Transport, magnetic, and structural properties of La0.7_{0.7}Ce0.3_{0.3}MnO3_3 thin films. Evidence for hole-doping

Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720 ^\circ$C and oxygen pressure $p_{O_2}=1-25$Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO$_2$ as a secondary phase could be avoided for $p_{O_2}\ge 8$Pa. However, transmission electron microscopy shows the presence of CeO$_2$ nanoclusters, even in those films which appear to be single phase in XRD. With O$_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La$_{0.7}$Ce$_{0.3}$MnO$_3$ with mixed Mn$^{3+}$/Mn$^{2+}$ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-to-insulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from 2+ to 4+. The dominating Mn valences in the films are Mn$^{3+}$ and Mn$^{4+}$, and only a small amount of Mn$^{2+}$ ions can be observed by XAS. Mn$^{2+}$ and Ce$^{4+}$ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.Comment: 8 pages, 10 figure

Importance of grain boundary Josephson junctions in the electron-doped infinite-layer cuprate superconductor Sr1−x_{1-x}Lax_xCuO2_2

Grain boundary bicrystal Josephson junctions of the electron-doped infinite-layer superconductor Sr$_{1-x}$La$_x$CuO$_2$ ($x = 0.15$) were grown by pulsed laser deposition. BaTiO$_3$-buffered 24\,^\circ [001]-tilt symmetric SrTiO$_3$ bicrystals were used as substrates. We examined both Cooper pair (CP) and quasiparticle (QP) tunneling by electric transport measurements at temperatures down to 4.2\,K. CP tunneling revealed an extraordinary high critical current density for electron-doped cuprates of $j_c > 10^3\,$A/cm$^2$ at 4.2\,K. Thermally activated phase slippage was observed as dissipative mechanism close to the transition temperature. Out-of-plane magnetic fields $H$ revealed a remarkably regular Fraunhofer-like $j_c(H)$ pattern as well as Fiske and flux flow resonances, both yielding a Swihart velocity of $3.1\cdot10^6\,$m/s. Furthermore, we examined the superconducting gap by means of QP tunneling spectroscopy. The gap was found to be V-shaped with an extrapolated zero temperature energy gap $\Delta_0 \approx 2.4\,$meV. No zero bias conductance peak was observed.Comment: 6 pages, 3 figure

Superconducting thin films of MgB2 on (001)-Si by pulsed laser deposition

Superconducting thin films have been prepared on Si-substrates, using pulsed laser deposition from a target composed of a mixture of Mg and MgB2 powders. The films were deposited at room temperature and post-annealed at 600 degrees C. The zero resistance transition temperatures were 12 K, with an onset transition temperature of 27 K. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. For this the optical emission in the visible range from the plasma has been used as indicator. Preventing Mg from oxidation was found to be essential to obtain superconducting films

Transmission Electron Microscopy on Interface Engineered Superconducting Thin Films

Transmission electron microscopy is used to evaluate different deposition techniques, which optimize the microstructure and physical properties of superconducting thin films. High-resolution electron microscopy proves that the use of an YBa2Cu2O buffer layer can avoid a variable interface configuration in YBa2Cu3O7 thin films grown on SrTiO3. The growth can also be controlled at an atomic level by using sub-unit cell layer epitaxy, which results in films with high quality and few structural defects. Epitaxial strain in Sr0 85La0 15CuO2 infinite layer thin films influences the critical temperature of these films, as well as the microstructure. Compressive stress is released by a modulated or a twinned microstructure, which eliminates superconductivity. On the other hand, also tensile strain seems to lower the critical temperature of the infinite layer

Properties of the electron-doped infinite-layer superconductor Sr1−x_{1-x}Lax_{x}CuO2_{2} epitaxially grown by pulsed laser deposition

Thin films of the electron-doped infinite-layer cuprate superconductor Sr$_{1-x}$La$_x$CuO$_2$ (SLCO) with doping $x \approx 0.15$ were grown by means of pulsed laser deposition. (001)-oriented KTaO$_3$ and SrTiO$_3$ single crystals were used as substrates. In case of SrTiO$_3$, a BaTiO$_3$ thin film was deposited prior to SLCO, acting as buffer layer providing tensile strain to the SLCO film. To induce superconductivity, the as-grown films were annealed under reducing conditions, which will be described in detail. The films were characterized by reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, Rutherford backscattering spectroscopy, and electric transport measurements at temperatures down to $T = 4.2\,$K. We discuss in detail the influence of different process parameters on the final film properties.Comment: 16 pages, 14 figure

New low-stress PECVD poly-SiGe layers for MEMS

Thick poly-SiGe layers, deposited by plasma-enhanced chemical vapor deposition (PECVD), are very promising structural layers for use in microaccelerometers, microgyroscopes or for thin-film encapsulation, especially for applications where the thermal budget is limited. In this work it is shown for the first time that these layers are an attractive alternative to low-pressure CVD (LPCVD) poly-Si or poly-SiGe because of their high growth rate (100-200 nm/min) and low deposition temperature (520/spl deg/C-590/spl deg/C). The combination of both of these features is impossible to achieve with either LPCVD SiGe (2-30 nm/min growth rate) or LPCVD poly-Si (annealing temperature higher than 900/spl deg/C to achieve structural layer having low tensile stress). Additional advantages are that no nucleation layer is needed (deposition directly on SiO/sub 2/ is possible) and that the as-deposited layers are polycrystalline. No stress or dopant activation anneal of the structural layer is needed since in situ phosphorus doping gives an as-deposited tensile stress down to 20 MPa, and a resistivity of 10 m/spl Omega/-cm to 30 m/spl Omega/-cm. With in situ boron doping, resistivities down to 0.6 m/spl Omega/-cm are possible. The use of these films as an encapsulation layer above an accelerometer is shown

Superconducting thin films of MgB2 on Si by pulsed laser deposition

Superconducting thin films have been prepared on Si-substrates, using pulsed laser deposition from a target composed of a mixture of Mg and MgB2 powders. The films were deposited at room temperature and annealed at 600°C. The zero resistance transition temperatures were 11-15.5 K, with an onset transition temperature of 27 K. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. For this the optical emission in the visible range from the plasma has been used as indicator. Preventing Mg from oxidation was found to be essential to obtain superconducting films