Evolution of electronic states in thin Ca(001) flms in strong electrostatic fields

The transformation of electronic states in Ca(001) films in strong electrostatic fields is studied using electron density functional theory. It is shown that an excess film charge of either sign pins the Fermi level (with respect to the conduction band edge) in a wide range of fields. For positively charged films, the change in the density of states at the Fermi level is small but the energy derivative of the density of states changes sign with increasing excess charge of the film. For negatively charged Ca(001) films, the change in the density of states at the Fermi level plays the main role in stabilizing the width of the occupied part of the conduction band; this should be manifested in the electronic thermodynamic and transport properties of negatively charged Ca(001) films with quantum confinement.     

Electronic transport in strained p-modulation Be-doped Ga0.8In0.2As/GaAs single quantum well

We report on the electronic transport properties of p-modulation Be-doped Ga_0.8In_0.2As/GaAs single quantum well. The experiments included the spectral photoluminescence between 8 and 300?K, and Hall effect measurements at temperatures between 14 and 300?K. The effect of strain which induces splitting of the valence band as light and heavy hole bands on transport is discussed. The calculated band alignment of the GaInAs sample using model-solid theory including strain effects indicates large conduction band discontinuities and a much smaller valance band discontinuity in GaInAs. The effect of the conduction and valance band discontinuity on the electronic transport properties is also discussed.     

Electronic structure of the organic semiconductor copper tetraphenylporphyrin (CuTPP)

The electronic structure of thin films of the organic semiconductor copper tetraphenylporphyrin (CuTPP) has been studied using synchrotron radiation-excited resonant soft X-ray emission spectroscopy (RSXE), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray photoemission spectroscopy (XPS). The C and N partial density of states for both the valence and conduction band electronic structure has been determined, while XPS was used to provide information on the chemical composition and the oxidation states of the copper. Good agreement was found between the experimental measurements of the valence and conduction bands and the results of density functional theory calculations.     

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

单空位缺陷对石墨纳米带电子结构和输运性质的影响

基于第一性原理电子结构和输运性质计算,研究了单空位缺陷对单层石墨纳米带(包括zigzag型和armchair型带)电子性质的影响.研究发现,单空位缺陷使石墨纳米带在费米面上出现一平直的缺陷态能带;单空位缺陷的引入使zigzag型半导体性的石墨纳米带变为金属性,这在能带工程中有重要的应用价值;奇数宽度的armchair型石墨纳米带表现出金属特性,有着很好的导电性能,同时,偶数宽度的armchair型石墨带虽有金属性的能带结构,但却有类似半导体的伏安特性;单空位缺陷使得奇数宽度的armchair石墨纳米带导电 关键词： 石墨纳米带 单空位缺陷 电子结构 输运性质     

On the electronic structure of Ag chalcogenides

The electronic structure of Ag chalcogenides in the α phase, which exhibit an interesting, electronic semiconducting behaviour as well as the fast ion transport, is discussed on the basis of an energy band structure calculation. As a simplest way of simulating the effect of the Ag ions on the electronic states, some hypothetical crystalline compounds are constructed such as the perovskite, the sodium chloride and the flourite structures. The absolute magnitude of the calculated conduction electron effective mass is quite small irrespectively of the structures, about 10% of the free-electron mass, in semiquantitative agreement with experiments. A deviation from an effective mass approximation near the conduction band bottom is found to be appreciable, and to explain reasonably well experimental results. An origin of these features of the conduction band is a rather strong hybridization of the Ag 5s band and the chalcogen s band. The calculation also shows that the hybridization of the Ag 4d band and the chalcogen p band can affect the absolute magnitude of the hole effective mass appreciably, and that the energy band gap depends sensitively on these s-s and d-p hybridization effects.     

Influence of correlation and temperature on the electronic structure of bulk and thin film GdN

The influence of correlation and temperature on the electronic structure of bulk and thin film GdN has been studied using the s-f model, which combines the one electron band structure with a many body procedure. The tight binding linear muffin tin orbital (TB-LMTO) method was used to obtain the one electron band structure of the system. The s-f exchange coupling constants for each band were obtained from the spin polarized band structure of the system using a mean field model. Correlation effects are found to be present in the system. However they are not sufficiently strong to cause a correlation induced splitting in the spectrum. Some bands of the thin films of GdN exhibit splitting at T=T_c and it is due to the combined effect of correlation and temperature. The conduction bands of both the bulk and the thin films of GdN exhibit a red shift with respect to temperature.     

Effect of nitrogen incorporation and oxygen vacancy on electronic structure and the absence of a gap state in HfSiO films

The effect of nitrogen (N) incorporation into HfSiO on the electronic structure and band alignment of HfSiO films was investigated. N depth profile data obtained by medium energy ion scattering (MEIS) showed that the concentration of N or the bonding or electronic state of N in the film was stable when the film was annealed at 950 °C, while the oxygen (O) in HfSiON films was present in dissociated form, as evidenced by the unoccupied electronic state of O. The valence band offsets of the HfSiO films were strongly affected by N incorporation due to the presence of N in a 2p state. Moreover, a reduction in the conduction band offset of a HfSiO film was confirmed after the film was annealed in an atmosphere of N₂. The unoccupied state of the O vacancy is responsible for the change in the conduction band offset. The results of ab-initio calculations for the density of states (DOS) of HfSiO and HfSiON supercells were in agreement with the experimental results. The incorporation on N into HfSiO prevents the formation of a gap-state inside the band gap despite the fact that an O vacancy is generated in the film.     

Electron transport in the carbon-copper nanocluster structure

The electron transport in hydrogenated amorphous carbon films a-C: H with copper nanocluster inclusions has been investigated. The conditions of cluster formation are derived. It is theoretically demonstrated that the energy band structure of the matrix substantially affects the conditions of cluster formation. The electron transport depends on the cluster structure. It is found that, below the percolation threshold (the case of isolated clusters), the transport current is governed by two components depending on the electric field strength. At low field strengths, the current is caused by electrons in the conduction band of amorphous carbon, which are thermally excited from copper clusters. At high field strengths, the transport current is provided by tunneling electrons from the Fermi level of copper clusters to the conduction band of a-C: H. The difference between the mobility edge of the conduction band of amorphous carbon and the Fermi level in copper clusters is determined from the temperature dependence of the resistance and proves to be equal to 0.48 eV. The temperature dependences of the resistance at low field strengths exhibit a fine structure. It is revealed that, above the percolation threshold, the electrical resistance of clusters is considerably contributed by the residual resistance, which is supposedly associated with the electron scattering by cluster surfaces. The temperature effect on the electron transport is examined using the spin-wave scattering technique at a frequency of 4.0 GHz. It is found that the spin wave in the yttrium iron garnet (YIG) film is predominantly affected by thermally excited electrons located above the mobility edge in the conduction band of a-C: H.     

Strong correlations in YH(3-delta) evidenced by Raman spectroscopy

Temperature dependent Raman measurements on insulating YH(3-delta) thin films are reported. With increasing temperature we observe a huge broadening of a line corresponding to an yttrium mode. This particular mode is assigned to a breathing vibration of the yttrium atoms around an octahedral hydrogen position. The line broadening is discussed in terms of a coupling between this breathing mode and the electron excited from an octahedral H vacancy into the 4d conduction band of Y, corroborating the strong correlation models for the electronic structure of YH3.     

Preparation and electronic structure of phase pure K3C60

Phase pure K₃C₆₀ films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.     

The electronic structure of InAs/GaSb(001) superlattices-two dimensional effects

Detailed calculations of the two dimensional effects in the electronic structure of InAs/GaSb(001)superlattices are presented for the first time. Comparison of the calculated thickness dependence of the superlattice band gap with optical absorption measurements shows that, at the Γ-point, the conduction band edge of InAs lies about 60 meV below the valence band edge of GaSb. Eigenfunctions of the highest light and heavy hole bands, and the lowest two conduction bands exhibit spatially confined nature in the GaSb and InAs regions respectively, thus establishing the two-dimensional nature of these bands. The calculated conduction band effective mass in the plane of the superlattice near the Γ-point is found to be enhanced by a factor of 2.5 over the bulk InAs value and compares very well with the appropriate mass extracted from recent magnetoresistance measurements.     

Amorphous germanium I. A model for the structural and optical properties

The structural properties (radial distribution function, small angle scattering, density), optical properties (complex dielectric function versus energy), and other characterizing parameters (scanning electron microscope examination, chemical analysis, recrystallization behaviour, transport phenomena), have been determined for a series of amorphous Ge films sputtered at different substrate temperatures. Various of these properties (e.g. refractive index, density, room temperature conductivity) show only slowly varying values for low and high substrate temperatures, with a rapid changeover in a critical temperature range. A model has been developed which relates this behaviour to the probability that an adatom configuration be able to change to a configuration of lower local free energy in the time for the deposition of a monolayer, and it is suggested that such behaviour may be a general feature of disordered films prepared by any deposition method. The structural and optical properties at a given deposition temperature, their differences from the properties of the FC-2 crystal, and the small changes in them as the deposition temperature is increased, are examined in terms of the Phillips spectroscopic theory of bonding. It is found that the changes in the gross optical properties from crystal to disordered film and among disordered films may be very satisfactorily explained in terms of changes in the bond length and the coordination number or density. The changes in these parameters are quantitatively related to the changes in the void density by the structural measurements. The special importance of the coordination number—and so of the void density distribution—both for the basic theory and for attempted extrapolation to the properties of the fully coordinated continuous random network without impurities is emphasized. The complex dielectric function is used, along with published data on the conduction band density of states and arguments concerning the dependence of the transition matrix element on energy, to derive densities of states for the valence band and in the pseudogap, and their changes with deposition temperature. These state densities are then used to calculate the conductivity due to transport in the delocalized valence band states using the random phase approximation and also the conductivity due to hopping near the Fermi level, and the results compared with preliminary data on conductivity. Finally, a review is given of pertinent earlier work on structure and its dependence on preparatory method and annealing treatment, the relation of the experimental structure measurements to atomic models, calculations of the electronic band structure, and earlier measurements of optical properties derived from photoemission, reflectivity and absorption edge data. An attempt is made to correlate the new data with earlier measurements, and to assess to what extent the properties reported here approach those of the hypothetical chemically pure, perfectly coordinated continuous random network.     

Bulk band gaps in divalent hexaborides

Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for many-body quasiparticle band calculations.     

First-principles study of structural,electronic, and optical properties of ZnSnO3

The structural, electronic, and optical properties of ZnSnO₃ were investigated using density functional theory within the generalized gradient approximation. The structure parameters obtained agree well with the experimental results. The electronic structures indicate that ZnSnO₃ is a semiconductor with a direct band gap of 1.0 eV. The calculated optical spectra can be assigned to contributions of the interband transitions from valence band O 2p levels to conduction band Sn 5s levels or higher conduction band Zn 3d levels in the low-energy region, and from O 2p to Sn 5p or Zn 4p conduction band in the high-energy region.     

卷曲效应对单壁碳纳米管电子结构的影响

利用计入卷曲效应的单壁碳纳米管(SWCNT)的能量色散关系,计算最低导带的电子速度及有效质量,并与不计入卷曲效应的结果进行了比较.计算结果表明:卷曲效应对电子速度及有效质量的影响与SWCNT的类型密切相关,金属锯齿型SWCNT对卷曲效应最为敏感,其次是扶手椅型SWCNT,最不敏感的是半导体锯齿型SWCNT.由此可以推断,卷曲效应对金属锯齿型SWCNT电子结构及低偏压输运特性影响最大,其次是扶手椅型SWCNT,影响最不明显的是半导体锯齿型SWCNT.这些结果与实验测量及密度泛函理论计算结果完全一致. 关键词： 单壁碳纳米管 卷曲效应 电子速度 电子有效质量     

The electronic structure of lithium metagallate

Herein we present a study of the electronic structure of lithium metagallate (LiGaO(2)), a material of interest in the field of optoelectronics. We use soft x-ray spectroscopy to probe the electronic structure of both the valence and conduction bands and compare our measurements to ab initio density functional theory calculations. We use several different exchange-correlation functionals, but find that no single theoretical approach used herein accurately quantifies both the band gap and the Ga 3d(10) states in LiGaO(2). We derive a band gap of 5.6 eV, and characterize electron hybridization in both the valence and conduction bands. Our study of the x-ray spectra may prove useful in analysing spectra from more complicated LiGaO(2) heterostructures.     

Electronic band structure and optical properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is fabricated by hydrothermally etching single crystal silicon (c-Si) wafers in hydrofluoric acid containing ferric nitrate. Microstructure studies disclosed that it is a typical micron/nanometer structural composite system with clear hierarchical structures. The optical parameters of Si-NPA were calculated by general light-absorption theory and Kramers–Kronig relations based on the experimental data of reflectance and the variations compared with the counterparts of c-Si were analyzed. The features of the electronic band structure deduced from the optical measurements strongly indicate that Si-NPA material is a direct-band-gap semiconductor and possesses separated conduction sub-bands which accords with conduction band splitting caused by silicon nanocrystallites several nanometers in size. All these electronic and optical results are due to the quantum confinement effect of the carriers in silicon nanocrystallites.     

Fabrication and electrical characterization of ZnO rod arrays/CuSCN heterojunctions

ZnO rod arrays/CuSCN heterojunctions are fabricated by depositing ZnO rod arrays films using two-step chemical bath deposition (CBD) and CuSCN thin films using successive ionic layer adsorption and reaction (SILAR) on ITO substrate successively. The structures and morphologies of ZnO films and CuSCN films, analyzed by X-ray diffraction (XRD) spectroscopy and metallurgical microscope, show that ZnO films are hexagonal wurtzite structure and consisted of vertical polycrystalline rods with diameter of 1 μm, CuSCN films are β-phase structure and consisted of elongated grains with length of 3 μm. Current–voltage (I–V) measurements of ZnO/CuSCN heterojunctions show good diode characteristics with rectification ratio about 48.3 at 3 V. The forward conduction is, respectively, determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of ZnO/CuSCN heterojunctions has been proposed to explain the transport mechanism.     

Growth of Fe films on Rh(001): a photoemission study

The growth mode and electronic structures of Fe thin films, epitaxially grown on Rh(001), were studied by various photoemission measurements for the film thickness of 0–12 monolayers. By comparing the valence band structures obtained from the angle-resolved photoemission spectra and the theoretical band calculations for the bulk Fe, we concluded that the valence band structure of the Fe film resembles that of fcc(001) in the low coverage region and that of bcc(110) in the high coverage region. This transformation of the electronic structure is considered to be related to the thickness dependence of the interlayer spacing of the films.