National Institute Of Materials Physics - Romania

The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead-lanthanum-zirconate-titanate system describes well the observed features of the graded ferroelectric structures. (C) 2003 American Institute of Physics.

IR photoconductive properties of PbS films can be controlled using two different methods: (1) chemical, controlling the film morphology and, thus, the physical film properties by adding small amounts of impurities in the deposition bath, (2) physical, using field effect in complex PbS/SiO2/Si pseudo-MOS structures to control the photoconductivity of PbS film. (C) 2003 Elsevier Science B.V. All rights reserved.

An iterative method is presented for the characterization of lossy piezoelectric materials in the thickness resonant mode, which allows to calculate the frequencies at which the electrical impedance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for processing thus simplifying the measurement procedure. The accuracy of the method was tested for several piezoceramic materials, covering a wide range of values of the thickness coupling factor and mechanical quality factor. The new method proved to be as accurate as other iterative methods, but it is easier to apply. (C) 2002 Elsevier Science Ltd. All rights reserved.

Asymmetric metal-ferroelectric-metal (MFM) structures were manufactured by sol-gel deposition of a lead zirconate-titanate (PZT with Zr/Ti ratio 65/35) film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 muC/cm2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarization-electric field (P-E), capacitance-voltage (C-V), and current-voltage (I-V) measurement results allowed us to estimate the near-electrode space-charge region thickness (roughly half of the film thickness at zero voltage), net doping concentration (around 10(18) cm(-3)), built-in potential (in the 0.4-0.8 V range, depending on the injecting electrode), and dynamic dielectric constant (5.2). The current logarithm-voltage dependence for the field-enhanced Schottky emission obeys a "1/4" law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290-800 nm range exhibits a cutoff wavelength at 370 nm and a maximum sensitivity at about 340 nm. The estimated band-gap energy of the PZT material is 3.35 eV. The MFM structure is discussed in terms of two back-to-back Schottky diodes with a ferroelectric material in between. It is concluded that the semiconductor properties of the films are not negligible and, in certain conditions, are dominating over the ferroelectric ones. (C) 2003 American Institute of Physics.

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co-60-gamma irradiation using the thermally stimulated current method. We have found that, for high irradiation doses, a second-order defect can be detected. This defect is largely suppressed in oxygen-enriched material while it is the main cause for the space charge sign inversion effect observed in standard float-zone material. (C) 2003 American Institute of Physics.

The paper refers to the study of patch antennas with a high permittivity dielectric substrate ((Zr-0.8 Sn-0.2)TiO4) obtained at National Institute of Materials Physics Bucharest. Because of the high value of the permittivity of the substrate material it was necessary to perform a comparative investigation of different prototypes and arrays. It was considered two substrate thickness h(1)=1mm and h(2)=2mm and two frequencies f(1)=2.4 GHz and f(2)=1.8 GHz. For each case we presented the radiation pattern in the two principal planes (E and H) and input impedance. Also, some radiation pattern for rectangular array with n=4, 16, 36 patches are presented. In the Conclusion section a discussion on the suitability of patch antennas with high permittivity substrate is done.

The paper describes the investigations of Nd doped Barium titanate (BNT) ferroelectric ceramic materials prepared by solid state reaction techniques. The decrease of the Curie temperature with the increase of the Nd2O3 concentration from 0 to 28 wt % was observed. High density values of BNT ceramics were obtained by adding 0.5 mol % PbO substitutional for BaO. The samples were sintered at temperatures T-s=1200divided by12600degreesC. At room temperature, the materials are in paraclectric phase, suitable for high frequency applications.

(Zr0.8Sn0.2)TiO4 ceramic (ZST) has been prepared and characterized. The effects of sintering parameters such as sintering temperature, sintering time and NiO addition on structural and dielectric properties were investigated. The material has a dielectric constant epsilon(r) similar to 36.0 and high values of the Q . f product from 32,170 to 50,000 at microwave frequencies. The tan delta values are decreased by low level doping, of NiO, while the temperature coefficient of the resonance frequency tau(f) takes values in the range (-2divided by+4) ppm/degreesC. Investigations on whispering gallery modes revealed low dielectric losses in millimeter wave domain. An intrinsic quality factor of 480 was measured at 136.3 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter wave applications such as filters, hybrid microwave integrated circuits, etc.

(Ba1-xSrx)TiO3 solid solution (BST), with different molar compositions (x = 0.25, 0.5, 0.75, 0.9) Were prepared by conventional solid-state reaction from raw materials. Structural parameters, crystallite sizes and bulk densities were determined. Perovskite type polycrystalline structure of the BST ceramics was revealed by X-ray diffraction (XRD) data. The dependence of permittivity and losses at low frequency (1 kHz) was analyzed. The microwave investigations at room temperature revealed dielectric constant around 1000 and loss smaller than 1% at 1.1 GHz. The results indicate that some BST dielectric ceramics are suitable in paraelectric phase for microwave devices. The dielectric constant can be adjusted by using a DC-bias field and such materials are appropriate for manufacturing electric field controlled components such as resonators, phase shifters etc.

As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z(1,2) defect with energy levels normally detected at about E-C-0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2-3) in the gas phase, was used to prepare the samples. The Z(1,2) defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5-2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z(1,2) defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z(1,2) defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z(1,2) deep-level defects could be confirmed. A 1:1 relation between the concentrations of Z(1) and Z(2) was obtained for the present as-grown epitaxial layers. (C) 2002 American Institute of Physics.