National Institute Of Materials Physics - Romania

Publications

847 articles found

801. Electrical behaviour of fresh and stored porous silicon films

Authors: Ciurea, ML; Baltog, I; Lazar, M; Iancu, V; Lazanu, S; Pentia, E

Published: JUL 18 1998, THIN SOLID FILMS, 325, 277, DOI: 10.1016/S0040-6090(98)00429-5

We have measured I-V and C-V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon, By storage in ambient, the low rectifying I-V curves become strong rectifying, and C-V curves become MIS-like. I-T characteristics for fresh samples have only one activation energy, in the 0.49-0.55 eV range. After storage, a slightly modified value, of about 0.50-0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20-1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50 degrees C induces small reversible changes in I-T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of porous silicon films. (C) 1998 Elsevier Science S.A. All rights reserved.

802. Characterization of carbon nitride thin films deposited by a combined RF and DC plasma beam

Authors: Dinescu, G; Aldea, E; Musa, G; van de Sanden, MCM; de Graaf, A; Ghica, C; Gartner, M; Andrei, A

Published: JUL 18 1998, THIN SOLID FILMS, 325, 129, DOI: 10.1016/S0040-6090(98)00509-4

Thin carbon nitride films have been deposited on silicon(100) substrates downstream of a nitrogen plasma beam generated in a combined RF (13.56 MHz, 40-50 W) and DC (voltage +/-200 V, power 1-10 W) discharge between a graphite electrode and a graphite nozzle. By combining the RF and DC sources the capability of rf field to create extended plasmas is used together with the enhanced sputtering and biasing effect of the DC source. The plasma characteristics (electron temperature, presence of molecular species) have been studied by optical emission spectroscopy. Deposition rates of 2.5-3 nm/s are obtained at the centre of the plasma beam and at a few centimetres distance from the nozzle. The films have been investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, scanning and transmission electron microscopy, and microhardness measurements. The films have an overall N:C ratio of 0.28 but the distribution of different nitrogen bonds depends upon the DC bias conditions. In the spectral range 0.3-0.7 mu m the refractive index increases slightly from 1.5 to 2.2. The films are amorphous, with morphology consisting of a columnar structure. The columns have a diameter of about 20 nm. A hardness of 24 GPa has been measured. (C) 1998 Elsevier Science S.A. All rights reserved.

803. Electron and hole trapping in PbCl2 and PbCl2 : Tl crystals

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

Published: MAY 1998, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 141, 541, DOI: 10.1016/S0168-583X(98)00051-2

Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb-2(3+) self-trapped electron (STEL) centers the PbCl2 : Tl crystals exhibit trapped-electron (PbTl)(+)-type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl-2 divided by centers the holes are self trapped at pairs of neighbouring Cl- anions resulting in V-k type centers with various orientation and length of the Cl-Cl axis. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

804. The molecular structure of some urea and thiourea derivatives

Authors: Bally, I; Simion, C; Mazus, MD; Deleanu, C; Popa, N; Bally, D

Published: APR 20 1998, JOURNAL OF MOLECULAR STRUCTURE, 446, 68, DOI: 10.1016/S0022-2860(97)00400-6

The molecular structures for N-phenyl-N'-acetylurea, N-phenyl-N'-capronylthiourea, and N - phenyl - N' - benzoyl thiourea have been characterized by X-ray diffraction and NMR spectroscopy. The relative positions of atoms in the parent molecular fragment, common to all three analyzed compounds, are discussed. (C) 1998 Elsevier Science B.V.

805. Electroluminescence of strained SiGe/Si selectively grown above the critical thickness for plastic relaxation

Authors: Stoica, T; Vescan, L; Goryll, M

Published: MAR 15 1998, JOURNAL OF APPLIED PHYSICS, 83, 3373, DOI: 10.1063/1.367104

Electroluminescence of strained Si0.80Ge0.20Si(001) pin diodes has been investigated experimentally and by quantitative modeling. The key aspect of this investigation was that by selective epitaxial growth the experimental critical thickness for plastic relaxation (80 nm at T-epi=700 degrees C and large areas) could be increased in finite pads. SiGe layers with thickness of 60, 72 or 370 nm have been grown within the intrinsic i region of pin structures. Samples free of misfit dislocations revealed electroluminescence with the SiGe no-phonon peak and its transversal optical-phonon replica corresponding to interband transitions. It was found that by increasing the thickness of the SiGe layer the drop in the electroluminescence with increasing temperature could be shifted to higher temperature, so that for the 370 nm thick SiGe sample the emission was observed to persist still at 300 K. Modeling based on drift-diffusion and carrier recombination equations was used to simulate the current-voltage characteristics of the pin diodes and their band gap electroluminescence. It was found that the modeling results can account for the temperature and thickness dependence of the electroluminescence. Hole and electron Shockley-Read-Hall recombination times could be evaluated. (C) 1998 American Institute of Physics.

806. Selective epitaxial growth of strained SiGe/Si for optoelectronic devices

Authors: Vescan, L; Stoica, T; Goryll, M; Grimm, K

Published: FEB 27 1998, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 51, 169, DOI: 10.1016/S0921-5107(97)00253-5

Two aspects of the selective epitaxial growth of Si and Si(1-x)Ge(x) will be discussed. First the facet formation as dependent on the oxide wall orientation and the lateral size of oxide openings. Besides the often cited {111}, {113} and {110} planes additional planes were observed, the {119} and the {0 1 12} planes. Second, the reduction of misfit dislocation density by reducing the area of the pads allows strained Si(1-x)Ge(x) layers to grow much thicker than the critical thickness. As an application for the latter the electroluminescence of forward biased PIN diodes with strained Si(0.80)Ge(0.20)/Si(001) will be discussed as being dependent on the thickness of the SiGe layer. It was found that in thicker strained samples the band edge electroluminescence persists up to room temperature. Quantitative modelling of the electroluminescence could explain the temperature and SiGe thickness dependence of the electroluminescence. (C) 1998 Elsevier Science S.A. All rights reserved.

807. X-ray-absorption features from multielectron excitations above Xe L edges

Authors: Ito, Y; Vlaicu, AM; Tochio, T; Mukoyama, T; Takahashi, M; Emura, S; Azuma, Y

Published: FEB 1998, PHYSICAL REVIEW A, 57, 878, DOI: 10.1103/PhysRevA.57.873

The x-ray-absorption cross section above the L edges in xenon gas has been measured using synchrotron radiation. Multielectron excitation effects are investigated over a few hundred eV region from the Xe L-I, L-II, and L-III edges and the contributions from the effects of shake-up and shake-off to the photoabsorption spectrum are elucidated. Several multielectron excitation features were detected and analyzed by the theoretical energy of the shake-up process. Previous observations of the [2(s,p)5(s,p)] and [2(s,p)4d] transitions have been confirmed. [2(s,p)4p] transitions are clearly identified in the present study. It is very difficult to detect the [2s3s] multielectron transition with the sensitivity of similar to 2.0 x 10(-4) at the L edge jump. The [2s3d] edge was distinguished using the first derivative of the absorption curve. A new transition is found between [2(s,p)4p] and [2(s,p)4d] transitions and identified as a three-electron transition [2(s,p)4d5p]. Moreover, the difference in the behavior of the multielectron excitation as a function of x-ray energy among the L-I and L-II, or L-III edges is confirmed and investigated. [S1050-2947(98)01802-2].

808. Theoretical model for carriers transport in nanocrystalline porous silicon films

Authors: Ciurea, ML; Iancu, V; Pavelescu, G; Baltog, I

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 112, DOI:

A quantum confinement model is proposed to explain the carriers transport in a nanowire network of porous silicon. The electron Hamiltonian is written as the sum of the longitudinal and the transversal contributions. The last one corresponds to a two-dimensional infinite cylindrical quantum well, whose energy levels determine the activation energies observed in the temperature dependence of the dark current. The model is in excellent agreement with the experimental data.

809. Change of the optical properties of porous silicon by post anodization treatments

Authors: Pavelescu, G; Ciurea, ML; Mihut, L; Galeata, G; Lengyel, E; Baltog, I; Roger, JP

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 458, DOI:

Optical properties of as prepared porous silicon (PS) were compared with those of thermally treated and chemically cleaned samples. The change of the photoluminescence (PL) spectra correlated with ellipsometric. reflectance and Raman measurements leads us to conclude that the adsorbed species in the PS nanocrystalline structure play an important role in the origin of the PI emission, at least for as - anodized samples.

810. 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.