National Institute Of Materials Physics - Romania

Publications

860 articles found

681. Investigation of nanocrystalline Pbs/n-Si heterostructures for optoelectronic applications

Authors: Buda, M; Stancu, V; Iordache, G; Pintilie, L; Pintilie, I; Buda, M; Botila, T

Published: FEB 15 2008, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 147, 288, DOI: 10.1016/j.mseb.2007.09.070

The nano-crystalline PbS/n-Si heterostructure was studied using photocurrent spectra, I-V and C-V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size. (c) 2007 Elsevier B.V. All fights reserved.

682. Characterization of high performance PbS photodetectors

Authors: Buda, M; Iordache, G; Stantcu, V; Botila, T

Published: FEB 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 310, DOI:

Dark noise plays an important role in electronic devices, especially in detectors. Performance of infrared detectors is reflected by parameters such as detectivity and specific detectivity, strongly related to the noise spectral density. This work summarizes the results on characterization of high-performance PbS photo detectors, using a certification pending method developed by the laboratory, in agreement to the standard ISO/CEI 17025:2005. The method is validated for a resistor of known resistance, by comparing the Johnson thermal noise using two different methods: first, the fast Fourier transform measured with a Stanford Research Systems SR785 Spectrum Analyzer and second using a Stanford SR830 lock-in amplifier which measures the rms noise for a certain frequency. We studied the influence of physical dimensions on the performance of thin film polycrystalline PbS photoresistors in the planar geometry. Measurements were performed to compare the dark noise of PbS photoresistors with that in high quality metal thin film resistors. The laboratory setup used for the measurements is appropriate to accurately determine typical parameters of infrared detectors such as spectral responsivity, blackbody responsivity, specific detectivity, and noise spectral density.

683. Capacitance spectroscopy study of InGaAs/GaAs quantum dot structures

Authors: Buda, M; Iordache, G; Mokkapati, S; Tan, HH; Jagadish, C; Stancu, V; Botila, T

Published: FEB 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 326, DOI:

Self assembled InGaAs/GaAs quantum dots (QD) have a great potential for high performance optoelectronic devices such as low threshold laser diodes, infrared detectors, modulators, memories. In order to characterize the behavior of the QD system, we use two p(+)-n structures grown epitaxially on GaAs under similar conditions. The first structure acts as reference while in the second structure a single QD self-assembled layer is introduced in the middle of the n-GaAs matrix layer. The structure is designed such that for OV applied bias the QD layer lies outside the depleted region. When the reverse bias is increased, the charge from the QD system is removed and the depletion layer moves further into the GaAs matrix material. The electronic structure of the QD is investigated using two methods: photoluminescence, in order to characterize the transition energies between electron and hole levels in the QD system and capacitance spectroscopy in order to study the electron levels in the conduction band only. In addition, admittance spectroscopy spectra are measured in order to characterize the carrier transport mechanism. There is no evident step in the capacitance versus frequency behavior at room temperature in the range 1 Hz-1 MHz, indicating a large carrier cross section caption and/or a low activation energy for the carrier transport between the dot system and the wetting layer and GaAs barriers. The plateau in the C-V behavior, due to charging or discharging of the QD system is modeled using the solution of a Poisson equation and the resulting energy of the electron states within the conduction band and QD size distribution are correlated with results from photoluminescence studies, which involve transitions between energy levels both from the conduction and valence bands.

684. Rough Bioglass films prepared by magnetron sputtering

Authors: Slav, A; Ianculescu, A; Morosanu, C; Saranti, A; Koutselas, I; Agathopoulos, S; Karakassides, MA

Published: 2008, BIOCERAMICS, VOL 20, PTS 1 AND 2, 361-363, +, DOI: 10.4028/www.scientific.net/KEM.361-363.245

Bioglass films were prepared by reactive magnetron sputtering from a 45S5 target and than annealed in air tip to 550 T in order to relax the mechanical stresses. The mixtures (0, 7 and 15%) of oxygen and argon were used in order to modify the films stoichiometry. Morphological and compositional studies were performed by SEM coupled with an EDAX system. FTIR and micro Raman measurements were performed in order to investigate the molecular vibrations of the BG structure. A modified composition respect to the target one Was found for all the films, with lower silicon dioxide concentrations. The films displayed rough surfaces with spherical particles of 10-20 mu m diameter embedded in the surfaces. The Raman results evidenced important carbonate lines besides the silica-phosphate bands.

685. Influence of polyvinylpyrolidone as an additive in electrochemical preparation of ZnO nanowires and nanostructured thin films

Authors: Enculescu, I; Matei, E; Sima, M; Enculescu, M; Sima, M; Ghica, C

Published: MAR-APR 2008, SURFACE AND INTERFACE ANALYSIS, 40, 560, DOI: 10.1002/sia.2749

Preparation of ZnO in different nano-morphological forms became a hot topic during the last few years. This tendency was motivated by the wide field of potential applications ranging from optics to electronics and spintronics. In the present work, we deal with the preparation of ZnO nanowires and nanostructured thin films using electrochemical deposition. ZnO nanowires were grown using the template approach, namely by replicating the nanopores of polycarbonate ion track membranes. The method yields uniform arrays of nanowires with the morphology controlled by the shape and size of the templates pores. Adding polyvinylpyrolidone as an additive in the growth bath became a necessity due to the fact that polycarbonate ion track membranes are usually hydrophobic and thus filling of the nanopores with the growth electrolyte is rather difficult. In order to better understand the influence of the additive on the electrochemical deposition of such nanostructures, similar experiments were performed for the deposition of ZnO thin films. Scanning electron microscopy, X-ray diffraction and optical spectroscopy measurements were performed for characterization of the deposited structures. Copyright (c) 2008 John Wiley & Sons, Ltd.

686. MODELING OF TRAP DISCHARGING PROCESSES IN MULTIPLE QUANTUM WELL STRUCTURES

Authors: Ciurea, ML; Iancu, V; Stavarache, I; Lepadatu, AM; Rusnac, E

Published: 2008, CAS: 2008 INTERNATIONAL SEMICONDUCTOR CONFERENCE, PROCEEDINGS, +, DOI: 10.1109/SMICND.2008.4703332

The paper presents the modeling of trap discharging processes in Multiple Quantum Well nanostructures. The coupling between trapping and detrapping phenomena, due to the CaF2 buffer layers is discussed. The relative role of tunneling and displacement currents is also analyzed. The model allows the determination of trap parameters that are not directly measurable. The results are in good agreement with the experimental data.

687. Functional ZnO thin films obtained by radiofrequency beam assisted pulsed laser deposition

Authors: Epurescu, G; Scarisoreanu, ND; Matei, DG; Dinescu, G; Ghica, C; Nistor, LC; Dinescu, M

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, +, DOI:

Piezoelectric and p type semiconductor ZnO thin films were deposited by radiofrequency beam assisted pulsed laser deposition. Optical, electrical and structural properties were found to depend on both deposition parameters and radiofrequency beam characteristics.

688. Defects in silicon: From bulk crystals to nanostructures

Authors: Ciurea, ML; Iancu, V; Lazanu, S; Lepadatu, AM; Rusnac, E; Stavarache, I

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, 748, DOI:

Defects in silicon are studied as function of the dimensionality of the investigated structures. Defects produced by strong irradiation in bulk crystals, like vacancies or interstitial defects, induce other defects, so that the irradiated devices are irreversibly damaged. Defects in nanostructures are shown to be produced mainly by surface/interface states and strains, and are therefore specific to the investigated structure. The modeling of the experimental methods allows the determination of defect parameters that are not directly measurable. The carriers capture on quantum confinement levels in OD systems has a similar behavior with the trapping phenomena.

689. Sensitivity to Reducing Gases of Polymer-Iron Nanocomposite Materials

Authors: Tomescu, A; Simion, CE; Alexandrescu, R; Morjan, I; Scarisoreanu, M

Published: 2008, ROMANIAN JOURNAL OF INFORMATION SCIENCE AND TECHNOLOGY, 11, 95, DOI:

Electrical properties of new Polymer-Iron nanocomposite materials obtained by laser pyrolysis were investigated, in order to find applications for the detection of reducing gases. The aim of this paper was to prove the necessity of complex experimental investigation in order to point out the contribution of different types of adsorption. Usually resistance measurement cannot fully describe the surface interactions. The electronaffinity changes are of real interest for in-field conditions which always involve humidity of surrounding gas atmosphere. On the other hand, the nanocomposite materials can be a solution for specific gas-sensing properties.

690. Structural investigations on yttria-doped zirconia nanopowdeirs obtained by sol-gel method

Authors: Vasile, BS; Ghitulica, C; Popescu-Pogrion, N; Constantinescu, S; Mercioniu, I; Stan, R; Andronescu, E

Published: DEC 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3780, DOI:

The attractive properties of zirconia, particularly fracture toughness, wear resistance and chemical stability, have led to vast research efforts designed to investigate, characterize and develop such materials. Applications vary from structural, mechanical, biomedical to electrical, especially the fuel cells field. The current study is aiming at preparation and structural characterization Of 10% Y2O3 stabilized ZrO2 nanometric powders. The sol-gel method is known to be easy to use for producing various kinds of materials, structured at nanometric level, so was chosen for the elaboration of powders. Different thermal treatments were applied to the obtained materials, which were consequently investigated through thermal analysis, X-ray diffraction (XRD) scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The characteristics Of Y2O3 doped zirconia powders are influenced by the processing parameters and proved promising for further applications in, for example, the Solid Oxides Fuel Cell field.