NW-based sample preparation for ultrahigh vacuum STM imaging

Nanowires (NWs) of III-V semiconductors provide a promising platform for the development of electronic and photonic components of integrated circuits. For the development of complex NW-based devices, it is crucial to precisely study structural, electronic, and optical properties at the nanoscale. Scanning tunneling microscopy (STM) is commonly used to achieve such precision. In this work we optimize the tunneling contact parameters in an ultrahigh vacuum STM (at room temperature) for reproducible high-quality topographic imaging of conductive GaP NWs, especially promising for photonic integrated circuits. Two methods were employed for transferring NWs onto auxiliary conducting substrates: ultrasonication in liquid (deionized water or isopropyl alcohol) followed by drop casting and mechanical scratching. Five substrate materials were tested: highly oriented pyrolytic graphite, single crystal silicon wafers, thin films of nickel, indium tin oxide and gold. The experimental results showed that the tunneling contact parameters, substrate material, and transfer method significantly affect the quality of STM images. It was found that bias voltages of 7-10 V, tunneling current up to 400 pA, and image recording rates in the range of 500-1500 nm s^-1were optimal, with nickel-coated substrates providing the best stability and image quality. Potentially harmful procedures for NW and substrate surfaces, such as ion treatment and high-temperature annealing, were avoided during the sample preparation. The results expand the understanding of STM studies of NWs and their applications in electronic and photonic devices.