Ultrafast Scanning Electron Microscopy (USEM) to probe charge dynamics at surfaces of oxide thin films

Photon—assisted Ultrafast Scanning Electron Microscopy (USEM) is a novel surface-sensitive stroboscopic pump-probe technique, to characterize charge carrier dynamics. USEM employs synchronized pulsed laser and electron beams, to excite optical transitions and to dynamically probe them in terms of the evolution in Secondary Electrons (SEs) contrast. It combines the ps time resolution, typical of ultrafast pump-probe techniques, with the nanoscale spatial resolution deriving from Scanning Electron Microscopy. The USEM technique will be introduced, as specifically tailored to visualize color center dynamics in insulating oxide thin films. The USEM apparatus here presented is excited by an UV laser beam and operates in Ultra-High Vacuum regime. By a lock-in demodulation scheme, fast dynamical SE signals in the picosecond and nanosecond temporal scale are detected by overcoming charging effects and optically induced CW contributions. In this way USEM becomes a characterization technique-complementary to time-resolved Photoluminescence and Cathodoluminescence also for oxide thin films and wide bandgap insulators, to visualize the SE dynamics proceeding from optically active defects and charge traps. Results for the case of Aluminum Oxide – on-Silicon thin film will be shown as example. In the case of alumina, the temporal evolution in SE contrast probes the dynamics of UV-excited oxygen vacancy states at surface, as well as hot electron photoemission from color centers excited by the electron beam. The observation returned fast dynamics, with components ranging from tens of ps to few ns, fitting within the timescales typical of the UV color centers evolution.