Electrons, photons, & the time in between: probing materials with pump-probe cathodoluminescence in an ultrafast SEM

Cathodoluminescence (CL) microscopy probes the optical properties of materials with the nanoscale spatial resolution of an electron beam. The development of ultrafast scanning electron microscopes (USEMs) has made it possible to study dynamic processes, making CL a quintessential tool for investigating III-V semiconductors, central to many modern technologies. With the addition of laser incoupling and electron energy spectroscopy, the USEM can be further expanded into pump-probe cathodoluminescence (PP-CL) and photon-induced near-field electron microscopy (PINEM) schemes, to explore fundamental interactions between electrons, light, and matter.

In this thesis, we first introduce a unique PP-CL setup and apply it to study to carrier recombination dynamics in Cu2ZnSnS4, an earth-abundant semiconductor of interest for thin-film photovoltaics. We then combine correlative CL spectroscopy and time-resolved-CL to investigate an InGaN/GaN quantum well structure, before and after focused ion beam (FIB) lamella preparation. While FIB thinning is widely used for TEM, we demonstrate how the optical properties of InGaN/GaN are significantly impacted. Next, we investigate the lasing properties of individual GaN nanowires, correlating emission to nanowire morphology and CL signal.

Finally, we reconfigure the pump-probe microscope to incorporate a home-built electron energy spectrometer based on a retarding field analyzer (RFA), enabling PINEM and EELS measurements in the USEM. We describe the technical implementation of the set-up and alignment, demonstrate near-field mapping of a dielectric metasurface, and use PINEM cross-correlation measurements and energy filtering with the RFA to measure the electron pulse lengths and energy spread as a function of different gun parameters.