National Institute Of Materials Physics - Romania

The medium-range order in covalent glasses was analyzed in the frame of the paracrystalline theory. This theory, earlier applied to lamellar and fibrous structures, finds in the chalcogenide glasses an interesting working case.

Trapping levels in fresh (one month) and naturally aged (one year) nanocrystalline porous silicon have been investigated using the optical charging spectroscopy method. Four significant maxima and/or shoulders were observed for fresh samples and five for aged ones. They have been attributed to five and six trapping levels, respectively. The trapping centers corresponding to the most shallow four levels are situated at or nearby the internal surface of the porous silicon films. (C) 2000 American Institute of Physics. [S0003-6951(00)01621-1].

The a-Si:H p-i-n photovoltaic cells photosensitized with an organic layer of 5,10,15,20-tetra (4-pyrydil) 21H,23H porphine (TPyP) have been prepared. The action spectrum of these cells was extended by approximately 30 nm to longer wavelengths, with respect to non-sensitized cells. The presence of the organic layer gives some modifications in the electrical and photovoltaic behavior of the cells, that is why an analysis of dark current-voltage characteristics of the cells at room temperature, is also presented. (C) 2000 Elsevier Science B.V. All rights reserved.

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns. (C) 2000 American Institute of Physics. [S0021-8979(00)04610-7].

Magnetic compounds of the type R2Fe17-xVx (R=Y, Gd and x=1, 1.5, 2) were synthesized by arc melting (with or without interstitial carbon) and studied by x-ray diffraction (XRD), Mossbauer spectroscopy, and hysteresis loop measurements. Compounds with the structure Y2Fe17-xVx (x=1, 1.5, 2) were found to be hexagonal, whereas the intermetallics containing carbon introduced by melting and those of the type Gd2Fe17-xVx (x=1, 1.5, 2) were found to be rhombohedral. For each symmetry, the lattice parameters a/c were obtained from the XRD spectra and a considerable expansion of the unit cell volume was observed in the Y2Fe17-xVxC (x=1, 1.5, 2) systems as effect of carbon additions. Room-temperature transmission Mossbauer spectra were analyzed considering the various inequivalent iron sites and yielded the average hyperfine magnetic field. This field was studied as a function of vanadium content x for all classes of compounds under investigation. The maximum hyperfine field values were obtained for the carbides. The hysteresis loops recorded at 4.2 K in an applied field of 1.5 T provided complementary information on the saturation magnetization as a function of vanadium substitution for each group of intermetallics involved. (C) 2000 American Institute of Physics. [S0021-8979(00)24408-3].

A correlated ESR and optical emission study on samples doped with different concentrations of Tl+ impurity ions shows the involvement of paramagnetic Pb-2(3+) self-trapped electron centers (STEL) and trapped hole A centers in the electron-hole recombination responsible for the 2.6 eV blue-green luminescence. (C) 2000 Elsevier Science B.V. All rights reserved.

We consider semiconductor devices that are composed of two parts: first, a small quantum structure constituting the active region, and second, a classical environment with larger typical length scales, While in general the classical environment should be represented by a drift-diffusion model, we consider here only simple contacts which we take as ideal metals with infinite conductivity, The transport through the quantum structure is described as in the Landauer-Buttiker formation through electronic scattering wave functions which define the electron density in the quantum system. Further sources of the self-consistent Coulomb field are layers of classical charges in the contacts at each of the interfaces to the quantum system. We present further a capacitance model that takes into account the openness of the quantum structure and the existence of finite contacts embedding the system. As particular quantum structures we study simple tunneling barriers. (C) 2000 Elsevier Science Ltd. All rights reserved.

We report experiments on conducting polymer polypyrrole leading to sensor applications. The resonance frequency, stability and sensitivity of AT-cut quartz crystals, electrochemically covered with polypyrrole tosylate: were tasted under various operating conditions, The interaction of this organic semiconductor with thin metallic films was analysed by photoelectron investigations. Shifts of the valence band spectra during silver deposition are explained by the neutralization of positive charged surface defects. Thus the existence of a space charge region in polypyrrole is demonstrated spectroscopically. (C) 2000 Elsevier Science Ltd. All rights reserved.

The molecular dynamics of the liquid crystals 8CB and 5CB confined to the mesopores of the molecular sieve Al-MCM-41 with pore diameter less than 2.5 nm was investigated by broadband dielectric spectroscopy. The phase transitions of the liquid crystal cannot be detected for the molecules confined to the pores. Moreover it has been found that even about 20 K below the melting temperature of the bulk material a relaxation process occurs for molecules confined to the mesoporous material which does not exist in the bulk liquid crystal state. The temperature dependence of the relaxation rate of that process obeys a Vogel-Fulcher-Tammann law.

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].