National Institute Of Materials Physics - Romania

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.

PbZr0.2Ti0.8O3-BiFeO3-PbZr0.2Ti0.8O3 (PZT-BFO-PZT) structure was prepared by sol-gel method on Pt/Si wafers. Three metals were tested as top electrod: Au, Cu, and Pt. A double hysteresis loop was observed in all cases, suggesting an antiferroelectric-like behavior. The values for remnant polarization and coercive field are about 3 mu C/cm(2) and 115 kV/cm, respectively. The saturation polarization is about 22 mu C/cm(2). Capacitance and leakage current show significant dependence on the top metal electrode. The results were explained considering the presence of a Schottky-type contact at the PZT-metal interface.

The purpose of this work is to compare the density of shallow interface states (D-it) at the interface of SiO2/SiC MOS capacitors as deducted by the conductance spectroscopy (CS) and thermally dielectric relaxation current (TDRC) techniques. Both capacitors of 4H- and 6H-SiC (n-type) arc investigated, and both ordinary dry oxidation and,in improved industrial procedure have been employed. The two techniques are found to give rather good agreement for interface states located >= 0.3 eV below the conduction band edge (E-c) while for more shallow states vastly different distributions of D-it are obtained. Different reasons for these contradictory results are discussed, such as strong temperature and energy dependence of the capture cross section of the shallow interface States.

Nitrogen doped 4H-SiC epitaxial layers grown by hot-wall chemical vapor deposition were investigated by deep level transient spectroscopy after irradiation with 6 MeV electrons or 1.6 MeV protons. The influence of silane and propane flows used during the epilayers growth on the behaviour of radiation induced EH6,7 levels is studied. Samples grown under different conditions were investigated: 1 sample grown in steps of different C/Si ratio obtained by changing the propane flow only; 1 sample grown in steps of different C/Si ratio obtained by changing the silane flow only; 2 samples grown with a C/Si ratio of 1.5 but with different flows of propane and silane. These investigations revealed that the low thermal stability of EH6,7 (the defects anneal out at temperatures as low as 750K) is due to the magnitude of silane now used during the growth irrespective of the C/Si ratio. A possible structure of the EH6,7 defect is discussed.

This work is focusing oil the effect of a high concentration of nitrogen (N) introduced by ion implantation at the SiO2/4H-SiC interface in MOS capacitors. The N implanted sample (N-interface similar to 1 x 10(19) cm(-3)) is compared with a non-implanted one (N-interface similar to 1 x 10(16) cm(-3)) by means of the electron interface trap density (D-it). The Dit is determined via High-Low frequency C-V method and Thermal Dielectric Relaxation Current (TDRC) technique. It is shown that the TDRC method, mainly used so far for determination of near interface oxide charges, call be exploited to gain information about the D-it too. The determined value of D-it in the N-implanted sample is nearly one order of magnitude lower than that in the sample without N implantation. Good agreement between the TDRC results and those obtained from High-Low frequency C-V measurements is obtained. Furthermore, the TDRC method shows a high accuracy and resolution of D-it,evaluation in the region close to the majority carrier band edge and gives information about the traps located into the oxide.

Soft ferrites in the CuZnTi system, having the chemical composition Cu1-x-yZnxTiyFe2O4 with 0.5 <= x <= 0.7 and 0.00 <= y <= 0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with different Curie temperatures, between 30 and 180 degrees C and very high change rate of permeability with temperature around their Curie points. Such materials are well suited to use as high sensitive magnetic temperature sensors and transducers for temperature control. Zn and Ti additions to copper ferrite change the Curie temperature in a controllable manner, thus being possible to produce ferrite material with a fine control of the Curie point at any desired temperature. Most important was the behavior of magnetic permeability with temperature around the Curie point, where it may change with about 60 %/degrees C by a proper choice of the cooling speed of samples from the sintering temperature to room temperature. This makes such materials extremely attractive as magnetic temperature sensors of high sensitivity. Two applications Of Such materials as temperature sensors, namely an ultrathermostat and an on-off switch type relay were designed. Their functionality and performances are presented and discussed.