Scanning Photocurrent Measurements of Degenerately Doped Si

Scanning photocurrent microscopy (SPCM) has shown much promise as a means for direct and efficient measurements of carrier lifetime and diffusion in novel semiconductors including topological insulators. High spatial resolution and electrical sensitivity in these setups has made detection of properties such as diffusion length routine. Degenerately doped Si with a surface oxide are often used as back-gating substrates for thin films or flakes of the materials. However, the high photocurrent of the silicon often complicates the elucidation of the same photo responses in thin films/flakes. Here, we present SPCM measurements of a degenerately doped silicon substrate. After buffered HF etching of the surface oxide, Cr/Au electrodes were defined via thermal evaporation through a shadow mask, resulting in a device of 25 μm channel width between two back-to-back Schottky contacts. An elevated photocurrent was measured in the regions near the electrodes at zero bias voltage, indicating a diffusion current. The experiment has shown the structure as a viable platform for SPCM measurements of thin topological insulators under continuous backgate tuning.