To study the effects of the surface treatments of c-plane GaN substrates on epitaxial growth, surfaces of c-plane GaN freestanding substrates grown by the Na flux method were treated in different ways: mechanical polishing (MP); chemical mechanical polishing after MP; or wet etching (WE) with the use of pyrophosphoric acid after MP. After each surface treatment, we grew Ga-face GaN substrates. Consequently, we concluded that the surface treatment by WE was one of the effective and easy ways to make a Ga-face GaN suitable for growth. Using the WE treatment, we successfully reduced the dislocation density of Ga-face GaN crystal from 2.4 x 10(7) cm(-2) to 3.9 x 10(3) cm(-2), after two times repeated growth. (C) 2013 Elsevier B.V. All rights reserved.