National Institute Of Materials Physics - Romania

The average density of zeros for monic generalized polynomials, P-n(z)=phi(z)+Sigma(k=1)(n)c(k)f(k)(z), With real holomorphic phi, f(k) and real Gaussian coefficients is expressed in terms of correlation functions of the values of the polynomial and its derivative. We obtain compact expressions For both the regular component (generated by the complex roots) and the singular one (real roots) of the average density of roots. The density of the regular component goes to zero in the vicinity of the real axis like \Im z\. We present the low- and high-disorder asymptotic behaviors. Then we particularize to the large-n limit of the average density of complex roots of monic algebraic polynomials of the form P-n(z)=z(n)+Sigma(k=1)c(k)z(n-k) with real independent, identically distributed Gaussian coefficients having zero mean and dispersion delta=1/root n lambda. The average density tends to a simple, universal function of xi=2n log \z\ and lambda in the domain xi coth(xi/2) much less than n \sin arg(z)\, where nearly all the roots are located for large n.

PbS films were chemically deposited on poled and unpoled ferroelectric ceramic substrates and on glass. The sensitivity of the PbS thin films and the concentration of trapping centers inside these films depend on the nature of the substrate and, in the case of poled ferroelectric substrates, depend on the polarity of the ferroelectric polarization as well. A considerable enhancement of the photoconductive signal at liquid nitrogen temperature was found in the case of PbS films deposited on the positive face of a poled ferroelectric substrate. (C) 1997 Elsevier Science S.A.

Infrared photoconductive detectors were prepared from three undoped n-type InSb single crystals with different electron concentration and mobility values. The single crystals were grown by the Czochralski method. The spectral responsivity and spectral detectivity of the detectors were determined as functions of the power dissipation density of the device. A remarkable improvement in performance was observed for the purest specimen in comparison with earlier reported data. (C) 1997 Elsevier Science S.A.

The frequency characteristic of the transverse photoelectric signal of a three electroded PbS/ferroelectric heterostructure is analyzed in this paper. The ferroelectric material is a lead titanate-zirconate ceramic obtained by hot pressing. It was found that the frequency characteristic of this signal is similar to a high-pass filter and depends on the properties of the ferroelectric substrates on which the PbS film was deposited, An equivalent circuit was proposed comparing the studied heterostructure with a metal-oxide semiconductor (MOS) structure. The circuit elements were determined fitting the obtained formula for the output signal with the experimental data. Some of the circuit elements were determined experimentally using various measuring techniques. It was found that the experimentally determined values are close to the theoretically determined values. The studied heterostructure can be changed into a band-pass filter adding some external circuit elements such as resistors and capacitors.

Pseudo-ternary magnetic compounds Y3Co11-xFexB4 (x = 2, 4, 5 and 6) were studied by Fe-57 Mossbauer spectroscopy at 293 and 4.2 K. The effect of Fe substitution on the hyperfine parameters and site preference is reported.

Cadmium sulphide (CdS) thin films were deposited on glass substrates using the homogeneous precipitation technique and different cadmium salts. The structure of the films was analysed using scanning electron microscopy (SEM). Thermally stimulated current (TSC) measurements had revealed the existence of several trapping levels in as deposited and annealed films. The activation energy of these traps was determined from TSC measurements. The following values were found for CdS films deposited from cadmium acetate: E-1 = 0.3 eV; E-2 = 0.45 eV, E-3 = 0.51 eV. For CdS films deposited from cadmium chloride several trapping levels were found with the activation energies between 0.04 and 0.33 eV. A considerable enhancement of photoconduction after annealing in air was found. The experimental results were explained considering a sensitizing process due to the chemisorbtion of the oxygen. (C) 1997 Elsevier Science S.A.

Semiconductor heterostructures with prescribed energy dependence of the transmittance can be designed by combining: a) Pade approximant reconstruction of the S-matrix; b) inverse scattering theory for Schrodinger's equation; c) a unitary transformation which takes into account the variable mass effects. The resultant continuous concentration profile can be digitized into an easily realizable rectangular-wells structure. For illustration, we give the specifications of a 2 narrow band-pass 12-layer AlcGa1-cAs filter with the high-energy peak more than twice narrower than the low-energy one.

The electronic structure of polycrystalline C-60 thin films has been investigated using surface photovoltage spectroscopy (SPS) and conductivity measurements. The films show n-type semiconductivity with an activation energy of approximate to 0.8 eV as found from the temperature dependence of the conductivity at high temperatures. The electronic structure emerging from our SPS results comprises a 1.6 eV photoconduction gap, a mobility gap of about 2.25 eV and two gap states, a donor and an acceptor, at 0.35 and 0.8 eV, respectively, below the photoconduction edge. The proposed model positions the Fermi level between these two localised levels. Thus, the high-temperature conductivity is due to electrons excited across the photoconduction gap.

A new structure is proposed for the PbS based photoresistors. This structure consists from a thin film of PbS (about 1 mu m) chemically deposited on a dielectric layer (SiO2 or Si3N4 Of about 2000 Angstrom) grown, using different methods, on a single crystal Si wafer. The drain and the source contacts are the two gold electrodes evaporated on the surface of the PbS film. The resulting active area is of 6x6 mm(2) and is subjected to an incident light with the wavelength in the domain 0.8 - 3 mu m (at room temperature). The gate contact is the aluminum electrode evaporated on the back side of the Si substrate. Spectral distribution measurements were performed for different voltages applied on the gate electrode. It was found that the shape of the spectral distribution remains the same but the amplitude of the signal depends on the value and on the polarity of the applied voltage. An improvement of about 40 % was obtained for the PbS photoconductive signal compared with the situation when the gate electrode is in air. The new structure offer the possibility to improve the photoconductive signal generated by the PbS film. This improvement is due to a field effect.

Orientational properties of rubbed thin polymer layers with vinylic backbone and different side groups are presented. Some of these polymers lead to the alignment of LC molecules parallel to the rubbing direction whereas other polymers impose a perpendicular orientation. The stability of the obtained alignment as function of the temperature was tested.