Defect characterization of Si-doped GaN films by a scanning near-field optical microscope-induced photoluminescence

We developed a tapping-mode-scanning near-field optical microscope to measure near-field photoluminescence (SNOM-PL) with nanometer spatial resolution using an ultraviolet laser, and we measured the defect distribution of a Si-doped GaN film. The obtained result was compared with one measured by high spatial resolution cathodoluminescence (CL) spectroscopy. Some dark spots with an average period of 100–300 nm were observed in the image of the peak intensity of near-field PL at about 362 nm, measured with a pyramidical cantilever, and in a plan-view CL image at the same wavelength. The near-field PL image agreed well with the plan-view CL image. The threading dislocations in GaN films were found to act as nonradiative recombination centers not only for band-edge emission but also for yellow luminescence around 580 nm. Furthermore, it was found that free carrier concentration decreased near the threading dislocations. The SNOM-PL we developed has at least a spatial resolution of about 100 nm. The SNOM-PL could potentially be applied to the estimation at a nanometer scale of defects in semiconductor films without requiring a vacuum environment.