Scientific Highlights 2015



The flip-over effect in pulsed laser deposition: Is it relevant at high background gas pressures?

Abstract:
In pulsed laser deposition the use of a rectangular or elliptical beam spot with a non 1:1 aspect ratio leads to the so called flip-over effect. Here, the longest dimension of the laser spot results in the shortest direction of plasma plume expansion. This effect has been mainly reported for vacuum depositions of single element targets and is particularly noticeable when the aspect ratio of the beam spot is large.

We investigate the flip-over effect in vacuum and at three relevant background-gas pressures for pulsed laser deposition using a La0.4Ca0.6MnO3 target by measuring the thickness dependence of the deposited material as a function of angle. The film thicknesses and compositions are determined by Rutherford backscattering and argon is used to reduce the influence of additional chemical reactions in the plasma. The results show the prevalence of the flip-over effect for all pressures except for the highest, i.e. 1 × 10−1 mbar, where the film thickness is constant for all angles. The composition profiles show noticeable compositional variations of up to 30% with respect to the target material depending on the background gas pressure, the angular location, and the laser spot dimensions.
Keywords: Flip-over effect; Angular distribution; Pulsed laser deposition; Composition; Congruent transfer; Laser spot size;

Facility: ENE, ETH, LMX, Thin Films and Interfaces

Reference: Alejandro Ojeda-G-P, Christof W. Schneider, Max Döbeli, Thomas Lippert, Alexander Wokaun, Appl. Surf. Science 357, 2055 (2015)

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Crystallization of zirconia based thin films

Abstract:
The crystallization kinetics of amorphous 3 and 8 mol% yttria stabilized zirconia (3YSZ and 8YSZ) thin films grown by pulsed laser deposition (PLD), spray pyrolysis and dc-magnetron sputtering are explored. The deposited films were heat treated up to 1000 °C ex situ and in situ in an X-ray diffractometer. A minimum temperature of 275 °C was determined at which as-deposited amorphous PLD grown 3YSZ films fully crystallize within five hours. Above 325 °C these films transform nearly instantaneously with a high degree of micro-strain when crystallized below 500 °C. In these films the t′′ phase crystallizes which transforms at T > 600 °C to the t′ phase upon relaxation of the micro-strain. Furthermore, the crystallization of 8YSZ thin films grown by PLD, spray pyrolysis and dc-sputtering are characterized by in situ XRD measurements. At a constant heating rate of 2.4 K min−1 crystallization is accomplished after reaching 800 °C, while PLD grown thin films were completely crystallized already at ca. 300 °C.
Keywords: pulsed laser deposition; sputtering; YSZ; recrystallization; ion conductors;

Facility: ENE, ETH, LMX, Thin Films and Interfaces

Reference: D. Stender, R. Frison, K. Conder, J. L. M. Rupp, B. Scherrer, J. M. Martynczuk, L. J. Gauckler, C. W. Schneider, T. Lippert and A. Wokaun, Phys. Chem. Chem. Phys., 17, 18613-18620 (2015)

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Interplay between magnetic order at Mn and Tm sites alongside the structural distortion in multiferroic films of o-TmMnO3

Abstract:
We employ resonant soft x-ray diffraction to individually study the magnetic ordering of the Mn and the Tm sublattices in single-crystalline films of orthorhombic (o−)TmMnO3. The same magnetic ordering wave vector of (0q0) with q≈0.46 is found for both ionic species, suggesting that the familiar antiferromagnetic order of the Mn ions induces a magnetic order on the Tm unpaired 4f electrons. Indeed, intensity variations of magnetic reflections with temperature corroborate this scenario. Calculated magnetic fields at the Tm sites are used as a model magnetic structure for the Tm, which correctly predicts intensity variations at the Tm resonance upon azimuthal rotation of the sample. The model allows ruling out a bc-cycloid modulation of the Mn ions as the cause for the incommensurate ordering, as found in TbMnO3. The structural distortion, which occurs in the ferroelectric phase below TC, was followed through nonresonant diffraction of structural reflections forbidden by the high-temperature crystal symmetry. The (0q0) magnetic reflection appears at the Mn resonance well above TC, indicating that this reflection is sensitive also to the intermediate sinusoidal magnetic phase. The model presented suggests that the Tm 4f electrons are polarized well above the ferroelectric transition and are possibly not affected by the transition at TC. The successful description of the induced order observed at the Tm resonance is a promising example for future element-selective studies in which “spectator” ions may allow access to previously unobtainable information about other constituent ions.
Keywords: pulsed laser deposition; soft resoant x-ray diffraction; multiferroics; thin films; magnetic ground state;

Facility: ENE, SLS, LMX, Thin Films and Interfaces

Reference: Y. William Windsor, Mahesh Ramakrishnan, Laurenz Rettig, Aurora Alberca, Elisabeth M. Bothschafter, Urs Staub, Kenta Shimamoto, Yi Hu, Thomas Lippert, and Christof W. Schneider , Phys. Rev. B, 91, 235144 (2015)

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Low-Temperature Micro-Solid Oxide Fuel Cells with Partially Amorphous La0.6Sr0.4CoO3-δ Cathodes

Abstract:
Partially amorphous La0.6Sr0.4CoO3-δ (LSC) thin-film cathodes are fabricated using pulsed laser deposition and are integrated in free-standing micro-solid oxide fuel cells (micro-SOFC) with a 3YSZ electrolyte and a Pt anode. A low degree of crystallinity of the LSC layers is achieved by taking advantage of the miniaturization of the cells, which permits low-temperature operation (300–450 °C). Thermomechanically stable micro-SOFC are obtained with strongly buckled electrolyte membranes. The nanoporous columnar microstructure of the LSC layers provides a large surface area for oxygen incorporation and is also believed to reduce the amount of stress at the cathode/electrolyte interface. With a high rate of failure-free micro-SOFC membranes, it is possible to avoid gas cross-over and open-circuit voltages of 1.06 V are attained. First power densities as high as 200–262 mW cm−2 at 400–450 °C are achieved. The area-specific resistance of the oxygen reduction reaction is lower than 0.3 Ω cm2 at 400 °C around the peak power density. These outstanding findings demonstrate that partially amorphous oxides are promising electrode candidates for the next-generation of solid oxide fuel cells working at low-temperatures.
Keywords: pulsed laser deposition; µ-SOFC; Anode materials; Amorphous thin films; fuel cell low-temperature operation;

Facility: ENE, ETH, LMX, Thin Films and Interfaces

Reference: Anna Evans, Julia Martynczuk, Dieter Stender, Christof W. Schneider, Thomas Lippert, and Michel Prestat , Adv. Energy Mater. 5, 1400747 (2015)

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