Visualizing Photoemission and Photocarrier Dynamics in Wide Bandgap Semiconductors with UV-Pumped Scanning Ultrafast Electron Microscopy

Wide bandgap semiconductors have a broad range of applications, including light-emitting diodes, photodetectors, and field-effect transistors. The performance of these devices strongly depends on the ultrafast carrier dynamics, referring to the generation, relaxation, and recombination processes of charged carriers. Therefore, a comprehensive microscopic understanding of carrier dynamics with simultaneously high spatial and temporal resolutions is vital to providing optimization strategies for the design of wide bandgap devices with enhanced performance.<br/><br/>Scanning ultrafast electron microscopy (SUEM) has emerged as a cutting-edge optical-pump-electron-probe technique for visualizing photocarrier dynamics with combined high spatial and temporal resolutions and surface sensitivity. Despite previous studies on the carrier dynamics of a broad range of inorganic and organic semiconductors with SUEM, investigations have been largely confined to narrow bandgap semiconductors due to limitations posed by the relatively long wavelength of the pump light used (typically around 500 nm), which precludes efficient one-photon excitation in wide bandgap materials. In this work, we have implemented an ultraviolet (UV) pump at 257 nm (4.8 eV) that enables the direct visualization of the hot photocarrier dynamics as well as the photoemission process via one-photon excitation in wide bandgap materials for the first time. The photoemission process of wurtzite gallium nitride and the photocarrier dynamics of silicon carbide will be discussed as an example. Our work deepens the understanding of the microscopic hot carrier transport and relaxation process near the surfaces of wide bandgap materials and offers valuable insights for future device design.<br/><br/>This work is based on research supported by AFOSR under the award number FA9550-22-1-0468 and ARO under the award number W911NF2310188.