Thevalence Band Research Articles

Native Point Defect States in ZnO

The native point defect states in ZnO have been calculated by using afull-potential linear muffin-tin orbital method. The results show that Znvacancy and O interstitial produce the shallow acceptor levels above thevalence band. The O vacancy produces a deep donor level, while Zn interstitialproduces a shallow donor level, both below the conduction band.The Zn interstitial is the main factor which induces the native n-typeconductivity in ZnO.

Quantum states and interband optical spectra in spherical GaAs-(Ga,Al)Asquantum dots doped with on-centre shallow donor impurities: a real-timewavepacket propagation method

A time-dependent approach for the calculation of eigenenergies, eigenfunctionsand optical spectra, within the effective-mass and single-hole-bandapproximations, of low-dimensional semiconductor systems is presented. In thismethod, energy and optical spectra are obtained by Fourier analysis of atime-autocorrelation function constructed from an appropriately chosenpropagated wavepacket. Eigenfunctions are determined by a direct Fourieranalysis of such a propagated wavepacket. The time evolution of the wavefunctionis obtained by numerical integration of the time-dependent Schrödingerequation. This approach shares the virtues of the more common numericalmethods (accuracy, flexibility and automatic enforcement of boundaryconditions) with the additional advantage that it can yield an entire eigen oroptical spectrum in a single calculation. The methodology is applied to thecalculation of all the eigenenergies and eigenfunctions of the conductionband, and the intensities of the transitions from the ground state of thevalence band to all the states of the conduction band, in sphericalGaAs-(Ga,Al)As quantum dots with finite confinement and a wide range of radii,without and with an on-centre shallow donor impurity.

Investigation of the electronic properties of substituted LaNi5 compounds used as material for batteries

The electronic properties of LaNi5 and substituted LaNi4M (M = Mn, Fe, Co, Cu and Al) and LaNi3.55Mn0.4Al0.3Co0.75 compounds have been studied by X-ray absorption (XAS) and photoemission (PES) spectroscopies in order to understand the influence of the substitutions on the electrochemical properties of these materials in rechargeable batteries. The experimental results are compared with band structure calculations. The valence band of LaNi5 is dominated by the Ni 3d band. Ni substitution by another 3d metal leads to the formation of an additional 3d subband which is above that of Ni for Mn, Fe and Co and below for Cu, in agreement with density of states (DOS) calculations. Al substitution leads to a progressive filling of the Ni d band associated with a decrease of the DOS at EF. In LaNi3.55Mn0.4Al0.3Co0.75 (LaNi3S), the XAS measurements indicate that the filling of the individual 3d subbands is different from the corresponding ones in LaNi4M single substituted compounds (M = Mn and Co). The total number of holes in the d bands of the transition elements of the LaNi3S compound is found to be the same as in LaNi4Co. The complete filling of these states by 4 H atoms per formula unit in LaNi4CoH4 as well as in the LaNi3S hydride is associated with a particular stability of the compounds up to this hydrogen concentration, a feature which has been observed in the pressure–composition isotherms and which appears to play an important role in the longer cycle life of these electrode materials.

TOTAL CURRENT SPECTRA STUDIES ON ELECTRONIC STATES OF Si(100) SURFACE

Electronic states of Si(100)2×1 clean surface and Si(100)1×1-2H surface are studied with total current spectrometer. The occupied surface states are at 0.25, 8.4 and near 12 eV below thevalence band maximum, while the unoccupied surface states are at 0.7 eV above the valence band maximum. Two induced surface states of Si(100)1×1-2H are observed below the valence band maximum.

The optical joint density of states in amorphous silicon nitride

Reflectivity measurements have been made in the range 0.5-12 eV on amorphoussilicon nitride films containing various concentrations of nitrogen. Films prepared by radiofrequency sputter deposition of silicon in an argon/nitrogen plasma, both with and withouthydrogen, were studied along with films prepared by the glow-discharge technique. Fromthe reflectivity data, ε1- and ε2-spectra were determined using Kramers-Krönig analysis and,from the latter, the optical joint densities of states (OJDOS) were derived. The behaviour ofthe OJDOS and its variation with nitrogen content in the films is discussed in terms of thevalence band density of states and its change on alloying.

Photoemission in SiO x alloys

We discuss the results of photoemission measurements in SiO x alloys for 0 ≦ x ≦ 2. The valence band spectra for x ap 2 are similar to previously reported results for SiO 2. For decreasing x, a widening of the valence band is observed, corresponding to the replacement of SiO bonds by SiSi bonds. An analysis of the broadening of the Si2p core level in terms of SiO n suboxides gives a chemical shift of 1.05 eV per SiO bond and a nearly statistical distribution of suboxides, supporting a random network model. From the measured shifts of the O1s, Si2p and O2s core levels we deduce a shift of the Fermi level of 1 eV toward thevalence band as x decreases to zero. Features of the valence band are discussed in the light of recent band structure calculations.

A Photoemission Study of the Epitaxial Growth of Si on Gap(110)

ABSTRACTWe have used angular resolved ultraviolet photoemission spectroscopy to study the epitaxial growth of Si on GaP(110). Surface state emission obscures the top of the valence band (TVB). The Fermi level for the clean GaP(110) surface is 1.20±0.05eV above the TVB. 1ML (monolayer) of Si pins the Fermi level position at 1.40±0.05eV above the TVB. Further deposition of Si leads to a valence band discontinuity ΔEv=1.07 ±0.10eV.