Showing papers on "Band gap published in 1971"

Electrochemistry of excited molecules: photo‐electrochemical reactions of chlorophylls*

Abstract: Semiconductors with a sufficiently large energy gap, in contact with an electrolyte, can be used as electrodes for the study of electrochemical reactions of excited molecules. The behavior of excited chlorophyll molecules at single crystal ZnO-electrodes has been investigated. These molecules inject electrons from excited levels into the conduction band of the electrode, thus giving rise to an anodic photocurrent. The influence of various agents on this electron transfer has been studied. In the presence of suitable electron donors (e.g., hydroquinone, phenylhydrazine) in the electrolyte chlorophyll molecules, absorbing quanta, mediate the pumping of electrons from levels of the reducing agents into the conduction band of the semiconductor-electron acceptor. The electron capture by the semiconductor electrode is irreversible, when an adequate electrochemical gradient is provided in the electrode surface. An experimental technique for the study of the kinetics of photoelectrochemical reactions of chlorophyll molecules is introduced and a theoretical approach for its calculation is given. Some properties of excited chlorophyll at semiconductor electrodes (unidirectional electron transfer, highly efficient charge separation, chlorophyll as electron pump and able to convert electronic excitation into electric energy) show similarity to the behavior of chlorophyll in photosynthetic reaction centers.

GaAs–AlxGa1−xAs Double Heterostructure Injection Lasers

Abstract: Double heterostructure GaAs–AlxGa1−xAs junction lasers which have very low thresholds and which have been operated continuously at and above room temperature have been fabricated by liquid phase epitaxial growth The threshold current density of these lasers decreases approximately linearly with the thickness of the active region from 3 to at least 05 μm This is interpreted as the result of near perfect carrier and optical confinement as the result of large steps in the energy gap and index of refraction at the heterojunctions in these diodes The gain in these lasers is very high and its dependence upon current density is superlinear Loss is very low and almost that expected from free carriers Complete polarization of the lasing mode was observed This latter is interpreted to be the result of an increased reflection coefficient for the T E mode

Liquid Phase Epitaxial Growth of InAs1 − x Sb x

Abstract: Single‐crystal epitaxial layers of have been grown using a steady‐state, liquid phase epitaxial growth technique. In the range of they were grown on InAs substrates from liquid compositions on the pseudobinary phase diagram and also from liquids in the In‐rich corner of the ternary phase diagram at temperatures between 580° and 720 °C. In the range of they were grown on InSb substrates from In‐rich liquids with growth temperatures ranging from 480° to 520°C. The grown crystals are homogeneous in the region more than 4µ from the substrate. The optical absorption edges are sharp, similar to the InAs absorption edge. The 300°K energy band gap as a function of composition may be described by the following expression The electrical properties of the alloys with are similar to those of pure , i.e., when typical mobilities are 30,000 cm2/V‐sec at 300°K and 50,000 cm2/V‐sec at 77°K. For alloys with typical mobilities are 67,000 cm2/V‐sec at 300°K and 50,000 cm2/V‐sec at 77°K. Incorporation of Sn during growth of from the pseudobinary liquid at 720 °C was found to give n‐type doping. Under these conditions the distribution coefficient was determined to be 0.01.

Electrical and photoconductive properties of SnS2 crystals

Abstract: Single crystals of SnS2 have been prepared by the vapour transport method. The electrical conductivity of these crystals has been measured along and perpendicular to their c direction by a probe method. The thermal activation energy associated with electrical conduction in the c direction exceeds that in the direction perpendicular to it by about 018 eV. The spectral response of photoconductivity measurements showed a band gap of 234 eV (at 300 K) with a temperature coefficient of -102×10−3 eV K−1.

Ti-Rich Nonstoichiometric BaTiO3: II, Analysis of Defect Structure*

Abstract: A defect structure which incorporates singly and doubly ionized nonstoichiometric oxygen vacancies and acceptor states, presumably associated with impurities, was inferred from the agreement between the theoretically derived relation and the experimentally observed O2-partial-pressure and temperature dependence of the conductivity. This relation describes the behavior over the entire O2 pressure and temperature range investigated. A simplified defect model gave good agreement for the limiting case where n> >p but was shown to be in error compared with the more comprehensive relation above. The standard enthalpies of formation for singly and doubly ionized O vacancies in BaTiO3, are 4.3 and 5.7 eV, respectively. The difference between these values, 1.4 eV, corresponds to a donor energy level which lies near the middle of the band gap.

Electronic Structure and Luminescence Processes in In1−xGaxP Alloys

Abstract: The conduction‐band structure of In1−xGaxP has been studied by cathodoluminescence (CL) and photoluminescence (PL). An accurate determination of the direct energy gap (± 10 meV) as a function of alloy composition (Δx = ± 0.025) is achieved by the simultaneous electron probe microanalysis of alloy composition and spectral measurements of the CL excited by the microprobe electron beam. Similar measurements in InP, GaAs, and GaAs1−xPx, as well as absorption edge measurements and the temperature dependence of the PL spectrum, indicate that essentially free‐carrier recombination is observed in the PL and CL measurements of lightly n‐type In1−xGaxP at 300 °K. These results indicate that the ``cross over'' between the direct and indirect conduction‐band minima occurs at the composition x = 0.74 and energy gap = 2.26 eV.

On the nature of 4ƒ bonding in the lanthanide elements and their compounds

Abstract: A new model is proposed for the electronic configuration of the trivalent lanthanide metals in the condensed state. It is suggested that there are two kinds of 4ƒ electrons—the “atomic” 4ƒ electrons and the 4ƒ “band” electrons. The former are the 4ƒ electrons which are commonly associated with the trivalent metals and ions. The number of 4ƒ atomic electrons varies from zero for La to 14 for Lu and accounts for the normal magnetic susceptibilities observed in the elements and their compounds. The 4ƒ band electron, as the name implies, is one of the valence or conduction band electrons. The 4ƒ band electron occurs because the lowest of the empty energy levels, which vary from 14 for La to one for Tm, lies very close to the Fermi level (and thus is partially occupied by the conduction or valence band electrons) and not well above the Fermi level as has been commonly assumed previously or perhaps completely ignored. The 4ƒ band electron varies from about 0.7 of an electron for the light lanthanides to about 0.1 or less for the heavy lanthanides (for Lu there is no 4ƒ band electron since all 14 atomic 4ƒ levels are filled). The 4ƒ band electron does not contribute to the magnetic susceptibility, except as a conduction electron. It does, however, contribute to the bonding and thus influences the melting point and heat of sublimation. An analysis of these two physical properties permits one to estimate the fractional 4ƒ electron concentration in the valence band. The influence of the bonding 4ƒ electron on ionic and covalent compounds is also briefly discussed.

Band-to-band auger recombination in indirect gap semiconductors

Abstract: Electron–hole pairs in semiconductors may recombine through an Auger process. The theory for this process was worked out by Beattie and Landsberg, for a semiconductor with a simple and direct band structure. They found that the conservation laws for energy and crystal momentum lead to a threshold energy for this type of recombination. In the present paper it is shown that an Auger process may equally well take place in an indirect gap semiconductor, without participation of phonons, and that the activation energy, depending on the band structure, may be very low and even accidentally zero. A survey of available band data for silicon and germanium shows that for both these materials the band structure is favourable for an Auger process with low threshold energy. A perturbational approach is applied for calculating the transition rates for silicon and germanium. The results are compared with recent experimental data.

Energy band structures of Cd3P2 and Zn3P2

Abstract: A pseudopotential energy band calculation is carried out for a hypothetical crystal which corresponds to Cd3P2 and Zn3P2. From these results, the conduction band effective masses as well as the interband transition assignments associated with the optical reflectivity peaks are deduced. The temperature dependence of the energy gap for these materials is estimated and dEg/dT is shown to be negative. Mit der Pseudopotential-Methode wird eine Energiebandberechnung fur einen hypothetischen Kristall, der Cd3P2 und Zn3P2 entspricht, durchgefuhrt. Mit diesen Ergebnissen werden die effektiven Massen des Leitungsbandes sowie die Zuordnungen der Interband-ubergange zu den Maxima des optischen Reflexionsvermogens hergeleitet. Die Temperatur-abhangigkeit der Energielucke wird fur diese Stoffe abgeschatzt und gezeigt, das dEg/dT negativ ist.

Hot Carriers in Si and Ge Radiation Detectors

Abstract: Characteristics of silicon and germanium radiation detectors are developed from an analysis of the energy‐loss mechanisms for hot carriers. The carrier behavior is described in terms of phonon and ionization scattering rates. A calculation of the probability of ionization vs primary electron energy in silicon, using Kane's characterization of the scattering rates, indicates an effective ionization threshold several times the band‐gap energy. Calculations of phonon losses and the carriers' residual kinetic energies are combined to give an average energy expended per created pair e of 4.24 eV. Closer agreement with the experimental value of 3.67 eV is achieved by lowering the phonon to ionization scattering‐rate ratio. Temperature effects, which enter through changes in the band gap, are calculated to be ∂e/∂Eg=1.73. The analysis also predicts a Fano factor of 0.059 for silicon. Discussion of the germanium situation is based on the premise of hot‐carrier behavior similar to that in silicon. Because germanium has a more complicated band structure, two sets of calculations are made using different effective band gaps. With e=2.96 eV at 90 °K, very similar results are obtained in both cases and indicate ∂e/∂Eg≈1.77 and F≈0.066.

Energy Band Structure of Lithium Fluoride Crystals by the Method of Tight Binding

Abstract: The method of tight binding has been applied to calculate the energy band structure of the lithium fluoride crystal As initial approximations to the ultimate self-consistent---field (SCF) calculations, two different overlapping atomic potentials were employed, one formed by a superposition of the potential of the neutral Li and F atoms, and the other by that of ${\mathrm{Li}}^{+}$ and ${\mathrm{F}}^{\ensuremath{-}}$ The resulting energy band gaps for these two potentials were 152 and 142 eV, respectively A minimal set of the ten Bloch sums of the SCF wave functions of the $1s$, $2s$, and $2p$ states of the free Li and F atoms, a set of 30 contracted-Gaussian Bloch sums, and a set of 50 single-Gaussian Bloch sums have been used as the basis functions, and our calculations show that the minimal set is quite adequate for computing the energies of the valence band and the lowest conduction band A computational procedure for incorporating the Hartree-Fock-Slater SCF scheme into the method of tight binding has been formulated and applied to carry out the energy band calculations of LiF to self-consistency The SCF band structure gives an energy band gap of 109 eV in comparison with the experimental value of 136 eV Our calculations place the top of the valence band 123 eV below the vacuum level, and the Li $1s$ core states 57 eV below the bottom of the conduction band, which may be compared with the observed onset of photoemission at 12 eV and photoabsorption structure at 60 eV

Surface structures and photoluminescence of molecular beam epitaxial films of GaAs

Abstract: Surface structures were observed during molecular beam epitaxial growth of GaAs in situ in an ultrahigh vacuum high energy electron diffraction (HEED) system. The surface structures were related to the relative populations of gallium and arsenic on the substrate surface when the intensities of gallium and arsenic in the molecular beam were varied during growth. On the GaAs ( 111 ) face a ( 111 )-2 surface structure is interpreted in terms of an arsenic-rich surface condition and a ( 111 )-√ 19 structure is interpreted in terms of a gallium-rich condition. Only the (111)-2 surface structure was observed on the (111) face. The presence of these different surface structures under different growth conditions is interpreted in terms of rearrangements of the unshared bonds of the atoms on the crystal surface. Photoluminescence measurements were used to investigate the radiative recombination characteristics of the films grown with either a ( 111 )-2 or a ( 111 )-√ 19 structure present on the surface. The results show that the near gap photoluminescence intensity from a film grown under the latter condition is much higher than that from the former. A broad photoluminescence peak ∼ 0.1 eV below the band gap energy is observed on all epitaxial layers grown with the ( 111 )-2 structure present on the surface. The peak may be reduced by annealing and may be attributed to centers associated with Ga vacancies.

Some properties of gapless semiconductors of the second kind

Abstract: The authors investigate the physical properties of gapless semiconductors of the second kind, i.e., substances in which the conduction band is in direct contact with the valence band and the energy exhibits a square-law dependence on the momentum in the vicinity of the contact point in all branches of the spectrum in the one-electron approximation. The methods normally applied to strong-coupling field theories (“scaling”) are used to analyze the singular energy-momentum domain in which the Coulomb carrier interaction becomes strong and intractable by perturbation theory. The surface impedance, conductivity, susceptibility in weak and strong fields, and the Hall coefficient are calculated. Some possible experimental approaches are explored.

Determination of Surface States at Clean, Cleaved Silicon Surfaces from Photoconductivity

Abstract: The spectral response of photoconductivity of clean, cleaved silicon surfaces shows at 80\ifmmode^\circ\else\textdegree\fi{}K a broad shoulder for photon energies lower than the band gap. This shoulder exhibits a sequence of minima, equally spaced with 59 meV. From the spectral dependence of photoresponse and the energetic position of the minima, it is concluded that the photoconductance of the clean, cleaved surface is caused by indirect transitions from the valence band to surface states 0.5 eV above the valence-band edge.

High Electron Mobility in Zinc Selenide Through Low‐Temperature Annealing

Abstract: Electron mobility in ZnSe has been measured between 40° and 400°K. It is shown that through repeated annealing in liquid Zn the mobility maximum can be increased to 12 000 cm2/V sec. This is one of the highest mobilities measured for semiconductors with band gaps as wide as that of ZnSe (2.7 eV). The increase in mobility is mainly due to elimination of doubly charged acceptor states. The residual scattering is believed to be due, in part, to charged isolated impurities and, in part, to paired impurity dipoles.

Electronic spectra of the surfaces of alkaline earth oxides

Abstract: The surfaces of fine powder samples of MgO, CaO and SrO have been characterized by the diffuse reflectance technique over the wavelength range 200–2000 nm. Spectra due to F+s centres (electron in surface anion vacancy), SH centres (F+s centre with nearby —OH group) and adsorbed O–2 species have been identified. In powders which have been freed from surface contaminants fluorescences exist which can be quenched by O2, H2O or CO2. These fluorescences are interpreted as due to the presence of surface states which create an “effective” band gap at the oxide surfaces at significantly lower energies than the band gaps typical of the bulk oxides.

The effect of copper and silver on the conductivity and photoelectric properties of glassy arsenic selenide

Abstract: A study was made of the effect of copper and silver on the electric properties, photoconductivity and its kinetics in glassy arsenic selenide. Introduction of copper and silver from 0 to 2.4 at% was shown to increase the conductivity from 5.6 × 10 −13 ohm −1 cm −1 to 5.8 × 10 −11 ohm −1 cm −1 and 2.3 × 10 −12 ohm −1 cm −1 , respectively At the same time, the activation energy decreases from 1.71 (for glassy As 2 Se 3 ) to 1.21 eV and 1.46 eV, respectively. The photoconductivity spectra shift to the infrared with an increase of the copper and silver content. The values of the drift mobility and their temperature dependences were calculated for all the compositions studied. A study of the photoconductivity kinetics showed the existence of trapping centres for the majority and minority current carriers in the band gap of the materials studied. We found depths of these centres for all compositions, and also found for the minority current carriers trapping centres the cross-sections for the trapping of electrons and holes.

The Valence Band Structure of Tellurium. III. The Landau Levels

Abstract: The Landau levels are calculated for two upper valence bands of tellurium. The peak positions and intensities of the cyclotron resonance and of the inter-valence band magneto-absorption are calculated. Effects of the k -linear terms on them are elucidated. For the magnetic field perpendicular to the c -axis, the Landau levels of the uppermost valence band are doubly degenerate at low magnetic fields and split as the field increases, while those of the lower band are equally spaced but bend downwards at high fields. The peaks in the magneto-absorption are all identified as the harmonic transitions. The agreement with the experiments is satisfactory, and all the band parameters are determined. It is shown that the k -linear terms make the anisotropy of the effective masses opposite for the two bands.

Absorption, Reflectance, and Luminescence of GaN Single Crystals

Abstract: Analysis of the low-temperature absorption, reflectance, and emission spectra of oriented single crystals of hexagonal GaN demonstrates that the features found at 3.62 eV ($E\ensuremath{\perp}C$) and 3.72 eV ($E\ensuremath{\parallel}C$) are due to the formation of free excitons associated with a direct energy gap. A close analogy exists between these results and similar data from ZnO, consistent with the relative position of Zn and Ga, and O and N atoms in the Periodic Table. Luminescence spectra present good, although not conclusive, evidence for the hypothesis that this direct gap is also the fundamental energy gap in GaN.