National Institute Of Materials Physics - Romania

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847 articles found

791. Multilayer structures deposited by laser ablation

Authors: Dinescu, M; Stanciu, C; Ghica, D; Dinu, R; Sandu, V; Nastase, N; Balucani, M; Bondarenko, V; Frachina, L; Lamedica, G; Ferrari, A

Published: APR 20 1999, SENSORS AND ACTUATORS A-PHYSICAL, 74, 30, DOI: 10.1016/S0924-4247(98)00331-8

TiN/Si structures were deposited on Si wafers by pulsed laser deposition technique. The highly conductive TiN films were grown on heated (100) Si substrates by laser ablation of a high purity Ti target in nitrogen reactive atmosphere. Subsequently, the Si layer was deposited by laser ablation of a Si target in vacuum (down to 10(-6) mbar) or in low pressure inert gas. The nitrogen gas pressure and the substrate temperature were found to strongly influence the TiN film structure and orientation. The degree of crystallinity of the Si layer grown on the TiN film was found to depend on Si/TiN collector temperature. Values below 550 degrees C (the threshold of TiN oxidation activation) were used in the experiments. Techniques as X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), High Resolution Transmission Electron Microscopy (HRTEM), Scanning Force Microscopy (SFM) have been used to characterize the deposited structures. The TiN/Si structure rectifying properties were tested. The obtained Si/TiN/Si structure could be suitable for the building of Permeable Base Transistor (PBT, vertical MESFET) devices. (C) 1999 Elsevier Science S.A. All rights reserved.

792. Growth of pure and doped Rb2ZnCl4 and K2ZnCl4 single crystals by Czochralski technique

Authors: Stefan, M; Nistor, SV; Mateescu, DC; Abakumov, AM

Published: APR 1999, JOURNAL OF CRYSTAL GROWTH, 200, 154, DOI: 10.1016/S0022-0248(98)01247-0

High-quality single crystals of Rb2ZnCl4 and K2ZnCl4, pure or doped with Cu, Mn, Cd, Tl, Sn, Pb and In cations, were grown by Czochralski technique in argon atmosphere, using an experimental setup that allows direct visual access to the whole growth zone. Slowly cooled crystals exhibit excellent cleavage properties. Fastly cooled crystals do cleave poorly. As shown by X-ray diffraction studies, such K2ZnCl4 samples exhibit inclusions of the high-temperature Pmcn phase with lattice parameters a = 7.263(2) Angstrom, b = 12.562(2) Angstrom and c = 8.960(4) Angstrom in the P2(1) cn room temperature stable phase. ESR and optical spectroscopy studies revealed the localization and valence state of the cation dopants. (C) 1999 Elsevier Science B.V. All rights reserved.

793. Low-loss low-confinement GaAs-AlGaAs DQW laser diode with optical trap layer for high-power operation

Authors: Buda, M; van der Vleuten, WC; Iordache, G; Acket, GA; van de Roer, TG; van Es, CM; van Roy, BH; Smalbrugge, E

Published: FEB 1999, IEEE PHOTONICS TECHNOLOGY LETTERS, 11, 163, DOI: 10.1109/68.740690

A low-confinement asymmetric GaAs-AlGaAs double-quantum-well molecular-beam-epitaxy grown laser diode structure with optical trap layer is characterized. The value of the internal absorption coefficient is as low as 1.4 cm(-1), while keeping the series resistance at values comparable with symmetrical quantum-well gradient index structures in the same material system. Uncoated devices show COD values of 35 mW/mu m. If coated, this should scale to about 90 mW/mu m, The threshold current density is about 1000 A/cm(2) for 2-mm-long devices and a considerable part of it is probably due to recombination in the optical trap layer. Fundamental mode operation is limited to 120-180 mW for 6.5-mu m-wide ridge waveguide uncoated devices and to 200-300 mW for 13.5-mu m-wide ones, because of thermal waveguiding effects, These values are measured under pulsed conditions, 10 mu s/l ms.

794. ESR of paramagnetic atom defects in CVD-grown diamond

Authors: Nistor, SV; Stefan, M; Ralchenko, V; Goovaerts, E; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 149, 307, DOI: 10.1080/10420159908230172

ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-standing polycrystalline diamond films grown by microwave plasma enhanced CVD from CH4/H-2 mixtures under variable deposition conditions. An exponential decrease in the concentration of the N-0 centres with increase in the H-2 flow rate was found. Neither H1 nor H2 centres could be detected.

795. ESR of trapped-electron Fe+ centres in chlorinated SrCl2 crystals

Authors: Nistor, SV; Stefan, M; Bouwen, A; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 149, 207, DOI: 10.1080/10420159908230156

Two Fe+ centres have been identified by ESR after X-ray irradiation at 80 K of chlorinated SrCl2 : Fe2+ crystals. Both centres, with tetragonal symmetry, consist of an Fe+ ion with a neighbouring charged defect along the [001] symmetry axis. The substitutional Fe+ ion of the Fe+(I) centre is strongly off-centre displaced in the Fe+(II) centres, in the middle of a square of four nearest neighbour chlorine ligands.

796. ESR of paramagnetic Tl2+-type centres in Rb2ZnCl4 crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 150, 368, DOI: 10.1080/10420159908226258

Tl2+(6s(1))-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4 : TICl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

797. Room-temperature SiGe light-emitting diodes

Authors: Vescan, L; Stoica, T

Published: DEC 1998, JOURNAL OF LUMINESCENCE, 80, 489, DOI: 10.1016/S0022-2313(98)00160-4

In the present paper the electroluminescence of PIN diodes with either strained SiGe/Si or Ge islands in the i-region has been investigated experimentally and by quantitative modelling. The modelling helped to improve the diode structure. Consequently, diodes with strained Si(0.80)Ge(0.20) could be improved so as to reveal emission up to room temperature, if the thickness was high enough. To overcome the thickness limitation due to plastic relaxation, we used selective epitaxy on small areas. We also present results for diodes with Ge islands in the active region. The internal quantum efficiency of light emitting diodes with strained SiGe was at room temperature similar to 10(-4), while diodes with islands emitted ten times less light. (C) 1999 Elsevier Science B.V. AII rights reserved.

798. Investigation of the W-74 L emission spectra and satellites

Authors: Vlaicu, AM; Tochio, T; Ishizuka, T; Ohsawa, D; Ito, Y; Mukoyama, T; Nisawa, A; Shoji, T; Yoshikado, S

Published: NOV 1998, PHYSICAL REVIEW A, 58, 3551, DOI: 10.1103/PhysRevA.58.3544

The L alpha and L beta spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the L alpha and L beta spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The L beta(2) satellite line observed by Salgueiro er al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the L beta(2) line. The origin of this satellite is discussed experimentally and theoretically. [S1050-2947(98)02011-3].

799. Quantum confinement model for electric transport phenomena in fresh and stored photoluminescent porous silicon films

Authors: Iancu, V; Ciurea, ML

Published: OCT 1998, SOLID-STATE ELECTRONICS, 42, 1896, DOI: 10.1016/S0038-1101(98)00160-9

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are Formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well? whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model. (C) 1998 Elsevier Science Ltd. All rights reserved.

800. Detailed structure of a Pb-doped Bi2Sr2CuO6 superconductor

Authors: Ito, Y; Vlaicu, AM; Mukoyama, T; Sato, S; Yoshikado, S; Julien, C; Chong, I; Ikeda, Y; Takano, M; Sherman, EY

Published: AUG 1 1998, PHYSICAL REVIEW B, 58, 2858, DOI: 10.1103/PhysRevB.58.2851

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.