F. Sánchez

Whiskerlike structure growth on silicon exposed to ArF excimer laser irradiation

The effects of ArF excimer laser irradiation on silicon single crystals in air have been studied. The etch rate versus fluence curve shows three well defined zones, with very different etch rates and dependences. In the intermediate zone (from 1.5 to 2.5 J/cm2), narrow (1–2 μm diameter) and tall columns (3–30 μm) start to grow after irradiation with some hundreds of laser pulses. These whiskerlike columns, with height between one and two orders of magnitude higher than the depth of the crater, have not been formed by preferential etching of the surrounding material, but through hydrodynamical processes.

Enhanced pinning in a magnetic-superconducting bilayer

Pinning of vortices in a high-temperature superconductor by the magnetic domain structure of a highly anisotropic ferromagnet is investigated by means of magnetic measurements in nanoscale period superconductor/ferromagnet (SC/FM) heterostructures. Two different samples consisting of highly epitaxial films of YBa2Cu3O7(SC) and BaFe12O19(FM) are analyzed relative to a pure superconducting YBa2Cu3O7 film. The irreversibility line obtained in the magnetic-field-reduced-temperature phase diagram for each heterostructure is found to shift upwards when compared to the line corresponding to the pure superconducting sample. This effect is interpreted as an evidence for the enhancement of pinning of vortices in the SC layer by the magnetic domain structure in the FM layer.

Pulsed laser deposition of epitaxial LaNiO3 thin films on buffered Si(100)

Abstract Epitaxial LaNiO 3 (LNO) thin films were deposited on Si(100) substrates by pulsed laser deposition. (100)-oriented films, without any other orientations, were obtained by using an intermediate double layer of CeO 2 /YSZ. We studied the influence of substrate temperature and oxygen pressure on the crystal structure and room temperature electrical resistivity of LNO films. Optimal processing conditions allow the growth of LNO films with surface morphology, crystal structure and electrical properties suitable for the use of the conductive layer as an electrode in ferroelectric capacitors.

ARF AND KRF EXCIMER LASER DEPOSITION OF YTTRIA-STABILIZED ZIRCONIA ON SI(100)

Yittria‐stabilized zirconia films have been deposited on Si(100) substrates by ArF (193 nm) and KrF (248 nm) laser ablation. By using KrF radiation, high‐quality (h00) epitaxial films are obtained on bare silicon. Epitaxy, although with slightly worse properties, is also obtained on substrates covered with a native oxide. Films have been deposited by ArF laser ablation over a wide range of substrate temperature and oxygen partial pressure and with variation in substrate surface cleaning. These films are polycrystalline and texture or epitaxy have not been obtained in any case. These results reveal that laser wavelength is a crucial factor in determining film properties.

CHARACTERIZATION OF THE PROGRESSIVE GROWTH OF COLUMNS BY EXCIMER LASER IRRADIATION OF SILICON

The growth mechanism of columns produced by excimer laser irradiation of a silicon target is investigated. The micrometric columns are formed by a cumulative process with well-defined steps. The first steps lead to silicon hillocks, which evolve to columns through a little-understood mechanism. To investigate it, we irradiated a Si(100) single crystal in air with the necessary pulses of an excimer laser beam to obtain two samples, one with a partially and one with a fully developed column structure. Then we alternated scanning electron microscopy observations with irradiation pulses on the same target area. The progressive evolution of the laser-generated structures is presented. Columns grow at notably high rates, from around 0.5 μm/pulse during the early stages to 0.2 μm/pulse on average for 10–20 μm tall columns. The experimental results lead us to suggest a hydrodynamic growth mechanism.

Transition from three- to two-dimensional growth in strained SrRuO3 films on SrTiO3(001)

The morphology of strained SrRuO3 films grown on SrTiO3(001) has been investigated as a function of thickness. A transition of growth mode has been observed. At the early stages, there is a fingerlike structure originated by three-dimensional (3D) islands that nucleated along the substrate steps. Afterward, adatoms stick preferentially in the valleys of the structure and the films become progressively smoother. At a thickness above 10–20 nm, the films are extremely flat and have a self-organized structure of terraces and steps, with the growth proceeding mainly by a step flow (two-dimensional) mechanism. Relevance on film properties and possible use of the initial, nanometric, 3D structures are discussed.

Impact of microstructure on transport properties of nanometric epitaxial SrRuO3 films

We report on structural and magnetotransport properties of SrRuO3 films—grown on SrTiO3—and its dependence on thickness t (4 nm⩽t⩽320 nm). At early stages of film growth, a self-ordered finger-like pattern of growth units is formed and gives rise to a prominent in-plane anisotropy of transport properties. We argue that this behavior originates from defective regions formed at grain-merging regions and we show that appropriate annealing allows eliminating these defects. In addition, there is a progressive shift towards a lower temperature of the characteristic feature associated with the onset of the ferromagnetism. Since the films are fully strained, we argue that both effects are not associated to homogeneous cell modifications, but reflects the microstructural disorder concomitant to the island growth observed at early stages of film growth, the resulting stress distribution and the polymorphism of SrRuO3.

Effects of wavelength, deposition rate and thickness on laser ablation deposited YSZ films on Si(100)

Abstract Laser wavelength has important effects on the crystalline properties of yttria stabilized zirconia (YSZ) films deposited on Si(100) by KrF and ArF excimer laser ablation. While KrF-deposited films have been found to be epitaxial on Si(100), only textured films can be obtained with ArF over wide pressure and temperature ranges. Studies on YSZ ablation showed that the deposition rate could be the main difference between the depositions with these wavelengths under the same conditions. The KrF deposition rate was reduced to the same level as ArF by changing the laser fluence and spot size, but the change had no effects on the films. Crystal quality decreases with film thickness, but the film texture and in-plane orientation ([100]YSZ∥[100]Si) do not depend on this parameter. Laser repetition rate was found to affect film properties. Best results are obtained at high repetition rates (over 10 Hz), while polycrystalline samples grow at 1 Hz. This behaviour is most probably related to YSZ degradation by exposure to high temperature.

Pulsed laser deposition of epitaxial PbZrxTi1-xO3 ferroelectric capacitors with LaNiO3 and SrRuO3 electrodes

It is commonly accepted that, in order to solve the fatigue problem in PbZr x Ti 1-x O 3 (PZT) ferroelectric capacitors, oxide electrodes have to be used. However, clear criteria for the choice of an electrode material have not given yet. Here we report on the deposition and ferroelectric properties of epitaxial pulsed laser deposited PZT capacitors with SrRuO 3 and LaNiO 3 perovskite bottom electrodes, and aluminium top electrodes. Remarkable improvement of the ferroelectric properties, i.e. remanent polarization increasing from 7.5 to 37 μC/cm 2 , is observed when the bottom electrode material is changed from SrRuO 3 to LaNiO 3 . This improvement is attributed to a smoother ferroelectriclelectrode interface and the formation of the tetragonal phase of PZT instead of the rhombohedral one.

High-quality epitaxial LaNiO3 thin films on SrTiO3(100) and LaAlO3(100)

Abstract.LaNiO3 thin films have been deposited by pulsed laser deposition on SrTiO3(100) and LaAlO3(100) substrates. The processing conditions have been investigated in order to optimize electrical resistivity, crystal quality, and surface morphology. Excellent properties are achieved at moderate substrate temperature and relatively low oxygen pressure, without the need for annealing. Thickness exerts an important influence on electrical transport, as the electrical resistivity increases quickly in films thicker than a few tens of nanometer. The surface of the films on LaAlO3 is very flat in all the studied thickness range, but the films on SrTiO3 develop a pattern of boundaries and even cracks as the thickness is higher. Below the critical thickness, high-quality epitaxial films with very smooth surface and low electrical resistivity are obtained under the optimum conditions of substrate temperature and oxygen pressure. The optimum processing conditions are different depending on the substrate, and control is especially critical in films deposited on SrTiO3.