Deep-UV photoemission electron microscopy for imaging nanoscale heterogeneity and defects in gallium nitride

Gallium nitride (GaN) is a promising wide-bandgap material for high-power electronics, where GaN-on-GaN homoepitaxy is being developed for fabrication of compact high-voltage vertical devices. However, variation in GaN substrate quality strongly influences the properties of epitaxial layers grown on top, which in turn affects device performance and reliability. Hence, better knowledge of the surface electronic properties is needed, especially after wafer processing steps that can introduce surface contaminants and oxide layers. Photoemission-based techniques provide chemical and electronic information but are surface-sensitive; therefore, the formation of native oxides or contamination from ambient conditions can affect findings. Here, we present the initial results of various surface treatment methods on the electronic properties of p-type GaN epitaxial layers grown via metal-organic chemical vapor deposition (MOCVD) in preparation for photoemission electron spectroscopy and microscopy characterization. We use X-ray photoelectron spectroscopy (XPS) to evaluate changes in residual contamination after treatment. We find that piranha-based cleaning methods have large reductions in surface carbon contamination, while NH4OH and HCl-based treatments remove surface oxide. The elemental core levels and valence band correspondingly exhibit binding energy shifts with the different treatment methods, indicating reduced surface band-bending. Both XPS and initial photoemission electron microscopy results of the photoelectron yield suggest a deeper valence band edge location with respect to the Fermi energy measured for the forming gas plasma-cleaned sample. These results demonstrate that combined ex-situ treatments for carbon and oxygen removal are more effective, yet further in-situ cleaning is necessary for more complete contaminant removal.