PHOTOASSISTED SCANNING TUNNELING MICROSCOPY AND TUNNELING SPECTROSCOPY OF N-TYPE TUNGSTEN DISELENIDE (N-WSE2) SINGLE-CRYSTALS

The van der Waals surface (perpendicular to the c axis) of a nondegenerate n-type WSe2 single crystal was imaged at negative substrate bias voltage (V) in the constant-current mode in air with a scanning tunneling microscope (STM) combined with optical excitation techniques. A current (i) image at a positive bias under steady-state illumination, obtained simultaneously with the topographic image, revealed that the photocurrent at the step edges was much smaller than that in the defect-free region. This reduction of the photocurrent at defect sites is attributed to a high surface recombination rate at defects. Tunneling spectroscopy (TS) performed in N2, including i vs V and di/d V vs V curves with the tip held over the n-WSe2 surface, was also carried out both in the dark and under laser irradiation. The results were interpreted in terms of the band locations and minority carrier injection through photoexcitation