The growth by hydride vapor phase epitaxy (HVPE) of high quality thin GaN layers (d=8 mu m) on c-plane sapphire substrates with dislocation densities lower than 2x10(8) cm(-2) is demonstrated using a two-step process similar to that of metal organic vapor phase epitaxy (MOVPE). Ex situ surface preparation and nucleation layer thickness are shown to be critical factors in achieving these high quality epilayers as they allow controlling the polarity and the dislocation density, respectively. Furthermore, we demonstrate that in situ reflectivity monitoring applied to HVPE is a powerful technique for rapidly optimizing the growth parameters. As a result, thin HVPE-grown GaN layers with state of the art MOVPE GaN quality are obtained as demonstrated through structural and optical characterizations. (c) 2006 American Institute of Physics.