National Institute Of Materials Physics - Romania

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886 articles found

851. On the photoluminescence decay in porous silicon films

Authors: Baltog, I; Ciurea, ML; Pavelescu, G; Mihut, L; Baibarac, M

Published: 1998, FIFTH CONFERENCE ON OPTICS (ROMOPTO '97), PTS 1 AND 2, 3405, 626, DOI: 10.1117/12.312818

The photoluminescence (PL) decay measurements were performed on porous silicon films. It was observed that the two components of FL, one of them fast (ns) and the other slow (mu s or ms sometimes) have different contributions to PL signal, depending on the wavelength of the excitation light. The slow component of PL was in details investigated. Time decay cures for different excitation (337.1 nm, 470 nm and 550nm) and emission (550, 650, 700, 800 and 860 nm) wavelengths and also for different excitation intensities were taken. All decay curves were fitted with a stretched exponential. The slow component of PL was proposed to be attributed to the radiative recombination on surfaces.

852. Physics of optimal resonant tunneling

Authors: Racec, PN; Stoica, T; Popescu, C; Lepsa, M; vandeRoer, TG

Published: AUG 15 1997, PHYSICAL REVIEW B, 56, 3597, DOI: 10.1103/PhysRevB.56.3595

The optimal resonant tunneling, or the complete tunneling transparence of a biased double-barrier resonant-tunneling (DBRT) structure, is discussed. It is shown that its physics does not rest on the departure from the constant potential within the barriers and well, due to the applied electric field, but on the effective symmetry of the rectangular-barrier profile, which approximates the real potential profile for the corresponding applied bias.

853. Localization and charge conversion of copper in Rb2ZnCl4:Cu crystals: An ESR and optical absorption study

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

Published: AUG 1997, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 202, 1011, DOI: 10.1002/1521-3951(199708)202:2<999::AID-PSSB999>3.0.CO;2-5

Electron spin resonance and optical studies reveal the presence of both Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagnetic Cu2+ centers with different concentrations and production properties have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedron. The less abundant Cu2+(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, part of it being afterwards converted to Cu+ precursor centers. The presence of a neighboring-charge compensating anion vacancy and its departure during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) centers, respectively.

854. Amorphous rf-sputtered Si100-xNix thin films with O<=x<=15 at.%: Structural, optical and electrical properties

Authors: BeluMarian, A; Serbanescu, MD; Manaila, R; Stoica, T; Dragomir, A; Zavaliche, F; Tanase, M; Devenyi, A

Published: JUN 1 1997, THIN SOLID FILMS, 301, 202, DOI: 10.1016/S0040-6090(97)00007-2

The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.

855. Electrical properties of porous silicon stabilised by storage in ambient

Authors: Ciurea, ML; Lazar, M; Lazanu, S; Pentia, E; Dragoi, V

Published: 1997, CAS '97 PROCEEDINGS - 1997 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 20TH EDITION, VOLS 1 AND 2, 180, DOI:

The electrical transport of stabilised porous silicon layers was investigated. The samples were stored in ambient for 1,5-2 years, A MIS-like C-V characteristic, a strong rectifying I-V curve and I-T dependence with two activation energies were obtained, Thermally stimulated depolarization currents have an activation energy of 0.81-0.87 eV. The electrical properties are discussed in the frame of a quantum confinement model, keeping into account the surface component.

856. Visible photoluminescence in porous silicon prepared in different conditions - Temperature dependence and decay

Authors: Ciurea, ML; Pentia, E; Manea, A; BeluMarian, A; Baltog, I

Published: JUN 1996, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 195, 645, DOI: 10.1002/pssb.2221950230

Measurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p-type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72 eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time alpha(-1) is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.

857. Electrical and structural properties of anodized porous silicon

Authors: Ciurea, ML; Pentia, E; Lazar, M; BeluMarian, A; Zavaliche, F; Manaila, R

Published: 1996, CAS '96 PROCEEDINGS - 1996 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 19TH EDITION, VOLS 1 AND 2, 236, DOI:

858. INTERFACE TRAPPING STATES IN MISIM STRUCTURES, WITH ZNS-MN

Authors: PETRE, D; PINTILIE, I; CIUREA, ML; BOTILA, T

Published: MAY 1 1995, THIN SOLID FILMS, 260, 57, DOI: 10.1016/0040-6090(94)06467-9

Optical charging spectroscopy and thermally stimulated depolarization measurements were performed on metal-insulator-semiconductor-insulator-metal structures with a ZnS:Mn active layer. Two groups of trapping centres situated at every insulator-semiconductor interface were in evidence. Their depths were found to be centred at 0.55 eV and 0.875 eV respectively in the forbidden gap of a ZnS:Mn film. A partial thermal healing of these interface states was observed.

859. Modified ruthenium exchanged zeolites for enantioselective hydrogenation

Authors: Parvulescu, VI; Parvulescu, V; Coman, S; Radu, C; Macovei, D; Angelescu, E; Russu, R

Published: 1995, PREPARATION OF CATALYSTS VI: SCIENTIFIC BASES FOR THE PREPARATION OF HETEROGENEOUS CATALYSTS, 91, 570, DOI:

Preparation of modified ruthenium molecular sieves has been investigated in two steps deposition of ruthenium and modifying of ruthenium molecular sieves in presence of ligands. As support sieve there were used two molecular sieves with large pore apertures and low acidity strength (zeolite L and APO-34). Correlation of in situ W-VIS ruthenium deposition measurements with catalyst characterisation revealed that ruthenium deposition takes place not only through ionic exchange but also through adsorption of ruthenium hydrolysed species. Modifying of the ruthenium molecular sieves catalysts with ligands has as effect a diminution of electronic charge on metal. The experimental data indicate that the presence of ligand favours an enantioselective hydrogenation of D-fructose to D-mannitol even if the yields are not very high.

860. EFFECTS OF ANNEALING ON THE CONDUCTIVITY OF C-60 THIN-FILMS

Authors: BELUMARIAN, A; MANAILA, R; STOICA, T; DRAGOMIR, A; MANCIU, M; DEVENYI, A; BRAUN, T

Published: 1995, FULLERENE SCIENCE AND TECHNOLOGY, 3, 509, DOI: 10.1080/153638X9508543803

Films of C-60, at different stages of annealing of T-c = 200 degrees and 300 degrees C have been electrical characterized over the temperature domain from -130 degrees C to T-c. X-ray diffraction revealed a random polycrystalline fee structure with stacking defects of an intrinsic nature, due to deposition conditions. The value of room-temperature conductivity was found to be in the range (6.3-1.0)*10(-10) (Omega cm)(-1). In the stable annealed state the conductivity showed an activated temperature dependence above 423 K and a non-activated dependence below 330-280 K. The activation energies E(2) 0.8 eV (film thickness 0.70 mu m) and E(2) = 1.0 eV (film thickness 2.40 mu m) were in good agreement with the energy gap values (1.63 eV and 2.08 eV) which were deduced from the al;sorption spectral dependence. Annealing decreased the non-activated contribution to conduction, extending the intrinsic conduction temperature range.