Microscopy Techniques for the Characterisation of Buried Interfaces

The object of this project is to develop a technique to image buried interfaces of spintronic devices using a decelerated electron-beam in a scanning electron microscope (SEM, JEOL JSM-7800F Prime). First, well-designed defects at the interface will be fabricated using the ultrahigh-vacuum molecular beam epitaxy (UHV-MBE). The penetration profile of the electron-beam in the SEM will be simulated using CASINO (monte CArlo SImulation of electroN trajectory in sOlids) software available at . The simulated decelerated electron-beam will then be used in the SEM to image the buried interfaces with well-defined defects. We have already developed a method to image buried interfaces using a decelerated beam [A. Hirohata et al., Nature Commun. 7, 12701 (2016)] and will adopt this method to develop a technique to image the precise depth profile in a series of buried junctions. For comparison, cross-sectional transmission electron microscope (TEM) imaging will be employed. This will allow the calibration of the depth and size of the defects for the project.

Conventional methods to image buried interfaces are based on microscopy, spectroscopy, scattering and reflection, and electrical methods. Microscopic techniques are the most powerful, such as cross-sectional transmission electron microscopy (TEM), and can reveal detailed information about atomic structures at the junction interfaces. However, the additional preparation required for a cross-sectional sample involves erosion and strain-induced damage of the junction and hence features that subsequently appear in the image may be due to the sample fabrication rather than being inherent in the original device. On the other hand, spectroscopic techniques can disclose the chemical composition in the vicinity of the junction interfaces and can be used in combination with other techniques, such as microscopic imaging. Secondary ion mass spectroscopy, Auger electron spectroscopy, energy dispersive X-ray spectroscopy and cathode luminescence have nanometric resolution but again they require destructive sample preparation, typically Ar-ion bombardment, to expose the desired interfaces for analysis. For junction evaluation techniques based on reflection and scattering, elipsometry has both nanometric resolution and is non-destructive. However, it requires an analytical model for fitting and is difficult to correlate to transport properties. In this project, a decelerated electron beam will be used to control the penetration depth into a multilayered junction. By combining the control of the electron-beam voltage and energy filters, these electrons generated at a specified layer can be collected to produce an image of conductance distributions across the buried junction.

The successful student will spend up to 3 months a year at Seagate Technologies in Derry, Northern Ireland, to perform supplementary measurements.

The research team currently has 7 PhD students, 1 Experimental Officer and 1 Post-Doctoral Research Associate. An enthusiastic PhD candidate will fit perfectly to our team. This project will involve collaboration with Japanese universities. PhD students are normally given an opportunity to present their work at the annual domestic Magnetism conference and at least once at an international conference, such as the International Conference on Magnetism or Magnetism and Magnetic Materials.