Microstructuring of materials with focused ion beam for charge transport studies

We use focused ion beam (FIB) microscopy and cleanroom techniques in order to tailor samples of investigated materials on a micron scale for high-precision measurements of charge transport properties. This approach allows greatly enhancing signal-to-noise ratio by optimising the aspect ratio of samples, enables accurately probing resistivity along specific crystallographic directions, and generally facilitates studies of novel compounds, which can only be synthesised as sub-millimetre-sized crystallites.

Scanning electron microscope image of a FIB-microstructured sample for resistivity anisotropy measurements. False colours are used: purple – probed crystal, beige – layer of gold acting as electrical leads. The crystal has a layered structure and the current path contains multiple sections where either the in-plane or the c-axis resistance can be probed.

Measurements of charge transport properties, such as electrical resistivity and Hall effect, are often used for gaining insights into the electronic structure of materials. However, conducting a high-quality measurement generally requires a careful preparation of the sample, which involves shaping the crystal into a well-defined current channel and making robust and low-resistance electrical contacts between the sample and the measurement leads. These tasks become increasingly difficult when crystals can only be synthesised as tiny sub-millimetre-sized grains, or have a particular morphology, making it hard to constrain the flow of current along a desired direction.

The aforementioned problems can be alleviated by employing microscopy-based techniques for sample preparation. The institute has two focused ion beam (FIB) microscopes which allow controlled ablation of materials down to a sub-micrometre scale. Using these devices, as well as the institute’s cleanroom facilities, we produce samples of well-defined and optimal geometries for high-precision measurements of charge transport properties. The particular examples include making narrow meandering samples of large effective length to cross-section ratio, thus greatly amplifying measured resistance, or defining current channels that extend along specific crystallographic directions, in order to study resistivity anisotropy. We also explore new ways of making use of FIB-based techniques, such as making miniature strain devices.

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