The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

Imaging from atomic structure to electronic structure

This paper discusses the possibility of retrieving the electron distribution (with highlighted valence electron distribution information) of materials from recorded HREM images. This process can be achieved by solving two inverse problems: reconstruction of the exit wave and reconstruction of the electron distribution from exit waves. The first inverse problem can be solved using a focal series reconstruction method. We show that the second inverse problem can be solved by combining a series of exit waves recorded at different thickness conditions. This process is designed based on an improved understanding of the dynamical scattering process. It also explains the fundamental difficulty of obtaining the valence electron distribution information and the basis of our solution.     

Electronic structure of silicon nitride according to ab initio quantum-chemical calculations and experimental data

Charge transfer ΔQ = 0.35e at the Si-N bond in silicon nitride is determined experimentally using photoelectron spectroscopy, and the ionic formula of silicon nitride Si₃^+1.4N₄^−1.05 is derived. The electronic structure of α-Si₃N₄ is studied ab initio using the density functional method. The results of calculations (partial density of states) are compared with experimental data on X-ray emission spectroscopy of amorphous Si₃N₄. The electronic structure of the valence band of amorphous Si₃N₄ is studied using synchrotron radiation at different excitation energies. The electron and hole effective masses m _e ^* ≈ m _h ^* ≈ 0.5m _e are estimated theoretically. The calculated values correspond to experimental results on injection of electrons and holes into silicon nitride.     

XANES study of 3d oxides: Dependence on crystal structure

XANES measurements are reported for a number of transition metal oxides. Oxide phases, in which the transition element could be widely varied (within the 3d series) while preserving the crystal structure, were systematically examined. The materials examined include monoxides, perovskites, zircons and spinels. For those samples of a given oxide phase, the near edge spectra are nearly identical but spectra for different phases are dissimilar. These observations are consistent with the simplest view of the x-ray absorption process, namely that dipole selection rules are obeyed and spectral features predominately result from transitions between the K shell and empty states with p-character.     

Small copper clusters: Study of the local atomic and electronic structure by the XANES and DFT methods

The Cu L ₃ edge X-ray absorption spectra of free Cu _n clusters containing 5–15 atoms were obtained for the first time. It is shown that the geometry of small clusters, including the bond length and bond angle, can be studied by analyzing the X-ray absorption spectra. The experimental X-ray absorption spectra of Cu₁₃ clusters were theoretically interpreted using the self-consistent method of full multiple scattering, the finite difference method in the muffin-tin potential approximation, and the full potential method for the icosahedral cluster structure. Good agreement between the theoretical and experimental spectra is achieved for the calculation in the full potential approximation.     

Chemisorption of atomic oxygen on silicon surface

We investigated the electronic structure of oxygen adsorbed on silicon surface in a head-on position, and showed that only the corresponding bonding structure is able to explain the features in UPS and ELS spectra that were not interpreted satisfactorily before. We also found that charge is transferred from backbonds to SiO bond and induces empty states localized between the first and second layers of the silicon surface.     

Oxidation of the porous silicon surface under the action of a pulsed ionic beam: XPS and XANES studies

The changes in the electronic structure and phase composition of porous silicon under action of pulsed ionic beams have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) using synchrotron radiation. The Si 2p and O 1s core photoemission spectra for different photoelectron collection angles, valence band photoemission spectra, and X-ray absorption near-edge fine structure spectrain the region of Si L _2,3 edges of the initial and irradiated samples have been analyzed. It has been found that, as a result of the irradiation, a thin oxide film consisting predominantly of higher oxide SiO₂ is formed on the porous silicon surface, which increases the energy gap of the silicon oxide. Such film exhibits passivation properties preventing the degradation of the composition and properties of porous silicon in contact with the environment.     

Core-hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure

The authors investigated theoretically core-hole effects on X-ray absorption near-edge structures (XANES) of Np and Am L_III in neptunium dioxide (NpO₂) and americium dioxide (AmO₂) with CaF₂-type crystal lattices using the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. The peak creation mechanism of XANES was shown by examining the electronic structures of these oxides, which indicated that core-hole screening was more marked for AmO₂ than for NpO₂ because of the difference in the charge transfer between these oxides. Furthermore, the results of charge density analysis suggested that the white line was assigned to the quasi-bound state composed of the localized Np d or Am d components and O components, and that the tail structure was created as a result of delocalized standing waves between the Np or Am atoms.     

Investigation of atomic and electronic structure of films generated on a cutting tool surface

It is established that oxygen-containing films named as secondary structures (SS) are developed on the surface of deformed composite powder material based on high-speed steel (HSS-based DCPM) tools during their operation at cutting. The composition, nearest atomic surrounding, and electronic structure features of these films were investigated by means of SEM, AES, ELS, and EELFS methods. It was found that structural features of these films differ from those of TiC, TiO₂ (rutile), and films formed on the surface of the TiC powder heated in air. Disordering of atoms at long interatomic distances (beyond the third coordination sphere) and low localisation of π-electrons make it possible to arrive at a conclusion on the amorphous-like structure of the films. Evolution of the atomic structure of DCPM tools at friction was revealed. It was discovered that favourable frictional properties and a high wear resistance of DCPM tools are followed from developing SSs that consist of the oversaturated solid solution of oxygen in titanium that possesses metal-like characteristics.     

Simulation of porous silicon formation and silicon epitaxy on its surface

Processes of porous silicon formation and silicon epitaxy on its surface are studied using the Monte Carlo method. The model for porous silicon formation under anode etching allows for non-uniformity of charge distribution over the silicon-electrolyte interface. Processes of diffusion, generation and recombination of holes, as well as dimensional quantization, are also considered. Gilmer's model, extended to the case of a rough surface, is used to study epitaxy. The structures obtained by simulations at different levels of doping of the crystal substrate and for various parameters (temperature, HF concentration, and anode current density) are presented. Analysis of nanoporous structures showed that the porosity changes with depth, and fractal dimensionality exists below 10 nm. It has been shown that epitaxy, developing by formation of metastable nuclei at the edges of pores, by their subsequent growth along the perimenter and by formation of a thin continuous overhanging layer, may be described within the framework of this model. Three-dimensional images of near-surface layers formed at different stages of epitaxy have been obtained. The dependence of the epitaxy kinetics on the amount of deposited silicon for different structure porosities has been revealed. Institute of Physics of Semiconductors, Siberian Branch, Russian Academy of Science. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 49–56, March, 1999.     

Temperature dynamics of triglycine sulfate domain structure according to atomic force microscopy and dielectric spectroscopy data

A hydrogen-containing ferroelectric triglycine sulfate (TGS) was comprehensively studied with an atomic force microscopy (AFM) and dielectric spectroscopy. The domain structure dynamics was in situ investigated with piezoresponse force microscopy (PFM) during heating and cooling the TGS crystal near phase transition. Relaxation dependencies of domain boundaries general perimeter and domain dimensions were obtained. TGS dielectric spectra measured at the frequency range from 10 to 10¹¹ Hz were analyzed on basis of significant contribution of conductivity into the dielectric response of ferroelectrics and a good agreement with the experimental data was received. It allows us to obtain more information about temperature dynamics of the domain structure.     

The electronic structure of alkali metal oxides

Crystalline phase total energies, band structures, the distributions of the total and partial densities of electron states, and atomic charges were calculated for lithium, sodium, and potassium oxides, peroxides, superoxides, and ozonides using the CRYSTAL 06 and GAMESS packages in the B3LYP parameterization. For the molecular phases, the geometry was optimized and molecular orbital energies calculated. The results obtained for metal oxides were compared with the experimental photoelectron spectroscopy data and used to analyze their formation and thermal decomposition.     

Investigation of electronic structure and associated nuclear quadrupole interaction of adsorbed fluorine atoms on silicon surface

The electronic energy levels and wave functions of fluorine and chlorine atoms adsorbed at the <111> surface of silicon are obtained by the Hartree-Fock procedure applied to clusters simulating the surface structure. Minimising the total energy with respect to the positions of the halogen atoms, the Si-Cl distance in the adsorbed system was found to be in good agreement with that from a recent SEXAFS measurement, there being no corresponding data presently available for fluorine on silicon surface. The calculated¹⁹ f ^* nuclear quadrupole coupling constants using the electronic wave functions for SiH₃F and the larger cluster Si₄H₉F, simulating the surface, differed substantially in contrast to the situation for the Si-F distance, indicating that the quadrupole coupling is a more sensitive indicator of the adequacy of the cluster chosen to represent the surface system. Experimental results for the coupling constants are awaited for comparison with theory.     

Kinetics of water molecule adsorption on the porous silicon surface

The kinetics of water vapor adsorption on the porous silicon surface is studied by the gas relaxometry method. The process multistepping which is reflected in sequential changes in H₂O molecule diffusivities into the porous matrix, is found. Phenomenological models describing the experiment are considered.     

XANES studies of unoccupied electronic states and local real structure of some antimony chalcogenides

Summary The X-rayL absorption edges of antimony and iodine in SbSI, Sb₂S₃, Sb₂S₅ have been measured with high resolution using synchrotron radiation. It is shown that the density of the unoccupied levels in the conduction band is adequate to interpret the edge structures between 0 and 10 eV. These structures and the chemical shifts of the three compounds are compared: a striking similarity of the electric charge of antimony and of the electronic structure in the bottom of the conduction band is found. The structures between 10 and 50 eV are analysed and interpreted in terms of stereochemical co-ordination. The nonexistence of Sb₂S₅ as a stoichiometric compound is confirmed.

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Riassunto Le soglie di assorbimentoL dell'antimonio e dello iodio sono state misurate ad alta risoluzione con luce di sincrotrone in SbSI, Sb₂S₃ e Sb₂S₅. La densità degli stati liberi nella banda di conduzione è risultata adeguata per interpretare le strutture di soglia tra 0 e 10 eV. Il confronto delle strutture di soglia e degli spostamenti chimici dei tre composti ha rivelato una notevole somiglianza nella carica dell'atomo di antimonio e nella struttura della parte bassa della banda di conduzione. Le strutture tra 10 e 50 eV sopra la soglia sono state analizzate ed interpretate in relazione alla coordinazione stereochimica. è stata confermata la non esistenza del Sb₂S₅ come composto stechiometrico.

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Резюме Были определны рентгеновскиеL края поглощения суьмы и йода в SbSI, Sb₂S₃, Sb₂S₅ с высоким разрешением, используя синхротронное излучение. Показывается, что плотность незаполненных уровней в зоне проводимости адекватно описывает структуры в области от 0 до 10 эВ. Для трех соединений проводится сравнение полученных структур и химических сдвигов. Обнаружено удивительное подобие электрического заряда сурьмы и электронной структуры на дне зоны проводимости. Анализируются структуры в области от 50 эВ и предлагается их интерпрпретация. Отмечается отсутствие Sb₂S₅, как стехиометрического соединения.

     

Modeling of electronic properties of silicon nanoparticles with dense atomic packing

Results of optimization of the atomic structure and calculation of electronic and transfer characteristics of Si₁₃ silicon clusters with dense atomic packing and silicon nanoparticles encapsulated by transitive metal atoms (Cu, Zn, Fe, and Ni) are presented. It is demonstrated that an external electric field and a silicon nanostructure charge change significantly their energy spectrum and kinetic properties.     

Oxidation of silicon surface with atomic oxygen radical anions

The surface oxidation of silicon （Si） wafers by atomic oxygen radical anions （O- anions） and the preparation of metal-oxide-semiconductor （MOS） capacitors on the O-oxidized Si substrates have been examined for the first time. The O- anions are generated from a recently developed O- storage-emission material of [Ca24Al2sO64]^4＋·4O^- （Cl2A7-O^- for short）. After it has been irradiated by an O- anion bean： （0.5 μA/cm^2） at 300℃ for 1-10 hours, the Si wafer achieves an oxide layer with a thickness ranging from 8 to 32 nm. X-ray photoelectron spectroscopy （XPS） results reveal that the oxide layer is of a mixture of SiO2, Si2 O3, and Si2O distributed in different oxidation depths. The features of the MOS capacitor of 〈Al electrode/SiOx/Si〉 are investigated by measuring capacitance-voltage （C - V） and current-voltage （I - V） curves. The oxide charge density is about 6.0 × 10^1 cm^-2 derived from the （C - V curves. The leakage current density is in the order of 10^-6 A/cm^2 below 4 MV/cm, obtained from the I - V curves. The O- anions formed by present method would have potential applications to the oxidation and the surface-modification of materials together with the preparation of semiconductor devices.