Charge Transport in Single-Crystalline GaAs Nanobars: Impact of Band Bending Revealed by Terahertz Spectroscopy

Arrays of ultimate-quality single-crystalline GaAs nanobars are prepared via electron-beam lithography in a molecular-beam-epitaxy-grown GaAs layer transferred onto an electrically insulating and optically and terahertz transparent sapphire substrate. Measurements of ultrafast terahertz photoconductivity at 300 and 20 K in an array of such aligned nanobars by time-resolved terahertz and multiterahertz spectroscopy and by time-resolved terahertz scanning near-field microscopy allow an in-depth understanding of the nanoscale electron motion inside the nanobars. A detailed analysis is performed in terms of quantum mechanical calculations of the mobility of carriers and in terms of plasmonic resonance controlled by photocarrier density. The investigations reveal a band bending close to the nanobar surfaces and its prominent effects on the picosecond charge carrier dynamics, leading to an enhanced localization of electrons at longer times. Terahertz spectroscopy thus proves to become an important tool for the investigation of the role of nanostructure surfaces.