Cathodoluminescence evaluation of subsurface damage in GaN substrate after polishing

The surface of HVPE grown GaN substrates were treated with two different polishing procedures. Both procedures were successful in producing highly smooth and featureless surfaces. However, subsurface damage was observed in the sample treated by one of the procedures. The subsurface damage was revealed by cathodoluminescence (CL) spectroscopy imaging but was not visible by scanning electron microscopy (SEM) or Atomic force microscopy (AFM). Thermal annealing at 950 °C with different gases was performed in an attempt to remove subsurface damages. Annealing in gas mixtures containing H₂ increased both the presence of surface scratches and overall surface roughness. On the other hand, annealing in mixtures of NH₃ and N₂ (with no H₂) led to the surfaces with significantly reduced subsurface damage. The surface roughness and optical properties of the sample after the annealing with such gas mixtures were slightly sacrificed. In order to evaluate subsurface damage depth, CL images were taken from the annealed surface for different acceleration voltages. The results suggest that the observed subsurface damages were within 1.48 μm of the surface.