Calculation of the electronic structure of crystalline and amorphous arsenic

Qualitative differences between crystalline and amorphous arsenic in spectroscopic data are reproduced in chemical pseudopotential calculations of, and a local environment approach to, the density of states of arsenic in the A7 structure and in a three-fold coordinated random network. The changes can be interpreted as a removal of the long interlayer bonds in the crystal, giving a local two-dimensional character to the amorphous density of states.     

Auger-spectroscopy studies of the electronic structure of amorphous carbon films

Information on the density of valence states (DOVS) of aC films has been determined by Auger spectroscopy. The films were prepared by magnetron sputtering of graphite and plasma decomposition of hydrocarbon gas. The relative contents of graphite-like and diamond-like components in aC have been determined by comparison of the area under the π-band of aC with that of graphite. The effect of annealing on the aC structure has benn studied.     

Microstructural changes in amorphous Si/crystalline Al thin bilayer films upon annealing

Microstructural changes occurring in a sputter deposited Si (150 nm, amorphous)/Al (50 nm, crystalline); {111} fibre textured bilayer, upon annealing at 523 K for 60 min in a vacuum of 2.0×10^-4 Pa, were analyzed employing X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, atomic force microscopy and focused-ion beam imaging. After the annealing the Al and Si sublayers had largely exchanged their locations in the bilayer; i.e. the Si layer was adjacent to the substrate after annealing. Simultaneously, the amorphous Si layer had crystallized into an aggregate of {111} oriented nanocrystals, with a crystallite size of about 15 nm. The Al layer, now adjacent to the surface, had formed a uniformly net-shaped layer in association with an increase of the surface roughness. Upon this rearrangement, the already initially present Al {111} fibre texture had become stronger, the Al crystallites had grown laterally and the macrostress in the Al layer had relaxed. An extensive analysis of thermodynamic driving forces for the transformation indicated that the largest gain in energy upon transformation is due to the crystallization of the amorphous Si. The only identifiable driving force for the layer exchange appears to be the release of elastic energy upon the rearrangement of the Si and Al phases in the layer. PACS 61.43.D; 61.72.C; 62.40; 65.70; 68.55.J; 68.60.BThis revised version was published online in September 2004. Due to technical problems the PDF of the previous version was incomplete.     

Effect of laser irradiation on the structure and properties of dielectric column clusters in the YBaCuO superconducting films

The effect of laser irradiation on the structure and properties of the YBa₂Cu₃O_{7 ? δ} epitaxial super-conducting films (T _c = 90–91 K) that are grown on the SrTiO₃ and LaAlO₃ substrates is studied. The films exhibit a system of pyramidal peaks that are incorporated in the single-crystal structure of the film whose system of the (00l) planes is parallel to the surface of the substrate. It is demonstrated that the peaks represent growth defects that result from the relaxation of the accumulating strain due to the mismatch of the crystallographic parameters of the growing layers of the film and substrate. The island structures that are formed owing to the relaxation of strains acquire the (11l) or (10l) orientation and penetrate through the film layers in the course of growth. It is demonstrated that the irradiation using relatively short laser pulses makes it possible to modify the structure of the dielectric clusters and allows the smoothing of the film surface at an insignificant (5–10%) decrease in the concentration of the superconducting phase. An increase in the energy density to a level of greater than 100 mJ/cm² when the number of pulses is greater than five causes an increase in the volume of dielectric phases and the worsening of parameters.     

Scale invariance of the structure upon explosive crystallization of amorphous films

Experimental results that are obtained in the process of the explosive crystallization of x-ray amorphous films Fe₂Tb and CoPd with magnetic anisotropy perpendicular to the film plane are reported. The internal bending of thus obtained “single crystals” reaches 100 deg/μm. An explanation of the mechanism and kinetics of explosive crystallization in the Fe₂Tb and CoPd films is proposed in the framework of the shear transformation zone theory.     

Method for computing the electronic structure of thin crystalline films with disordered surface atoms

We present a Green's function method for computing the electronic structure of a crystalline film in which the surface atoms are disordered. The disorder is taken into account by considering an ensemble of random displacement configurations. We use a uninodal approximation to construct a general formalism for the method and obtain a determinant equation and expression for the density of states, permitting a numerical solution to the spectral problem. We discuss which features of the surface electronic structure are due to displacement disorder in the surface region of the crystal, and in what systems such disorder can be realized.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 81–88, August, 1988.We thank G. V. Vol'f for some useful discussions.     

Electronic structure of the Si-C-N amorphous films

The results of the investigation of amorphous a-SiC _x N _y films of various compositions by the ultra-soft X-ray emission spectroscopy are presented. The Si L _{2, 3}, C K _α, and N K _α emission spectra, which reflect the partial densities of states Si 3s3d, C 2p, and N 2p, respectively, have been measured and analyzed. It has been established that the Si-N chemical bond is similar in type to the Si-C chemical bond and completely substitutes for the Si-C bonds in thin a-SiC _x N _y films as the oxygen concentration increases. A similar effect is associated with the local agglomeration and subsequent clusterization of carbon atoms in the SiN-enriched regions of the internal volume of the film. A high ability of the a-SiC _x N _y films to oxidation in air has been established. This has been confirmed by a satisfactory approximation of the Si L _{2, 3} X-ray emission spectra of a-SiCN with the use of the superposition of the spectra of γ-Si₃N₄ and Si₂N₂O. The relative weight coefficients of the spectra of γ-Si₃N₄, which have been used to approximate the spectra of thin a-SiC _x N _y films, are proportional to the values of the Young’s modulus for different values of compositions (different values of x and y).     

The electronic structure of carbon films deposited in rf argon–hydrogen plasma

The electronic structure of C films deposited by sputtering a graphite target in rf. Ar–H₂ plasma is investigated by photoemission, Auger emission and electron energy loss spectroscopy (EELS) as a function of the H₂ concentration in the feed gas, referred to as [H₂]. Adding hydrogen to the plasma causes the films to change from a graphite-like unhydrogenated structure to a non-graphitic hydrogenated structure. The film mass density, as derived from the π + σ plasmon energy, decreases upon H₂ addition to the gas mixture, goes through a minimum at low [H₂] and increases with increasing [H₂]. It reveals a non-monotonous behavior of the film H content as a function of [H₂], the maximum H incorporation occurring at low [H₂]. This appears to be a characteristic of C deposition via graphite sputtering in Ar–H₂ plasma and it is discussed in connection with previous results on the subject.     

Atomic and electronic processes of switching in amorphous thin films

It is proposed that the conversion of C₃⁰ from C₂⁰ leads to switching. Since C₃⁰ has more connection points, the conductivity of C₃⁰ is higher and large current favours the C₃⁰ species; thus increasing the current increases the C₃⁰ density and less voltage is needed to sustain the same amount of current. This explains the S-shaped negative resistance. The concept is simple and the mathematics for quantitative calculation is manageable.     

Effect of the amorphous thin layer on the surface growth of amorphous/crystalline binary multilayer films

The effect of the amorphous thin layer on the surface growth of amorphous/crystalline binary multilayer films has been studied by using a continuum model. It is shown that both the surface roughness and the growth exponent of amorphous/crystalline binary multilayer films decrease with increasing thickness ratio between amorphous and crystalline layers. Our simulations have also revealed, in contrast to the monotonous rise in surface roughness observed in single-layer films grown on flat substrates, the surface growth of a multilayer film consists of two processes: interface smoothing and roughening, namely the film roughness decreases during the growth of amorphous thin layers but increases monotonously during the growth of crystalline thin layers. The observed interface smoothing and roughening can be obviously influenced by the change in the thickness ratio between amorphous and crystalline layers. The rise in thickness ratio between amorphous and crystalline layers enhances the interface smoothing effect but lowers the interface roughening effect and consequently shows a marked smoothing effect on the surface roughness.     

Electron structure investigations of homogeneous hydrocarbon films formed in plasma conditions of T-10 tokamak

Electron structure investigations of smooth hydrocarbon thick films with a high atomic D/C ratio, redeposited from deuterium plasma discharge onto the vacuum vessel regions of the T-10 tokamak, are continued. For the first time, the investigations of both film sides and in a valence band region were performed by X-ray photoemission spectroscopy, combined with “conventional” measurements of survey spectra, C1s, O1s core levels, and by using Auger X-ray electron spectroscopy for sp ²/sp ³ ratio determination. The distinctions of electron structure of both sides of flakes which were noticed earlier using another techniques, were found, differing from the structure of thin films deposited in cleaning discharges of a low-temperature plasma. It is shown how these differences are connected with different processes of film deposition in tokamaks.     

New probe of the electronic structure of amorphous materials

Here we show that electrochemical equilibrium voltage curves of amorphous WO3 and TiO2 coatings exhibit fine structure in striking agreement with the density of states in the conduction bands, as obtained by ab initio calculations for the crystalline counterparts. We suggest that localization of the band states is essential for observing the electronic structure. Our highly sensitive electrochemical method opens new vistas for studying the electronic structure of nonmetallic disordered materials that can be intercalated with an ionic species.     

Effect of cation composition on the superconducting properties and on the microstructure of YBaCuO thin films

The surface morphology and superconducting properties of YBaCuO epitaxial films prepared through magnetron sputtering from targets of different cation composition were systematically studied. It was shown that small changes in the growth conditions and relatively small variations in the cation composition of the condensate noticeably affect the surface morphology of the films and their structural and superconducting properties, thus offering an efficient way of controlling the YBCO film parameters. It was found that the 90° off-axis configuration of the magnetron sputtering system permits realization of growth conditions in which the grown films do not contain CuO precipitates and exhibit good superconducting properties (T_c≥88 K, j_c(77 K)≥4×10⁶ A/cm²).     

Photoemission study of the electronic and chemical structure of amorphous CuxY100-x films

Valence band and core level photoelectron spectra are reported for clean Cu_xY_100-x amorphous thin films grown in situ by sputtering of targets with different stoichiometry. The density of occupied states of these amorphous metallic alloys is shown not to be a simple linear superposition of the constituent densities of states. Experimental evidence shows overlap of Cu and Y d bands with significant electron transfer from Y to Cu atoms which induce large bonding shifts, narrowing and intensity modifications of the valence band. The same conclusion is reached from the Cu core level shift and asymmetry. An estimation of the effective Coulomb interaction on Cu sites from Auger results is attempted.