National Institute Of Materials Physics - Romania

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.

The morphology and structure of a La0.6Y0.07Ca0.33MnO3 thin film is studied by transmission electron microscopy. The film was grown by pulsed laser deposition (PLD) on an MgO-buffered silicon <100> wafer. The as-deposited films are polycrystalline and show a columnar morphology. We present a detailed high-resolution transmission electron microscopy structural study concerning a possible monoclinic distortion of the orthorhombic LYCMO crystal cell. (Received February 22, 2000; accepted March 10, 2000).

This work is a comparative study of the structural and magneto-transport properties of La0.60Y0.07Ca0.33MnO3-delta (LYCMO) as bulk and thin film. The bulk samples were prepared by solid-state reaction between the corresponding metallic oxides mixed in stoichiometric ratios. The thin film was deposited by pulsed laser deposition on an MgO single crystal using an excimer laser. We show that the structure and stoichiometry of the bulk target are perfectly reproduced in the thin film. We measured the magnetoresistive effect on both the LYCMO pellet and the thin film by using the four-probe technique. The maximum of the MR effect is 680% on the polycrystalline thin film in a 2T magnetic field. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Atmospheric pressure chemical vapour deposition films of borophosphosilicate glass with different contents of phosphorus and boron can be successfully used as masks for etching of silicon in KOH solution. The optical properties of these films were pointed out by spectroellipsometric measurements, using different theoretical models (such as Cauchy and Wemple-DiDomenico models). (C) 2000 Elsevier Science Ltd. All rights reserved.

An X-band electron paramagnetic resonance (EPR) study of nominally pure, diamond-like cubic boron nitride (c-BN) crystalline powders, has led to the first identification of a spectrum attributed to two related paramagnetic species. The composite EPR spectrum can be observed only in dark brown colored powders known to contain excess of boron. It consists of two superimposed lorentzian components, called D1 and D2, centered at g1 = 2.0063 and g2 = 2.0084, with peak-to-peak linewidths of 3.3 and 17.9 mT, respectively. The temperature dependence of the integrated intensities, their linewidths and intensity ratio D2/D1 allows one to conclude that the narrow line D1 originates from EPR transitions inside a S = 1/2 ground doublet and the broad line D2 from transitions inside the excited levels of another center. Evidence suggests that both centers are boron related paramagnetic species. (C) 2000 Elsevier Science Ltd. All rights reserved.

Conduction properties as well as trapping phenomena in nanocrystalline porous silicon films are investigated in relation to their microstructure. Theoretical models to describe the phenomena are proposed. The main results presented in this paper are: the two-scale porosity of our porous silicon films, the decisive role of the quantum confinement to the conduction phenomena and the prominent part played by the oxidation on the surface effects.

ITO thin films were deposited by spin coating method starting from indium and tin propoxide in the frame of a sol-gel process. Films with the thickness corresponding to 68 nm for one layer deposition were produced and characterized by X-ray diffraction, transmission electron microscopy and infrared reflection spectroscopy. Low conductivity polycrystalline films exhibiting cubical structure (bixbyite-type) and good adherence were obtained.

There is an increasing interest in Si-based optoelectronics using Si1-xGex nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for possibilities to increase the contribution of the radiative recombination to the emission. One possible approach involves self-organised growth of lattice-mismatched layers. In the present paper, p-i-n structures, using one layer with Ge islands and which emit in the near infrared up to room temperature were fabricated. The self-organised growth of Ge was performed at 700 degreesC with a small coverage (9 ML) so as to avoid plastic relaxation of the islands, but with a high growth rate (0.3 ML/s) which leads to the formation of a broad bimodal island distribution (small- and medium-sized islands). The diode structure including the Ge islands was deposited in the form of mesas using selective epitaxial growth by low-pressure chemical vapour deposition. The mesa areas were Varied with the aim of demonstrating the influence of size distribution of the islands on the light emission. At low current density the emission is dominated by islands with smaller band gap (larger valence band offset) while at higher currents emission from islands with larger band gap takes place. From the comparison of single diodes with arrays of small-area diodes with the same total area it is found that the arrays emit three times more light due to the lower total number of deep traps in each diode. (C) 2001 Published by Elsevier Science Ltd.

A microstructural investigation of porous silicon films, related with a study of carriers transport. is presented. Scanning electron microscopy and conventional transmission electron microscopy with selected area electron diffraction were performed. The films show a double scale porosity: the macroporosity forms an alveolar structure at a micrometric scale and the nanoporosity forms a silicon nanowires network in the alveolar walls and at their surface. Current-voltage characteristics in the -5 to +5 V range and the temperature dependence of dark current in the 150-330 K interval at low voltage were measured. The transport properties are explained by means of a simplified quantum confinement model. Two symmetries of the nanowires, cylindrical and square, are investigated and the differences between them are shown to be insignificant. (C) 1999 The Electrochemical Society. S0013-4651(98)12-109-2. All rights reserved.

Hall effect, magnetoresistance, and electrical conductivity measurements, carried out on ZnO surface wells created by a large variety of methods, are analyzed in the frame of the weak-localization theory. The ZnO surface wells have some unique features that allow the investigation of the weak-localization effects: ZnO has a single valley conduction band; the Thouless length is much larger than the elastic mean-free path even at room temperature; the well accumulates the largest surface electron concentration obtained up to now in a surface quantum well; there are a large variety of preparation methods, some of them making it possible to modify independently both the width and the depth of the surface wells. These features allowed us to investigate: the presence of the weak-localization effect in the largest range of temperatures(1.6-300 K) reported up to now for a quantum well; the influence on the transport properties of the increase in the number of subbands in the well; the effect of the presence of more inelastic scattering mechanisms and their weights in the entire scattering process; and the passage from a quasi-two-dimensional system to a three-dimensional one. [S0163-1829(99)01932-3].