Showing papers on "Band gap published in 1982"

Detailed photocurrent spectroscopy of the semiconducting group VIB transition metal dichalcogenides

Abstract: Summary of Band Gaps Measured for Various Compounds in This Studya The Journal of Physical Chemistty, Vol. 86, No. 4, 1982 467 sitions to a pseudo-two-dimensional system with d-d transitions. It is unclear whether the parabolic band as- sumption which is used in the three-dimensional materials is valid for these systems.lg The angle of incidence and the polarization of the in- cident light were varied in hopes that the change in the electric vector of the incoming radiation would couple the radiation more or less to the electronic transitions in the layered compound. Gerischer has observed an effect of the polarization and the angle of incidence on the photo- current spectxa of GazSePm The angular dependence was studied for p-polarized light with a normalization to s- polarized light at normal incidence for several doping levels in the semiconductor. The large angular dependences that were observed were explained by a large anisotropy in the absorption coefficient and by different diffusion lengths associated with the impurity doping. We were unable to measure significant changes in the photocurrent spectra as a function of the polarization

Energy gap versus alloy composition and temperature in Hg1−xCdxTe

Abstract: We have used the data from 22 different studies to derive a new empirical expression for the energy band gap (Eg) of Hg1−xCdxTe: Eg =−0.302+1.93x+5.35(10−4)T(1−2x) −0.810x2+0.832x3. This expression is valid over the full composition range and for temperatures from 4.2 to 300 K. The standard error of estimate is 0.013 eV, which is at least 15% better than that of previously reported expressions.

A photoelectrochemical determination of the position of the conduction and valence band edges of p‐type CuO

Abstract: Lithium‐doped p‐type CuO is a low mobility semiconductor with an indirect band gap of 1.35 eV and a flatband potential of +0.55 V, with respect to the saturated calomel electrode (SCE), when in contact with an electrolyte at a pH of 9.4. Its valence band lies 5.42 eV below the vacuum level and is made up mainly from the Cu2+−3d wavefunctions. An oxygen‐2p type band is at 7.33 eV, in agreement with a semi‐empirical estimate. Its performance as a photoelectrode for the solar photoelectrolysis of water is rather poor, due to the presence of recombination centers in the band gap and its chemical instability. As photoelectrolysis electrodes, oxides with 3d‐type valence bands may have advantages over the more common oxides with 2p‐type valence bands.

Semiconductors for high‐voltage, vertical channel field‐effect transistors

Abstract: The influence of material parameters upon the characteristics of vertical channel power field effect transistors is examined. It is demonstrated that for devices with the same breakdown voltage and device structure, the on‐resistance is inversely proportional to the third power of the energyband gap and inversely proportional to the mobility. In addition the frequency response of these devices increases in proportion to the mobility and the energyband gap. Calculated device parameters for III–V semiconductor compounds, as well as their alloys, have been compared to those of a silicondevice with the same breakdown voltage. It is found that devicesfabricated from GaAs,InP, and GaP are expected to have a current handling capability which is a factor of 12.7, 5, and 1.85 better than that of the silicondevice with the same breakdown voltage. In addition, the current handling capability of devicesfabricated from the alloy semiconductors GaAlAs, GaAsP, and InGaP are even superior to those of a GaAsdevice with the same breakdown voltage.

Strained-layer superlattices from lattice mismatched materials

Abstract: Results are presented from the first theoretical study of the electronic properties of strained‐layer semiconductor superlattices made from lattice mismatched materials. The energy gaps and electronic states of GaAs‐GaAs0.2P0.8 (100) superlattices are studied as a function of layer thicknesses using a tight binding model. The superlattice band gaps are found to depend on the layer thicknesses through quantum mechanical effects and through the strains in the layers.

Diluted magnetic semiconductors: An interface of semiconductor physics and magnetism (invited)

Abstract: This paper reviews the electrical, magnetic, and optical properties of diluted magnetic semiconductors (sometimes also referred to as ‘‘semimagnetic’’ semiconductors). These materials are ternary semiconductor alloys whose lattice is made up in part of substitutional magnetic ions. Cd1−xMnxTe and Hg1−xMnxTe are examples of such systems. As semiconductors, these alloys display interesting and important properties, such as the variation of the energy gap and of effective mass with composition. They also exhibit magnetic properties which are interesting in their own right, e.g., a low temperature spin glass transition and magnon excitations. Most importantly, however, the presence of substitutional magnetic ions in these alloys leads to spin–spin exchange interaction between the localized magnetic moments and the band electrons. This in turn has rather important consequences on band structure and on donor and acceptor states, leading to dramatic effects in quantum transport, impurity conduction, and magneto‐optics. Specifically, the presence of exchange interaction results in extremely large and temperature dependent g‐factors of electrons and holes; in gigantic values of Faraday rotation; in anomalously large negative magnetoresistance; and in the formation of the bound magnetic polaron.

Structural Determination of the Symmetry-Breaking Parameter in trans-(CH) x

Abstract: The magnitude of the symmetry-breaking dimerization distortion has been determined by analysis of x-ray scattering data from trans-${(\mathrm{CH})}_{x}$ ${u}_{0}\ensuremath{\simeq}0.03$ \AA{}.This distortion can account for the magnitude of the energy gap, implying that electron-electron interactions do not dominate the physics of long-chain polyenes.

Dynamical aspects of correlation corrections in a covalent crystal

Abstract: A central problem in one-electron band calculations is the proper inclusion of exchange and correlations. We have performed a first-principles calculation by utilizing the Green's-function method with an energy-dependent nonlocal self-energy operator obtained by replacing the Coulomb potential in the exchange operator by a dynamically screened interaction. Diamond was chosen as a prototype of covalent materials. To be consistent with a variety of experimental facts, we have taken the dielectric matrix of the medium within the time-dependent screened Hartree-Fock approximation, thereby including both local-field and electron-hole (excitonic) effects. Previous calculations along similar lines have been restricted either to a random-phase-approximation frequency-independent dielectric function or to a plasmon-pole approximation. We have investigated for the first time the role of a realistic frequency and wave-vector-dependent dielectric matrix, and examined the relative importance of the electron-hole excitations and of the plasma resonance across the range of the valence and conduction bands. The correlated band structure was calculated by diagonalizing the quasiparticle equation of motion in a local orbital basis in order to exploit the local character of both the self-energy operator and the orbitals spanning these bands. We have found that the plasma resonance does not contribute appreciably in the energy range about the band gap while it contributes significantly to the valence bandwidth. Our values of 7.4 eV (band gap) and 25.2 eV (valence band-width) are in good agreement with reflectivity and photoemission experiments. Implications for the local-density and the energy-independent Coulomb-hole plus screened-exchange approximations are discussed. In addition, our method, by utilizing an energy-dependent self-energy, has also enabled us to calculate quasiparticle damping times (specifically, intraband Auger decay rates) that are consistent with photoemission spectra.

Ab initio effective Hamiltonian study of the electronic properties of conjugated polymers

Abstract: The valence effective Hamiltonian technique is applied to a series of polymers to compute ionization potentials, bandwidths, and band gaps. The polymers considered represent systems of interest to the conducting polymers area and include various derivatives of polyacetylene and polyphenylene, polydiacetylene, polyacene, polybenzyl, and polyyne. The theoretical results for relative ionization potentials are in excellent agreement with available experimental estimates, as well as with the observed behavior of the electrical conductivity of these systems on exposure to weak (I2) versus strong (AsF5) electron acceptors. The bandwidths of the highest occupied band show a qualitative correlation to the conductivities achieved with acceptor doping. Band gaps for the planar systems considered are also in good agreement with experiment.

Optical properties of In1−xGaxP1−yAsy, InP, GaAs, and GaP determined by ellipsometry

Abstract: Refractive indices and absorption coefficients of In1−x Gax P1−yAsy for y = 0 to 1 lattice matched to InP and of GaAs and GaP have been measured by ellipsometry in the wavelength range between 365 and 1100 nm. The high‐purity layers (7×1014

New model dielectric function and exchange-correlation potential for semiconductors and insulators

Abstract: We propose a new model frequency and wave-vector-dependent dielectric function for systems with an energy gap in their electronic excitation spectrum. The function is homogeneous, isotropic, and causal and satisfies two sum rules relating to particle-number conservation. Moreover, it has an analytic representation, reduces to the Lindhard function in the limit of zero gap, and compares well numerically with the $\ensuremath{\epsilon}(q,\ensuremath{\omega})$ of Si from band-structure calculations. With the model irreducible polarizability, we extend the theory of Singwi et al. [Phys. Rev. B 1, 1044 (1970)] to obtain a one-parameter family of exchange-correlation potentials appropriate for semiconductors and insulators. The existence of a band gap is found to enhance the exchange contributions but reduces the correlation contributions to the exchange-correlation potentials resulting in an overall potential which deepens as the average band gap of the system increases. A band calculation of silicon in the present theory shows a slight improvement of the band gaps over previous work using the metallic exchange-correlation potential.

Compositional dependence of band‐gap energy and conduction‐band effective mass of In1−x−yGaxAlyAs lattice matched to InP

Abstract: The band‐gap energy and the electron effective mass of In1−x−yGaxAlyAs lattice matched to InP have been determined as a function of Al content. From photoluminescence measurements we obtain Eg(eV) = (0.76±0.04)+(1.04±0.10)y+(0.87±0.13)y2. The electron effective mass is determined from the plasma frequencies measured with Raman scattering in n‐type samples. Its compositional dependence is given by m* = (0.0427±0.0015)+(0.0683±0.0007)y.

A GaAsxP1−x/GaP strained‐layer superlattice

Abstract: Strained−layer superlattices form a broad new class of semiconductor materials with tailorable electronic properties. We have succeeded in growing a GaAsxP1−x/GaP(100) strained−layer superlattice (SLS). The structure was grown by alternate metalorganic chemical vapor deposition of thin (60 A)layers (20 each) of GaAs0.4P0.6 and GaP. These layers were grown onto a GaAsxP1−x layer which was graded in composition from x = 0 (composition of underlying GaP substrate)to x = 0 (average composition of the SLS). Photoluminescense studies of the SLS were carried out to determine the optical band gap. At T = 78 K, the spectrum shows a dominant band−edge peak at 2.03 eV as well as weaker peaks at higher energies. Tight binding and effective mass calculations, also carried out, predict a direct band gap (due to zone folding) of 2.02 eV and higher lying transition energies which are in good agreement with these data.

Thermal Oxidation of SiC and Electrical Properties of Al–SiO2–SiC MOS Structure

Abstract: Silicon dioxide layers have been thermally grown on the (000)C face of 6H–SiC at 850–1100°C in wet O2 and studied by Auger analysis and ellipsometry. These oxide layers are quite homogeneous with a narrow interface width of \lesssim80 A. The oxide thickness vs, the oxidation time follows the general relationship used for the thermal oxidation of Si. Temperature dependencies of the oxidation rate constants were obtained. C-V characteristics of Al–SiO2–SiC MOS structures were measured at 10 Hz–1 MHz. The accumulation, depletion and inversion regions are clearly observed under illumination. In the dark, the inversion does not occur, probably owing to the absence of minority carriers because of the large band gap. Frequency dispersion is not observed. The minimum surface-state density is ~2×1012cm-2eV-1. The oxide resistivity and the breakdown strength are 2×1012 Ωcm and 2×106V/cm, respectively.

Intrinsic density ni (T) in GaAs: Deduced from band gap and effective mass parameters and derived independently from Cr acceptor capture and emission coefficients

Abstract: Prior attempts at determination of the intrinsic density in GaAs are reviewed, and this quantity is then deduced anew from the temperature dependences of the intrinsic gap and of the valence and conduction band system statistical weights. The nonparabolicity of the lowest conduction band, and the effects of the next two conduction bands, are taken into account in deducing ni (T) for the range 250–1500 K. That procedure gives a room‐temperature value ni (300) = 2.1×106 cm−3, which can be compared with prior values from various experimental methods. The magnitude and temperature dependence of ni are then calculated by a different and entirely new method, which utilizes experimental data of the electron and hole emission and capture coefficients associated with Cr2+?Cr3+ transitions of the substitutional CrGa deep‐level impurity in GaAs. Recent data of Martin et al. concerning these coefficients permits a deduction of ni(T) = 1.05 ×1016 T3/2 exp(−0.802/kT) cm−3 for 300

Electronic Structure of Black Phosphorus in Self-Consistent Pseudopotential Approach

Abstract: The energy band structure of black phosphorus is calculated by using self-consistent pseudopotential method. The resulting band structure has the direct minimum gap at the point Z in the Brillouin zone in agreement with the result of the tight-binding approach. Effective electron and hole masses and the level shift of the band edge by pressure are calculated from the bands obtained. The pressure dependence of the energy gap is in good agreement with experiment, but the anisotropy of the effective masses contradicts that of the electrical conductivity measured for the single crystal. The nature of the optical absorption edge is discussed in terms of the calculated band structure.

Determination of the optical bandgap of amorphous silicon

Abstract: The optical bandgap of glow-discharge and sputter-deposited a-Si has been deduced from measurements of the absorption coefficient α using a linear extrapolation of (αhvn)1/3 as a function of the photon energy h v. The exponent 1/3 is used instead of 1/2, resulting in a much better fit to the data. The influence of the method of extrapolation on the resulting value for the optical bandgap has been assessed.

Band tail recombination limit to the output voltage of amorphous silicon solar cells

Abstract: Recombination mediated by band tail states is shown to substantially reduce the maximum achievable output voltage in amorphous silicon hydride solar cells. The maximum open circuit voltage calculated from measured density of states parameters and reasonable estimates for the localized state capture rates is 1.0±0.1 V.

Tunable absorption coefficient in GaAs doping superlattices

Abstract: The optical-absorption coefficient $\ensuremath{\alpha}(\ensuremath{\omega})$ of GaAs "NIPI" crystals [a new type of semiconductor superlattice consisting of $n$- and $p$-doped layers, possibly separated by intrinsic ($i$) regions, in an otherwise homogeneous bulk] is measured at photon energies below the forbidden gap of the unmodulated material We use the extremely strong photoconductive response of these NIPI crystals as a very sensitive method for the detection of very weak absorption signals in the tail far below the gap of bulk GaAs Another peculiarity of NIPI crystals expected from theory is the tunability of $\ensuremath{\alpha}(\ensuremath{\omega})$ by variation of the energy gap, which is no longer a constant quantity for these systems The experimental results on molecular-beam-epitaxy-grown GaAs NIPI crystals reported in this paper show that both the frequency dependence and the dependence on modulation of the band gap are in excellent agreement with theory

Direct evidence for the site of substitutional carbon impurity in GaAs

Abstract: Direct evidence that substitutional carbon in GaAs is predominantly on the As sublattice is obtained from Fourier transform infrared spectroscopy absorption measurements of the carbon‐induced localized vibrational mode (LVM). The previously reported LVM absorption band of carbon is measured under high resolution conditions and is found to be the near superposition of at least four bands. It is shown by comparison with similar measurements of silicon‐doped GaAs and by physical arguments that the only satisfactory explanation is that the bands arise from carbon on As sites with different nearest‐neighbor configurations of the two Ga isotopes. There is no experimental indication of carbon on the Ga sublattice. These are the first observations of such shifts in LVM spectra for simple substitutional impurity defects in semiconductors.

Effect of electronic structure of the diamond surface on the strength of the diamond-metal interface

Abstract: Recent electron spectroscopic investigations have shown that the diamond surface undergoes a transformation in its electronic structure by a vacuum anneal at ∠900 °C. The polished surface has no electronic states in the band gap, whereas the annealed surface has both occupied and unoccupied states in the band gap. In addition, the annealed surface exhibits some electrical conductivity. The effect of this transformation on the strength of the diamond–metal interface is investigated by measuring the static friction force of an atomically clean metal sphere on a diamond flat in ultrahigh vacuum. The friction force is due to interfacial bonding. It is found that low friction (weak bonding) is associated with the diamond surface devoid of gap states whereas high friction (strong bonding) is associated with the diamond surface with gap states. Exposure of the annealed surface to excited hydrogen also leads to weak bonding. The interfacial bond will be discussed in terms of interaction of the metal conduction band electrons with the band gap states on the diamond surface. Effects of surface electrical conductivity on the interfacial bond will also be considered.

Bandgap control in amorphous semiconductors

Abstract: Method of producing amorphous semiconductor hydrides (hydrogenated amorphous semiconductors) with specified bandgaps. The desired bandgap is achieved by controlling the temperature and partial pressure of higher order semiconductanes which are introduced into, for example, in a hotwall epitaxial reactor, to create a deposit on a substrate held in the reactor, said deposit being created by pyrolytic decomposition of said semiconductanes.

Photoemission and photon‐stimulated ion desorption studies of diamond(111): Hydrogen

Abstract: The interaction of hydrogen with the diamond (111) surface is examined using our new results in photoemission spectroscopy (PES) and photon‐stimulated ion desorption (PSID) yield at photon energies near the carbon K edge. Also discussed in the light of our new results are previous studies using PES and low energy electron diffraction (LEED). PSID verifies that the mechanically polished 1×1 surface is hydrogen terminated and finds that the reconstructed surface is hydrogen free. Atomic hydrogen is found to be reactive with the reconstructed surface, while molecular hydrogen is relatively inert. Exposure of the reconstructed surface to atomic hydrogen results in chemisorption of hydrogen and removal of the intrinsic surface state emission in and near the band gap region.

Determination of the InAs–GaAs(100) heterojunction band discontinuities by x‐ray photoelectron spectroscopy (XPS)

Abstract: The valence‐ and conduction‐band discontinuities have been determined for the InAs–GaAs (100) heterojunction (HJ) by means of XPS measurements. The values are ΔEv = 0.17±0.07 eV and ΔEc = 0.90±0.07 eV, respectively. The InAs‐GaAs HJ has a rather large lattice mismatch (7%) but high quality epitaxial heterojunctions may be grown by molecular beam epitaxy (MBE). We have carried out XPS measurements on MBE grown InAs (100) films and on the InAs‐GaAs (100) HJ by using a recently developed arsenic cap transfer technique which permits XPS measurements on atomically clean and ordered heterojunctions. XPS is used to obtain the In4d and As3d to valence‐band maximum binding‐energy differences for bulk InAs by fitting the experimental XPS valence‐band density of states (VBDOS) to an instrumentally broadened theoretical VBDOS. The values are (EInAsIn4d−EInAsv) = 17.43 ±0.02 eV and (EInAsAs3d−EInAsv) = 40.77±0.02 eV, respectively. The core‐level separation ΔECL between the In4d and Ga3d core levels for a thin InAs–GaA...

Fermi-Level Pinning and Chemical-Structure of Inp-Metal Interfaces

Abstract: We have used soft x‐ray photoemission spectroscopy (SXPS) to investigate the dependence of Fermi‐level pinning on chemical structure at InP–metal interfaces. SXPS core level spectra of Al, Ti, Ni, Au, Pd, Ag, and Cu on UHV‐cleaved InP(110) surfaces reveal evidence for semiconductor outdiffusion, metal indiffusion, metal‐anion bonding and metal‐cation alloying. Corresponding Fermi‐level movements indicate a range of pinning positions at significantly different energies within the n‐type InP band gap. These results demonstrate that the Schottky barrier heights depend sensitively on changes in interface chemical bonding and diffusion, which strongly affect the type of electrically active sites and interfacial layers formed.