Showing papers on "Band gap published in 1995"

Polymer Light-Emitting Electrochemical Cells

Abstract: A device configuration for light emission from electroactive polymers is described. In these light-emitting electrochemical cells, a p-n junction diode is created in situ through simultaneous p-type and n-type electrochemical doping on opposite sides of a thin film of conjugated polymer that contains added electrolyte to provide the necessary counterions for doping. Light-emitting devices based on conjugated polymers have been fabricated that operate by the proposed electrochemical oxidation-reduction mechanism. Blue, green, and orange emission have been obtained with turn-on voltages close to the band gap of the emissive material.

Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite

Abstract: Using the self-consistent orthogonalized linear-combination-of-atomic-orbitals method in the local-density approximation, the electronic structure and the optical properties of three phases of titanium dioxide have been studied. For rutile, the calculated band structure, equilibrium lattice constant, and bulk modulus are in good agreement with other recent calculations and with experimental data. The results on the ground-state properties of anatase and brookite are reported. Compared with the rutile phase, anatase has similar ground-state properties except for a larger band gap, whereas brookite has relatively smaller bulk modulus. The optical properties of these three phases are also calculated using the band-structure results and compared with the available measurements. For the rutile phase, the anisotropic properties of the dielectric function are in good agreement with the reflectance spectroscopy. For the anatase phase, there are very limited experimental optical data for comparison. For the brookite phase, no experimental data are available. Our calculations show subtle differences in the optical properties of these three phases.

Quasiparticle band structure of bulk hexagonal boron nitride and related systems

Abstract: The quasiparticle band structure of bulk hexagonal boron nitride is studied within the GW approximation for the self-energy operator. The influence of the interlayer distance on the band structure is investigated both within the local density approximation and the quasiparticle approach, and the importance of an interlayer state in determining the gap is demonstrated. Also, the quasiparticle band structure for an isolated sheet of boron nitride is calculated. We show that the equivalent of the interlayer state in the case of the isolated boron nitride sheet plays the same role as in the bulk case in determing the band gap.

Photoinduced effects and metastability in amorphous semiconductors and insulators

Abstract: Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Band offsets and optical bowings of chalcopyrites and Zn‐based II‐VI alloys

Abstract: Using first‐principles band‐structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence‐ and conduction‐band offsets for three mixed‐anion (CuInX2, X=S, Se, Te) and three mixed‐cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed‐anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn‐based II‐VI alloys in the CuAu, CuPt, and chalcopyrite structures are present...

Ga2O3 films for electronic and optoelectronic applications

Abstract: Properties of Ga2O3 thin films deposited by electron‐beam evaporation from a high‐purity single‐crystal Gd3Ga5O12 source are reported. As‐deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≊100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to ...

New Materials and Performance Limits for Thermoelectric Cooling

Abstract: 34.4 Introduction Systematic Search Historical Review New Materials Phonon "Glasses" Desired Performance PGEC Relaxation of A,,. Assumption Band Gaps of Semiconductors Carrier Mobilities Background Explanation Electronegativities and Good Thermoelectric Semiconductors Dopant and Mixed-Crystal Effects on Mobilities Dopant Effects on the Weighted Mobility Mixed-Crystal Effects on Mobility Weighted Mobility Lattice Thermal Conductivity Phonon Scattering Mechanisms New Amin Crystals and New Thermoelectrics Conclusions From the Analysis Appendix: Calculation of ZT for PGEC References

Optical investigations of defects in Cd1-xZnxTe.

Abstract: We investigated the optical properties of defects in CdTe and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te (0x1). Residual impurities give rise to specific far infrared absorptions, while metal vacancy-donor complexes (A centers), identified by optically detected magnetic resonance, are characterized by their near infrared (1.4 eV) photoluminescence (PL) properties. The specific zero-phonon-line positions and phonon couplings are worked out for these complexes involving different group-VII (F, Cl, Br, In) or group-III (In, Al) donors. In addition to the A center PL two emission bands are found at 1.135 and at 1.145 eV. The temperature dependences of the PL show that the 1.145 eV luminescence follows the temperature dependence of the band gap, while the energy position of the 1.135 eV band emission shifts to higher energies with increasing temperature. The A-center PL and the luminescence bands at 1.1 eV are investigated throughout the complete alloy composition range form x=0 to 1. The A center and the 1.135 eV band were found to follow the band-gap shift from CdTe to ZnTe, whereas the 1.145 eV luminescence keeps its emission energy constant.

Band gap and stability in the ternary intermetallic compounds NiSnM (M = Ti, Zr, Hf): A first principles study

Abstract: The structural stability and electronic properties of the ternary intermetallic compounds NiSnM (M=Ti,Zr,Hf) and the closely related Heusler compounds ${\mathrm{Ni}}_{2}$SnM are discussed using the results of ab initio pseudopotential total-energy and band-structure calculations performed with a plane-wave basis set using the conjugate gradients algorithm. The results characterize the lowest-energy phase of NiSnM compounds, with a SnM rocksalt structure sublattice, as narrow-gap semiconductors with indirect gaps near 0.5 eV, while the ${\mathrm{Ni}}_{2}$SnM compounds are described as normal metals. Two other atomic arrangements for NiSnM in the MgAgAs structure type result in energetically unfavorable compounds that are metallic. The gap formation in the lowest-energy structure of NiSnZr and relative stability of the three atomic arrangements are investigated within a tight-binding framework and by considering the decomposition of each ternary compound into a binary substructure plus a third element sublattice. The stabilization of the lowest-energy phase of NiSnZr is found to be mainly due to the relative stability of the SnZr rocksalt substructure, while the opening of the gap induced by the addition of the symmetry-breaking Ni sublattice makes a relatively minor contribution. The results from the theoretical calculations for the NiSnM compounds are compared with the existing experimental data. From analysis of structural and chemical trends in the NiSnM compounds, CoVSn is predicted to be a semiconducting intermetallic compound in the MgAgAs structure type. Preliminary first-principles calculations suggest an indirect gap of 0.8 eV.

Electronic structure of the semimetals Bi and Sb

Abstract: We have developed a third-neighbor tight-binding model, with spin-orbit coupling included, to treat the electronic properties of Bi and Sb. This model successfully reproduces the features near the Fermi surface that will be most important in semimetal-semiconductor device structures, including (a) the small overlap of valence and conduction bands, (b) the electron and hole effective masses, and (c) the shapes of the electron and hole Fermi surfaces. The present tight-binding model treats these semimetallic properties quantitatively, and it should, therefore, be useful for calculations of the electronic properties of proposed semimetal-semiconductor systems, including superlattices and resonant-tunneling devices.

Defect‐enhanced electron field emission from chemical vapor deposited diamond

Abstract: Diamond samples with varying defect densities have been synthesized by chemical vapor deposition, and their field emission characteristics have been investigated. Vacuum electron field emission measurements indicate that the threshold electric field required to generate sufficient emission current densities for flat panel display applications (≳10 mA/cm2) can be significantly reduced when the diamond is grown so as to contain a substantial number of structural defects. The defective diamond has a Raman spectrum with a broadened peak at 1332 cm−1 with a full width at half maximum (FWHM) of 7–11 cm−1. We establish a strong correlation between the field required for emission and the FWHM of the diamond peak. The threshold fields are typically less than 50 V/μm and can reach as low as 30 V/μm for diamond with a FWHM greater than 8.5 cm−1. It is believed that the defects create additional energy bands within the band gap of diamond and thus contribute electrons for emission at low electric fields.

Green luminescence from copper doped zinc sulphide quantum particles

Abstract: Free‐standing powder of zinc sulphide quantum particles has been synthesized using a chemical route. X‐ray diffraction analysis shows that the diameter of the particles is ∼21±2 A which is smaller than the Bohr exciton diameter for zinc sulphide. UV absorption shows an excitonic peak centered at ∼300 nm corresponding to an energy gap of 4.1±0.1 eV. These particles show a luminescence band at ∼424 nm. The quantum particles could be doped with copper during synthesis without altering the UV absorption or x‐ray diffraction pattern. However, doping shifted the luminescence to 480 nm, green wavelength in the visible region.

Electronic spectroscopy and photophysics of Si nanocrystals. Relationship to bulk c-Si and porous Si

Abstract: The structural characterization, electronic spectroscopy, and excited-state dynamics of surface-oxidized Si nanocrystals, prepared in a high-temperature aerosol apparatus, are studied to gain insight into the emission mechanism of visible light from these systems. The results are compared with direct-gap CdSe nanocrystals, indirect-gap AgBr nanocrystals, bulk crystalline silicon, and porous silicon thin films. As the size of the Si crystallites decreases to 1-2 nm in diameter, the band gap and luminescence energy correspondingly increase to near 2.0 eV, or 0.9 eV above the bulk 1.1-eV band gap. The absorption and luminescence spectra remain indirect-gap-like with strong transverse optical vibronic origins. The quantum yield increases to about 5% at room temperature, but the unimolecular radiative rate remains quite long, approximately 10{sup {minus}3}-10{sup {minus}4} s{sup {minus}1}. The luminescence properties of Si nanocrystals and porous Si are consistent, in most respects, with simple emission from size-dependent, volume-quantum-confined nanocrystal states. Room-temperature quantum yields increase not because coupling to the radiation field is stronger in confined systems, but because radiationless processes, which dominate bulk Si emission, are significantly weaker in nanocrystalline Si. An analogous series of changes occurs in nanocrystalline AgBr. 42 refs., 8 figs.

Valence-band physics and the optical properties of GaN epilayers grown onto sapphire with wurtzite symmetry

Abstract: We report on a quantitative analysis of the band gap of hexagonal GaN epilayers in terms of the joint contributions of the actual wurtzite symmetry on the one hand and of residual strain fields on the other hand. This investigation leads to revision of the previous modelings based on quasicubic descriptions of the valence-band physics and gives ${\mathrm{\ensuremath{\Delta}}}_{1}$=10\ifmmode\pm\else\textpm\fi{}0.1 meV, ${\mathrm{\ensuremath{\Delta}}}_{2}$=6.2\ifmmode\pm\else\textpm\fi{}0.1 meV, and ${\mathrm{\ensuremath{\Delta}}}_{3}$=5.5\ifmmode\pm\else\textpm\fi{}0.1 meV. Last we propose a set of deformation potentials for the hexagonal GaN semiconductor.

Size dependence of band gaps in silicon nanostructures

Abstract: The size dependence of the energy band gap for hydrogen saturated silicon clusters, wires and slabs are calculated using all electron density functional theory. The hydrogen saturation is considered as a model for a wider band gap insulator enclosing the silicon structures. With this perspective in mind, an effective mass model with finite barriers for both valence and conduction band is found to semiquantatively account for the numerical findings.

Optical properties of rutile near its fundamental band gap.

Abstract: We report low-temperature absorption, time-integrated photoluminescence, and resonant-Raman-scattering spectra near the fundamental band gap of ${\mathrm{TiO}}_{2}$ single crystals. The photoluminescence spectrum comprises a first peak at \ensuremath{\Elzxh}\ensuremath{\omega}=3.031 eV, followed by several peaks at lower energies. A polarization study of the emission spectrum indicates that the highest energy peak corresponds to 2${\mathit{p}}_{\mathit{x}\mathit{y}}$ dipole-allowed second-class excitonic transitions while the lower-energy peaks are phonon replicas of the 1s quadrupolar exciton. This result is corroborated by time-resolved photoluminescence measurements. The near-band-gap optical response of ${\mathrm{TiO}}_{2}$ is thus controlled by two distinct exciton states. The Raman-scattering intensity is found to increase slowly for excitation energies in the range 2.7--3 eV. This indicates that the Raman cross-section enhancement is dominated by virtual transitions involving the first dipole-allowed direct gap at 4.2 eV.

Guided and defect modes in periodic dielectric waveguides

Abstract: The nature of guided modes and defect modes in periodic dielectric waveguides is investigated computationally for model systems in two dimensions. It is shown that defect states that exist within the band gap of guided modes can be excited to form tightly localized high-Q resonances.

First-principles studies of the structural and optical properties of crystalline poly( para -phenylene)

Abstract: We present first-principles local-density band structure calculations for one-dimensional and three-dimensional crystalline poly (para-phenylene) (PPP) using the full-potential linearized augmented-plane-wave and the pseudopotential methods. Optimized structural parameters for PPP chains and for orthorhombic crystalline phases with space groups Pbam and Pnnm are determined. A torsion angle of 27\ifmmode^\circ\else\textdegree\fi{} is predicted in PPP chains and 17\ifmmode^\circ\else\textdegree\fi{} in the crystals. The dielectric tensor and the absorption coefficient are calculated. We find very good agreement with experimental data, indicating that the excitations are extended band states. The interchain coupling leads to energy band splittings of the order of 0.5 eV. It is shown that the energy gap can be varied over a wide energy range by relatively small structural modifications.

Insulated-gate transistor having narrow-bandgap-source

Abstract: A structure of a semiconductor device and a method of manufacturing the same is provided wherein a leakage current can be reduced while improving a drain breakdown voltage of an Insulated-Gate transistor such as a MOSFET, MOSSIT and a MISFET, and a holding characteristic of a memory cell such as a DRAM using these transistors as switching transistors can be improved, and further a reliability of a gate oxide film in a transfer gate can be improved. More particularly, a narrow band gap semiconductor region such as Six Ge1-x, Six Sn1-x, PbS is formed in an interior of a source region or a drain region in the SOI.IG-device. By selecting location and/or mole fraction of the narrow band gap semiconductor region in a SOI film, or selecting a kind of impurity element to compensate the crystal lattice mismatching due to the narrow-bandgap semiconductor region, the generation of crystal defects can be suppressed. Further the structure that the influences of the crystal defects to the transistor or memory characteristics such as the leakage current can be suppressed, even if the crystal defects are generated, are also proposed.

Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2

Abstract: Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three‐parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

Thin film II–VI photovoltaics

Abstract: With the exception of HgSe and HgTe, II–VI compounds are direct gap semiconductors with sharp optical absorption edge and large absorption coefficients at above bandgap wavelengths. Device quality polycrystalline films of II–VI compounds can be prepared from inexpensive raw materials by a number of low-cost methods. They are well-suited for thin film solar cells and provide an economically viable approach to the terrestrial utilization of solar energy. Thin film II–VI solar cells are usually of the heterojunction type consisting of a high bandgap window (or collector) and a lower bandgap absorber. The grain boundary effects in polycrystalline II–VI films are considerably less pronounced than those in III–V films and can be passivated, at least partially, by chemical treatment. The use of CdS, ZnO, ZnSe and Cd1 − xZnxS as the window and the use of CdTe and Cd1 − xZnxTe as the absorber are reviewed in this paper. The fabrication and characteristics of a number of the thin film solar cell structures are discussed with emphasis on the thin film CdS CdTe solar cell.

Photoemission studies of high-tc superconductors: the superconducting gap.

Abstract: Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature (T(c)) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-T(c) superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity.

SmB6: Kondo insulator or exotic metal?

Abstract: High pressure resistivity and Hall effect measurements on Sm${\mathrm{B}}_{6}$ show that the activation gap \ensuremath{\Delta} vanishes discontinuously between 45 and 53 kbar, yielding at high pressures a mass-enhanced Fermi liquid phase. The low temperature transport is dominated by extended states in the gap with unusual superunitarity scattering properties, and a carrier density which grows almost exponentially with reduced \ensuremath{\Delta}, saturating below 45 kbar near 0.15 electron per unit cell. Our results are inconsistent with hybridization gap models and suggest a striking parallel to Mott-Hubbard insulators.

Electronic structure of NiO in the GW approximation.

Abstract: We present a method for calculating the self-energy in the $\mathrm{GW}$ approximation that can be applied to systems containing $3d$ and $4f$ electrons. The method is applied to NiO and a gap of $\ensuremath{\sim}5.5$ eV is obtained, which is in reasonable agreement with the experimental value of 4.0 eV. The local density O $p$ band is also improved. The high binding energy satellite at 8 eV, however, is not obtained and there is no substantial increase of O $p$ character at the top of he valence band compared to the local density result. Based on our results, we discuss to which extent the $\mathrm{GW}$ approximation is capable of describing highly correlated systems such as NiO.