Electron dynamics in aluminum oxide thin film revealed by Ultrafast Scanning Electron Microscopy (USEM)

We report about a time resolved measurement of electron dynamics in an aluminum oxide thin film on silicon done by Ultrafast Scanning Electron Microscopy (USEM). This novel technique aims to join the time resolution typical of fs lasers to the spatial lateral and depth resolution of electron microscopy in a single tabletop tool. The USEM apparatus operates in Ultra-High Vacuum regime, and in our pump-probe setup the sample under test is excited both by an UV fs laser pulse and by a delayed electron pulse, triggering the emission of low energy electrons to the detector.   Optical properties in the visible and near UV of wide bandgap insulators are influenced by defects that act as traps for electrons; lifetime and energy of excited states have been studied by time resolved photoluminescence [1]. It’s known from cathodoluminescence measurements that defects may be excited also by the electron beam of a Scanning Electron Microscope (SEM) [2]. Our goal is to evaluate the excitation dynamics of the color centers in an insulator from the secondary electron contrast typical of the SEM images.   The laser‑induced electronic contrast of the alumina film shows a relevant CW contribution, that we attribute tentatively to the photo‑induced carriers within the insulator. Nonetheless, by employing lock-in detection of secondary electrons, it is possible to observe also a fast dynamics with components ranging from hundreds of picoseconds to few nanoseconds. We observed an enhancement of secondary electron emission rising at zero delays and vanishing for positive delay with a time constant of tens of nanoseconds attributed to the excitation of an oxygen vacancy by the laser beam. It was possible to observe a depletion mechanism acting at positive delays and decaying with a tau of tens of picoseconds that was attributed to laser induced surface charging [3]. A third enhancement acting at negative delays and decaying with a time constant of few nanoseconds was attributed to the photoemission of electrons from color centers excited by the electron beam.   [1] B.D. Evans, G.J. Pogatshnik and Y. Chen, Nuclear Instruments and Methods in Physics Research B, 91, 258-262 (1994), Optical properties of lattice defects in α-Al2O3   [2] E.C. Kouroukla, I.K. Bailiff, I. Terryand L. Bowen, Radiation Measurements, 71, 117-121 (2014), Luminescence characterisation of alumina substrates using cathodoluminescence microscopy and spectroscopy   [3] R. Stoian, A. Rosenfeld, D. Ashkenasi, I.V. Hertel, Phyical Review Letters 88, 097603 (2002) Surface Charging and Impulsive Ion Ejection During Ultrashort Pulsed Laser Ablation