National Institute Of Materials Physics - Romania

Publications

847 articles found

781. EPR observation of first point defects in cubic boron nitride crystalline powders

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

Published: 2000, SOLID STATE COMMUNICATIONS, 115, 44, DOI: 10.1016/S0038-1098(00)00135-6

An X-band electron paramagnetic resonance (EPR) study of nominally pure, diamond-like cubic boron nitride (c-BN) crystalline powders, has led to the first identification of a spectrum attributed to two related paramagnetic species. The composite EPR spectrum can be observed only in dark brown colored powders known to contain excess of boron. It consists of two superimposed lorentzian components, called D1 and D2, centered at g1 = 2.0063 and g2 = 2.0084, with peak-to-peak linewidths of 3.3 and 17.9 mT, respectively. The temperature dependence of the integrated intensities, their linewidths and intensity ratio D2/D1 allows one to conclude that the narrow line D1 originates from EPR transitions inside a S = 1/2 ground doublet and the broad line D2 from transitions inside the excited levels of another center. Evidence suggests that both centers are boron related paramagnetic species. (C) 2000 Elsevier Science Ltd. All rights reserved.

782. Electrical properties of nanocrystalline porous silicon

Authors: Ciurea, ML; Iancu, V

Published: 2000, 2000 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, CAS 2000 PROCEEDINGS, 64, DOI: 10.1109/SMICND.2000.890188

Conduction properties as well as trapping phenomena in nanocrystalline porous silicon films are investigated in relation to their microstructure. Theoretical models to describe the phenomena are proposed. The main results presented in this paper are: the two-scale porosity of our porous silicon films, the decisive role of the quantum confinement to the conduction phenomena and the prominent part played by the oxidation on the surface effects.

783. Characterization of ITO thin films prepared by spinning deposition starting from a sol-gel process

Authors: Stoica, TF; Stoica, TA; Zaharescu, M; Popescu, M; Sava, F; Popescu-Pogrion, N; Frunza, L

Published: 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 688, DOI:

ITO thin films were deposited by spin coating method starting from indium and tin propoxide in the frame of a sol-gel process. Films with the thickness corresponding to 68 nm for one layer deposition were produced and characterized by X-ray diffraction, transmission electron microscopy and infrared reflection spectroscopy. Low conductivity polycrystalline films exhibiting cubical structure (bixbyite-type) and good adherence were obtained.

784. Room-temperature light-emitting diodes with Ge islands

Authors: Vescan, L; Chretien, O; Stoica, T; Mateeva, E; Muck, A

Published: OCT-DEC 2000, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 3, 387, DOI: 10.1016/S1369-8001(00)00059-7

There is an increasing interest in Si-based optoelectronics using Si1-xGex nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for possibilities to increase the contribution of the radiative recombination to the emission. One possible approach involves self-organised growth of lattice-mismatched layers. In the present paper, p-i-n structures, using one layer with Ge islands and which emit in the near infrared up to room temperature were fabricated. The self-organised growth of Ge was performed at 700 degreesC with a small coverage (9 ML) so as to avoid plastic relaxation of the islands, but with a high growth rate (0.3 ML/s) which leads to the formation of a broad bimodal island distribution (small- and medium-sized islands). The diode structure including the Ge islands was deposited in the form of mesas using selective epitaxial growth by low-pressure chemical vapour deposition. The mesa areas were Varied with the aim of demonstrating the influence of size distribution of the islands on the light emission. At low current density the emission is dominated by islands with smaller band gap (larger valence band offset) while at higher currents emission from islands with larger band gap takes place. From the comparison of single diodes with arrays of small-area diodes with the same total area it is found that the arrays emit three times more light due to the lower total number of deep traps in each diode. (C) 2001 Published by Elsevier Science Ltd.

785. Microstructural aspects related to carriers transport properties of nanocrystalline porous silicon films

Authors: Ciurea, ML; Teodorescu, VS; Nistor, LC; Blanchin, MG

Published: SEP 1999, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 146, 3521, DOI: 10.1149/1.1392507

A microstructural investigation of porous silicon films, related with a study of carriers transport. is presented. Scanning electron microscopy and conventional transmission electron microscopy with selected area electron diffraction were performed. The films show a double scale porosity: the macroporosity forms an alveolar structure at a micrometric scale and the nanoporosity forms a silicon nanowires network in the alveolar walls and at their surface. Current-voltage characteristics in the -5 to +5 V range and the temperature dependence of dark current in the 150-330 K interval at low voltage were measured. The transport properties are explained by means of a simplified quantum confinement model. Two symmetries of the nanowires, cylindrical and square, are investigated and the differences between them are shown to be insignificant. (C) 1999 The Electrochemical Society. S0013-4651(98)12-109-2. All rights reserved.

786. Weak localization effects in ZnO surface wells

Authors: Goldenblum, A; Bogatu, V; Stoica, T; Goldstein, Y; Many, A

Published: AUG 15 1999, PHYSICAL REVIEW B, 60, 5838, DOI: 10.1103/PhysRevB.60.5832

Hall effect, magnetoresistance, and electrical conductivity measurements, carried out on ZnO surface wells created by a large variety of methods, are analyzed in the frame of the weak-localization theory. The ZnO surface wells have some unique features that allow the investigation of the weak-localization effects: ZnO has a single valley conduction band; the Thouless length is much larger than the elastic mean-free path even at room temperature; the well accumulates the largest surface electron concentration obtained up to now in a surface quantum well; there are a large variety of preparation methods, some of them making it possible to modify independently both the width and the depth of the surface wells. These features allowed us to investigate: the presence of the weak-localization effect in the largest range of temperatures(1.6-300 K) reported up to now for a quantum well; the influence on the transport properties of the increase in the number of subbands in the well; the effect of the presence of more inelastic scattering mechanisms and their weights in the entire scattering process; and the passage from a quasi-two-dimensional system to a three-dimensional one. [S0163-1829(99)01932-3].

787. Off-center displacement of Fe+ ions in irradiated SrCl2 : Fe crystals grown in chlorine

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

Published: AUG 1999, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 214, 236, DOI: 10.1002/(SICI)1521-3951(199908)214:2<229::AID-PSSB229>3.0.CO;2-H

Chlorinated SrCl2:Fe2+ crystals exhibit, after X-ray irradiation, two trapped-electron Fe+(I) and Fe+(II) centers both with axial [001] symmetry, but different EPR spectrum parameters and production properties. The analysis of the experimental data strongly suggests a substitutional eight-fold coordination of the Fe+ ion in the Fe+(I) center and a strong [001] off-center displacement to a fourfold coordinated site in the Fe+(II) center resulting in S = 3/2 and S = 1/2 ground states, respectively.

788. 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.

789. 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.

790. 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.