Showing papers on "Band gap published in 1977"

Photoelectrolysis and physical properties of the semiconducting electrode WO2

Abstract: The behavior of semiconducting electrodes for photoelectrolysis of water is examined in terms of the physical properties of the semiconductor. The semiconductor‐electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate since large‐band‐gap semiconductors have an intrinsic oxygen overpotential which removes the electrode reaction kinetics as the rate‐limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO3 and allows a determination of the band gap, optical absorption depth, minority‐carrier diffusion length, flat‐band potential, and the nature of the fundamental optical transition (direct or indirect). It is shown for WO3 that minority‐carrier diffusion plays a limited role in determining the photoresponse of the semiconductor‐electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is determined by diffusion‐controlled bulk recombination processes rather than the more common trap‐limited recombination. It is also shown that the fundamental optical transition is indirect and that the band‐gap energy depends relatively strongly on applied potential and electrolyte. This effect seems to be the result of field‐induced crystallographic distortions in antiferroelectric WO3.

A new semiconductor superlattice

Abstract: We treat theoretically, through the use of Bloch functions, a new semiconductor superlattice where the interaction of the conduction band in one host material with the valence band of the other host material plays an important role. The result indicates that this superlattice offers new intriguing features, realizable with the In1−xGaxAs‐GaSb1−y Asy system. In addition, the tunneling probability is calculated across a barrier involving this system.

Electronic band structure of titanium dioxide

Abstract: The electronic band structure of titanium dioxide is calculated by a combined tight-binding and pseudopotential method in order to interpret the numerous experimental data. The gap anisotropy is clearly shown and the values of parallel and perpendicular gaps are in good agreement with the measured ones.

Thermodynamic Potential for the Anodic Dissolution of n‐Type Semiconductors A Crucial Factor Controlling Durability and Efficiency in Photoelectrochemical Cells and an Important Criterion in the Selection of New Electrode/Electrolyte Systems

Abstract: We show that the standard potential for anodic dissolution, , of n‐type semiconductors (eg, for is + 008V vs SCE) plays a key role in ultimate efficiency of thermodynamically stable n‐type semiconductor‐based photoelectrochemical cells The selection of redox active substances that can be used for competitive capture of photogenerated holes is limited to those systems where the product does not have the potential to oxidize the semiconductor Where and represent the position of the valence and conduction band positions at the interface, respectively, we must conclude that is at a more negative potential (vs a reference) than , if the semiconductor undergoes photoanodic dissolution Quenching of the photoanodic dissolution by competitive hole capture by some electrolyte component, say A, is possible if lies at a potential more negative than But additionally, if the A+, the oxidation product, is to be incapable of oxidizing the semiconductor, must be more negative than Consequently, for the semiconductor to be thermodynamically stable in the A/A+ electrolyte the maximum photovoltage output, , can be no greater than for more negative than Naturally, kinetic inertness of the semiconductor to an oxidant with more positive than may allow its presence and/or use in a more efficient cell N‐type and semiconductors in aqueous electrolytes are treated in detail Some preliminary comments are made concerning p‐type materials

Ionicity and the theory of Schottky barriers

Abstract: We have investigated the role of ionicity in metal-semiconductor Schottky barriers by examining interfaces of increasing semiconductor ionicity. The electronic structure of four separate interfaces consisting of jellium (of Al density) in contact with the (111) surface of Si and the (110) surfaces of GaAs, ZnSe, and ZnS is investigated through the use of a self-consistent pseudopotential method. The barrier height and the surface density of states in the semiconductor band gap are determined. The phenomenological index of interface behavior S (studied by Kurtin, McGill, and Mead for semiconductors of different ionicity) is discussed in terms of a simple model involving metal-induced states in the semiconductor gap.

Optical and photoconductive properties of discharge‐produced amorphous silicon

Abstract: Optical and photoconductive properties of discharge‐produced amorphous silicon (a‐Si) of the type used in efficient thin‐film solar cells have been studied as a function of a wide range of deposition conditions The optical absorption, optical band gap, photoconductivity, hydrogen content, and the characteristics of the Si‐H vibrational mode in a‐Si were determined Both substrate temperature in the range ∼200–400 °C and the type of discharge used are found to be important factors in determining the measured optical and photoconductive properties of a‐Si For films produced at substrate temperatures near 200 °C, dihydride bonding occurs, and the optical band gap is about 17 eV As the substrate temerature increases, monohydride bonding is favored, the optical band gap decreases, the optical absorption increases, and the photoconductive properties improve These properties are, in part, associated with the presence of bonded hydrogen For substrate temperatures between 300 and 400 °C, the photoconductive

Semiconducting oxide anodes in photoassisted electrolysis of water

Abstract: The electrochemical properties of semiconducting anodes of TiO2, SrTiO3, BaTiO3, Fe2O3, CdO, CdFe2O4, WO3, PbFe12O19, Pb2Ti1.5W0.5O6.5, Hg2Ta2O7, and Hg2Nb2O7 in photoassisted electrolysis of water were determined. All of these oxides formed a rectifying junction with the electrolyte and anodic photocurrents were generated only with larger‐than‐band‐gap illumination. For Fe2O3, the optical absorption spectrum was different from the photoelectrochemical spectrum due to crystal field transitions. These oxides were found to be stable over certain range of pH. In a given electrolyte, the flatband potential Vfb varied linearly with the band gap. A good correlation was obtained between Vfb and the heat of formation of the oxide per metal atom per metal‐oxygen bond, but not between Vfb and the calculated Fermi energy of the oxide. This suggests that a semiconductor‐electrolyte interface may be approximated by a semiconductor‐metal junction where the barrier height is determined by the heat of formation of the me...

Deep Level Impurities in Semiconductors

Abstract: Solids are usually divided into metals and nonmetals. In a simplified scheme we may say that nonmetals consist of insulators and semiconductors. Both can be described by energy bands separated by a forbidden energy gap Eg• Whether a nonmetal is considered to be an insulator or a semiconductor often depends on the temperature at which its properties are to be investigated or used. In this chapter we use "semi­ conductor" to mean a nonmetal with a band gap from at least a few tenths of an eV up to a maximum of 10 eV. The considerable interest in semiconductors since the 1930s has been stimulated by their technical significance. Unlikl� metals, the physical properties of semi­ conductors can be considerably modified by introducing small amounts of foreign atoms. In this manner the resistivity of a silicon crystal can be changed by about seven orders of magnitude when one-millionth of the atoms in the crystal are replaced by suitable foreign atoms. Note that impurity substitution of this magni­ tude does not affect the band gap or other basic characteristics of the host material. Moreover, depending on the kind of foreign atom, the electrical current through the crystal is carried by electrons or holes. Hence multilayer structures with different types of conductivity can be constructed by doping a semiconductor crystal in­ homogeneously, resulting in devices such as diodes, transistors, lasers, etc. Experimentally it is found that replacing an atom of the host lattice by a foreign atom results in lattice defects with physical properties depending considerably on the particular atom introduced. Good understanding of such impurity centers has been achieved at least for Si and Ge when the foreign atom belongs to the groups of the periodic table closest to that of the semiconductor. They introduce localized donor and acceptor levels in the otherwise forbidden energy gap and are often described by the effective mass theory of Kahn & Luttinger (KL-EMT) (1-3), giving a hydrogen-like spectrum of levels with binding energies En' which can be written as

An isothermal etchback-regrowth method for high-efficiency Ga/1-x/Al/x/As-GaAs solar cells

Abstract: High-efficiency p-Ga(1-x)Al(x)As, p-GaAs, n-GaAs solar cells are made by isothermally soaking n-GaAs substrates in an undersaturated Zn-doped Ga-Al-As melt. This one-step growth procedure produces a graded band gap p-Ga(1-x)Al(x)As layer 0.2-0.4 micron thick. Efficiencies of 18.5% AM0 and 21.9% AM1 have been measured.

Self-consistent numerical-basis-set linear-combination-of-atomic-orbitals investigation of the electronic structure and properties of TiS2

Abstract: A fully-self-consistent numerical-basis-set linear-combination-of-atomic-orbitals calculation of the electronic structure of Ti${\mathrm{S}}_{2}$ is reported using the method described previously. The calculated band structure differs considerably from those previously obtained by non-self-consistent muffin-tin models. Comparison with experiment shows that the calculated optical properties for energies below 16 eV and the various characteristics of the valence and conduction bands agree very well with optical-absorption and electron-energy-loss data as well as with photoemission, x-ray absorption, and appearance-potential spectra. A small indirect gap (0.2-0.3 eV) occurs at the points $M$ and $L$ in the Brillouin zone with a larger direct gap (0.8 eV) at $\ensuremath{\Gamma}$. We suggest that the characteristic semi-metallic large $g$ value observed experimentally originates from a near coincidence of the band gap with the enhanced spin-orbit splitting which is consistent with the soft-x-ray data and our band model. The bonding mechanism in Ti${\mathrm{S}}_{2}$ is discussed in detail; it is shown by a direct calculation of the self-consistent charge density and the transverse effective charge that the system is predominantly covalent with small static ionic character and large dynamic ionicity. In contrast with muffin-tin $X\ensuremath{\alpha}$ models, the bonding is found to be largely due to Ti $4s4p$ to S $3p$ bonds and a much weaker Ti $3d$ to S $3p$ bond. The effects of muffin-tin approximation and self-consistency are discussed in detail. Extrapolation of these results to the case of Ti${\mathrm{Se}}_{2}$ is made and the possible origin of its charge-density wave is discussed.

Growth and properties of liquid‐phase epitaxial GaAs1−xSbx

Abstract: A detailed study is presented of the Ga‐As‐Sb ternary phase diagram. Liquidus temperatures have been measured between 700 and 900 °C by direct observation of the solutions, and layers then grown epitaxially on GaAs substrates. The nonequilibrium effect of constitutional supercooling on the compositions of the grown layers is investigated. Solidus compositions, from layers slowly grown to minimize this effect, are used together with the liquidus data to fit a calculated phase diagram. These results lead to the conclusion that GaAs1−xSbx is a borderline case between miscible and immiscible solid solutions. An accurate determination of the ternary band gap has been made as a function of composition over the range 0⩽x⩽0.2. The room‐temperature electrical properties of undoped, Te‐doped (n‐type), and Ge‐doped (p‐type) epilayers have also been determined.

The Rate of the Photoelectrochemical Generation of Hydrogen at p‐Type Semiconductors

Abstract: The current-potential relations with and without illumination, quantum efficiency-wavelength relations at several potentials, the flatband potentials, the transient behavior, and the stability of seven p-type semiconductors, i.e., ZnTe, CdTe, GaAs, InP, GaP, SiC, and Si, have been measured in 1N NaOH and 1N H/sub 2/SO/sub 4/. The position of the photocurrent-potential relations are related to the flatband potential and the energy gap of the semiconductor. The existence of the maximum in quantum efficiency-wavelength relation is analyzed by considering surface recombination. The stability and the transient behavior are analyzed.

Electronic Structures of Water and Ice

Abstract: Electronic structures of water and ice are investigated by means of XPS, UPS and VUV absorption spectroscopy. All the bands found in the XPS spectra of ice show a fairly good agreement with the electronic band structure theoretically calculated for cubic ice using molecular tight binding approximation. From the UPS spectra the vacuum level of ice is found at 10.5 eV above the top of the valence band, and a conduction band level of high density of states at 3.5 eV above the vacuum level. In VUV spectra D 2 O ice film shows a well separated absorption band at 8.75 eV, whose shape is almost independent of temperature, and it is suggested that ice is essentially a kind of molecular crystal. The absorption coefficients in the tail regions of the fundamental absorption bands of water and ice show nearly exponential dependence on the photon energy, and they seem to be well expressed as the tail of a Gaussian band.

Optical properties, band gap, and surface roughness of Si3N4

Abstract: The optical constants, surface roughnesses, and band gap Eg of amorphous Si3N4 films on quartz substrate are determined from optical measurements. The results of the Si3N4 samples with different preparation are compared and discussed. Absorption tails at the band edges indicate localized states in the forbidden gap. A model for continuous and symmetric state density is presented, and the localized volume V0 and the total concentration of the states NT are estimated.

Polymer Primary Structures Studied by ESCA and EHCO Methods

Abstract: The electronic structure (core and valence levels) of various simple polymers has been investigated by ESCA (Electron Spectroscopy for Chemical Analysis), with the aid of original EHCO (Extended H?ckel Crystalline Orbitals) calculations giving corresponding density of states and band structure schemes. Simple monosubstituted homopolymers derived from polyethylene ([CH2-CHX]n) have been chosen in order to study the influence of X substituents (X = -F, -Cl, -OH, -CH3, -CH2CH3, -C6H5, -C6H11 corresponding respectively to poly(vinyl fluoride), poly(vinyl chloride), poly(vinyl alcohol), poly(propylene), poly(1-butene), poly(styrene), and poly(vinyl cyclohexane)) through the chemical shifts measured for the core levels, and the alteration of the valence band structures. Assignment of the valence molecular orbitals is made with the aid of the theoretical calculations and with comparison of spectra recorded for molecules similar to the monomeric units. Emphasis is put on the very significant information provided by the valence band spectra for polymers containing only carbon and hydrogen atoms. Valence band data are used to calculate physical parameters (first ionization potential, energy gap, work function) of importance to deduce electrical properties for the polymers.

Surface and interface states on GaAs(110): Effects of atomic and electronic rearrangements

Abstract: Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surfaces of III–V semiconductors. In this paper we will briefly review these new developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band‐gap materials (e.g., GaSb, InAs, and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can produce first‐order effects in the valence‐band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage, the Ga 3d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first‐order changes in the valence‐band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3d chemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating practical surfaces. In particular, the effect of mixed As and Ga oxides, the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain‐induced interface states are discussed.

Measurement of the energy gap in tellurium under pressure

Abstract: From photoconductivity and transmission spectra the dependence of the forbidden band width ϵg of tellurium on pressure (up to 10 kbar) at 300, 77, and 1.5 to 4.2 K is determined. This dependence is shown to be nonlinear, well described by the exponential relation ϵg(P) = ϵg0, e - ϵaP where the α-values differ not more than 15% at a temperature variation from 1.5 to 300 K. [Russian Text Ignored.]

Optical constants ofε-GaSe

Abstract: The optical constants for both polarizations of light (E‖C andE⊥C) in e-GaSe have been studied between 30 cm−1 and 22000 cm−1, at room temperature. In the region of the band gap absorption edge, data are given down to 2 K. Infra-red data forE⊥C andE‖C in the (200÷300) cm−1 range have been obtained by reflectivity measurements and Kramers-Kronig analysis. TheE⊥C polarization has been compared with interferometric measurements between 30 cm−1 and 600 cm−1. Verification for the high-frequency range (σ>300 cm−1) has been done by comparison with interference patterns, analysed by the same method as for the visible region.

Superconductivity and semiconductor-metal phase transition in the system BaPb1-xBixO3

Abstract: Detailed infrared absorption, superconductivity, and structural investigations are made in the perovskite system BaPb1–xBixO3 to explain the semiconductor–metal phase transition in this system. It is proposed that the electronic states of Bi3+ in BaPb1–xBixO3 are localized, with a sudden delocalization of them at x = 0·4 where the semiconductor energy gap collapses, thereby giving rise to a Fermi surface with a relatively high density of states in the conduction band at 0 K. It is shown that these materials in the superconducting composition region are type-II s–p superconductors.

Photoelectrolysis with YFeO3 electrodes

Abstract: YFeO3 is shown to work as a photoanode for photoelectrolysis of water Conduction is n type as indicated by its use as an anode The optical band gap is an indirect transition of 26 eV, somewhat larger than in Fe2O3 However, the smaller electron affinity of YFeO3 means that it requires less bias for photoelectrolysis than Fe2O3

Photoelectrolysis of Water with Semiconductor Materials

Abstract: In an effort to find a semiconductor electrode with a bandgap in the visible part of the spectrum that can serve in the catalytic photodecomposition of water, different systems were explored. The first was sputtered thin films of wide bandgap semiconductor materials such as TiO/sub 2/, SnO/sub 2/, Nb/sub 2/O/sub 3/, Al/sub 2/O/sub 3/, and Si/sub 3/N/sub 4/ on low bandgap, n-type semiconductors such as GaAs and GaAlAs. Scanning electron micrographs showed that corrosion is greatly reduced but continues by diffusion of the electrolyte through the film, undermining the film by pit formation in the low bandgap semiconductor. There was no evidence for hole conduction through the film. The second system employed p-type GaP as the cathode and Pt as the anode. It was observed that this cell catalytically photoelectrolyzes water with conversion efficiency of 0.1%. The efficiency of the device is a strong function of crystal orientation, surface treatment, and purity of the crystal. The results are explained in terms of an energy diagram of the entire system. Areas for possible improvement are mentioned.

Absorption, refractive index, and birefringence of AlAs‐GaAs monolayers

Abstract: The band structure of artificial periodic crystals grown by molecular‐beam epitaxy and consisting of sequential AlAs and GaAs monolayers differs slightly from AlxGa1−xAs random alloys having a comparable average Al content. The measured direct band gaps are up to 110 meV higher than the alloys, and the indirect gaps up to 70 meV higher. The measured dispersion of the refractive index agrees best with the theoretical dispersion curve of an alloy composition having a higher Al content than in the monolayer. This is consistent with the higher band gap of the monolayer. The alloy and monolayer exhibit a birefringence resulting from strain‐induced anisotropy of the lower and upper cutoffs of the complex part of the dielectric constant. The strain results from the differential thermal contraction of the layers and the GaAs substrate on cooling from the growth temperature to room temperature. The birefringence of the monolayer sample contains an additional contribution resulting from the periodic structure.

A Comparative E.S.C.A. Study of the Electronic Structure of Solid Acenes: benzene, naphthalene, anthracene, and tetracene

Abstract: By comparison of the E.S.C.A. valence band spectra recorded in the solid phase for polyacenes (benzene, naphthalene, anthracene, and tetracene) with gas phase results, important energy parameters (energy gap, work function) of these compounds have been obtained. The trends observed in the evolution of the main C1s shake-up positions and intensities serve to assign the π satellites to transitions of the type nb1 → b1*.