National Institute Of Materials Physics - Romania

This paper describes the pulsed constant current anodization of n(+) and p(+)-GaAs in ethylene glycol-water-tartaric acid (AGW) mixture, up to 17 mA cm(-2). A 0.26 mu m thick film is obtained for a final voltage of 135V at 4.3 mA cm(-2) pulsed current density. Cyclic voltammetry showed that the initial growth of a monolayer anodic pride can be described by a charge transfer with uncompensated cell resistance model. The relationship between peak current, peak voltage and scan rate has been verified for this process, based on the above model.

The magnetic properties of the amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons are studied using temperature dependence of the specific magnetization. A two-phase ferromagnetic behaviour is obtained for the magnetization curves. Gd addition tends to stabilize the interfacial (grain boundary) phase by forming a metastable Gd-Fe-B phase, leading to the coexistence of two phases up to thermal treatments as high as 993K.

The high-T-c superconducting system YBa2Cu3Odelta:Li (123) was investigated by microwave absorption. The effect of neutron irradiation on the 123 phase as well as the dependence of the resulting modification both on the neutron fluence and on the doping behavior with Li are discussed. The results show a significant improvement of the superconducting characteristics: critical temperature, phase homogeneity, and critical current densities for a certain neutron fluence. We have found that the optinum neutron fluence for irradiation is 4.98 x 10(21) m(-2).

Formation conditions, optical nonlinear properties and applications of F2- color centers in LiF monocrystals are investigated. The efficiency of F-2(-) centers generation depending of irradiation conditions and content of OH- impurities of the LiF monocrystal are also presented. Thermal stability of the centers generated by electron irradiation was studied. It was measured the variation of absorption factor of the LiF: F-2(-) crystal in the -40 degrees C to 60 degrees C temperature range. The LiF: F-2(-) crystals obtained by an efficient method of irradiation with electrons generated by a linear accelerator were employed in Q-switching a Nd:YAG laser cavity with output energies of 18 mJ and fluctuations less than 5% in the -40 degrees C to 60 degrees C temperature range.

We have investigated the magnetic and structural properties of polycrystalline Ni-Zn ferrite thin films with thicknesses of about 1000 Angstrom. The films were prepared by R F sputtering deposition in 1.5x10(-3) mbar argon atmosphere, using targets with nominal composition Ni0.7Zn0.3Fe2O4. The effects of substrate temperature and post deposition heat treatment of thin films were examined. The X-ray spectra of Fe-Ni-Zn oxides deposits reveal the formation of spinel phase of Ni-Zn ferrite. that is characteristic for these materials. only for temperatures higher than 500 degrees C. TEM and ED indicate that the initially amorphous thin films. deposited at 100 degrees C, become polycrystalline by heat treatment. The grain size of the crystallites is 800 Angstrom to 1000 Angstrom for Ni-Zn ferrite thin films annealed at 900 degrees C. V.S.M. measurements show coercive fields of 12 kA/m for thin films annealed at 900 degrees C. which are larger than those found for Ni-Zn ferrite targets. This value of coercive field is related to the shape anisotropy originated by the film microstructure and with the increase in the number of low angle grain boundaries, which act as pinning sites for domain walls. Saturation magnetization values of 220-250mT for Ni-Zn ferrite thin films are higher than half of bulk values.

Dc conductivity measurements for Z-cut natural quartz plates, at different temperatures and values of the applied electric field were made. The evolution of I-V characteristics shapes is studied for electrodiffusion times up to 100 h (in interstitial alkali ions transport region). The non-ohmic behavior is discussed and is considered to be due to the space charge produced by the uncompensated Al3+ ions substitutional for Si4+.

Crystallized Fe-(Cu)-Sm-B ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry and Mossbauer spectrometry. The bcc alpha-Fe, tetragonal Fe3B and tetragonal Fe2B phases were found to crystallize after appropriate annealing. The relative proportion of alpha-Fe decreases with increasing Sm content, in agreement with the increase of the crystallization point. The metastable Fe3B phase decomposition into alpha-Fe and Fe2B was observed within a wide range of annealing temperature. The Sm ions randomly accommodate the Fe sites in the Fe3B lattice. After cumulative annealing at higher temperatures, the tetragonal Sm1.1Fe4B4 paramagnetic phase is found.

The polarization behaviour of Armco iron in solutions of HCl with and without inhibitors has been studied by potentiostatic method. At lower overvoltage values the dissolution process is controlled by activation, while at higher overvoltage values the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using three surfactants. Under the critical micelle concentration the inhibition of those three surfactants is negligible. At a concentration higher than critical micelle concentration, the inhibiting action of surfactants increases rapidly. The process of inhibition was attributed to the formation of the adsorbed film on the metal surface, that protects the metal against the corrosive agents.

Simultaneous measurements of electrical resistance R and Seebeck e.m.f. V-s, of thick film SnO2, gas sensors were performed in air containing different concentrations p(i) of reducing gas (ethanol vapours, CO, CH4, C2H6 were successively used). The graphical representations of the V-s,versus In h values both for the identification of the reducing gas (selectivity) and for the determination of gas concentration. (C) 1998 Elsevier Science S.A. All rights reserved.

GaN thin films were deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. An Nd-YAG laser (lambda = 1.06 mu m, tau(FWHM) = 10 ns) of 0.35 J energy/pulse was used as laser source. The nitrogen pressure was varied in the range of 10(-2) to 10(-1) mbar. As substrates, we used (0001) sapphire plates and (100) Si wafers, coated or uncoated with ZnO as buffer layers, and heated below 300 degrees C. Different analysis techniques evidenced the characteristics of the deposited films. SIMS profiles corresponding to N and Ga in-depth distribution carried out the presence of layers of the order of 130-150 nm, with uniform distribution of Ga and N inside the layer. XPS studies evidenced the Ga-N bonding. The Nls signal contains as main peak the one centered at 397.3 eV and corresponding to Ga-N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV-visible range evidenced a high transparency (over 80% transmission) for the deposited films. The energy band gap obtained from the absorption spectra was found to be larger than 3.6 eV. (C) 1998 Elsevier Science B.V.