Study on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatments

The etching of sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4:H3PO4 mixture, as a function of temperature and etching time, was systematically studied using atomic force microscopy. The sapphire preparation by liquid-based etchings was compared with H2 etching at 1100°C and air-annealing at 1400°C. In liquid-based treatments, the smoothest, pit-free surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3⋅17H2O crystalline deposits on the surface.

Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture, which is the routinely employed chemical treatment for sapphire preparation, etched the sapphire preferentially at defect sites and resulted in pit formation on the surface. Sapphire treatment using H2 at 1100°C did not remove the surface damage. Air annealing the sapphire at 1400°C for 1 h produced an atomically smooth surface consisting of a terrace-and-step structure. The results of this study were described in terms of the chemistry of the sapphire etching process.


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