National Institute Of Materials Physics - Romania

Electron states at the SiO2/4H-SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of similar to 10(19) cm(-3) and 10(16) cm(-3), respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35-295 K. Three main categories of electron states are disclosed, true interface states (D-it), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of D-it and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstrated to be responsible for the parallel shifts and hysterezis effects occurring in the C-V characteristics. All three categories of electron states are reduced in concentration in the implanted samples. This holds particularly for NIToxfast with a peak at similar to 0.1 eV below the conduction band edge of 4H-SiC that is suppressed by at least two orders of magnitude relative to the nonimplanted samples. The decrease for D-it is also substantial (a factor of similar to 10) while the loss for NIToxslow is considerably smaller (only similar to 30%). The results provide firm evidence that NIToxfast and NIToxslow do not originate from the same kind of defect center. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457906]

Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E(g), and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap. (C) 2009 Elsevier B.V. All rights reserved.

The short-circuit photocurrent was measured in ferroelectric capacitors of polycrystalline and epitaxial quality. The interest was to study the possible relation between photocurrent and back-switching phenomena due to ferroelectric polarization imprint, as suggested by Pintilie [J. Appl. Phys. 101, 064109 (2007)]. An interesting relation between the shape of the ferroelectric hysteresis loop and the shape of the photocurrent spectral distribution was found. In polycrystalline samples, the shape of spectral distribution and the sign of photocurrent are changing in time, although the hysteresis is almost symmetrical. However, the hysteresis is not rectangular as in the case of epitaxial films. This behavior suggests a subtle relation between polarization back-switching and photocurrent. In epitaxial samples a peculiar dependence between photocurrent and polarization imprint was found. All these are explained assuming the presence of an internal field, possibly generated by charged defects, which can change its direction and magnitude under illumination, with consequence on the orientation and magnitude of the ferroelectric polarization, and on the sign/shape of the short-circuit photocurrent spectral distribution. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3445877]

Functionalized implants represent an advanced approaching in implantology, aiming to improve the biointegration and the long-term success of surgical procedures. We report on the synthesis of hydroxyapatite (HA) thin films on polymethylmetacrylate (PMMA) substrates - used as cranio-spinal implant-type structures - by two alternative methods: pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (MS). The deposition parameters were optimized in order to avoid the substrate overheating. Stoichiometric HA structures were obtained by PLD with incident laser fluences of 1.4-2.75 J/cm(2), pressures of 30-46.66 Pa and 10 Hz pulses repetition rate. The MS depositions were performed at constant pressure of 0.3 Pa in inert and reactive atmospheres. SEM-EDS, XRD, FTIR and pull-out measurements were performed assessing the apatitic-type structure of the prepared films along with their satisfactory mechanical adhesion. Cell viability, proliferation and adhesion tests in osteosarcoma SaOs2 cell cultures were performed to validate the bioactive behaviour of the structures and to select the most favourable deposition regimes. For PLD, this requires a low thence of 1.4 J/cm2, reduced pressure of water vapours and a 100 degrees C/4 h thermal treatment. For MS, the best results were obtained for 80% Ar + 20% O-2 reactive atmosphere at low RF power (similar to 75 W). Cells grown on these coatings exhibit behaviour similar to those grown on the standard borosilicate glass control: increased viability, good proliferation, and optimal cell adhesion. In vitro tests proved that HA/PMMA neurosurgical structures prepared by PLD and MS are compatible for the interaction with human bone cells. (C) 2010 Elsevier B.V. All rights reserved.

Implant type coatings were prepared by magnetron sputtering (MS) technique onto medical grade Ti6Al4V alloy substrates starting from biological 45S5 glass system powders. The as-deposited thin layers were annealed 2 h at 700 degrees C in ambient air, followed by a slow cooling (2 degrees C/min) in order to induce crystallization. The behavior of the coatings was investigated by soaking the samples in simulated body fluids (SBF) and extracting them after 24 and 72 h. The changes in the films' structure were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) analysis and by Fourier transform infrared spectroscopy (FTIR). A pitched, pore free microstructure with 6 mu m agglomerates of submicron grains in a continuous matrix was revealed by the SEM images of the annealed samples. Complex silicates as Na(4)Ca(4)Si(6)O(18)-combeite and phosphates NaCaPO(4) as crystalline phases were identified in the XRD diffraction patterns. In-growths after 24 h show the enrichment in Si-O (s) non-bonding oxygen (NBO). The polymerization reaction in the surface layer appears for the sample immersed 72 h in SBF. No crystalline hydroxyapatite (HA) was evidenced for those samples. Subsequent dissolution processes of the surface layers were noticed from XRD patterns and SEM images. (c) 2010 Elsevier B.V. All rights reserved.

The to study of the total oxide (SiO2+SiOx) thickness, SiO2 and SiOx (e.g. Si2O, SiO, Si2O3) thicknesses on Si(100) crystalline substrate with take-off angles ranging from 30 degrees to 80 degrees has been carried out by spectrometric method. The d(s) X-ray Photoelectron Spectroscopy (XPS) thicknesses were compared with d(EL) thicknesses obtained by fitting the Spectroscopic Ellipsometry (SE) spectra. A qualitatively good correlation is revealed. However, from these estimations of film thicknesses it results that ellipsometry analysis cannot be as accurate as in XPS evaluation. This is due to uncertainty of used optical constants as well due to very thin oxide films used in this work.

Radiofrequency magnetron sputtering deposition at low temperature (150A degrees C) was used to deposit bioactive glass coatings onto titanium substrates. Three different working atmospheres were used: Ar 100%, Ar + 7%O(2), and Ar + 20%O(2). The preliminary adhesion tests (pull-out) produced excellent adhesion values (similar to 75 MPa) for the as-deposited bio-glass films. Bioactivity tests in simulated body fluid were carried out for 30 days. SEM-EDS, XRD and FTIR measurements were performed. The tests clearly showed strong bioactive features for all the prepared films. The best biomineralization capability, expressed by the thickest chemically grown carbonated hydroxyapatite layer, was obtained for the bio-glass coating sputtered in a reactive atmosphere with 7% O(2).

Plasma polymerization has been carried out in a parallel plate RF discharge reactor in mixtures of argon with pentacyclic precursors, namely thiophene, furan and pyrrole. The plasma characterization showed the presence of emission bands peculiar to each monomer. The deposition of highly transparent films with partial conservation of the pentacyclic structure, inclusion of various oligomers, roughness and surface energy depending on the precursor nature was demonstrated by material investigations results. The material conductivity is significantly improved upon iodine doping in the case of polythiophene and polypyrrole-like films, while an opposite behavior was observed for the polyfuran-like films.

Ba(Mg1/3Ta2/3)O-3 (BMT) ceramic is a dielectric material possessing an extremely high Qxf product value (up to 400 THz) at microwave frequencies. This makes it suitable for applications as a dielectric resonator for wireless communications. BMT samples, prepared by solid-state reaction and doped with Nb5+, V5+ or Ga3+, were employed for compositional, structural morphological characterization. In the present study, we report on the influence of dopants (type and quantity) and sintering temperature on the dielectric properties. It was found that the quality factor is maximum when Nb5+ is used as dopant.

In the aim to improve the adherence of the nanostructured gold layers onto glass substrate, gold deposition has been first time performed onto a polystyrene (PS) layer. The PS layer of ca. 50nm was spin-coated onto glass plates from a toluene solution. Gold layers of 10-20nm were vacuum deposited at small incidence angle. We found that at variance from the plates with gold deposited directly on the glass, the plates with an intermediate PS layer do not peel under overnight treatment in ethanol solution. The layers were characterized by several methods. X-ray diffraction (XRD) measurements showed that gold peaks have the position corresponding to the face-centered cubic structure: However, the crystallites on the sample with PS layer seem to be a little bit smaller than those with gold deposited directly onto the glass. XR reflectometry measurements have given the thickness of the gold layer in good agreement with the value estimated from quartz monitor readings and with the ellipsometric data as well, Liquid crystal cells were obtained to observe the molecular alignment imposed by the nanostructured gold layer deposited onto PS film. A rather strong interaction of gold atoms with the substrate molecules can be considered on the basis of the XRD and ellipsometry measurements.