MODULATED PHOTODETECTION WITH SEMICONDUCTOR TIPS IN A SCANNING TUNNELING MICROSCOPE

We report on the detection of modulated light power irradiated into the tunnel junction of a scanning tunneling microscope. When semiconductor tips are used we can distinguish three contributions to the measured current: photocurrent due to electron-hole pair generation at the apex of the tip, a contribution of thermal tip-sample distance modulation, and a displacement current due to the surface photovoltage that develops on the tip surface. Using a simple model we can understand the phases of the detected signals with respect to the photoexcitation, and extract values for several capacitances and conductances involved in the tunnel junction. This is relevant for applications of high-frequency photoexcitation in scanned probe microscopies.