Nanoscale Electron Motion in GaAs Nanobars Studied by THz Spectroscopy

Mr. Hynek Nemec1, Mr. Vladimir Pushkarev1, Mr. Tomas Ostatnicky2, Mr. Petr Kuzel1
1Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic. 2Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic


Time-resolved THz far-field spectroscopy and THz-SNOM measurements are combined to investigate electron motion in a photoexcited array of single-crystalline GaAs nanobars prepared via electron-beam lithography in a molecular-beam-epitaxy-grown GaAs layer. A comparison with quantum mechanical calculations of the mobility of carriers complemented by effective medium approximation confirms that charges move within entire nanobars at room temperature. Analogical calculations demonstrate that at low temperature charges are localized in a volume smaller than the nanobar width. We conclude that this is due to the band bending close to the nanobar side surfaces, since charges have not high enough thermal energy to overcome the potential energy. A rapid depletion of electrons close to the nanobar top surface is evidenced by the dynamics after photoexcitation measured using the THz SNOM. The main message is two-fold: (i) various forms of terahertz spectroscopy can be used for the investigation of the role of surfaces in nanostructures, and (ii) surface effects may play a crucial role for the THz response and ultrafast dynamics of charge carriers confined in nanostructures.