Laser assisted Ultrafast Scanning Electron Microscopy (USEM) probing surface 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 [1]. The sample is excited by two ultrashort pulses, an optical one and an electronic one, and the effect on SE emission is measured as a function of the delay between the two pulses. We have developed a USEM setup for optimized surface sensitivity, by coupling a UV emitting laser with a customized SEM operating in Ultra-High Vacuum regime [2]. The nanometer escape depth of the SE probe enables to address dynamics at surfaces and interfaces of today’s nano-scale devices, where many applications rely on the interplay between semiconductors and insulators. In principle, SEM analysis is best performed onto conductive samples, since slow electrical charging effects are detrimental to reliable and reproducible results. In the case of dynamical USEM, it is also possible to probe charge dynamics at the surface of widegap semiconductors and oxides. We have demonstrated the potentialities of USEM by recording the charge dynamics of Al2O3-on-Silicon thin films. Results can be explained in terms of optical and electronic excitation of color centers associated with oxygen vacancies [2]. The laser-induced SE contrast shows a relevant CW contribution. Nevertheless, by a lock-in detection scheme, fast dynamical SE signal in the picosecond and nanosecond timescale can also be retrieved. The enhancement of SE emission at positive pump-probe delays is associated with optical excitation of F2+ center, with a τ = 12ns decay time; at negative delays the dynamics of the F+ centers, excited by the e-beam and decaying over τ ~ 7ns is recorded, by probing hot-electron photoemission. SE depletion at zero delay and with decay constant τ ~ 70ps was attributed to surface charging due to two-photon-photoemission. In this way USEM becomes a successful tool, complementary to time resolved PL [4] and CL [5], to visualize the dynamics of optically active defects and charge traps, at the surface and interfaces of wide bandgap and insulating thin films. [1] Zewail 1 - J. van der Geer, J.A.J. Hanraads, R.A. Lupton, J. Sci. Commun. 163 (2010) 51–59. [2] M. Zani et al., Ultramicroscopy 187 (2018) 93–97 [3] P.W. Levy, Phys. Rev. 123 (1961) 1226–1233. [4] B.D. Evans, G.J. Pogatshnik, Y. Chen, , Nucl. Instrum. Methods Phys. Res. B 91 (1994) 258–262. [5] E.C. Kouroukla, I.K. Bailiff, I. Terry, L. Bowen, Radiat. Meas. 71 (2014) 117–121.