4 are reported. The main contribution to the temperature‐independent susceptibility observed is attributed to the Van Vleck susceptibility of Co3+ or Rh3 ions in the low spin state t26.

Journal ArticleDOI
TL;DR: In this article, the magnetic and electrical properties of Fe1−xCuxCr2S4 were interpreted on the basis of a model for sulphospinels given by Lotgering and Van Stapele, with the additional assumption that the Fe2+ levels are situated in the energy gap between the valence and conduction band.

Journal ArticleDOI
TL;DR: In this article, the authors studied the optical absorption, photoconductivity, contact photovoltage, electrical conductivity, and Hall coefficient of single crystals of WSe2 over the temperature range 77°-295°K.
Abstract: The optical absorption, photoconductivity, contact photovoltage, electrical conductivity, and Hall coefficient of single crystals of WSe2 have been studied over the temperature range 77°–295°K. It was found that the forbidden energy gap Eg was 1.35 eV at 295°K and that the temperature dependence of Eg was given by dEg/dT=−4.6×10−4 eV/°K. The material as grown by iodine vapor transport in a sealed ampule is p‐type with hole mobility μh∼80 cm2/V·sec and p∼1016/cc at 295°K. The carrier concentration could be reduced by pumping out excess selenium. Doping with rhenium during the crystal growth process resulted in n‐WSe2 with a carrier concentration n∼1017/cc and electron mobility μn∼100 cm2/V·sec at 295°K.

Journal ArticleDOI
TL;DR: In this paper, surface electric-field-induced and resonanceenhanced Raman scattering by LO phonons in the spectra obtained from InSb surfaces has been observed and it has been shown that excitons are responsible for the resonance enhancement.
Abstract: We have observed surface electric-field-induced and resonance-enhanced Raman scattering by $q\ensuremath{\approx}0$ LO phonons in the spectra obtained from InSb surfaces. The data indicate that excitons are responsible for the resonance enhancement. The effects of temperature and externally applied electric fields are discussed. The results suggest the possibility of studying the surface electronic properties by Raman spectroscopy.

Journal ArticleDOI
TL;DR: The band structures of GaS and GaSe near the Fermi levels are derived in a semi-empirical way in this paper, and the properties of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis.
Abstract: The band structures of GaS and GaSe near the Fermi levels are derived in a semiempirical way. In view of the anisotropic structures, the π band structures are mainly studied. The valence band consists of a π band with the heavy effective masses. The conduction band at the center of the Brillouin zone has the light effective masses for the both directions parallel and perpendicular to the layers, while its minimum at the zone edge has a two-dimensional character. The characteristics of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis. Most of the structures in reflectivities in GaS and GaSe and the binding energy of an exciton in GaSe are explained fairly well.

Journal ArticleDOI
TL;DR: In this paper, a linear polymer of acetylene was prepared in which the oxygen content was as low as 0.7 % and the electrical conductivity and Seebeck voltage but not the energy gap depended on the extent of oxidation of the samples.
Abstract: Linear polymer of acetylene was prepared in which the oxygen content was as low as 0.7 %. The electrical conductivity and Seebeck voltage but not the energy gap depended on the extent of oxidation of the samples. Adsorbed donor and acceptor gases affected electrical conductivity but not e.s.r. spin assay. It was concluded that the degree of crystallinity in these polymers is of equal importance with chemical purity to achieve high electrical conductivity.

Journal ArticleDOI
R.E. Nahory1, J. L. Shay1
TL;DR: In this paper, reflectance spectra in GaAs modulated by a second, intense light beam were studied and it was shown that these structures result from a neutralization of the builtin surface field by free carriers created by the intensity of the light beam.
Abstract: By studying reflectance spectra in GaAs modulated by a second, intense light beam, we have observed oscillatory structures in the spectra near the energy gap as well as near 3 eV. We conclude that these structures result from a neutralization of the builtin surface field by free carriers created by the intense light beam. The experimental line shape near the band gap is qualitatively very similar to the theoretical prediction for the Franz-Keldysh effect but is shifted to lower energies presumably due to exciton effects.

Journal ArticleDOI
TL;DR: In this article, an analysis is presented of two variants of the superposition of spectra of boron and nitrogen of hexagonal BN into one energy diagram of electron composition in the crystal BN hex.

Journal ArticleDOI
TL;DR: In this article, it is assumed that optical excitation of a surface state practically coincident with the top of the valence band corresponds to a set of states 0.16 eV below the bottom of the conduction band.
Abstract: Optical absorption of a cleaved surface of germanium shows a band at energies smaller than the gap, that is removed when the surface is oxidized. The band is assumed to be due to optical excitation of a surface state practically coincident with the top of the valence band to a set of states 0.16 eV below the bottom of the conduction band.


Journal ArticleDOI
TL;DR: In this article, the first relativistic first-principle band calculations for IV-VI compounds are presented, which lead to physically realistic energy band models which are sufficiently accurate to account for most of the characteristic features of the experimental reflectivity spectra.
Abstract: : Relativistic OPW band calculations have been carried out at key points in the reduced zone for several IV-VI compounds. The band structure in the remainder of the zone has been filled in with the aid of an interpolation scheme. To our knowledge, these are the first fully relativistic first-principles band calculations for IV-VI compounds (in contrast to non-relativistic calculations supplemented by relativistic and spin-orbit coupling corrections). Our first-principles band calculations lead to physically realistic energy band models which are sufficiently accurate to account for most of the characteristic features of the experimental reflectivity spectra. It is difficult to interpret the electroreflectivity spectra unambiguously in terms of these band models but a number of plausible spectral assignments can be made. Refined energy band and optical spectrum calculations are in progress and will be reported elsewhere. (Author)

Journal ArticleDOI
TL;DR: In this article, the expected change in resistivity, effective Hall mobility and band gap has been calculated from the known influence of stress on the electrical properties of bulk semiconductors, and has been compared with the experiments.
Abstract: Silicon films on single crystal magnesium-aluminium spinel slices and similarly on sapphire are strained under compressive stress. This is due to the difference in thermal expansion and to the high growth temperature, causing a curvature of the slices. The coefficients of thermal expansion have been measured by a dilatometer. From the curvature measured by a stylus tracing method a residual stress of about 8 × 10 3 kp/cm 2 has been determined. The expected change in resistivity, effective Hall mobility, and band gap has been calculated from the known influence of stress on the electrical properties of bulk semiconductors, and has been compared with the experiments. The measured mobility as a function of carrier concentration has a maximum between 10 16 and 10 17 carriers per cm 3 . The highest mobilities measured were 600 cm 2 /V-sec for n -type and 350 cm 2 /V-sec for p -type silicon films. The observed decrease of carrier mobility with decreasing carrier concentration (for n , p ≤ 10 16 cm −3 ) and the relatively low minority carrier lifetime are explained by the assumption of space-charges localized at crystal defect accumulations. In accordance with this assumption an anomalous dependence of mobility on temperature has also been found. The existence of localized space charge regions was further confirmed by the change of photo-voltage on scanning the film with a finely focused laser beam. First characteristics of MOST's on relatively thick silicon films on spinel do not show any remarkable difference from that of transistors on bulk silicon, both having been produced by applying the usual MOS technology.

Journal ArticleDOI
TL;DR: In this article, the superconductivity of electron beam evaporated tungsten films by X ray and electron diffraction techniques is discussed, and temperature dependence of energy gap is discussed.
Abstract: Superconductivity of electron beam evaporated tungsten films by X ray and electron diffraction techniques, discussing temperature dependence of energy gap

Journal ArticleDOI
TL;DR: In this paper, the dominant mechanism for electrical conduction in NiO and similar materials is by means of holes in the oxygen $2p$ band, and a band-structure diagram which takes into account the primarily localized $3d$ states is suggested.
Abstract: It is proposed that the dominant mechanism for electrical conduction in NiO and similar materials is by means of holes in the oxygen $2p$ band. A band-structure diagram which takes into account the primarily localized $3d$ states is suggested.

Journal ArticleDOI
TL;DR: In this article, the spin-orbit splitting at a selected point was found to agree well with experiment, and a comparison with results of k.p calculations was made, showing that the results at other points in the zone were consistent with experiment.

Journal ArticleDOI
TL;DR: In this paper, calculated Debye-Waller factors have been combined with a modified Lin-Kleinman pseudopotential and formalism to compute the temperature dependence of the PbTe direct band gap.
Abstract: Experimentally verified, calculated Debye-Waller factors have been combined with a modified Lin-Kleinman pseudopotential and formalism to compute the temperature dependence of the PbTe direct band gap. The band-gap increase of 0.008 Ry between 100 and 300\ifmmode^\circ\else\textdegree\fi{}K is in fair agreement with an experimental estimate of the explicit increase.

Journal ArticleDOI
TL;DR: In this article, a phase diagram for various electronic states in the space nd, b, T, where nd is the number of electrons per d orbital per atom, b is the transfer energy, and T is the temperature.
Abstract: Spontaneous band magnetism occurs as a transitional electron state, in the thermodynamic sense, between a localized‐electron (or small‐polaron) state and a conventional collective‐electron state. The localized‐electron state is well described by crystal‐field theory, together with superexchange and double‐exchange theories. Conventional band theory neglects electron correlations, except in the superconducting state. In the transitional state, electron correlations introduce not only the exchange interactions responsible for spontaneous magnetism, but also a deep minimum, if not an energy gap, in the density‐of‐states vs energy for half‐filled bands. Since Umklapp processes stabilize antiferromagnetic vs ferromagnetic order as the Fermi surface approaches a Brillouin‐zone boundary, it is possible to construct a semiempirical phase diagram for various electronic states in the space nd, b, T, where nd is the number of electrons per d orbital per atom, b is the transfer energy, and T is the temperature. Magnetic data that illustrate a few significant features of this diagram are discussed briefly, and it is pointed out that spontaneous band magnetism is a relatively rare phenomenon because the transitional state occurs over only a small range of b.

Journal ArticleDOI
TL;DR: In this paper, the negative spin-orbit splitting of the valence band at k = 0 in ZnO has been confirmed experimentally by measuring the effect of static uniaxial compression along the c-axis on the exciton spectrum at 77°K.

Journal ArticleDOI
TL;DR: In this article, the Green's function method was used to calculate relativistic energy bands for CsI and other Cs halides, and the value of the potential outside the muffin-tin spheres was adjusted so as to duplicate the observed value for the band gap.
Abstract: Relativistic energy bands have been calculated for CsI by means of the Green's function method. The value of the potential outside the muffin-tin spheres is adjusted so as to duplicate the observed value for the band gap. d -like conduction bands originating mainly in the d -states in the alkali ion are found to be overlapping with the s -like band more strongly than in the potassium halides. The complex structure in the intrinsic optical spectra can be explained by the excitons associated with the s -like and d -like conduction-band states at the point \(\varGamma\). Spectra of the other Cs halides can be understood by analogy with CsI. Discussion is given on the multiplet structure of the F -centre absorption band of the Cs halides.

Journal ArticleDOI
TL;DR: In this paper, single crystals of Cd 1- x Mn x S (x ≤ 0.4) were prepared by the Bridgman-Stockbarger method under high gas pressure and electrical and optical properties of these crystals were measured.
Abstract: Single crystals of Cd 1- x Mn x S ( x ≤0.4) were prepared by Bridgman-Stockbarger method under high gas pressure and electrical and optical properties of these crystals were measured. The mobility of electrons was found to be significantly influenced by the alloy scattering. In the optical absorption spectra, three absorption peaks due to Mn 2+ ions were observed at 455, 480, and 510 mµ, similar to the case of Zn 1- x Mn x S crystals. The band gap of Cd 1- x Mn x S varies anomalously with x ; it shows a minimum at x ∼0.03 and increases linearly with x for x ≥0.1. The reason for the appearance of this minimum at small x has not been understood.