National Institute Of Materials Physics - Romania

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.

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.

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.

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.

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.

A new type of dual mode filter is proposed for mobile communications systems in 900 MHz frequency bandwidth such as GSM or GPRS. The planar filters were developed on a 0.635 mm substrate with a 10.8 dielectric constant. Several methods of modes coupling were investigated. Each dual mode resonator has a square-shape with the edge 20.5 mm and contributes with two poles to the filter response.

Epitaxial Pb(Zr0.2Ti0.8)O-3 (PZT) thin films were grown on single crystalline Nb-doped SrTiO3 (STON) substrates by Pulsed Laser Deposition (PLD). The top electrode was SrRuO3. The investigated electric and ferroelectric properties have revealed a strong asymmetry of the two PZT-electrode interfaces. A qualitative explanation based on the possible existence of a n(+) - p junction at the STON-PZT interface is suggested.

Lead zirconate-titanate (PZT) thin films with Zr/Ti ratio of 92/8 were deposited by sol-gel on Pt(111)-coated Si(100) and single crystal MgO(100) substrates. The films crystallize differently on the two type of 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 some preferred orientation that is substrate dependent. Average values of 2.1 muC/cm(2) and 78 kV/cm were obtained for remnant polarization and coercive field, respectively. The I-V characteristic at low to medium voltages is dominated by field enhanced Schottky emission, with Ln(J)similar to(V + V-bi)(1/4) dependence of the current density. A value of about 3.85 eV was estimated for the band-gap from optical measurements.

The paper describes the investigations of (Zr0.8Sn0.2)TiO4 compounds prepared by solid-state reaction techniques. The effect of La2O3, ZnO and NiO addition on microwave dielectric properties was investigated. The samples were sintered at temperatures T-s = 1280 divided by 1400 degrees C The inaterials exhibit a dielectric constant epsilon(r) similar to 36 and high values of the Q f productfroin 30,000 to 55,000 at microwave frequencies. The measured temperature coefficient of the resonance frequency tau(f) takes values in the range (-2 divided by+4) ppm/degrees C.

Nanocrystalline powders of a soft FeNbLi-doped PZT material have been prepared by a novel mechanochemical process consisting of mixing the stoichiometric oxides in a planetary ball mill for prolonged times up to 80 h. The constituent oxides were reacted in a tungsten carbide vial with balls of 5, 10 and 20 mm in diameter and a ball/powder ratio of 15/1. The chemical reaction between the component oxides was triggered after 20 h of energetic milling and was completed after 80 h. The XRD of the reacted nanopowder showed the well known perovskite structure. Compacted samples of this powder were sintered between 800-1300degreesC for 3 h and the main piezoelectric properties were determined. The density of the sintered samples reached nearly 99% of the theoretical density at 1100degreesC and showed good piezoelectric characteristics: planar coupling factor of 0.66, dielectric displacement constant d(33) of 550 pm/V, mechanical quality factor of 85, and relative dielectric constant of 3800. The possible mechanisms for solid state reaction of mechanically activated nanopowders such as local heating and pressure at collision as well as defects diffusion are discussed. (C) 2004 Kluwer Academic Publishers.