We studied processes of cleaning GaN(0 0 0 1) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition(MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 degrees C for 12 h + 400 degrees C for 1 h + 500 degrees C for 5 min) without sputtering, after which the surface contamination of oxide and carbide was reduced to similar to 20% of that before annealing. Clear GaN(0 0 0 1)1 x 1 patterns were obtained by LEED and RHEED. STM images showed flat terraces of similar to 10 nm size and steps of similar to 0.5 nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (> 550 degrees C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar+ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear 1 x 1 structure. Upon annealing at 500-600 degrees C, the surface hydrogen was removed and a 3 x 3 reconstruction structure partially appeared, although a 1 x 1 structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure. (C) 2010 Elsevier B.V. All rights reserved.