Papers by Keyword: Epitaxial Thin Films

Abstract: La_1-xCa_xMnO₃ (x=0.0-1.0) epitaxial thin films were deposited on both tilted and untilted LaAlO₃ (0 0 1) single crystalline substrates using PLD (pulsed laser deposition) technique, in which the employed qualified polycrystalline La_1-xCa_xMnO₃ targets were synthesized by sol-gel method. The structural properties and the ultraviolet LITV (laser induced thermoelectric voltage) effect of the as-grown films on the tilted substrates were studied. The results demonstrate that the intensities of the LITV signals depend non-monotonously on Ca content, that is, the peak values of the LITV signals firstly increase and then decrease with Ca content increasing. The maximal peak value of LITV was found in the film with Ca=0.33.

Abstract: We report on the preparation and investigation of epitaxial thin films of the magnetic shape memory alloy Ni2MnGa. For samples close to the stoichiometric composition we find that the phase transformation temperature is affected by the crystallographic orientation. Changes in the crystal structure due to the transformation are observed using temperature-dependent X-ray diffraction. Films with higher manganese content are in the martensitic state at room temperature. Those samples on Al2O3(11-20) reveal the 7-layered orthorhombic structure that allows strains up to 10 %. To avoid blocking of magnetostrictive effects by the substrate, free-standing films are prepared using water-soluble NaCl(100) single crystals as substrate.

Abstract: Epitaxial Single-crystal ZnO thin films have been grown on c-plane (0001) sapphire by Pulsed Laser Deposition process at different substrate temperatures (300 – 800 °C) with 10-1 mbar oxygen pressure. The thicknesses of the films have been varied by varying number of pulses with a repetition rate of 10 pulse/sec. It is found that the sheet resistivity of ZnO thin films grown on c-plane sapphires are in the order of 10-2 Ω-cm and it increases with increasing substrate temperatures and film thickness. The carrier concentrations and Hall mobility are found to be in the order of 1017 cm-3 and ~195 cm2/V-s, respectively. The Hall mobility slightly decreases with increase of substrate temperature and thickness of the films. It is also found that the ZnO films are structurally uniform and well oriented with perfect wurtzite structure with c/a ratio 5.1. We have also deposited non-epitaxial ZnO films on (100) p-Silicon substrates at the same conditions. From HR FE-SEM micrographs, surface morphology of ZnO films grown at lower substrate temperature are found to be uniform compared to the films grown at higher temperatures showing non-uniformity and misoriented wurtzite structures. However, the surface morphology of ZnO flims grown epitaxially on (0001) sapphire are found to be more uniform and it does not change much with growth temperature. The resistivity of the films grown on p-Silicon at higher temperatures is in the order of 103 Ω-cm whereas films grown at lower substrate temperatures show comparatively lower resistivities (~ 102 Ω-cm). From the recorded UV-Visible absorption spectrum the band gap of the film has been estimated to be 3.38 eV.

Abstract: In this work we present results of contact resistance between La2/3Ca1/3MnO3 (LCMO) and Pt system. LCMO epitaxial thin films have been grown by rf sputtering on top of LaAlO3 (LAO) substrate. The contact between the manganite film and the metal have been prepared by exsitu deposition of a 30 nm thick Pt layer on top of the manganite film by e-beam evaporation at room temperature. Different nanostructured contact geometries have been defined by using a focus ion beam system and then transport properties have been tested by means of AFM system working on the current sensing mode with a doped diamond tip. We show that the top-most LCMO layers do play a very relevant role on the transport properties acting as an insulating barrier. AFM measurements in the current sensing mode exhibit typical features of tunneling conduction. Ex-situ annealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier. X-ray absorption spectroscopy measurement at the Mn Ledge has been performed to gain a deeper insight into the properties of the top-most LCMO layers.

Abstract: Magnetotransport properties of very high quality fully strained epitaxial thin films of La2/3Ca1/3MnO3 (LCMO), grown on top of SrTiO3 (STO) (001) substrates, are analyzed. As-grown fully strained epitaxial films are ferromagnetic and metallic but exhibit depressed transport and magnetic properties (low TC and MS). Detailed analysis of the structural and magnetotransport properties after high temperature annealing processes, have revealed a progressive structural relaxation, as detected by the variation of the c/a tetragonal distortion, as the annealing temperature rises up. Nevertheless, no changes in the in-plane cell parameters are observed. Simultaneously, the magnetic transition temperature, TC, and saturation magnetisation, MS, substantially increase approaching values similar to that of bulk materials.