Publications

In this paper we focus on supersaturation and migration of the atomic species involved in the growth of oxide films in order to search for new efficient methods for growth of flat, uniform and precipitate-free complex thin films with high quality. In this respect we have applied MOCVD to HTS thin films, 'interrupted growth' (consisting of on/off vapour deposition cycles, keeping the temperature and atmospheric conditions constant), similar to 'pulsed laser interval deposition' reported in the literature. We show that MOCVD can be easily and successfully applied to suppress island growth, contrary to the currently accepted idea that this approach is unique to PLD. This suggests that principles of supersaturation-migration-supersaturation cycles are universal for the films' growth. By this approach, high-quality Bi-2223 thin films with a uniform flat surface, having roughness less than half the c-axis unit cell, were grown by MOCVD. Using substrates with artificial steps of predefined width equal to 'twice the migration length' totally precipitate-free thin films were obtained: precipitates-segregates of impurity phases (Cu rich for HTS materials) are gathered at the step edges where the free energy is lowest. As-prepared films were used to fabricate intrinsic Bi-2223 and Bi-2212/Bi-2223 SIS-type Josephson junctions (IJJs).

Electron-trapped Fe+-type centers, produced by x-ray irradiation at 80 K and further annealing at higher temperatures in iron-doped SrCl2 single crystals grown in chlorine gas, have been investigated by electron paramagnetic resonance. The Fe+(III) and Fe+(IIIa) centers, produced by annealing at temperatures higher than 200 K, exhibit monoclinic local symmetry with the two g down arrow- tensor principal axes situated in the (110) plane slightly tilted away from the [001] and [1-10] directions, respectively. The Fe+(IV) center, observed after several cycles of irradiation and annealing to 700 K, exhibits tetragonal local symmetry around and a well-resolved four-component structure, attributed to the superhyperfine interaction with a neighboring monovalent impurity ion. The presence and properties of the low symmetry radiation-induced Fe+ paramagnetic centers are attributed to trapping and the thermally activated movement of chlorine interstitials. Both precursor Fe2+ and resulting Fe+ centers are perturbed by these interstitials, which are introduced in SrCl2:Fe crystals during growth under a chlorine atmosphere.

Pulse shaping introduces the method that makes possible the production of tunable arbitrary shaped pulses. We extend this method to control the prevalent growth of cubic SiC films on Si (10 0) substrates by pulsed laser deposition at temperatures around 973 K from a SiC target in vacuum. We used a laser system generating 200 fs pulses duration at 800 nm with 600 mu J at 1 kHz. The obtained structures are investigated by electron microscopy, X-ray diffraction and profilometry. We observed grains embedded in an amorphous texture, characteristic in our opinion to the depositions obtained with very short pulses. We present a comparison of deposited films with and without pulse shaping. Pulse shaping promotes increased crystallization and results in the deposition of thin structures of cubic SiC with a strongly reduced density of particulates, under similar deposition conditions. (c) 2005 Elsevier B.V. All rights reserved.

We achieved the growth of cubic silicon carbide (SiC) films on (100)Si substrates by pulsed laser deposition (PLD) at moderate temperatures such as 750 degrees C, from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The morphology of deposited films is dominated by columns nucleated from a thin nanostructured beta silicon carbide (beta-SiC) interface layer. The combined effects of columnar growth, tilted facets of the emerging columns and the presence of particulates on the film surface, lead to a rather rough surface of the films. (c) 2005 Elsevier B.V. All rights reserved.

We report novel pulsed laser deposition conditions that were used to obtain superconducting epitaxial YBCO thin films, grown in situ using an oxygen pressure lower than the usual one during the cool-down time. We studied the influence of the PLD conditions as substrate temperature, oxygen pressure, laser fluence, and number of laser pulses on the crystallographic and morphological features, and on the superconducting properties of the films. Good superconducting properties were obtained without a high temperature post-deposition annealing process. A maximum critical temperature of 88.6 K was obtained. (c) 2005 Elsevier B.V. All rights reserved.

Epitaxial Pb(Zr,Ti)O-3 (PZT) thin films with thicknesses in the range of 50-200 nin and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD). The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0)SrTiO3 (STON). SrRuO3 (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the ext,ended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT. (c) 2005 Elsevier B.V. All rights reserved.

Thermal, magnetic and transport measurements on CePd2-xNixAl3 alloys within the 0 <= x <= 1 range are reported, including applied pressure (p) and magnetic field on some selected samples. The low temperature results indicate that long range antiferromagnetic order is robust up to x = 0.2, whereas between 0.25 and 0.5 magnetic fluctuations give rise to non-Fermiliquid (NFL) behaviour. In this critical region, the low temperature specific heat can be described as due to two components, the major showing a NFL C-p/T = gamma(0) - gamma(1) root T dependence, while the minor one includes a decreasing fraction of short range order degrees of freedom. The latter is only observed close to the critical point, x(cr) approximate to 0.35. Electrical resistivity (rho) studies performed under pressure for x = 0.5 allow us to investigate the evolution of the NFL state around and beyond x(cr), where the exponent of rho proportional to T-n increases from n = 1 (for p = 0) up to n = 2 (for p = 12 kbar). This exponent is also observed at normal pressure on the x = 1.0 sample. indicating the onset of the Fermi liquid behaviour. Doping and pressure effects are compared by fitting high temperature resistivity data employing a unique function which allows us to describe the evolution of the characteristic energy of this series along a large range of concentration and pressure.

Experimental results on spontaneous emission rates from InGaAs quantum dot lasers that can be explained theoretically by considering the influence of nonradiative mixed state recombinations in the quantum dot-wetting layer system are presented. Our model qualitatively explains the experimental results such as an increase in the threshold current density, temperature stability, and a narrower gain spectrum due to doping the quantum dot active region with the acceptors. Our model also predicts that moderate acceptor concentrations can improve the laser performance at higher carrier injection densities; but high acceptor concentrations deteriorate the laser performance due to the nonradiative Auger recombination that counteracts the benefits of increased spontaneous emission rates.

The possibility of using direct pumping into the emitting level of the Nd3+ ion in magnesium-compensated strontium lanthanum. aluminate (Sr1-xLax-yNdyMgxAl12-xO19) to improve 900 nm F-4(3/2) -> I-4(9/2) laser emission is discussed. Selection of the composition parameters x and y for optimization of laser emission and reduction of heat generation is based on the spectroscopic and crystal growth characteristics. Pumping in the 865.5 nm absorption band I-4(9/2)(Z(1)) F-4(3/2)(R-1) transforms the laser process into a quasi-two-level scheme of very low (below 4%) quantum defects. A very high slope efficiency (over 84%) for 901 nm continuous-wave laser emission is demonstrated with Ti:sapphire laser pumping in this band for a crystal with x = 0.4 and y = 0.05. (c) 2006 Optical Society of America.

La2/3Ca1/3MnO3 (LCMO) nanowires were prepared using a sol-gel process and an anodically oxidized aluminum template. Scanning electron microscopy (SEM) shows that the nanowires are almost parallel and their diameter is around 40 nm. The temperature dependence of the electron spin resonance (ESR) linewidth is similar to that observed for the case of nanostructured LCMO. From this dependence, the determined magnetic transition temperature, T-C, of the LCMO nanowires, is of the order of 120-130 K.