Monoclinic gallium oxide (β-Ga₂O₃) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5-4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga₂O₃ has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright – 50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

To investigate this PL emission with submicron resolution, a PL map was generated with a Klar Mini Pro UV microscope equipped with a 355 nm CW laser and Ocean Insight Maya2000 Pro spectrometer. A hydrogen-annealed sample was mapped at a spatial resolution of 700 nm, for a total of 1.8 × 10⁷ spatial (x,y) points, while the reference was mapped at a spatial resolution of 600 nm for a total of 2.7 × 10⁶ points. The integration time was 10 ms per point. The 3.27 eV peak was fit using a bi-Gaussian function using GPU acceleration.

The figure shows a PL map of the entire hydrogenated sample. Weak defect emission was observed everywhere, but especially bright regions were detected as well.