National Institute Of Materials Physics - Romania

Publications

847 articles found

681. (Te/SnSe2)(3) multilayers deposited by pulsed laser deposition. Structure and gas sensing properties

Authors: Lorinczi, A; Sava, F; Tomescu, A; Simion, C; Socol, G; Mihailescu, IN; Popescu, M

Published: NOV 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3492, DOI:

Multilayers made of alternative layers of composition SnSe2 and tellurium (three pairs) have been prepared by pulsed laser deposition (PLD). The complex films have been investigated by X-ray diffraction. After a special annealing in oxygen fluence at various temperatures, up to 620 degrees C, the films were investigated both by X-ray diffraction and for gas sensing. Gas sensing properties were tested for water vapours, carbon monoxide (CO) and methane (CH4). After annealing at high temperature, the structure of the multilayer samples show oxide phases like TeO2, SnO2 and a mixed oxide ternary phase SnTe3O8. The annealed multilayers show good sensing properties to water vapours and CO gas.

682. Ge dots embedded in silicon dioxide using sol-gel deposition

Authors: Stoica, TF; Gartner, M; Teodorescu, VS; Stoica, T

Published: OCT 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3274, DOI:

A versatile method, that of sol-gel deposition was used to produce Si1-xGexO2 films on Si substrates. Samples after annealing at different temperatures are investigated by different techniques including high-resolution transmission electron microscopy and spectral ellipsometry. It was revealed that, by annealing at high temperatures, Ge accumulates in small clusters (5-10nm) inside of the oxide layer. By ellipsometric investigations, valuable data about the temperature evolution of the composition and optical constants were obtained.

683. Trapping phenomena in silicon-based nanocrystalline semiconductors

Authors: Ciurea, ML; Iancu, V; Mitroi, MR

Published: OCT 2007, SOLID-STATE ELECTRONICS, 51, 1337, DOI: 10.1016/j.sse.2007.07.002

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.

684. Thickness dependence of crystallization process for hydroxyapatite thin films

Authors: Mercioniu, I; Ciuca, S; Pasuk, I; Slav, A; Morosanu, C; Bercu, M

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2538, DOI:

Hydroxyapatite (HAP) thin films (0.2 pm, 0.3 pm and 1.2 mu m thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 mu m/h). Crystalline HAP films were obtained using a low temperature (100 degrees C) followed by post-deposition annealing at 300 degrees C, 450 degrees C, 500 degrees C and 550 degrees C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.

685. Phototransport and photoluminescence in nanocrystalline porous silicon

Authors: Iancu, V; Ciurea, ML; Stavarache, I; Teodorescu, VS

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2643, DOI:

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

686. Percolation phenomena in Si-SiO2 nanocomposite films

Authors: Stavarache, I; Ciurea, ML

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2647, DOI:

Current-voltage characteristics of samples containing silicon nanodots embedded in an amorphous silicon dioxide matrix were investigated. A percolation mechanism was evidenced by the dependence on the concentration of the initial differential conductance and by the appearance of several thresholds in the I-V characteristics.

687. Nanocrystalline Er : YAG thin films prepared by pulsed laser deposition: An electron microscopy study

Authors: Stanoi, D; Popescu, A; Ghica, C; Socol, G; Axente, E; Ristoscu, C; Mihailescu, IN; Stefan, A; Georgescu, S

Published: JUL 31 2007, APPLIED SURFACE SCIENCE, 253, 8272, DOI: 10.1016/j.apsusc.2007.02.113

We report the synthesis of well crystallized stoichiometric Er:YAG thin films by pulsed laser deposition from Er:YAG targets in low pressure oxygen, followed by a post-deposition treatment at 1400 degrees C in environmental air for 19 h. The structure and morphology of the films were examined by transmission electron microscopy and selected area electron diffraction. We demonstrated that the subsequent application of pulsed laser deposition and heat treatment resulted in the formation of well crystallized single phase cubic Er:YAG nanostructured coatings. (c) 2007 Elsevier B.V. All rights reserved.

688. Structure, properties and gas sensing effect of SnSe2 films prepared by pulsed laser deposition method

Authors: Popescu, M; Sava, F; Lorinczi, A; Socol, G; Mihailescu, IN; Tomescu, A; Simion, C

Published: JUN 15 2007, JOURNAL OF NON-CRYSTALLINE SOLIDS, 353, 1869, DOI: 10.1016/j.jnoncrysol.2007.02.055

Amorphous SnSe, films were prepared by pulsed laser deposition (PLD) from solid polycrystalline targets. The atomic scale structure has been revealed by X-ray diffraction. Hardness and electrical properties were measured. Strong gas sensing properties have been evidenced for the first time in heat treated films. (c) 2007 Elsevier B.V. All rights reserved.

689. Conduction mechanism involved in sensing mechanism of SnSe2

Authors: Tomescu, A; Simion, CE

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1897, DOI:

Simple investigation of the electrical resistance does not permit to draw up the conduction mechanism of thin SnSe2 layers. The presence of reducing gas atmospheres, respectively CH4, CO and H2O could change the concentration of the free charge carriers, beside electron affinity changes induced by the appearance of surface dipoles. Work function and electrical resistance were simultaneous investigated for thin SnSe2 films. The experimental results permit the evaluation of electron affinity and band-bending behavior. Conduction mechanism can be discussed in terms of surface or bulk contribution.

690. Varieties of nanostructured carbon grown by expanding radiofrequency plasma beam

Authors: Vizireanu, S; Mitu, B; Dinescu, G; Nistor, L; Ghica, C; Maraloiu, A; Stancu, M; Ruxandra, G

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1652, DOI:

Results regarding the growth of carbon nanostructures by using a low-pressure expanding radiofrequency plasma jet are reported. The growth was performed in an argon plasma injected with acetylene precursor, in the presence of ammonia or hydrogen, on a substrate covered with nickel or cobalt catalysts. Various forms of nanostructured carbon material were obtained, depending on the catalyst size, ammonia or hydrogen / acetylene ratio and substrate temperature. The results exemplified in the paper describe nickel nanoparticles coated in highly ordered graphite shells, granular carbon, two dimensional carbon nanoflakes, nanowires and nanorods.