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

The etching of sapphire substrates using H 2 SO 4 , H 3 PO 4 , and a 3:1 H 2 SO 4 : H 3 PO 4 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 H 2 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 H 2 SO 4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al 2 ( SO 4 ) 3 and Al 2 ( SO 4 ) 3 ⋅ 17 H 2 O crystalline deposits on the surface. Phosphoric acid and the 3:1 H 2 SO 4 : H 3 PO 4 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 H 2 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.