(Invited) Advanced Electronic Characterization of Wide Band Gap Semiconductor Defects at the Nanoscale
Authors: Winchester Andrew , Pookpanratana Sujitra 
Published: August 2024

Wide bandgap materials such as gallium nitride (GaN) and gallium oxide (Ga2O3) are currently under development for realizing next-generation high power electronic devices beyond silicon. Vertical structures produced through homoepitaxial growth are preferred for scaling high-voltage devices, which places strict demands on the quality of bulk substrates. In particular, extended defects can propagate during epitaxial growth into the active areas of devices. Therefore, there is a need for identifying defects and determining their electronic properties at relevant nano- to micro- scale dimensions in order to disentangle their combined effects on macroscopic devices. Here, we use a combination of laser-based photoemission electron microscopy (PEEM) and electrical atomic force microscopy (AFM) methodologies to investigate the local electronic properties of surface defects in epitaxially grown GaN and β-Ga2O3. PEEM is a non-scanning electron microscopy technique where photoelectrons emitted by a surface illuminated by deep-ultraviolet light are imaged with nanometer-scale spatial resolution. The emitted photoelectrons are sensitive to the local electronic structure, making PEEM a powerful tool for evaluating electronic changes at length scales relevant for devices, and provides valuable information for identifying “killer” defects which must be minimized during device growth and processing.

The difficulty in producing large, low dislocation density bulk GaN substrates has led to longstanding issues with leakage current in diodes and transistor devices. We identify surface defects with different electronic properties in GaN epitaxy on different substrates using PEEM. We observe star-shaped defects on strain-patterned GaN substrates and elongated dark patches on growth ridges on ammonothermally grown GaN substrates. In both cases, the observed electronic changes in PEEM correlate directly with local electrical AFM measurements, revealing that local changes in the electronic structure at these defects results in altered electrical conduction.

In contrast, the development of large bulk Ga2O3 substrates has been more successful, yet the identification and influence of various defects on device performance is still largely unknown at this point. The topography of β-Ga2O3 epitaxy varies widely between growth methods and substrate orientations. For example, β-Ga2O3 by hydride vapor physical epitaxy lacks fine structure, yet has surface defects that regularly appear in parallel orientations. In β-Ga2O3 grown on different substrate orientations, we observe variations in the surface electronic properties using PEEM. For certain substrate orientations and growths, we find nanoscale variations in doping, while other orientations display micron-scale electronic inhomogeneity and defects. Our results demonstrate that laser-based PEEM can be a powerful tool for detecting electrically active defects, as well as evaluating the nanoscale electronic homogeneity of wide bandgap semiconductors.