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

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.


Supplementary information
is fitted to the data to obtain the activation energy and the pre-exponential factor as ( 0 ).

Remarks on the Multi-beam Optical Stress Sensor (MOSS)
The MOSS is a stress measurement technique based on the in-situ measurement of the sample curvature during the growth of a thin film. The working principle of the MOSS is described in detail elsewhere 1 . In brief, the MOSS is a laser deflectometer that illuminates the substrate surface with an array of parallel beams that are reflected from the substrate surface toward a CCD camera which records their relative position. As a consequence of the growth of a thin film in compressive or tensile stress, a force is applied parallel to the substrate surface inducing a curvature in the substrate. The stress-induced curvature introduces a divergence into the laser array, changing the mean differential spacing ( − 0 )/ 0 (see Fig. 1 in manuscript) of the reflected beams. The relative change of the mean differential spacing is proportional to the relative change of substrate curvature which is in turn proportional to the stress-thickness product of the growing film through the Stoney equation. To our knowledge MOSS, RHEED and PLD are combined the first time in our laboratory.
The angle of incidence of the laser beams or their orientation with respect to the crystallographic orientation of the substrate does not affect the measurement of the curvature (see Fig. 1 in manuscript), thus using the Stoney equation the stress measurement is always exactly in-plane, i.e. parallel to the substrate surface.
The stress thickness product is used as a unit, as this is commonly done in papers where a MOSS is used 2-8 , stress being the average stress over the film thickness. The advantages of this unit are: -It is directly proportional to the raw measurement data, i.e. the curvature -As compared to a curvature plot, the stress values can be read out relatively easily. More importantly, while for one particular strain value of the film the resulting curvature depends the elastic constant of the substrate (see Stoney's equation), the stress*thickness product does not. -As the stress is the average over the whole thickness, immediate changes are better visible in the stress*thickness product, e.g. the point where the curvature, and thus stress*thickness becomes constant (red line in Supplementary Fig. 2). In a stress plot this is only visible as a slight decrease in stress (orange square in Supplementary Fig. 2). After this point, the total elastic energy of the entire film remains constant while the thickness increases, which can be related to models for thin film relaxation 1,9,10 .

Electrode Geometry
The electrode schematics are shown in Supplementary Fig. 3 with a distance between the electrodes of 1 or 0.5 mm and a film thickness of tens of nm, the electric field can be safely assumed to be purely in-plane and uniform across the film thickness. Interdigitated electrodes were only used for the electrical characterization of the template platform MgO + BaZrO 3 + SrTiO 3 due to its high resistance.