Showing papers on "Band gap published in 1980"

Comments on the Moss Formula

Abstract: An analysis is given of the well-known Moss formula connecting the energy gap to the refractive index of semiconductors. Based on Penn's model for the dielectric function of semiconductors and the treatment of Wemple, the refractive index is related to the energy gap. For small gaps, the relation is shown to reduce to the earlier predicted linear form.

Energy gap in Si and Ge: Impurity dependence

Abstract: The energy gap in silicon and germanium is calculated as a function of the concentration of donor impurities. The results are compared with the available data from optical experiments and devices. Previous theories are critically reviewed.

Accurate analysis of temperature effects in I/SUB c/V/SUB BE/ characteristics with application to bandgap reference sources

Abstract: The inaccuracy of the analyses commonly used for predicting the temperature behavior of the I/SUB C/-V/SUB BE/ characteristics of transistors and the output of bandgap reference sources is pointed out. The problem is traced to a basic assumption implicit in such analyses, namely that the variation of the bandgap voltage of silicon with temperature is linear; this assumption is shown to be of poor accuracy. By taking into account the nonlinearity in this variation, new accurate formulas are derived. Both the previous analyses and the proposed analysis are compared to experiment; a valuable improvement is demonstrated. Equations which should prove to value in the design of bandgap reference sources and bipolar transistor temperatures transducers are given. Higher order effects are discussed.

Principles of photoelectrochemical, solar energy conversion

Abstract: Photoelectrochemical devices for conversion of solar energy into both electrical energy and chemical energy are discussed with emphasis on how the various material properties of the photoactive electrodes influence device efficiency and stability. The similarity between photoelectrochemical cells (PECs) and solid state devices is used to model their behaviour and optimize such parameters as band gap, doping level, minority carrier lifetime, etc. A model is presented which calculates the electron affinity of any semiconductor and allows the prediction of the open circuit voltage of wet photovoltaic cells and optimum biasing forchemical producing cells. The effects of absorbed ions at the semiconductor/electrolyte interface are reviewed. The temperature dependence of the energy levels in the semiconductor and the electrolyte are considered and the implications of these results to operation of PECs at elevated temperature are discussed. The major differences between PECs and solid state devices are the stability considerations. The thermodynamics of this problem is discussed. Other important degradation mechanisms and some solutions to these problems are reviewed. Finally, a prognosis of the future of this field is presented.

Electrical and optical properties of n-type semiconducting chalcogenide glasses in the system Ge-Bi-Se

Abstract: The glass‐forming region was determined for the system Ge‐Bi‐Se; 13 at.% Bi was found to be incorporated at its maximum content into glasses, the Ge content of which was 20 at.%. Electrical resistivity, thermoelectric power, and optical absorption coefficient were measured mainly on a series of Ge20BixSe80−x glasses (x=0 –13). The resistivity decreased by about four orders of magnitude between x=9 and 10, and remained almost constant for x?10. The thermoelectric measurement showed the change of conduction type from p to n, accompanied by the above‐mentioned abrupt decrease of resistivity. In n‐type glasses, hopping conduction in the tail of localized states was proposed in parallel with conduction in extended states. The optical band gap was very slightly changed with the incorporation of more than 2.5 at.% Bi, in contrast to the remarkable decrease in the activation energy for conduction between x=9 and 10. The discussion based on the concentration of covalent bonds formed in the glasses led to the conclusion that the formation of a fairly large number of Bi‐Se bonds and the disappearance of Se‐Se bonds near x=10 were closely related to the composition dependence of the electrical and optical properties of the glasses in the present system.

The crystal structure of IV-VI compounds. I. Classification and description

Abstract: It is shown that the crystal structures of the binary IV-VI compounds can be classified by parameters derived from a Pauli-force model potential, which has previously been used successfully for the octet and suboctet materials (ANBP-N with P=8 and 2

Method for optimizing photoresponsive amorphous alloys and devices

Abstract: The production of improved photoresponsive amorphous alloys and devices, such as photovoltaic, photoreceptive devices and the like; having improved wavelength threshold characteristics is made possible by adding one or more band gap adjusting elements to the alloys and devices. The adjusting element or elements are added at least to the active photoresponsive regions of amorphous devices containing silicon and fluorine, and preferably hydrogen. One adjusting element is germanium which narrows the band gap from that of the materials without the adjusting element incorporated therein. Other adjusting elements can be used such as tin. The silicon and adjusting elements are concurrently combined and deposited as amorphous alloys by vapor deposition, sputtering or glow discharge decomposition. The addition of fluorine bonding and electronegativity to the alloy acts as a compensating or altering element to reduce the density of states in the energy gap thereof. The fluorine bond strength allows the adjusting element(s) to be added to the alloy to adjust the band gap without reducing the electronic qualities of the alloy. Hydrogen also acts as a compensating or altering element to compliment fluorine when utilized therewith. The compensating or altering element(s) can be added during deposition of the alloy or following deposition. The addition of the adjusting element(s) to the alloys adjusts the band gap to a selected optimum wavelength threshold for a particular device to increase the photoabsorption efficiency to enhance the device photoresponsive without adding states in the gap which decrease the efficiency of the devices. The adjusting element(s) can be added in varying amounts, in discrete layers or in substantially constant amounts in the alloys and devices.

High‐quality transparent conductive indium oxide films prepared by thermal evaporation

Abstract: Highly transparent (over 90% transmission in the visible range) and highly conductive (resistivity ≃2×10−4 Ω cm) indium oxide (undoped) films have been produced by thermal evaporation from an In+In2O3 source in a vacuum chamber containing low pressures of O2 These properties are comparable or superior to the best tin‐doped indium oxide films ever reported, and excellent reproducibility has been achieved Hall effect measurements have revealed that the observed low resistivity is primarily a result of the excellent electron mobility (≃70 cm2/V sec), although the electron concentration is also rather high (⩾4×1020/cm3) X‐ray diffraction measurements show distinctly polycrystalline In2O3 structure with a lattice constant ranging from 1007 to 1011 A Electrolytic electroreflectance spectra exhibit at least four critical transitions, from which we have determined the direct and indirect optical band gaps (≃356 and 269 eV, respectively) Burstein shifts due to the population of electrons in the conduction

Photoluminescence in heavily doped GaAs. I. Temperature and hole-concentration dependence

Abstract: The temperature and hole-concentration dependence of the photoluminescence from the ${E}_{0}$ and ${E}_{0}+{\ensuremath{\Delta}}_{0}$ energy gaps have been measured in $p$-type GaAs. In heavily doped GaAs the emission spectra across the ${E}_{0}$ gap can be described by taking into account band-band optical transitions with and without $\stackrel{\ensuremath{\rightarrow}}{k}$ conservation. The thermal shift of the band gap is the same as for pure GaAs and can be represented by the Varshni equation. The energy-gap shrinkage was measured and compared with the previously determined values of Casey and Stern. The luminescence across the ${E}_{0}+{\ensuremath{\Delta}}_{0}$ gap gives evidence for impurity states associated with the spin-orbit-split band. At 2.1 K this luminescence contains two peaks which are interpreted to arise from band-band and band-impurity optical recombination. The energy of these peaks does not depend on hole concentration, hence the ${E}_{0}+{\ensuremath{\Delta}}_{0}$ gap is not influenced by the shrinkage affecting the ${E}_{0}$ gap. The luminescence from the sample with 1.7 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ holes ${\mathrm{cm}}^{\ensuremath{-}3}$ displays at 2.1 K a series of lines corresponding to the emission of LO phonons in the energy range between the ${E}_{0}$ and ${E}_{0}+{\ensuremath{\Delta}}_{0}$ gaps. This observation indicates that the holes relax to the top of the valence band by hole-optical phonon scattering.

Angle-resolved photoemission studies of the band structure of Ti Se 2 and Ti S 2

Abstract: The electronic structure of high-quality Ti${\mathrm{Se}}_{2}$ and Ti${\mathrm{S}}_{2}$ crystals has been investigated using angle-resolved photoemission with HeI, HeII, and NeI resonance radiation. Results compare well with recent self-consistent energy-band calculations although differences occur which may be due to the three- rather than two-dimensional nature of specific bands. Occupied $d$ states at the zone edge are observed in both materials. A small overlap with the $\frac{s}{p}$ valence band at $\ensuremath{\Gamma}$ is observed in the case of Ti${\mathrm{Se}}_{2}$ in approximate agreement with other workers. Ti${\mathrm{S}}_{2}$ appears to be a defect semiconductor with a band gap of 0.3 \ifmmode\pm\else\textpm\fi{} 0.2 eV.

Photoreduction at illuminated p-type semiconducting silicon photoelectrodes. Evidence for Fermi level pinning

Abstract: Studies of p- and n-type Si electrodes are reported which show that semiconducting Si electrode surfaces do not allow efficient H/sub 2/ evolution in the dark (n type) or upon illumination with band gap or greater energy light (p type). The key experiment is that N,N'-dimethyl-4,4'-bipyridinium (PQ/sup 2 +/) is reversibly reduced at n-type Si in aqueous media at a pH where H/sub 2/ should be evolved at nearly the same potential, but no H/sub 2/ evolution current is observable. The PQ/sup 2+/+/.system may be useful as an electron-transfer mediator, since PQ/sup +/.can be used to effect generation of H/sub 2/ from H/sub 2/O using a heterogeneous catalyst. The PQ/sup +/.can be produced in an uphill sense by illumination of p-type Si in aqueous solutions. Studies of p-type Si in nonaqueous solvents show that PQ/sup 2 +/, PQ/sup +/., Ru(bpy)/sub 3//sup 2 +/, Ru(bpy)/sub 3//sup +/, and Ru(bpy)/sub 3//sup 0/ are all reducible upon illumination of the p-type Si. Interestingly, each species can be photoreduced at a potential approx. 500 mV more positive than at a reversible electrode in the dark. This result reveals that a p-type Si-based photoelectrochemical cell based on PQ/sup 2+/+/., PQ/sup +/l/sup 0//, Ru(bpy)/sub 3//sup 2+/+/,more » Ru(bpy)/sub 3//sup +/0/, or Ru(bpy)/sub 3//sup 0/-/ would all yield a common output photovoltage, despite the fact that the formal potentials for these couples vary by more than the band gap (1.1 V) of the photocathode. These data support the notion that p-type Si exhibits Fermi level pinning under the conditions employed.Surface chemistry is shown to be able to effect changes in interface kinetics for electrodes exhibiting Fermi level pinning.« less

Scattering-theoretic method for defects in semiconductors. II. Self-consistent formulation and application to the vacancy in silicon

Abstract: A self-consistent-field method for calculation of the electronic structure of localized defects in semiconductors is described. The method is based on Green's-function theory and follows the original idea of Koster and Slater and its developments by Callaway and coworkers. The Wannier functions of the original formulations are, however, replaced by a more flexible set of linear combination of atomic orbitals. This choice and an accurate evaluation of the perfect-crystal Green's function bring this method to the level of sophistication, accuracy, and rigor characteristic of state-of-the-art band-structure and surface calculations. The efficiency of the method stems largely from the fact that it exploits both the translational symmetry of the host crystal and the short range of the defect potential. Thus, all bulk properties (e.g., band gaps, bandwidths, etc.) are built in from the start via a band-structure calculation and are preserved. One then focuses on the changes produced by the defect potential, so that the interpretation of the results is straightforward and unambiguous. In this paper, we report an application of this method to an isolated vacancy in Si assuming no lattice relaxation. The unrelaxed vacancy introduces a bound state of ${T}_{2}$ symmetry at 0.7 eV above the valence-band edge and a number of resonances and antiresonances within the valence bands. A detailed analysis of these states in terms of their origin, orbital content, and of state and charge densities is presented. We find that, while many of these states are individually quite extended, they combine destructively to produce a very localized net change in the charge density. We also find that the resulting localized potential can be well approximated by a negative of an atomic silicon potential extracted from a self-consistent bulk calculation. Finally, we compare the relative merits of the three increasingly more sophisticated, but also more costly, approaches to the defect problem, namely, (1) tight-binding, (2) non-self-consistent, and (3) self-consistent calculations.

Semiconductor diode laser array

Abstract: An integrated laser array is disclosed in which a plurality of semiconductor lasers are integrated on a semiconductor multi-layer crystal that includes an active layer in which the band gap energy varies in one direction By means of this arrangement a plurality of semiconductor lasers, which differ in their respective oscillating wavelengths over a relatively broad range, can be formed on a common substrate

Interpretation of the conductance and capacitance frequency dependence of hydrogenated amorphous silicon Schottky barrier diodes

Abstract: We present a general model of the frequency dependence of conductance and capacitance in a‐Si:H Schottky diodes. In order to circumvent several questionable assumptions required in the analysis of capacitance voltage characteristics, the frequency dependence of sputtered a‐Si:H devices is measured with no applied dc voltage. We obtain independent, consistent values of the depletion width and of the density of states at the Fermi level and below from both conductance and capacitance at both low and high modulation frequencies. We show that the linear frequency dependence of conductance cannot be attributed to hopping conductance, but rather to the interaction of gap states with free carriers. Our study shows that the interaction kinetics of the states around the Fermi level with the conduction‐band carriers is so fast that the response of the diode is limited by the band transport of these carriers, which rapidly thermalize and distribute themselves through the continuum of states from the conduction band ...

Semiconductor properties of polyacetylene p‐(CH)x : n‐CdS heterojunctions

Abstract: Trans:(CH)x:n‐CdS heterojunctions have been fabricated and used to study the properties of undoped trans‐polyacetylene. The I‐V data show rectifying behavior characteristic of a p‐n junction diode, thus confirming that as‐grown polyacetylene is p type. From C‐V characteristics we infer a residual acceptor concentration in as‐grown films of p‐(CH)x of NA?2×1018 cm−3. Depletion has been studied in the polymer by using CdS of different doping concentrations. Detailed studies of the photovoltaic response at energies below Eg for (CH)x imply the existence of a meta‐stable deep trap in the polymer with energy near the center of the gap. The threshold energy for pumping into this level provides an independent measurement of the energy gap, Eg=1.45 eV.

Conduction-electron screening in metallic oxides: Ir O 2

Abstract: Valence-band spectra from single crystals of Ir${\mathrm{O}}_{2}$ reproduce the general features of a recent band-structure calculation. Good agreement with the unusual asymmetric core-electron line shapes is obtained by a perturbation-theory treatment of the $d$-electron many-body screening response.

Cohesive properties of CaF2 and UO2 in the atomic sphere approximation

Abstract: The zero-temperature equations of state of UO2 and CaF2 have been computed from semi-relativistic LMTO calculations. The calculated lattice parameters are 1/2% above and 5% below the experimentally determined equilibria for UO2 and CaF2 respectively. The computed bulk moduli are 3.0*1012 and 0.71*1012 dyn cm-2 and the band gap of UO2 is found to be 5.35 eV.

Optical properties of the band-edge exciton in GaSe crystals at 10 K

Abstract: Optical constants (absorption coefficient and refractive index) of unstrained GaSe samples at low temperature have been measured in the vicinity of the band-edge and exciton series. A quantitative fit with Elliott's model is given for the position and intensity of the levels. This does not necessitate any variation of the apparent oscillator strength with thickness (or temperature). Parameters of the tridimensional hydrogenic series obtained from this fit give values of the band gap, the exciton binding energy, the damping of the excited levels, the oscillator strength of the transition, and the reduced effective masses of the exciton.

Charged dislocations and the plastic deformation of II-VI compounds

Abstract: The flow of charge associated with the plastic deformation of monocrystals of ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe and CdTe has been studied both in the dark and under illumination. It ariscs from the motion of charged dislocations on either the shuffle (Aa) or the glidc (aB) set of (111) or (0001) planes. The magnitude of the charge has been determined for each material. Most of the materials are n-type and have negatively charged dislocations, but in p-type ZnTe the dislocations are positively charged. The ionic charge of these dislocations is discussed, but this charge will be changed by the capture or loss of electrons. The experimental results are explained by a modification to the model of Kirichenko. Petrenko and Uimin (1978) in which there is n dynamic equilibrium between electrons in levels on dislocations, at point defects and in the conduction band. The dislocation charge in the dark increases with the energy gap in a manner that is consistent with this theory, Both tee dislocation charge a...

Energy Band Structure and Lattice Constant Chart of III-V Mixed Semiconductors, and AlGaSb/AlGaAsSb Semiconductor Lasers on GaSb Substrates

Abstract: Variations in energy gap, band structure, and relative dielectric constants were examined in 18 ternary and 15 quaternary III-V mixed semiconductor materials for optical devices. A semiconductor laser constructed with AlGaSb/AlGaAsSb on GaSb emitting a wavelength in the range from 1.3 to 1.7 µm is described. Uniform layers of mixed semiconductors, AlGaSb and AlGaAsSb, were successfully grown on a (111)B-oriented GaSb substrate. Lasing oscillation near 1.2 µm was observed at 77 K, slightly beyond 4.0 kA/cm2, in a hetero-isolation stripe laser with an active layer 1.6 µm thick.

A Unified Theory of Magnetism in Narrow Band Electron Systems

Abstract: A unified description is given for magnetism of narrow band electron systems on the basis of a functional integral formalism within the static approximation. A method is developed to calculate the free energy functional from a given band structure and its closed form expression is given for arbitrary amplitudes of the spin and charge density fluctuations, correponding to arbitrary strength of the electron-electron interaction. This expression is shown to lead to essentially correct results in both the weak and strong coupling limits. The metal-insulator (Mott) transition and related magnetic properties for the case of half-filled band and the magnetism of metals in the case of non-integral occupation number per atom are discussed with a numerical example for a tight-binding band in a simple cubic lattice.

Influence of the image force on the band gap in semiconductors and insulators

Abstract: The change in potential energy of electrons and holes and the resulting band‐gap change caused by the image force have been calculated close to the insulator‐semiconductor interface. Some examples are given from which it appears that in metal‐insulator‐semiconductor (MIS) structures, as in Schottky diodes, the effects of the image force should not be overlooked.

The Applicability of Semiconducting Layered Materials for Electrochemical Solar Energy Conversion

Abstract: Semiconducting transition metal dichalcogenides with layer structure belong to the most stable electrode materials in electrochemical solar cells. The photoelectrochemical reactions however, are complicated by the surface anisotropy. Results of photoelectrochemical cells with n-type MoSe2 and WSe2 demonstrate the importance of surface morphology for the energy conversion efficiency. — The optimal energy conversion yield of these layered semiconductors has turned out being lower than theoretically expected from their band gaps. It is shown that this is connected with the existence of an indirect band gap which restricts light absorption in a considerable energy quantum range above the band gap. Besides this, surface defects and inhomogeneities reduce the quantum yield by increased surface recombination. — Photoelectrochemical hydrogen evolution has been investigated with p-type WSe2. It is shown that a considerable energy gain can be reached by the catalytic action of platinum deposits on the surface of the electrode. This is explained in terms of surface state catalysis. Halbleitende Ubergangsmetalldichalkogenide mit Schichtgitterstruktur gehoren zu den bestandigsten Elektrodenmaterialien fur elektrochemische Solarzellen. Photoelektrochemische Reaktionen werden jedoch durch die Oberflachenanisotropie kompliziert. Ergebnisse von photoelektrochemischen Zellen mit n-leitendem MoSe2 und WSe2 zeigen die Bedeutung der Oberflachenmorphologie fur den Wirkungsgrad der Energieumwandlung. — Der optimale Energieumwandlungswirkungsgrad dieser Schichtgitterhalbleiter erwies sich als geringer, als man gemas der Bandlucke theoretisch erwarten konnte. Es wird gezeigt, das dies mit der Existenz einer indirekten Bandlucke in Zusammenhang steht, welche eine sehr schwache Lichtabsorption in einem betrachtlichen Energiebereich oberhalb der Bandkante bedingt.

Photoelectrochemical Reactions and Formation of Inversion Layers at n-Type MoS2-, MoSe2-, and WSe2-Electrodes in Aprotic Solvents

Abstract: Capacity measurements and cyclic voltammetry have been used to conduct a systematic investigation of the potential distribution at the semiconductor-organic electrolyte interface in the dark and under illumination with the n-type synthetic layered compounds MoS2, MoS2, and WSe2. From the obtained flat band potentials it could be concluded that the valence band edges vs. Ag/Ag+ are located in acetonitrile at ca. 1.0 V (MoS2), 0.7 V (MoSe2), and 0.4 V (WSe2). As theoretically expected, the oxidized component of redox systems with Fermi levels near or below the valence band edge can inject holes thus inducing an inverted region and generating a p-n junction in the solid. The cathodic current in the cyclic voltammograms of such redox systems occurs via hole injection with consecutive recombination of the injected holes while less oxidizing redox systems can only be reduced via the conduction band. — Since the semiconductors used in these experiments do not corrode in presence of a high surface concentration of holes, such a contact with an oxidizing redox electrolyte offers most favourable conditions for the application in a photoelectrochemical solar cell. The inversion layer has a maximal band bending and promises to obtain the highest possible photovoltage, which is however limited by the width of the band gap. Mit Hilfe von Kapazitatsmessungen und zyklischer Voltametrie wurde die Potentialverteilung an der Grenzflache Halbleiter/organischer Elektrolyt im Dunkeln und unter Belichtung untersucht. Aus der Lage der Flachbandpotentiale synthetischer Schichtgitterkristalle von MoS2, MoSe2 und WSe2 in Acetonitril-Elektrolyten konnten die Lagen der Valenzbandkanten ermittelt werden. Sie liegen bei 1,0 V (MoS2), 0,7 V (MoSe2) und 0,4 V (WSe2) gegenuber einer Ag/Ag+-Elektrode in Acetonitril. Entsprechend der theoretischen Erwartung konnen Redoxsysteme mit Fermi-Niveaus nahe oder unterhalb der Valenzbandkante Defektelektronen in der Oberflache dieser Halbleiter injizieren. Dadurch entsteht eine Inversionsschicht und somit ein p-n-Ubergang im Festkorper. Der kathodische Strom in den zyklischen Voltamogrammen mit solchen Redoxsystemen erfolgt durch Injektion von Defektelektronen, wonach diese in der Inversionsschicht mit Elektronen rekombinieren. Dagegen konnen weniger oxidierende Redoxsysteme nur uber das Leitungsband reduziert werden. — Die Halbleiter, die hier untersucht wurden, korrodieren in diesem Medium nicht, obwohl die Oberflache stark p-leitend ist. Solch ein Kontakt mit einem oxidierenden Redoxsystem bietet daher besonders gunstige Voraussetzungen fur die Verwendung in photoelektrochemischen Solarzellen. Die Inversionsschicht besitzt eine maximale Bandverbiegung und fuhrt daher zur Erzeugung der grostmoglichen Photospannung, die allerdings durch die Weite der Bandlucke begrenzt wird.