In situ stress observation in oxide films and how tensile stress influences oxygen ion conduction

Working principle of the multi-beam optical stress sensor (MOSS): 10 × 10 mm2 MgO substrate on the sample holder of the PLD system equipped with MOSS and RHEED. A 3 × 3 array of parallel laser beams (visible as bright spots on the substrate surface) is reflected by the substrate towards a CCD camera that records the relative distance between the laser spots. The paths of two laser beams of the MOSS and of the electron beam of the RHEED are sketched. The growth of a strained layer induces a change of curvat…
Working principle of the multi-beam optical stress sensor (MOSS): 10 × 10 mm2 MgO substrate on the sample holder of the PLD system equipped with MOSS and RHEED. A 3 × 3 array of parallel laser beams (visible as bright spots on the substrate surface) is reflected by the substrate towards a CCD camera that records the relative distance between the laser spots. The paths of two laser beams of the MOSS and of the electron beam of the RHEED are sketched. The growth of a strained layer induces a change of curvature (1/ρ) of the substrate and a change of the direction of the reflected laser beams. The effect of a stress-induced curvature of the substrate is illustrated in cross-section for the case of an in-plane tensile strained film (ρ>0). The relative curvature change is obtained by measuring the change in the relative distance (D−D0)/D0 between the laser beams; D0 being the distance at the beginning of the growth, L the optical path length and α the incident angle. From the CCD image, the MOSS software calculates the average of (D−D0)/D0 using all nine beams.

Many properties of materials can be changed by varying the interatomic distances in the crystal lattice by applying stress. Ideal model systems for investigations are heteroepitaxial thin films where lattice distortions can be induced by the crystallographic mismatch with the substrate. Here we describe an in situ simultaneous diagnostic of growth mode and stress during pulsed laser deposition of oxide thin films. The stress state and evolution up to the relaxation onset are monitored during the growth of oxygen ion conducting Ce0.85Sm0.15O2-δ; thin films via optical wafer curvature measurements. Increasing tensile stress lowers the activation energy for charge transport and a thorough characterization of stress and morphology allows quantifying this effect using samples with the conductive properties of single crystals. The combined in situ application of optical deflectometry and electron diffraction provides an invaluable tool for strain engineering in Materials Science to fabricate novel devices with intriguing functionalities.

Keywords: strain; thin films; pulsed laser deposition; optical wafer curvature measurements; oxygen ion conductors;

Facility: ENE; LMX, Thin Films and Interfaces

Reference: Aline Fluri, Daniele Pergolesi, Vladimir Roddatis, Alexander Wokaun & Thomas Lippert, Nature Communications 7, 064421 (2016)

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