Charge carrier dynamics by secondary electron detection in ultrafast scanning electron microscopy

Abstract Ultrafast Scanning Electron Microscopy (USEM) aims at combining the temporal resolution of femtosecond laser spectroscopy and the nanometer spatial resolution of electron microscopy to characterize the dynamics of photo induced processes at surfaces and in ultra-thin films. Our USEM apparatus works in pump-probe mode exploiting a field-emission electron source coupled to a femtosecond fiber laser, in a ultra-high vacuum environment. The UV third harmonic (TH) laser pulses works as the optical pump at the sample, while the fourth harmonic (FH) pulses optically promote the emission of the ultrafast pulsed electron probe beam from the SEM tip. We present time resolved secondary electron images providing information on the dynamics of optically excited charge carrier and defects at surfaces of semiconductors and oxide overlayers. Overview Secondary electrons (SE) imaging is sensitive to a variety of properties of a few nanometer thick volume at the surface of a bulky specimen. In Scanning Electron Microscope (SEM), SE imaging allows routine access, over a wide range of view fields and down to the nanometer lateral resolution, to morphology and topology of surfaces and shallow buried layers. In a controlled environment, SE detection can unveil information on local surface fields, charge carriers and defects [ , ]. The pump-probe technique allows to achieve a temporal resolution from sub-millisecond down to the ultrafast regime in imaging the dynamics of repeatable phenomena. The pulsed modulation of the electron beam can realized by driving the beam blanker as a gated shutter, with a time resolution of a fraction of microsecond, depending on the beam blanker speed and the sensitivity of the detection [ ]. An ultrafast temporal resolution can be achieved by photoemitting the electron pulses directly at the tip source by laser pulses, as originally suggested by A. Zewail and coworkers [ , ]. Our USEM apparatus based on a modified UHV (10-9÷10-10 Torr) SEM, equipped with a ZrO coated W Field-Effect electron tip. The time resolved operation in pump-probe mode is achieved by means of a pulsed fs laser, optically coupled to the SEM. The laser source (300 fs , 1030 nm) is operated at 10 MHz repetition rate and optical feedthroughs allow to focus the FH optical beam directly onto the tip and the TH on the sample at the center of the view field. The pump-probe relative delay is tuned with sub-ps resolution by a mechanical delay stage, set on the beam optical path. The pump path is fully optical, while time of flight along the hybrid probe path depends on the electron kinetic energy within the SEM column, from few keV up to 30 keV typically. A signal rise time of about 10 ps has been demonstrated. We will present the temporal evolution of SE contrast as a function of the relative pump-probe delay, providing information on the charge transport and its timescales within a few nm thick layer at the sample surface. The SE signal is detected by an Everhart-Thorley detector, either in current mode for time resolved imaging or by lock-in demodulation for time spectroscopy on selected areas. Examples of surface charge dynamics under strong UV optical pumping regime are given on Si based systems and alumina overlayers. We will discuss the constraints and artifacts resulting from the choice of the experimental conditions, the sample structure and the key experimental apparatus parameters, such as pointing stability, beam intensity and modulation regime. References 1) A. P. Janssen, P. Akhter, C. J. Harland, and J. A. Venables, Surf. Sci. 93, 453 (1980). 2) J. T. Heath, C.-S. Jiang, and M. M. Al-Jassim, J. Appl. Phys. 111, 046103 (2012). 3) N. C. MacDonald, G. Y. Robinson, R. M. White, J. Appl. Phys. 40, 4516 (1969). 4) D. Yang, O. F. Mohammed, A. H. Zewail, PNAS 107, 14993 (2010). 5) E. Najafi, T. D. Scarborough, J. Tang, A. H. Zewail, Science 347, 164 (2015)