High Spatio-Temporal Resolution Low-Dose Phase Characterisation in STEM Using Detector Signal Digitisation

The capability of resolving electric and magnetic fields within materials using the differential phase contrast (DPC) technique in scanning transmission electron microscopy (STEM) has been demonstrated thoroughly. Due to the high sensitivity of this technique, it is becoming commonly used to image both long range fields inside samples and the electric fields surrounding atomic nuclei. This enhances STEM materials characterisation beyond the structural characterisation available using high-angle annular dark field (HAADF) imaging. Additionally, DPC detectors are placed within the bright field region and the high collection efficiency achieved results in a much higher dose efficiency than HAADF. For characterisation of beam sensitive materials, reduction of beam current is crucial to avoid sample damage and degradation during imaging, and DPC is a promising low-dose imaging technique since it allows significantly reduced beam currents.<br/><br/>Furthermore, dose fractionation by multi-framing has been previously demonstrated to significantly reduce sample damage. By acquiring multiple frames with a lower dwell time, as opposed to a single frame with a higher dwell time, the dose is delivered to the sample in shorter bursts, delaying sample degradation. However, at very high scan speeds a scintillator detector is typically too slow to keep up, resulting in artefacts from the temporal response of the detector in the final image. In this work we investigate the practicability of in-hardware digitisation of scintillator detector signal from a segmented annular all-field (SAAF) detector used for STEM-DPC. Live digitisation of the detector signal yields a purely digital image where each electron is detected equally, and the noise-floor of the image is true zero. Most importantly, digitisation retains the precision of the temporal position of electron detection events, and as such they show up in one pixel only in the final image.<br/><br/>Using in-hardware, live digitisation of four segments on a SAAF detector, we have demonstrated experimental imaging of STO at a very fast scan speed: with a dwell time of only 50 ns. At this speed the images from using a scintillator detector exhibit significant loss of information in the fast scan direction due to severe streaking artefacts. On the other hand, the digitised images retain precision of the atomic columns. Finally, binning of the multi-frame stacks in the time-direction allows us to sacrifice some of the signal-to-noise ratio for temporal resolution, paving the way for in-situ phase characterisation in STEM.