Local defect and mid-gap state analysis of GaN using monochromated EELS combined with nanodiffraction and atomic-resolution imaging

Enhancement of energy resolution due to evolution of monochromators has made electron energy loss spectroscopy (EELS) in transmission electron microscopy a practical approach for evaluating mid-gap states of semiconductor materials at the nanoscale. However, experimental studies on how actual defective structures in local regions relate to mid-gap states are limited. Here, we performed high energy resolution EELS (HR-EELS) measurements with an energy resolution of less than 100 meV to detect mid-gap states of GaN where various defects were induced by Ga-ion implantation and the defect concentration varies in the depth direction. The formation of mid-gap states was verified by synchrotron hard X-ray photoelectron spectroscopy and the origin of the mid-gap states was investigated via nano-to-atomic-scale structural analysis based on scanning transmission electron microscopy (STEM). The HR-EELS measurements elucidated depth dependence of valence-loss spectra in which intensities corresponding to mid-gap states gradually increase toward the surface, while slope at onsets corresponding to interband transition decreases. The structural analysis including 4D-STEM and atomic-resolution observation revealed the presence of structural disorder and defective structures, suggesting that the defective structures include extended defects such as stacking faults and domain boundaries. These defective structures were abundant near the surface and obviously less in the deeper region. On the basis of these experimental results, we conclude that variations of valence-loss spectra have the capability to qualitatively evaluate crystal imperfections at the nanoscale.