Assessing the insulating properties of an ultrathin SrTiO3shell grown around GaAs nanowires with molecular beam epitaxy.

We have studied electronic transport in undoped GaAs/SrTiO3core-shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/µm. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO3shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3shells as gate insulators.