Terahertz Scanning Tunneling Microscopy of Semiconductor Nanowires
Creators: Stratmann Meouchi, Max
Publisher: name: University of Alberta Library
Year: 2024

<p>Terahertz-Scanning Tunneling Microscopy (THz-STM) is an exciting combination of THz experiments and STM. In conventional STM, an atomically sharp tip is used to measure the topography of a surface with sub-nanometer spatial resolution. Meanwhile, terahertz spectroscopy experiments can be used to study picosecond electron dynamics, but most experiments have a spatial resolution limited by the spot-size of the terahertz pulse. THz-STM distinguishes itself by being able to do both simultaneously - ultrafast measurements on a sub-nanometer scale. This makes it an ideal candidate to study the electron dynamics of individual nanostructures as explored in this thesis.</p>

<p>In this thesis, the theoretical backgrounds of STM, THz generation and detection, THz-STM, optical-pump/THz-STM-probe (OPP-THz-STM) experiments are presented, as well as the experimental setup. Photoemission experiments are then shown. These experiments use a high laser fluence to photoexcite electrons from the tip and sample and can be used to sample the THz electric field locally at the tip, and can demonstrate the ability of THz-STM to measure sub-picosecond electron dynamics. The local THz electric field at the THz-STM tip is described in detail, having been experimentally sampled at the tip and modeled using antenna coupling.</p>

<p>Having established the theory behind THz-STM and proven its capabilities, this thesis presents THz-STM of Cadmium Sulfide (CdS) nanowires. CdS nanowires are exciting semiconductor nanostructures with useful photocatalytic applications and a long-lived charge separation when optically excited. This thesis presents the first THz-STM study of a semiconducting nanowire. STM of undoped CdS nanowires is shown, indicating that there is a significant amount of broadening of the image due to a convolution between the tip and sample. A bundle of undoped CdS nanowires is shown, including several ways of separating the bundle into individual nanowires, with a novel approach using a machine learning algorithm. Current-voltage characteristics are presented, showing a contrast between the undoped CdS nanowire and substrate, but not between different points on the nanowire. Differential current-voltage characteristics are also presented, which do not show a contrast between the undoped CdS nanowire and substrate. Bias-dependent scans show how the semiconducting nature of the nanowires and tip-induced band bending can lead to the STM being unable to detect the nanowires at certain biases. THz-STM results are shown on the substrate, showing an electric field dependence on the measured THz-STM signal. Algorithms showing the convolution between tip and sample are presented, along with simulations that prove the nanowire width broadening effect. Mn-doped CdS nanowires are presented in combination with simulations showing the tip-sample convolution. From these simulations, tip dimensions are fitted which match scanning electron microscopy images of the tips. For Mn-Cds nanowires, the current-voltage and differential current-voltage characteristics do not show a contrast. The first THz-STM results on Mn-CdS nanowires are presented, showing the ability of THz-STM to obtain a signal on this material. Finally, an OPP-THz-STM result on GaAs (110) is shown, as well as an attempt to obtain this result on a doped CdS nanowire at 50 K.</p>