October 2, 2014

Scientists from NPL, the XMaS beamline and the I16 beamline at Diamond Light Source Ltd have carried out the first experiments using a new technique to measure crystallographic lattice parameters and their distribution (intrinsic strain) in thin film heterostructures.

The technique which is also applicable to single crystals involves shinning synchrotron light through thin film samples where it undergoes Diffuse Multiple Scattering (DMS). The DMS gives rise to lines of weak scatter which are similar to the Kikuchi features seen in diffuse electron scattering, and arise from an extended secondary source within the sample. In this case this source is the crystal truncation rods of the substrate in conjunction with the thin film truncation rods.

This opens up the possibility of simultaneous measurements in a single snapshot of the substrate as well as the film lattice parameters and their distribution. This technique could have a resolution on the order of 10-4 Ångstroms, which is comparable with standard techniques, without requiring the sample to be moved. Measurements could also be taken under the influence of external stimuli, allowing the determination of strain in ferroelectric materials as well as the deformation of the substrate induced by the piezoelectric material.

The preliminary experiments carried out in May used a Gold single crystal as an exemplar to calibrate these calculations. The top two panels of Figure 1 show the X-ray data collected on a single-photon counting area detector around the (2.6 2.6 1.6) reciprocal lattice point. The second panel uses red dotted lines to highlight the weak DMS line.

 

The bottom two panels show the corresponding DMS lines for a sample of technological relevance, such as thin piezoelectric heterostructure on a ceramic substrate.  In this case the red circle highlights a crystal truncation rod which, in this instance, is used like a torchlight to enhance the DMS signal. For this sample the data were collected around the (0 2.2 0) reciprocal point in the pseudo-cubic crystallographic unit cell of the piezoelectric thin film.

This data, and subsequent data recorded with electric filed induced changes in strain, represents a first for ferroelectric thin films and presents a novel simultaneous method to explore the static and potentially dynamic response of functional films and their substrates.