Investigation of defects and surface polarity in GaN using hot wet etching together with microscopy and diffraction techniques

The availability of reliable and quick methods to determine defect density and polarity in GaN films is of great interest. Photo-electrochemical (PEC) and hot wet etching using both H₃PO₄ and molten KOH have been used to estimate the defect density in GaN by producing at the surface defect sites freestanding whiskers and hexagonal pits, respectively. Using Atomic Force Microscopy (AFM), we found the whisker density to be similar to etch pit density for samples etched under precise conditions. Additionally, Transmission electron microscopy (TEM) analysis confirmed dislocation densities obtained by etching which increased our confidence in the consistency of methods used. Hot wet etching was used also to investigate the polarity of GaN films together with Convergent beam electron diffraction (CBED) and AFM imaging. We found that hot H₃PO₄ etches N-polar GaN films very quickly resulting in the complete removal or drastic change of surface morphology. On the contrary, the acid attacks only defect sites in Ga-polar films producing pits but leaving the defect-free GaN intact and the morphology unchanged. Additionally, the polarity assignments were related to the as-grown morphology and to the growth conditions. We have found that the GaN films grown on high temperature AlN and GaN buffer layers on sapphire by molecular beam epitaxy (MBE) show Ga and N-polarity, respectively. However, the polarity of GaN films grown on low temperature AlN or GaN buffer layers depends critically on the growth conditions.