Light-illuminated STM studies on photo-absorption in InAs nanowires

We fabricated surface InAs nanowires along GaAs giant step structures on GaAs (1 1 0) vicinal substrates, and their photo-absorption properties were studied by scanning tunneling microscopy (STM) under light illumination. In photo-induced current detection, it was difficult to identify the photo-absorption signals in the InAs nanowires because of the electrical isolation of the surface nanostructures from the conductive substrate. On the other hand, optical response measurements of differential conductance under small AC bias modulation in STM enabled us to obtain the photo-absorption signals from the surface nanostructures with avoiding such an electrical isolation problem. We successfully observed optical response images of differential conductance with a clear contrast between InAs nanowires and surrounding GaAs region by the appropriate tuning of the illuminating photon energy to excite the InAs nanowires selectively.