Detection limit of defect-induced strain in GaN evaluated by valence EELS and correlated structural analysis

Crystal defects are intrinsically linked to the electrical and optical properties of semiconductor materials, making their nanoscale detection essential across all phases (from research and development to manufacturing). Electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) has emerged as a promising technique for detecting even point defects due to the shape modulation in valence-loss spectra induced by defects. However, previous studies have primarily focused on qualitative detection, leaving the detection limit, i.e. the minimum detectable concentration, insufficiently explored. To experimentally evaluate the detection limit of defects and clarify the application scope of valence EELS, we prepared GaN samples with controlled defect concentrations along the depth direction using multi-step He-ion implantation and acquired valence-loss spectra at each depth. Based on the simulated depth profile of defects, we evaluated the detection limit from the depth at which significant modulation in the spectral shape was observed. The detection limit fundamentally depends on the signal-to-noise ratio of the valence-loss spectra. Under typical STEM conditions with an electron dose of 5 × 105 e-/Å2, the detection limit of defects in GaN was determined to be 0.35% (3500 ppm). Detailed structural analysis revealed that GaN contains implantation-induced defects and their clusters, and exhibits lattice strain and local disorder while retaining its wurtzite structure. The shape modulation in the valence-loss spectra was attributed to the indirect detection of defects through the surrounding strain fields.