Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy
Four-dimensional scanning ultrafast electron microscopy is used to investigate doping- and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured time-resolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration. When electrons undergo sufficient energy loss on their journey to the surface, dark contrast becomes dominant in the image. At negative times, however, when the electron pulse precedes the optical pulse (electron impact), the dynamical behavior of carriers manifests itself in a dark contrast which indicates the suppression of secondary electrons upon the arrival of the optical pulse. In this case, the loss of energy of material's electrons is by collisions with the excited carriers. These results for carrier dynamics in GaAs(110) suggest strong carrier–carrier scatterings which are mirrored in the energy of material's secondary electrons during their migration to the surface. The approach presented here provides a fundamental understanding of materials probed by four-dimensional scanning ultrafast electron microscopy, and offers possibilities for use of this imaging technique in the study of ultrafast charge carrier dynamics in heterogeneously patterned micro- and nanostructured material surfaces and interfaces.
Additional Information© 2014 National Academy of Sciences. Contributed by Ahmed H. Zewail, January 3, 2014 (sent for review December 24, 2013). Published online before print January 27, 2014. This work was supported by the National Science Foundation Grant DMR-0964886 and the Air Force Office of Scientific Research Grant FA9550-11-1-0055 for research conducted in The Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology. Author contributions: J.C., T.Y.H., and A.H.Z. designed research, performed research, analyzed data, and wrote the paper. The authors declare no conflict of interest.
Published - PNAS-2014-Cho-2094-9.pdf