Metalorganic chemical vapor deposition-grown GaN on 6H-SiC substrates were cleaned by annealing in an NH, flux. Oxygen contamination was removed by thermal desorption, and carbon removal was facilitated by reaction with NH3. The GaN(0 0 0 1) surface after NH3 beam cleaning at 730 degreesC was smooth with distinct atomic steps. The roughness (0.20 nm RMS) was only slightly greater than that of the untreated substrate (0.17 nm RMS), Carbon and oxygen concentrations were reduced to background levels (similar to1 at%) by annealing in an NH3 flux at 800 degreesC. The surface step structure was destroyed by annealing in an NH3 flux of 4 x 10(15) cm(-2) s(-1) from a seeded supersonic beam; however, annealing in an NH3 flux of 7 x 10(15) cm(-2) s(-1) from a leak valve inhibited surface roughening and produced a relatively smooth surface (0.28 nm RMS) with a root3 x root3 R30 degrees reconstruction. We infer from the effects of annealing temperature and NH3 flux that the observed surface roughening is due to GaN decomposition.