National Institute Of Materials Physics - Romania

As-grown n-4H-SiC/SiO2 capacitors exhibit anomalous capacitance-vs.-voltage (C-V) characteristics at low temperatures. Abrupt minima appear in the C-V curves at specific values of the gate voltages independent of the sample temperature, strongly suggesting the presence of resonant electron tunneling. We put forward a qualitative model where neutral donor states present at the SiC/SiO2 interface enable electron tunneling into distinct energy levels in the oxide. Numerical simulations based on this model show close agreement with the anomalous C-V characteristics observed experimentally. The model implies that under given conditions, i.e., the existence of a sufficient density of neutral donors at the semiconductor/oxide interface and empty electron states in the oxide layer, abrupt minima are in general to be expected during C-V measurements of metal-oxide-semiconductor capacitors. (C) 2013 Elsevier B. V. All rights reserved.

Electric and magnetic properties of symmetric and asymmetric Pb(Zr-0.2 Ti-0.8)O-3-CoFe2O4 (PZT/CFO) heterostructures, grown by pulsed laser deposition on SrTiO3 (100) substrates with a 25 nm SrRuO3 (SRO) buffer layer as bottom electrode, were investigated by using hysteresis and capacitance measurements. X-ray diffraction, and transmission electron microscopy investigations reveal the high quality crystalline structure and the epitaxial relationship between SRO, PZT and CFO. The electric polarization-voltage hysteresis reveals that the remnant polarization and the coercive field are significantly affected by the CFO layer. The frequency dependence of capacitance suggests a Maxwell-Wagner type relaxation at low frequencies and is also affected by the presence of the PZT/CFO interface(s). The magnetic hysteresis measurements infer the possible presence of another spinel phase (Co3O4) in the CFO film, due to the lattice mismatch at the PZT/CFO interfaces, and with direct influence on the magnetic response of the structure. According to the electric and magnetic characterization, better room temperature multiferroic properties would be expected for the symmetric heterostructure. (C) 2013 Published by Elsevier B. V.

The paper investigates the dependence of the optical properties on cation concentration of amorphous-like indium gallium zinc oxide thin films (Inx-wGawZn1-xO1+0.5x-delta) with various (In + Ga)/(In + Ga + Zn) and Ga/(In + Ga) ratios obtained by pulsed laser deposition. X-ray reflectivity and spectroscopic ellipsometry thickness results were in good agreement. The proportionality between density and the refractive index in the transparency range is evidenced. The extracted physical parameters are clearly influenced by the variation of cation concentration. (C) 2013 Elsevier B. V. All rights reserved.

Bioglasses (BG) are the inorganic materials exhibiting the highest indices of bioactivity. Their appliance as films for bio-functionalization of metallic implant surfaces has been regarded as an optimal solution for surpassing their limited bulk mechanical properties. This study reports on magnetron sputtering of alkali-free BG thin films by varying the target-to-substrate working distance, which proved to play an important role in determining the films' properties. Post deposition heat-treatments at temperatures slightly above the glass transformation temperature were then applied to induce inter-diffusion processes at the BG/titanium substrate interface and strengthening the bonding as determined by pull-out adherence measurements. The morphological and structural features assessed by SEM-EDS, XRD, and FTIR revealed a good correlation between the formations of inter-metallic titanium silicide phases and the films' bonding strength. The highest mean value of pull-out adherence (60.3 +/- 4.6 MPa), which is adequate even for load-bearing biomedical applications, was recorded for films deposited at a working distance of 35 mm followed by a heat-treatment at 750 degrees C for 2 h in air. The experimental findings are explained on the basis of structural, compositional and thermodynamic considerations. (C) 2013 Elsevier B.V. All rights reserved.

We study the ternary clathrate Pr3Pd20Si6 in specific heat and ac susceptibility measurements on a high-quality single crystal, distinguishing antiferromagnetic and antiferroquadrupolar ordering, as well as a hitherto unknown magnetic low-temperature transition. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the antiferromagnetic ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. Pr3Pd20Si6 is, therefore, one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.

We report on specific heat and resistivity measurements under hydrostatic pressure on the quasi-one dimensional metal TlxV6S8. We studied the interplay between the low temperature superconducting (SC) ground state and a high temperature charge density wave (CDW) instability. We observed a clear dependency of the physical properties of TlxV6S8 on the Tl concentration x. The CDW anomaly is present in all investigated samples that are strongly enhanced at half Tl filling, x = 0.47. This is also the only composition for which no signature of superconductivity is observed. The specific heat results regarding the SC phase in Tl0.63V6S8 suggest that this compound is a highly anisotropic, weak coupling superconductor. Pressure suppresses both SC and CDW transitions to lower temperatures. Nevertheless, as the CDW gap is closed at a critical pressure p(c), the increase in the density of states leads to a small enhancement of T-c suggesting that SC and CDW compete for parts of the Fermi-surface.

Epitaxial SrCuO2 thin films were grown on (0 0 1) SrTiO3 substrates by pulsed laser deposition using a stoichiometric target. X-ray diffraction indicated that the SrCuO2 films undergo a structural phase transition as a function of the substrate temperature. Films deposited at temperatures below 600 degrees C exhibit a tetragonal phase with the c-axis oriented along the growth direction while films deposited at temperatures above 700 degrees C exhibit an orthorhombic phase with the b-axis oriented along the growth direction. Atomic force microscopy indicated that the as-grown film surfaces are rather smooth and the roughness increases with increasing substrate temperature. Energy dispersive X-ray spectroscopy in agreement with X-ray diffraction intensity ratio data revealed that all films are non-stoichiometric and contain Sr vacancies (Sr/Cu similar to 0.8). The influence of film-substrate lattice matching and substrate temperature on the structural phase transition is discussed. (C) 2012 Elsevier B.V. All rights reserved.

Ba(X1/3Ta2/3)O-3 (X = Mg, Zn) thin films were grown on commercial Pt-coated Si substrates by radiofrequency plasma beam assisted pulsed laser deposition (RF-PLD) method. Single phase Ba(Mg1/3Ta2/3)O-3 and Ba(Zn1/3Ta2/3)O-3 ceramic targets having an ordered hexagonal structure (with P (3) over bar m1 space group) were used for deposition. Structural, morphological and surface characterizations of the Ba(X1/3Ta2/3)O-3 (BXT) films were performed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction studies evidenced a cubic (disordered) perovskite structure, with unit cell typical for Pm3m space group. Scanning electron microscopy investigations showed that BXT films have a columnar microstructure, which is oriented perpendicular to the substrate. The temperature dependence of the dielectric permittivity of the films was recorded at 100 kHz. Unlike targets, the BXT films exhibit positive values of the temperature coefficient of the dielectric permittivity. BaMg1/3Ta2/3O3 and BaZn1/3Ta2/3O3 thin films with dielectric constant of about 22.5 and 25, respectively have been obtained. (C) 2012 Elsevier B.V. All rights reserved.

The formation of an interface between Bi2O3, Fe2O3, BiFeO3, Bi0.5Na0.5TiO3, and the high work function metallic RuO2 is studied using photoelectron spectroscopy with in situ RuO2 deposition. Schottky barrier heights are derived and the valence band maximum energies of the studied materials are aligned with respect to each other as well as to other functional oxides like SrTiO3 and PbTiO3. The energy band alignment follows systematic trends compared to a large number of oxides, and can be understood in terms of the contribution of Fe 3d and Bi 6s/6p (lone pair) orbitals to electronic states near the valence band maximum. The results indicate that the valence band maxima are largely determined by the local environment of the cations, which allows to estimate valence band maximum energies of oxides with multiple cations from those of their parent binary compounds. The high valence band maximum of BiFeO3 is consistent with reported p-type conduction of acceptor doped material, while the high conduction band minimum makes n-type conduction unlikely.

Barium titanate-substituted bismuth titanate ((Bi0.5Na0.5)(0.92)Ba0.08TiO3, BNT-BT0.08) nanotubes were fabricated using sol-gel chemistry, spin casting, and a porous polycarbonate membrane template. The structure and morphology of the tubes have been investigated by scanning electron microscopy and transmission electron microscopy. The diameter and length of these tubes were about 650 nm and 20 mu m, respectively, and their wall thickness was about 50 nm. The tubes were polycrystalline with average grain size of similar to 40 nm. The BNT-BT0.08 nanotubes exhibited ferroelectric behavior as evidenced by saturated piezoresponse hysteresis loops and phase switching. BNT-BT0.08 piezoelectric nanotubes promise novel device architectures and enhanced electric properties.