Photoelectrical properties of semiconductor tips in scanning tunneling microscopy

We describe a model as well as experiments on the electrical properties of a photoexcited tunnel junction between a metal and a semiconductor material, as is established in a scanning tunneling microscope. The model treats the case in which carrier transport is mediated by capture and relaxation in the semiconductor surface states. In the semiconductor, majority carrier transport is determined by thermionic emission over the Schottky barrier and subsequent surface recombination. By optical excitation an additional minority carrier current is generated. The voltage that develops on the semiconductor surface is determined by the balance between majority and minority carrier current in the semiconductor, and the current across the tunnel barrier We present model calculations of the (nonplanar) band-bending profile in the semiconductor, which indicate that the subsurface electric field operates as an electrical lens that can focus or defocus the current. Measurements were performed with moderately doped GaAs tips or samples prepared by cleavage. Continuous as well as modulated photoexcitation was used. Relationships are determined between tunnel current, applied voltage, incident optical power, and tip-sample distance. The experimental results are well described by the model that includes carrier capture in the semiconductor surface states. It is shown that the sensitivity of the tunnel current to small variations in optical power is determined by the ratio of the tunnel barrier conductance to the Schottky barrier conductance. The implications for near-field optical imaging and spin-polarized tunneling with semiconductor tips are discussed.