Ultrafast Scanning Electron Microscopy (USEM) to probe charge dynamics in oxide thin films

Photon—assisted Ultrafast Scanning Electron Microscopy (USEM) is a novel stroboscopic pump-probe technique to probe charge carrier dynamics, featuring ps time resolution, nanoscale spatial resolution and surface sensitivity. It employs synchronized pulsed laser and electron beams, respectively to excite optical transitions and to probe their dynamical effects in terms of Secondary Electron (SE) contrast. A USEM setup has been devised, excited by an UV laser beam and operating in Ultra-High Vacuum regime. By a lock-in detection scheme, fast dynamical SE signal in the picosecond and nanosecond scale can overpass charging effects and optically induced CW contributions. In this way USEM becomes a successful tool -complementary to time resolved PL and CL- also for oxide thin films and wide bandgap insulators, to visualize the SE dynamics proceeding from optically active defects and charge traps. We show USEM dynamics in the case of color centers in Al2O3 – on-Silicon thin film. UV excitation of an oxygen vacancy state at surface as well as hot electron photoemission from color centers excited by the electron beam are probed by SE contrast evolution. SE depletion, acting at short positive delays, was attributed to fast laser-induced surface charging in the tens of ps range.