National Institute Of Materials Physics - Romania

Graded ferroelectric (GF) structures were obtained by successive deposition of Pb1-3y/2Lay(Zr0.4Ti0.6)O-3 films with different La contents on Pt/TiO2Si substrates. The method used for deposition was sol gel. The final structure consists of four layers, starting from the Pt electrode to the top gold electrode: (1) no La; (2) 4% La; (3) 8% La; and (4) 12% La. Ferroelectric properties of the GF structure were investigated, and an anomalous shift of the hysteresis loop along the polarization axis was found. The sense of this shift depends on the initial direction of the ferroelectric polarization. The presence of hysteresis displacement "up" or "down" can be explained if the presence of a built-in charge in the GF structure is admitted. The built-in potential produced by this charge is computed using a simple theoretical model. (C) 2000 American Institute of Physics. [S0003-6951(00)01040-8].

p-type lead sulphide (PbS) thin films were chemically deposited onto Si3N4/n-Si substrates. A three-contact pseudo-metal-oxide-semiconductor structure was designed in order to investigate the possibility of enhancing the signal-to-noise ratio in the PbS film by field effect. It was proved that on the entire sensitivity domain of PbS (1-3 mu m at room temperature), the magnitude of the photoconductive signal generated by the PbS film depends on the polarity and value of the gate voltage. It is shown that the signal-to-noise ratio can be enhanced in this way with by at least 25% compared with the case when the gate electrode is in air (standard configuration for photoconductive measurements). (C) 2000 American Institute of Physics. [S0003-6951(00)01514-X].

The trapping levels induced in p-n Si junctions by irradiation with 24 GeV proton were investigated using Thermally Stimulated Currents (TSC) methods in the 90-300 K temperature range. Several trapping levels, with activation energies between 0.27 and 0.57 eV, were put into evidence. The spatial distribution of the traps was investigated using different wavelengths for the light used to fill the traps. The results lead to the conclusion that the deepest trapping levels are not uniformly distributed in the volume of the sample. For the most important trapping level the average introduction rate was estimated to 0.9-1.2 cm(-1). The activation energy and the capture cross-section of this trapping level seems to depend on the impurity element introduced in Si. (C) 2000 Elsevier Science B.V. All rights reserved.

Optical charging spectroscopy (OCS) is first time reported as applied to p-n junctions. The existence of one deep trapping level for electrons and two deep trapping levels for holes was put into evidence, using this method, in proton irradiated p(+)-n-n(+) silicon structures. An analytical formula for the OCS discharging current for this type of structures was deduced. (C) 2000 Elsevier Science B.V. All rights reserved.

Lead titanate (PT) is well known as a good piezoelectric material with a high Curie temperature (similar to 490 degreesC) and a low dielectric constant (similar to 200). But pure lead titanate cannot be sintered as ceramic bodies, for practical purposes, due to its large crystal anisotropy (tetragonality c/a=1.064). However, dense PT ceramic bodies can be obtained if small additions of other elements are used. Ca and Sm modified PT were succesfully tried, but such dopings proved rather expensive for mass production. Therefore some other doping elements must be tried. In this regard we investigated the effect of Bi and Y additives on the piezoelectric properties of PT ceramics. The general formula of our Bi, Y doped lead titanate ceramics was: (Pb1-3x/2Mex)(Ti0,98Mn0,02)O-3, with Me=Bi,Y and x=0.04, 0.06, 0.08. The modified PT ceramics were prepared by conventional ceramic technique, using p.a.purity raw materials. The mixed powders were sintered at temperatures between 1000-1300 degreesC. Poling was done in fields of about 70kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degreesC. It was found that Bi,Y modified lead titanate ceramics have a very high Curie point (T-c>450 degreesC), a low dielectric constant (<140) and a large electromechanical anisotropy (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4), which make them competitive to the rare-earth doped PT ceramics, for high temperature and high frequency transducers applications.

Piezoelectric properties of four tungsten doped samples of PZT type were determined as a function of composition. The samples were prepared by the usual ceramic technique from raw oxide of 99.9% purity. All compositions exhibited maximum values for the density at a sintering temperature of 1180 degrees C with a soaking time of 4h, so that the piezoelectric properties were determined on the samples sintered at this temperature only. The results showed that the piezoelectric properties were not drastically modified by the doping level, but dopings higher than 5% at. sightly alter the properties. The best values obtained for the piezoelectric parameters were: density rho= 7.7 g/cm(3); dielectric constants, epsilon(T)=1410; electromechanical coupling factors, k(p) = 0.63, and k(T) = 0.55; mechanical quality factor Q(m) approximate to 90; piezoelectric voltage constant d(33) = 470.10(-12) m/V; piezoelectric charge constant g33 = 25.10(-3) Vm/N. These values show that tungsten doped materials are piezoelectric soft type ceramics of high quality that can be successfully used for manufacturing ultrasonic transducers.

Metal/ Ferroelectric/ Semiconductor (MFS) heterostructures have been of interest in the last decade due to possibility of their potential integration in classical silicon technology and because their potential use in memory devices. Research in this field has lead to new generations of sensors and actuators, laser modulators and deflectors, non-volatile random access memories (NRAM's). Unfortunately many at these types of devices remain in the prototype phase because of the impossibility to know and control phenomena which appear at the ferroelectric/ semiconductor interface. The present paper presents some results on the investigation of a PbTiO3/ Si heterostructure. A study of the film occurring at the interface and how it influences the ferroelectric properties of the heterostructure can clarify some problems of the reproducibility of the parameters of a MFS heterostructure.

Pb(ZrxTil .x)O-3 (x = 0 divided by 0.8) graded thin films were fabricated by sol-gel method on Pt/ SiO2/ Si substrates using metal alkoxide solution. The samples were crystallized by conventional thermal annealing in air for 30 min. at 700 degreesC. The ferroelectric properties of the graded structure were investigated Specific unusual behavior in the ferroelectric loops like large offset on the polarization axis was observed and disscused.

Cadmium sulphide films were prepared by Chemical Bath Deposition (CBD) technique from CdCl2, CdSO4, Cd(CH3COO)(2) respectively, as cadmium precursor salts, and thiourea in ammoniacal medium. In this paper we study the changes in the morphology and properties of the films prepared from different cadmium salts, although the starting reaction parameters as concentration of the reactants, pH (denoted as pH(i)), temperature, and time of reaction for all precursor cadmium salts are identical. It is proposed that, during the deposition process, depending upon the anion type of cadmium salt the pH of reaction mixture decrease with different rates. The pH(i) decreases in the deposition bath could allow the change of the layers growth mechanism and therefore the change in the morphology and properties of the films. Above a certain value of starting pH (pH(i) >12.5), the pH decrease did not affect the growth mechanism and properties of the layers. The structure and morphology of the films were investigated by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and X-ray diffraction (XRD). Also the optical, electrical and photoelectrical characteristics of the layers and their dependence on films morphology were presented. These studies have allowed us to establish the optimum conditions for fabrication of quality CdS films using CdCl2, CdSO4, or Cd(CH3COO)(2) as cadmium precursor salts.

Annealing studies of a proton irradiated Si test diode (1MeV- neutron equivalent fluence is phi (eq) = 1.82x10(13) cm(-2)) were performed. The macroscopic characterization shows that during the annealing experiment the type inversion effect happened. The changes in the microscopic features during the annealing treatment were investigated using Thermally Stimulated Current method.