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The authors have applied contactless electroreflectance (CER) spectroscopy to study the epi readiness of m-plane GaN substrates obtained by the ammonothermal method. It has been clearly observed that the CER resonance, which is related to the energy gap transition, appears for samples with a well-polished surface. The sharpness of this resonance is directly related to the surface quality. The broadening of energy gap transition can be used as a parameter to quantify the surface quality. For samples polished with optimal conditions, this broadening (γpol) is close to the broadening observed for the cleaved GaN surface (γclev) with m-plane orientation (150-190 vs 135 meV). The quality of the polishing process can be evaluated by analyzing the γclev/γpol ratio, where γclev/γpol=1 corresponds to an excellent polishing process. In the authors' case, this ratio has been determined to be close to 1 for well-polished samples. [ABSTRACT FROM AUTHOR]