Publications

We investigate the far-infrared (FIR) absorption of a two-dimensional electron gas in a periodically modulated quantizing magnetic field. The magnetic field varies along only one spatial direction and the external time-dependent electric field is linearly polarized along that axis. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. The effects of the magnetic material on top of the heterostructure as a grating coupler is included in the time-dependent incident FIR electric field. We show that, similar to an electric modulation, the absorption can be directly correlated to the underlying electronic energy bands. In addition, the magnetic modulation leads to absorption spectra with a richer structure due to the quite different static response of the electron density to the modulation. (C) 1998 Academic Press Limited.

The electronic structure of polycrystalline C-60 thin films has been investigated using surface photovoltage spectroscopy (SPS) and conductivity measurements. The films show n-type semiconductivity with an activation energy of approximate to 0.8 eV as found from the temperature dependence of the conductivity at high temperatures. The electronic structure emerging from our SPS results comprises a 1.6 eV photo-conduction gap, a mobility gap of about 2.25 eV and two gap states, a donor and an acceptor, at 0.35 eV and (1.0-1.1) eV, respectively, below the photo-conduction edge. The results indicate the possibility of the existence of band tails, extending into the optical gap of these films, as well as other deep gap states.

The structure of two trinuclear iron acetates [Fe3O(CH3COO)(6)(H2O)(3)]Cl . 6H(2)O (I) and [Fe3O(CH3COO)(6)(H2O)(3)][FeCl4]. 2CH(3)COOH (II) was determined by X-ray diffraction analysis. Crystals I and II are ionic and belong to the orthorhombic system with parameters a = 13.704(3), b = 23.332(5), c = 9.167(2) Angstrom, R = 0.0355, space group P2(1)2(1)2 for I and a = 10.145(4), b = 15.323(6), c 22.999(8) Angstrom, R = 0.0752, space group Pbc2(1) for II. The complex cation [Fe3O(CH3COO)(6)(H2O)(3)](+) has a mu(3)-O-bridged structure typical for trinuclear iron(III) compounds. As shown by Mossbauer spectroscopy, the iron(III) ions are in the high-spin state. In trinuclear cations, antiferromagnetic exchange interaction takes place between the Fe(III) ions with the exchange parameter J = -26.69 cm(-1) for N (Heisenberg-Dirac-Van Vleck model for D-3h symmetry).

The effect of rapid thermal annealing on the structural properties of hydrogenated amorphous silicon (a-Si:H) as a function of annealing time and temperature is studied by spectroscopic ellipsometry in the spectral region of 300-1200 nm, X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) methods. After annealing at 850 and 1050 degrees C, the a-Si:H films are predominantly amorphous and consist of two sublayers which contain a mixture of amorphous and crystalline silicon and voids, the volume fractions of which are temperature-dependent. Annealing of these a-Si:H samples at 1250 degrees C results in a completely crystallized and void-free structure. (C) 1998 Elsevier Science B.V. All rights reserved.

A comparatively study of pulsed - laser and alpha particle irradiation effects in metallic glasses has been performed in order to understand the relationship between magnetic behaviour and select variations in the structural characteristics of alloy phases. Samples of Fe78B13Si9 and Fe66CO18B15Si metallic glasses were irradiated with a pulsed excimer laser (lambda=308nm, tau=10ns) and alpha particle beams (W = 2.8 MeV) using radiation doses of 10(16) and 10(17)cm(-2). Irradiation - driven changes in the magnetic anisotropy and phase equilibrium of alloy samples were studied by Mossbauer Spectroscopy and scanning electron microscopy. The evolution of phases and microstructure during the radiation-induced amorphous-to-crystalline transformations or revitrification depend on the specific irradiation and sample composition.

Electrical properties of adherent, low stressed aluminum nitride films prepared by magnetron sputtering have been studied. Arrhenius plots in the 100 degrees-170 degrees C range of temperature and transient current curves with different bias steps (af room temperature and 150 degrees C) have been performed. The conduction activation energy has been found and ifs dependence on subsequent annealings has been studied. A phenomenological conduction model at the ITO-AIN interface based on the shape of the transient curves has been elaborated The conditions of behaviour of AlN as a highly insulating layer (sputtering conditions, thicknesses, anealing temperatures) have been established.

Six new mu 3-oxo trinuclear iron(III) complexes of the type [Fe3O(R-COO)(6)(H2O)(3)] . X, where X = NO3- or Cl- and R = C-5, C-7, C-8, C-9 were synthesized and investigated by Infra Red, Mossbauer Spectroscopy and magnetic measurements. The data pointed to a high spin state (S = 5/2) for iron and for antiferromagnetic interactions. The asymmetrical shape of the Mossbauer doublet and its temperature dependence was treated using the Plume relaxation model. The relaxation process is influenced by both X and R components.

Based on Tiersten's analysis an accurate calculation of the coupling coefficient dependence on geometrical dimensions and mass-loading for quartz and langasite monolithic structures was performed(I). We have studied the effect of mass-loading on the coupling coefficient of langasite and quartz monolithic filter and we have found that rite affect on langasite filters is lower than on quartz filters. A better agreement between input data and experimental attenuation characteristics is obtained when the rigorous theoretical approach is used in the calculation of the coupling coefficient. The relation between the coupling coefficient and the material constants is discussed on crystals grown by various laboratories.

Interstitial Y(2)Fe(17-x)M(x)C(y) compounds, with M=A1 or Si and x less than or equal to 2, y less than or equal to 1, were investigated by (57)Fe Mossbauer spectroscopy at room temperature. In the analysis of the spectra, the preferential occupation by Al or Si of the Fe (12k) sites in the hexagonal R(2)Fe(17) phase was taken into account. The hyperfine parameters variations are discussed on the grounds of local crystallographic details at the inequivalent iron sites, altered by the composition modifications. The low values of the isomer shifts and quadrupole interactions suggest small electronic charge redistribution in Y(2)Fe(17-x)M(x)A(y) as compared to Y(2)Fe(17). However, the isomer shift variations upon Wigner-Seitz cell volume changes evidence for screening and electron transfer effects. In order to provide a more consistent picture of the local effects of interstitial versus substitutional atoms in R(2)(Fe,M)(17)A(y) compounds, the discussion of the present results is extended to (57)Fe Mossbauer data obtained on the parent compounds and their nitrides. (C) 1997 American Institute of Physics.

The growth of thin platinum silicide layers on Si(111) and (001) surfaces was studied with in situ grazing incidence diffraction and X-ray reflectivity measurements during the annealing process. The interface roughnesses, layer thicknesses, structures and orientations of the three phases, Pt, Pt2Si and PtSi, were identified and their temperature dependencies observed. In the case of PtSi on Si(111), a plane reaction front was found, growing from the Si substrate towards the sample surface and forming an epitactic PtSi layer. In contrast, PtSi on Si(001) grows upwards locally and forms 'islands' of polycrystalline material on the Si surface.