Showing papers on "Band gap published in 1976"

Nonlocal pseudopotential calculations for the electronic structure of eleven diamond and zinc-blende semiconductors

Abstract: An empirical nonlocal pseudopotential scheme is employed to calculate the electronic structure of eleven semiconductors: Si, Ge, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$, GaP, GaAs, GaSb, InP, InAs, InSb, ZnSe, and CdTe. Band structures, reflectivity spectra, electronic densities of states, and valence charge densities are presented and compared to experimental results. Improved optical gaps, optical critical-point topologies, valence-band widths, and valence charge distributions are obtained as compared to previous local pseudopotential results.

Measurements of bandgap narrowing in Si bipolar transistors

Abstract: Theory predicts appreciable bandgap narrowing in silicon for impurity concentrations greater than about 1017 cm−3. This effect influences strongly the electrical behaviour of silicon devices, particularly the minority carrier charge storage and the minority carrier current flow in heavily doped regions. The few experimental data known are from optical absorption measurements on uniformly doped silicon samples. New experiments in order to determine the bandgap in silicon are described here. The bipolar transistor itself is used as the vehicle for measuring the bandgap in the base. Results giving the bandgap narrowing (ΔVg0) as a function of the impurity concentration (N) in the base (in the range of 4.1015–2.5 1019 cm−3) are discussed. The experimental values of ΔVg0 as a function of N can be fitted by: δV g0 = V 1 ln N N 0 + ln 2 N N 0 +C where V1, N0 and C are constants. It is also shown how the effective intrinsic carrier concentration (nie) is related with the bandgap narrowing (ΔVg0).

Luminescence in amorphous semiconductors

Abstract: A review of the luminescence properties of amorphous semiconductors is presented. The materials covered are chalcogenide glasses, silicon and arsenic. Luminescence spectra, excitation spectra, temperature dependences and lifetimes are described. The radiative transition in chalcogenides is the recombination of an electron in the conduction band tail and a trapped hole. A strong electron-phonon coupling distorts the lattice near the trapped hole, lowering its energy. This interaction is responsible for the broadness of the luminescence band and its position at about half the band gap energy. The recombination centre is thought to be a charged dangling bond with density 1017 cm-3 in arsenic chalcogenides and 1016 cm-3 in selenium. The same centre is observed in the hole drift mobility, and thermally stimulated conductivity. Luminescence in amorphous silicon also originates from recombination between the band tails and deep centres, with three separate transitions identified. In contrast to chalcoge...

Tungsten trioxide as a photoanode for a photoelectrochemical cell (PEC)

Abstract: THE PEC1 has acquired popularity among researchers in the past years, mainly because of its possible use in solar energy conversion2,3. The major problem at present is to find a semiconductor electrode with an optical band gap (written as just band gap below) small enough to allow it to absorb a reasonably large portion of the solar spectrum, and at the same time being stable against photocorrosion. We here report that WO3 meets these criteria although it is not very efficient at solar wavelengths.

Optical properties of hexagonal boron nitride

Abstract: Optical absorption, reflectivity, and photoconductivity in the near-uv range (1950-3200 \AA{}) of a thin film of hexagonal boron nitride were measured. The main absorption peak was observed at 6.2 eV. A sharp fall at about 5.8 eV was attributed to the direct band gap. The temperature dependence of the band gap was found to be less than 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ eV/\ifmmode^\circ\else\textdegree\fi{}K. Self-consistent tight-binding band-structure calculations were performed on a two-dimensional hexagonal crystal model, using Hamiltonian matrix elements calculated by semiempirical LCAO (linear combination of atomic orbitals) methods. The calculated value for the band gap of hexagonal BN was in reasonably good agreement with the experimental value obtained in the present work, as well as with values reported earlier from electron-energy-loss and photoelectron-emission measurements. The calculations also predicted a very small change in the band gap with temperature, in agreement with the experimental observations.

Semiconductor liquid junction solar cells based on anodic sulphide films

Abstract: ALTHOUGH good performance has been achieved with established solid state junction solar cells, their cost remains well above that required for large scale terrestrial applications. New concepts1,2 for photovoltaic energy conversion have been introduced which involve the junction between a semiconductor and an electrolyte. These schemes may lead to new possibilities of considerably reduced materials and fabrication costs, providing that many technical problems could be solved. We report here metal–semiconductor–liquid electrolyte junctions in which an n-type sulphide semiconductor is anodically formed in situ on its metal and, under photoexcitation, drives a sulphide–polysulphide redox couple when connected to a suitable cathode. The original contribution in this area was made by Gerischer2 who demonstrated that the cell (single crystal n-CdS/Fe(CN)64−, Fe(CN)63−/SnO2) has an initial conversion efficiency > 5% (ref. 3) for sunlight to electric power, in spite of the 2.4-eV band gap of CdS, which implies that wavelengths > 550 nm (the greater part of the incident solar photons) are ineffective.

Temperature dependence of the optical properties of Au, Ag and Cu

Abstract: The optical properties of Au and Ag are measured between 0.5 and 5.4 eV for temperatures ranging from 40 to 840K. The analysis of these spectra-also extended to the case of Cu-permits the assessment of energy separations associated with interconduction-band transitions in the vicinity of L over a wide temperature range. Notably the L4- to L4+ (nonrelativistic: L2' to L1) energy gap at 295K is found to be 4.81 eV for Cu, 4.11 eV for Ag and 4.20 eV for Au. The temperature dependence of this gap (around 295K) typically is -6.5*10-4 eV K-1 for all noble metals, i.e., considerably larger than predicted on the basis of band calculations. The present analysis also shows the interband absorption edge of Ag to arise from transitions in extended regions of the Brillouin zone. A comparison of experimental and theoretical spectra suggests lattice expansion to be the major source of temperature dependence of the band structure. It also yields information regarding the influence of dipole matrix elements on the epsilon 2 spectra.

Some optical properties of the AlxGa1−xAs alloys system

Abstract: Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0

Electrons in bismuth

Abstract: A review of the electronic properties of pure bismuth is given. Theoretical ideas on the band structure of bismuth and the dispersion relation for electrons near the bottom of the conduction band are briefly outlined. The experiments considered are those which give the most precise information on the Fermi surface (quantum effects in conductivity, cyclotron resonance, size effects, magnetoplasma waves, etc.) and on the electron energy spectrum near the bottom of the conduction band (studies in the quantum limit, infra-red magneto-reflection). The validity of the Cohen model as a first approximation to the electron spectrum in bismuth is established, and deviations from this model are described. Experiments are proposed which may give additional information on electrons in bismuth.

Electrical and Optical Properties of GexSe1-x Amorphous Thin Films

Abstract: Measurements of electrical and optical properties have been made on GexSe1-x amorphous thin films of about 1 µm thickness. From the experiment of electrical conductivity at various temperatures, two processes of the d.c. conductivity in GexSe1-x films are observed in plots of ln σ against 1/T. The thermoelectric power exhibits a maximum value of p-type conduction at pure Se and a change of the sign from positive to negative at 0.4

Critical behavior of the electrical resistivity in magnetic systems

Abstract: The effect of critical fluctuations on the resistivity near magnetic and order-disorder phase transitions is discussed. It is shown that in all magnetic and electronic systems the asymptotic high-momentum spin correlation functions dominate the temperature dependence of the resistivity sufficiently close to the transition. The dependence on the parameters of the system of the critical behavior and the way in which this behavior is approached is discussed in detail. In particular, the importance of the transition from a classical to a proper critical behavior, for the interpretation of experimental results, is emphasized, and recent renormalization-group results for the form of the correlation functions are utilized. For semiconductors the effects of fluctuations on the band gap are also calculated. It is suggested that the Fisher-Langer relation between the temperature derivative of the resistivity and the specific heat should be valid over a considerable temperature range outside the critical region. Theoretical predictions are compared with available experimental results on the resistivity and band gaps. It is shown that these results can all be understood, at least qualitatively, in the Born approximation by using only the most general known properties of the system.

Effect of electron-hole pairs on phonon frequencies in Si related to temperature dependence of band gaps

Abstract: The direct and indirect gaps between valence and conduction bands in semiconductors usually decrease with temperature. This effect is related by thermodynamic identities to the influence of electron-hole pairs on the lattice vibration frequencies. We show that the surprisingly large magnitude of the effect in Si and similar semiconductors is related to the sensitivity of the transverse-acoustic modes to covalent bonding. We are able to account for the magnitude of the effect from zero to the melting temperature. We also account for anomalous temperature variation in HgTe and related cases and mention other applications of the theory.

ZnxCd1−xS films for use in heterojunction solar cells

Abstract: ZnxCd1−xS films suitable for use in solar cells have been formed by simultaneous evaporating of ZnS and CdS. Lattice parameter and band gap are found to change almost linearly with composition. Electrical resistivity increases from <1 to ≳1010Ω cm as x increases from 0 to 1. Nonseries‐resistance‐limited ZnxCd1−xS‐CuyS photovoltaic cells have exhibited open‐circuit voltages greater than 0.7 V, apparently due to a better match between the ZnxCd1−xS and CuyS electron affinities.

Auger recombination and junction resistance in lead‐tin telluride

Abstract: The rate of Auger recombination in materials with a many‐valley band structure such as that of PbTe is calculated in this paper. It is found that recombination comes principally from the collision of current carriers in different valleys; the recombination rate can be large at quite low temperatures if the ratio of transverse‐to‐longitudinal effective masses in a valley is far from unity. The calculated rate sets an upper limit to the resistance of a simple p‐n junction in such materials; for material of band gap 0.1. eV, this limit is 6 Ω cm2 at 77 °K.

Electron energy loss studies in solids; The transition metal dichalcogenides

Abstract: The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed. The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX2, where M=Zr, Hf, Nb, Ta, Mo and W and X=S and Se, have been measured in the range of 0–50 eV. In the experiments, a beam of 50 keV electrons is incident along the c-axis of the crystals and electrons inelastically scattered through an angle of 1 m radian are selected for energy analysis. This ensures that the momentum transfer and hence the electric vector for the excitations lies in the basal plane of the crystal (E⊥c). Kramers-Kronig analysis has been applied to the energy loss data to deduce the complex dielectric function of each material. From this function, all other ‘optical’ constants, such as the reflectivity, and t...

Observation of athermal defect annealing in GaP

Abstract: Capacitance spectroscopy studies of lattice defects intentionally introduced into GaP diodes by 1‐MeV electron irradiation have shown a very large enhancement in the defect annealing rate under conditions of e‐h recombination. Six deep levels are observed and all exhibit recombination enhanced annealing. Four of the levels anneal athermally during recombination having the same rate at 100 K as at 300 K. Comparisons of the thermal annealing activation energy and the energy released during hole capture indicate that the observed athermal enhancement is a natural consequence of the wide GaP band gap.

Superconductivity and electron-phonon interaction in impure simple metals

Abstract: Nonmagnetic impurities change the electron-phonon interaction in a metal and therefore they influence the superconducting properties. This is demonstrated for an impure simple metal by deriving the Eliashberg equation. It turns out that (1) this equation is of the same formal structure as in the clean case, (2) there exists an additional coupling of low-frequency transverse phonons to the electrons, whereas (3) the interaction between longitudinal phonons and electrons is reduced. Since the former effect is much larger than the latter, the effective phonon density of states α2F(ω) is considerably enlarged in the low-frequency regime, which generally leads to an increased transition temperature and energy gap. Calculated values of the change in the transition temperature are compared for various impurity concentrations with experimental results and satisfactory agreement is obtained.

Localized defects in III-V semiconductors

Abstract: A Green's-function approach, closely related to that of Koster and Slater, is developed, within the one-electron pseudopotential formalism. This is applied to single vacancies, the divacancy and vacancy-oxygen pairs in GaAs, and single vacancies in GaP and InSb. A number of localized states are found. It is shown that the properties of the host-crystal band structure, the electron-electron interaction, and lattice relaxation play an important part in the formation of these states. In contrast with the Koster-Slater calculations, our results are not very sensitive to the strength of the vacancy potential. Our calculations indicate that As and Ga vacancies in GaAs should behave as single donors and acceptors, respectively. The ${V}_{\mathrm{Ga}}\ensuremath{-}\mathrm{O}$ complex behaves as a deep neutral center. The wave functions associated with the levels introduced into the band gap are highly localized. Since they contain admixtures of both valence- and conduction-band functions, and exhibit large lattice distortion, they are likely to take part in nonradiative recombination processes. Finally, we note that nonradiative recombination via some deep neutral centers is likely to be very efficient since our calculation also indicates a possibility of localized excited states introduced by these centers.

Contact potential differences for III–V compound surfaces

Abstract: Contact potential difference (CPD) measurements are reported for GaAs, GaP, and InAs UHV‐cleaved single crystals, and for GaAs epitaxially grown layers of (110) orientation. For the GaAs and InAs single crystals the CPD between n‐ and p‐type samples of the same material was practically equal to the band gap. This means that the intergap surface state density must be <1012/cm2. For epitaxially grown GaAs layers the CPD appeared to be independent of the volume dope, i.e., the Fermi level is stabilized due to surface states in the forbidden zone. For GaP single crystals the energy bands appear to be flat up to the surface in p‐type material, but for an n‐type sample a band bending of 0.45 eV occurs.

Prediction of semiconductor heterojunction discontinuities from bulk band structures

Abstract: We present a first attempt to predict heterojunction band lineups from the bulk band structures of the participating semiconductors, without invoking free surface properties Band structures and electrostatic potentials are calculated by a self‐consistent pseudopotential A simple electrostatic matching scheme lines up the electrostatic potentials, and through them the band structures Predicted lineups are in good qualitative agreement with known lineups, and even in some cases [particularly GaAs– (Al,Ga)As] in good quantitative agreement

X-ray photoelectron spectroscopy of silica in theory and experiment

Abstract: Fourier deconvolved X-ray photoelectron spectroscopy (XPS) valence band spectra obtained from crystalline and amorphous silica, used in conjuction with the results of quantum chemical calculations of the SiO4 tetrahedral unit and other spectrometric measurements (soft X-ray emission, UV absorption and reflectivity, photoconductivity, photoinjection and energy loss spectroscopy), suggest a reinterpretation of the electronic band structure of silica that is consistent with all the data. A unique method for pinning the Fermi level of insulators to that of a metal calibrant is described, resulting in the ability to obtain absolute binding energies of the electronic levels in wide bandgap insulators. Observe peaks in UV reflectivity and energy loss spectra of silica are all assigned to direct interband transitions, and no excitonic states need be involved to explain the data. Upper and lower limits for the bandgap of dry crystalline (α-quartz) and amorphous (Corning Code 7940 glass) silica are adjusted downward from the 8.9 eV bandgap proposed by DiStefano and Eastman [1] to 7.8–5.55 eV for α-quartz and 7.3–5.05 eV for fused silica, respectively. This in no way compromises the obvious insulating properties of silica in MOS devices, since the conductivity is governed by the high barrier height (∼3.8 eV in the case of gold) for metal-insulator electron transfer. The lowered bandgap results from increased low-energy electron density in the valence band, which we ascribe to the 1t1 molecular orbital predicted by various quantum formalisms, but heretofore not detected experimentally in bulk (thick) silica. Disappearances of this orbital and rearrangement of the non-binding 5t2 and 1e orbitals in silicas rich in silanols (OH), as may be the case for thin-film silica on Si metals, would increase the bandgap to 8.3 eV, in better agreement with previous determinations.

The effect of electron interaction on variable-range hopping

Abstract: The effect of interaction between electrons on variable-range hopping is investigated. It is shown that interaction between electrons on localized states produces a band gap for single-particle excitations, but not for those of the many-electron system. Variable-range hopping with σ proportional to exp (– B/T1/4) still occurs in the limit of low T, but interaction may lead to deviations at higher T, and decreases the pre-exponential factor.

Electron mobility and energy gap of In0.53Ga0.47As on InP substrate

Abstract: Very uniform In0.53Ga0.47As was grown on InP by liquid phase epitaxy. The electron mobility is 8450 cm2/V sec at 300 K and 27700 cm2/V sec at 77 K. The mobility increases with decreasing temperature from 300 to 77 K in contrast to the results of In1−xGaxAs grown directly on GaAs by vapor phase epitaxy. The energy gap of this high‐mobility material is 0.750 eV at room temperature.

An InP MIS diode

Abstract: Wet chemical anodic oxidation of InP in a 1% by weight solution of sodium salicylate in ethyl alcohol is shown to result in the growth of a high‐quality dielectric layer. Resistivities in excess of 1013 Ω cm at room temperature have been realized with breakdown fields ∼106 V/cm. Apparent surface state densities as determined from 1‐MHz capacitance‐voltage plots are in the range 3–4×1011 cm−2 eV−1 over a large portion of the band gap.

The effect of electron interaction on the band gap of extrinsic semiconductors

Abstract: The change in the band gap of a semiconductor due to high doping is considered in the limit where the free carriers form a degenerate gas. In contrast to previous treatments, the effect of the change in the electron interaction on the whole band structure is considered within a self energy framework so that the difference between valence and conduction bands can be found more accurately. The results obtained are significantly different, in some aspects, from earlier work. This is discussed and the application to experiment reviewed.