Metalorganic chemical vapor deposition growth of GaN films using trimetylgallium (TMGa) and triethylgallium (TEGa) sources exhibited the different growth mechanisms and properties of GaN films. In situ normal incidence reflectance measurements during the growth of GaN with TEGa showed an increased coalescence time and a lower growth rate compared with TMGa, resulting in defect free columnar domains, a lower density of dislocation at the domain boundary, a rough surface, and good electrical properties. These results indicate that the growth mode for TEGa-grown GaN slowly changes from three-dimensional to quasi-two dimensional lateral growth via a coalescence stage and that the roughening and coalescence processes are repeated with increasing film thickness. A broad yellow emission peak was also observed at around 550 nm, in the case of the TEG-grown GaN. This yellow emission can be attributed to an increase in the concentration of nitrogen vacancy-related complexes or extended defects in the TEGa-grown GaN. This situation led also to an increase in the background electron concentration and a rough surface, compared with those of TMG-grown GaN.