National Institute Of Materials Physics - Romania

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849 articles found

741. Doped TGS crystals for IR detection and sensors

Authors: Alexandru, HV; Berbecaru, C; Stanculescu, F; Pintilie, L; Matei, I; Lisca, M

Published: AUG 16 2004, SENSORS AND ACTUATORS A-PHYSICAL, 113, 392, DOI: 10.1016/j.sna.2004.03.046

Triglycine sulfate crystals (TGS) doped With L- and D-alanine, respectively, where grown in paraelectric phase at 52 degreesC. They show stable parameters versus pure TGS crystals, lower permittivities and losses, i.e. higher pyroelectric figure of merit. L- and D-alanine-doped crystals show a mirror asymmetry of the growth habit along the ferroelectric axis. Hysteresis loop of pure and doped samples were automatically recorded. Internal bias field of about I kV/cm, induced by the two dopants, stabilizes the polarization components P+ and P- in opposite directions. Spontaneous polarization of doped crystals shows two unequal P+/P- components, having a peculiar dependence on the ac applied electric field. Dopage simulation of pure TGS samples, using several bias fields, shows even more complicated pattern. Pyroelectric coefficients were evaluated from pyroelectric current measurements. A remarkable asymmetry of the pyroelectric coefficient peak values for the enantiomorphous-doped crystals was noticed, crossing up and down the Curie point. This fact suggests a non-equivalent L/D-alanine dopant substitution of the glycine group GI in the host lattice. It has been shown that the pyroelectric coefficient can be increased at room temperature, under an optimized dc electric field applied on the pyroelectric wafer. Doped crystals are suitable for IR detector and for more stable gas sensors applications in pollution monitoring. (C) 2004 Elsevier B.V. All rights reserved.

742. Properties of Pb(Zr-0.Ti-92(0).(08))O-3 thin films deposited by sol-gel

Authors: Pintilie, L; Boerasu, I; Gomes, M; Pereira, M

Published: JUN 30 2004, THIN SOLID FILMS, 458, 120, DOI: 10.1016/j.tsf.2003.12.057

Lead zirconate-titanate (PZT) thin films with Zr/Ti ratio of 92/8 were deposited by sol-gel on Pt coated silicon and single crystal MgO(100) substrates. In case of the films deposited on Pt the perovskite structure is established only after oxygen annealing, while for the films deposited on MgO annealing in air is enough. The films are polycrystalline with preferred orientation that is substrate dependent ((110) on Pt and (100)/(200) on MgO). Average values of 2.10 muC/cm(2) and 78 kV/cm were obtained for remnant polarization and coercive field, respectively. The I-V characteristic at low-to-intermediate electric fields is controlled by thermionic emission, but the Schottky representation is Ln(current)-(voltage)(1/4), implying that the film behaves like a semiconductor and not like an insulator. From transmission-reflectance measurements a value of 3.85 +/- 0.05 eV was estimated for the band gap, while the refraction index in the transparency region was found to be 2.6. A pyroelectric signal was detected on as grown films, making them attractive for infrared detection. (C) 2003 Elsevier B.V. All rights reserved.

743. BNT ceramics synthesis and characterization

Authors: Ioachim, A; Toacsan, MI; Banciu, MG; Nedelcu, L; Alexandru, H; Berbecaru, C; Ghetu, D; Stoica, G

Published: JUN 15 2004, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 109, 187, DOI: 10.1016/j.mseb.2003.10.116

Investigations on barium neodymium titanate (BNT) prepared by solid state reaction techniques are presented in this paper. The samples were sintered at temperatures, T-s, in a range 1200-1300degreesC. High density values and adequate microwave parameters were obtained by adding 0.5 mol PbO substitutional for BaO. A decrease of the Curie temperature with the increase of the Nd2O3 concentration was confirmed. For the sample with 28 wt.% Nd2O3, 0.5 mol PbO pyroelectric data indicate Curie temperature around 75 K, therefore, at room temperatures, the materials are in paraelectric phase. BNT ceramics with high Nd2O3 concentration (28 wt.%), sintered at 1250degreesC for 2 h appear to be suitable for dielectric resonators for frequencies up to 3 GHz (epsilon(r)=80-90 and Q = 2000). (C) 2003 Elsevier B.V. All rights reserved.

744. Structural and electrical properties of sol-gel deposited Pb(Zr0.92Ti0.08)O-3 thin films doped with Nb

Authors: Pintilie, L; Boerasu, I; Pereira, M; Gomes, WJM

Published: JUN 15 2004, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 109, 177, DOI: 10.1016/j.mseb.2003.10.043

Pb(Zr, Ti)O-3 (PZT) thin films with Zr/Ti ratio of 92/8 and doped with up to 4 at.% Nb were deposited by sol-gel on Pt coated Si substrates. The scope was to investigate the influence of Nb additive on the structural, optical and electrical properties of Zr-rich PZT phase. it was found that a residual pyrochlore phase stabilizes with increasing the Nb content. The remnant polarization, the coercive field, and the dielectric losses increase with increasing Nb content. The presence of the pyroelectric effect was also evidenced on as-deposited layers. The films have potential for light detection in the infrared region of the electromagnetic spectrum (pyroelectric effect). (C) 2003 Elsevier B.V. All rights reserved.

745. Electrical properties of metal-oxide-silicon structures with LaAlO3 as gate oxide

Authors: Mereu, B; Sarau, G; Dimoulas, A; Apostolopoulos, G; Pintilie, I; Botila, T; Pintilie, L; Alexe, A

Published: JUN 15 2004, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 109, 98, DOI: 10.1016/j.mseb.2003.10.054

In this paper, electrical investigations performed on complex metal-oxide-semiconductor (MOS) structures with amorphous LaAlO3 deposited as gate oxide on a rapid thermal processed SiO2 buffer layer are presented. Current-voltage (I-V) measurements at different temperatures and high frequency room-temperature capacitance-voltage (C-V measurements indicate the presence of a non-equilibrium state in p-type Si (100) substrate under reverse polarization of the structure. The asymmetry in I-V characteristics is opposite to the expected one for the high-kappa/interfacial layer stack behavior. Different indications for the non-equilibrium MOS state are inferred from the experimental data, and the extraction of MOS physical parameters by using the classical C-V method is shown to be unreliable. For small and large applied forward voltages defect-related charge transport and tunneling currents dominate, respectively. (C) 2003 Elsevier B.V. All rights reserved.

746. Field-effect transistor based on nanometric thin CdS films

Authors: Mereu, B; Sarau, G; Pentia, E; Draghici, V; Lisca, A; Botila, I; Pintilie, L

Published: JUN 15 2004, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 109, 263, DOI: 10.1016/j.mseb.2003.10.077

Cadmium sulphide (CdS) thin films were deposited by chemical bath deposition (CBD) method on SiC2/Si (n-type) substrates. Approximately, 70 nm thick nano-crystalline CdS layers were obtained. Thin film field effect transistors were realised by deposition of two coplanar electrodes of Au (drain and source) on the US surface. The gate contact is aluminium deposited on the backside of the Si substrate. The drain current-drain voltage characteristics (I-d - V-d) were performed in dark. Normal field effect transistor characteristics are obtained in case of positive gate and drain voltages, the device acting as an n-channel transistor in the accumulation mode. For negative drain voltages the characteristic is dominated by space charge limited currents (SCLC). An on/off current ratio of about 10(2) is reported, this being limited in our case by geometry. (C) 2003 Elsevier B.V. All rights reserved.

747. Oxides ferroelectric (Ba, Sr)TiO3 for microwave devices

Authors: Alexandru, HV; Berbecaru, C; Ioachim, A; Toacsen, MI; Banciu, MG; Nedelcu, L; Ghetu, D

Published: JUN 15 2004, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 109, 159, DOI: 10.1016/j.mseb.2003.10.034

Barium-strontium titanate (BST), ternary ceramic compounds with the molar formula (Ba1-xSrx)TiO3, were prepared by solid-state reaction from raw materials. Three compositions with x = 0.25, 0.50 and 0.75, have been checked. Samples with two type of thermal treatment at 1230 and 1260degreesC have been manufactured and analyzed. Perovskite type, polycrystalline structure of the BST ceramics was revealed by the X-ray diffraction data at room temperature. Strontium ion (Sr2+) enters substitutionally on the Ba2+ site. A small decrease in the unit cell volume with the Sr content increase was observed. Dielectric measurements have been performed at low and high frequencies. The temperature dependence of low frequency permittivity shows large interval of ferroelectric transition, which appears to be merely due to dispersion of crystallites dimensions. The increasing concentration of Sr ion leads to a shift of the Curie point below room temperature. Experimental data show a linear decrease of the Curie point T-C (degreesC); approximate to 120-360x, versus x Strontium fraction. Room temperature permittivity and dielectric loss have been measured in microwaves by using the Hakki-Coleman method. BST samples prepared using water as a milling medium exhibit lower porosity and lower dielectric loss in microwaves than samples prepared using acetone as a milling medium. Dielectric loss Of 10(-3) at 1 GHz shows that this ceramic material is quite suitable for high frequency device applications. (C) 2003 Elsevier B.V. All rights reserved.

748. Chemical growth of calcium phosphate layers on magnetron sputtered HA films

Authors: Verestiuc, L; Morosanu, C; Bercu, M; Pasuk, I; Mihailescu, IN

Published: MAR 15 2004, JOURNAL OF CRYSTAL GROWTH, 264, 491, DOI: 10.1016/j.jcrysgro.2004.01.015

Chemical growth of calcium phosphate films on top of apatite structures is an essential step for promoting implants integration in human bones. We used for chemical growth silicon wafers either covered with crystalline and amorphous hydroxyapatite films or uncoated. To promote growing, the structures have been immersed in simulated body fluids for 21 days at 37degreesC. The growth stages were monitored by atomic spectroscopy of Ca and P ions in solution. The calcium phosphate deposits were investigated by X-ray diffraction, optical and atomic force microscopy and FTIR spectroscopy. The grown structures onto sputtered hydroxyapatite were amorphous and smooth. The depositions performed directly on silicon substrates are mild. (C) 2004 Elsevier B.V. All rights reserved.

749. Toward super radiation tolerant semiconductor detectors for future elementary particle research

Authors: Lindstroem, G; Fretwurst, E; Kramberger, G; Pintilie, I

Published: MAR 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 38, DOI:

The principal of a semiconductor particle detector is briefly outlined and applications in several fields of fundamental and applied physics mentioned. Starting from the basic physics motivation in High Energy Physics (HEP) research, the further discussion then focuses on the use of these devices in present and future HEP experiments outlining especially the requirements for precise measurements of elementary particle tracks, a task ideally fulfilled by segmented silicon detectors. These developments have also initiated further applications in different fields as e.g. material analysis, medical imaging and space missions. In future HEP-experiments such devices will however face an unprecedented challenge imposed by extremely intense radiation fields. Basics of hadron and lepton induced radiation damage in silicon are described together with their implications on the detector quality. The following part of the report is then dealing with several approaches to improve the radiation tolerance of silicon detectors. Defect engineering of the semiconductor material, modified detector geometries and specific operational conditions are outlined. First results toward a complete understanding of detector degradation as caused by point and cluster defects are given and these promising successes may finally pave the way for developments of superior devices.

750. Effect of ytrium substitution on the structure and piezoelectric properties of lead titanate ceramics

Authors: Miclea, C; Amarande, L; Tanasoiu, C; Miclea, CF; Sandu, V; Gheorghiu, A

Published: 2004, 2004 International Semiconductor Conference, Vols 1and 2, Proceedings, 414, DOI:

Lead titanate (PT) is well known as a good piezoelectric material with high Curie temperature (similar to 490 degrees C) and low dielectric constant (similar to 200). Pure lead titanate cannot be sintered as ceramic bodies, due to its high crystalline anisotropy (c/a=1.064), but dense ceramic bodies can be obtained if small additions of other elements are used. In this regard we investigated the effect of Y addition on the structure and piezoelectric properties of PT ceramics, with general formula (Pb1-3x/2Yx)(Ti0.98Mn0.02)O-3, and x=0.04, 0.06, 0.08 and 0.10. Density and coupling factors were determined as a function of sintering temperatures, and doping level. Temperature dependence of the electromechanical behaviour was also investigated for temperatures as high as 500 degrees C. The Y modified PT ceramics have low dielectric constant (< 140), high Curie point (T-C > 450 degrees C), and large electromechanical anisotropy.