National Institute Of Materials Physics - Romania

Publications

860 articles found

801. Colloidal sol-gel ITO films on tube grown silicon

Authors: Stoica, TF; Stoica, TA; Vanca, V; Lakatos, E; Zaharescu, M

Published: JUL 6 1999, THIN SOLID FILMS, 348, 278, DOI: 10.1016/S0040-6090(99)00136-4

Electrical and optical properties of colloidal PTO sol-gel films on glass substrates have been investigated. The optical gap of 3.14 eV and room temperature conductivity of 4 Ohm(-1) cm(-1) were obtained on ITO layers of about 500 Angstrom thickness. Heterostructures of ITO/n-Si have been obtained by colloidal sol-gel deposition of ITO. In this deposition method, the oxidation of silicon during the heterostructure formation can be avoided. The silicon substrates were cut out of silicon tubes. Electrical and photoelectric properties of these structures have been studied in comparison with those of p-n tube-Si junctions obtained by thermal boron diffusion. On ITO/n-Si heterojunctions, the spectral quantum efficiency has a maximum value of about two times higher and a blue shift of the maximum position from 0.75 to 0.6 mu m, than those for p-n junctions. A theoretical investigation of the I-V curves for ITO/n-Si heterojunctions under various illumination levels has been performed and contribution of the tunneling and series resistance phenomena were evaluated. (C) 1999 Elsevier Science S.A. All rights reserved.

802. Growth of carbon nickel multilayer for X-ray-UV optics by RF reactive magnetron sputtering

Authors: Georgescu, G; Nelea, V; Ulmeanu, M; Ghica, C; Nastase, N

Published: JUL 1999, APPLIED SURFACE SCIENCE, 148, 146, DOI: 10.1016/S0169-4332(99)00219-6

Amorphous C/Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown on float-glass substrates by magnetron sputter deposition in Ar discharge. A comprehensive set of characterization techniques has been applied: grazing X-ray reflection (0.154 nm), atomic force microscopy and transmission electron microscopy in order to determine the quality of the structure. By comparing the results, it could be concluded that RF-magnetron sputtering technique is a good choice for growing such layered synthetic microstructures. (C) 1999 Elsevier Science B.V. All rights reserved.

803. Transmission electron microscopy study of thin films and surface implanted wafers of LiNbO3

Authors: Teodorescu, VS; Ghica, C; Blanchin, MG; Moretti, P

Published: JUN 1999, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 1, 48, DOI:

Thin films prepared by pulsed laser deposition and single crystal implanted LiNbO3 wafers used in optoelectronic technology were studied by transmission electron microscopy. The use of high electron beam density for high resolution imaging induced a decomposition effect in the implanted LiNbO3 lattice. It is shown that the most probable decomposition leads to the formation of NbO2 nuclei by a topotactic solid state reaction along the pseudo-cubic directions of the lithium niobate matrix. It is also shown that powdered LiNbO3 specimens or samples obtained by preparation of cross section of LiNbO3 thin films are more stable in the electron beam, compared with the samples obtained from implanted LiNbO3 wafers.

804. 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.

805. 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.

806. 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.

807. 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.

808. 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.

809. 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.

810. 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.