The development and implementation of variable tip-sample separation scanning tunnelling spectroscopy to enhance spectroscopic measurement

Scanning Tunnelling Microscopy (STM) and Spectroscopy (STS) are valuable surface science techniques in the characterisation of a wide variety of surfaces on the atomic scale. When STM and STS are performed concurrently, both the topological and electronic properties of a surface can be investigated. Conventionally STS is performed with the tip-sample separation fixed, which leads to a limited dynamic range in the measurement. This limitation can be overcome by varying the tip-sample separation during STS. Varying the tip-sample separation improves the dynamic range by exploiting the exponential relationship between tunnel current and tip-sample separation. To accomplish variable tip-sample separation STS with Omicron Micro STM and STM/SEM-HC Scanning Tunnelling Microscope systems, external instrumentation was designed and fabricated. This instrumentation operated in conjunction with the Omicron microscopes to create the tip-sample contour and measure conductivity during variable tip-sample separation STS. The instrumentation performed as expected, although problems were encountered making conductivity measurements using a Lock-in amplifier. Conductivity spectra obtained using a Lock-in amplifier contained perturbations related to the tip-sample capacitance. Future work is required to compensate for the effects of the tip-sample capacitance in conductivity measurements based around a Lock-in amplifier. To comparatively evaluate variable and fixed tip-sample separation techniques, STS was performed consecutively on Si(111) 7x7 and GaAs(110) surfaces. For both surfaces an improvement in the definition of spectral features is observed. In the case of GaAs(110), surface states indistinguishable from noise in fixed tip-sample separation STS are clearly observed utilising variable tip-sample separation.