National Institute Of Materials Physics - Romania

The leakage current in epitaxial BiFeO3 capacitors with bottom SrRuO3 and top Pt electrodes, grown by pulsed laser deposition on SrTiO3 (100), SrTiO3 (110), and SrTiO3 (111) substrates, is investigated by current-voltage (I-V) measurements in the 100-300 K temperature range. It is found that the leakage current is interface-limited and strongly dependent on the orientation of the substrate. The potential barriers at the electrode interfaces are estimated to about 0.6, 0.77, and 0.93 eV for the (100), (110), and (111) orientations, respectively.

The structural peculiarities of titanium oxide films formed by pulsed laser deposition have been investigated. A pure titanium target was irradiated by a Nd:YAG laser (355 and 532 nm). The work studied the influence of the beam wavelength, and oxygen pressure (20, 40, 80, and 160 mTorr) upon the layer structure. The films deposited at room temperature were partly crystalline. The crystalline fraction was a mixture of titanium sub-oxides. The 532 nm wavelength seems to favour the oxidation of titanium leading to TiO(2) formation even at room temperature. After annealing, crystalline TiO(2-) anatase formed at both irradiation wavelengths, but only at low oxygen pressures (20 and 40 mTorr). The best crystallization occurs in the layers deposited at the lowest oxygen pressure (20 mTorr), at both laser beam wavelengths; most anatase formed in the sample obtained at 20 mTorr, 532 nm. At 20 mTorr the films were compact, for both wavelengths, but also tilted or randomly distributed columnar grains were observed at higher pressure. The films deposited with 355 nm, were thinner than those with 532 nm; at 355 nm the thickness decreased at higher pressure, while at 532 nm the dependence was opposite.

The characteristics of carbonic materials obtained by downstream deposition in a low pressure argon plasma beam injected with acetylene are reported. The influence of substrate temperature, presence of Ni catalyst and hydrogen in gas composition on the material properties is described. By increasing the substrate temperature, an enhanced order in the material is revealed by Raman spectroscopy, while FTIR measurements show a decreasing of the hydrogen content and the disappearing of sp(1) hybridized carbon in the deposit. The SEM and Raman investigation show a clear tendency of crystalline phases formation when hydrogen is assisting the deposition. (C) 2008 Elsevier B. V. All rights reserved.

Thin films of complex oxides have been obtained by pulsed-laser deposition (PLD) from glass targets belonging to the system Li2O-Al2O3-P2O5-(RE)(2)O-3, with RE = Nd, Pr, Er. The films were deposited on quartz, silicon and ITO/glass substrates using a F-2 laser (lambda = 157 nm, iota approximate to 20 ns) for ablation in vacuum. The structural, morphological and optical properties of the oxide films were investigated through IR and UV-VIS spectroscopy, Atomic Force Microscopy (AFM), Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy (SEM-EDX) and Spectroscopic Ellipsometry. The laser wavelength was found to be the key parameter to obtain thin films with very smooth surface. In this way new possibilities are opened to grow multilayer structures for photonic applications. (C) 2008 Elsevier B. V. All rights reserved.

Mixtures of transparent and conductive oxides such as ITO-ZnO have been grown by a combinatorial pulsed laser deposition technique from two targets that were located 15 mm apart. The films were deposited on (1 0 0) Si and quartz substrates that were heated at temperatures from 300 to 500 degrees C. Measurements of the In to Zn ratios along the transversal axis of the substrates, which passes through the maximum thickness points corresponding to each target position were performed using energy dispersive X-ray spectroscopy and spectroscopic ellipsometry. From simulations of the X-ray reflectivity spectra, collected with a 2 mm mask on different locations along the transversal axis of the samples, the density and thickness of the deposited films were calculated and then the In to Zn ratios. The crystalline structure and electrical properties of the deposited films were also investigated along the same axis. Changes in the ratio of In/Zn along this axis resulted in changes of the film lattice constant and texture. (C) 2008 Elsevier B. V. All rights reserved.

Ferroelectric materials in thin film form, largely used in a broad variety of high-tech applications, are characterized by performing electrical measurements on capacitor-like structures. A complete characterization should include hysteresis, capacitance, and current measurements performed at different bias voltages, frequencies and temperatures. The analysis of the experimental data should be made considering that the real test structure is metal-ferroelectric-metal, and taking care of the direct impact of the microstructure on the macroscopically measured quantities. Further on, the theoretical models developed to simulate the experimental results should be able to explain simultaneously the data obtained from different types of electrical measurements. The main type of electrical measurements performed on ferroelectric capacitors will be discussed in detail. A special attention will be given to some important problems such as: the electrode-ferroelectric interfaces; calculation of the dielectric constant; intrinsic-versus-extrinsic contributions to the value of the dielectric constant--fake hysteresis loops and the question "Is the presence of the hysteresis cycle solid evidence for the presence of ferroelectricity?"; non-conventional contributions to the polarization charge and their effect on the frequency dependence of the hysteresis loop; conduction mechanisms in different ferroelectric materials. Some "hot" topic will be also discussed, as for example the validity of the serial model in the case of ferroelectric multilayers, and the coexistence of ferroelectric and antiferroelectric behavior in some structures and multilayers. The presentation is supported by experimental material collected by the author in the last 7 years, especially during his extended stay at the Max Planck Institute from Halle, Germany. The author is very grateful to V. Stancu, I. Vrejoiu and K. Boldyreva for providing most part of the samples, and to M. Alexe, D. Hesse and U. Goesele for the useful discussions and funding during the stay in Halle. Part of the experimental work was made in the frame of the contract CEEX-44-DINAFER, financed by the Romanian Ministry of Education and Research.

Imaging experiments at the XFEL pose unprecedented requirements to the detectors in terms of radiation tolerance: Fluxes of up to 10(16) (12 keVphotons/cm(2)) corresponding to approximately 10(9) Gy in silicon, are expected. An irradiation station has been set up in the DORIS beam line F4, MOS test structures have been irradiated, and first results on the dose dependence of the CN-characteristics, surface current density, and interface trap density have been obtained.

The Ba(Zn1/3Ta2/3)O-3 (BZT) ceramic samples were prepared by solid-state reaction and sintered in the range 1550-1650 degrees C for 2 h. Several methods-X-ray diffraction (XRD) and scanning electron microscopy (SEM)-were used for structural and morphological characterization. The unit cell distortion and the presence of the secondary phase content were studied by XRD. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was noticed with the increase of the sintering temperature. SEM investigations revealed polyhedral well-faceted grains and large grain size distribution. The dielectric properties in the microwave range were measured at room temperature and at 1 kHz on a large temperature interval (+/- 150 degrees C). The dielectric parameters were correlated with morphological and structural properties. Ceramic samples were annealed at 1410 degrees C for 30 h to improve the microwave properties. The dielectric constant of BZT samples measured at 6 GHz and at 1 kHz was between 27 and 28 on the whole temperature range, that is, typical values for BZT material. The temperature coefficient of the resonance frequency at 6 GHz exhibits positive values less than 6 ppm/degrees C.

Ba1-xSrxTiO3 solid solutions with x = 0.4, 0.5, 0.6 and 0.75 were prepared by solid state reaction at 1260 degrees C. The complex dielectric constant dispersion was investigated in the frequency range 1Hz divided by 10 MHz for temperatures between -100 degrees C and 100 degrees C. Microwave measurements, performed at room temperature, show amonotonically decrease of the permittivity from about 3000 to 300 with the increase of the strontium content. Moreover, the variation of the dielectric loss from 0.8% to less than 0.2% with the increase with x was observed. The results indicate that the paraelectric Ba1-xSrxTiO3 materials are suitable for microwave applications.

Hydroxyapatite (HAp) films, widely used as a biocompatible material for hard tissues repairing. are studied in this paper for a better control and understanding of the crystallization and densification of sol-gel deposited layers. Calcium nitrate and triethyl phosphite diluted in alcohols were used as calcium and phosphorus precursors. HAp coatings were obtained by spinning method, followed by drying and annealing in the range 130-750 degrees C. Specific temperatures of the chemical reactions of HAp formation have been revealed by thermogravimetric analysis (TG-DTA). Structural. chemical and optical characterizations of the films were performed by Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Analysis (EDX) facilities, X-ray diffraction (XRD). Fourier Transform Infrared Spectroscopy (FTIR) and Spectral Ellipsometry (SE). The film crystallinity increases with file annealing temperature above 550 degrees C. XRD lines of HAp crystallographic planes become narrow at 750 degrees. and correspond to a single crystallographic phase. The Film morphology depends not only on annealing history but also on the synthesis parameters. Thus. layers with large grains of well sinterized HAp nano-crystals can be obtained. Using EDX measurements, the Ca/P ratio was found close to the stoichiometric value in the well sinterized HAp grain regions and slightly increased 1.82 for boundary regions between grains. The density of pores usually observed in sol-gel derived films was estimated for different samples using modeling of SE data.