Multi-modal microscopy and spectroscopy of wide band gap semiconductors

The heart of materials science is the link between materials structure and properties. Traditionally, the small-scale structure of materials is often assessed using microscopy techniques, which may provide access to topography, crystallography, composition etc. The physical properties of material are more commonly assessed at a macroscopic level, often using spectroscopy techniques. Increasingly, however, advanced microscopes allow spectroscopy or other physical property measurements with nanoscale resolution.
One established technique that allows direct correlation of microscopy and spectroscopy is cathodoluminescence (CL) in the scanning electron microscope (SEM), in which the recombination of electron-hole pairs excited by the electron beam leads to emission of light that is collected and analysed spectroscopically. CL can be collected alongside structural/compositional data recorded in the SEM. This approach is often simply used for the collection of panchromatic CL images, mapping the total intensity of light emitted at each pixel. However, sophisticated CL systems allow not only the collection of time-integrated spectra, but also time-resolved measurements on the tens of picosecond timescale and data collection across a range of temperatures.
I will discuss the application of temperature- and time-dependent CL to a range of problems in wide-bandgap semiconductors, particularly defects and quantum wells in gallium nitride, addressing both the standard hexagonal form of this material and metastable cubic GaN. However, SEM only provides a limited subset of the structural information a materials scientist desires. Thus, I will introduce methods that allow different microscopy techniques to be brought to bear on exactly the same nanoscale structure, revealing more about its characteristics than any individual technique could achieve alone. In particular, I will discuss methods for combining SEM-CL with transmission electron microscopy, and scanning probe microscopy.