The results of an experimental and numerical study on the effect of carrier gas on the HVPE growth of GaN in a horizontal reactor are reported. A surprising result is that both the maximum and the total growth rate under 85 vol% H-2 are higher than under N-2 carrier gas. Computational fluid dynamics studies show that an alteration of the flow pattern caused by buoyancy effects is responsible for the increase in growth rate. A second factor influencing the growth rate is the diffusion coefficient; the net effect of the diffusion coefficient depends on the carrier gas. The deposition rate of GaN depends on both the carrier gas and the position relative to the GaCl inlet. The experiments show that the quality of the grown layers (as measured with XRD and PL) increases with increasing H-2 content. The calculated and experimental data show a very good qualitative correspondence. (c) 2005 Elsevier B.V. All rights reserved.