The diffusion of Ga species in gas phase as well as on the surface are studied in selective area growth of AlGaN in low pressure metalorganic vapor phase epitaxy. The experiments were performed on a trapezoidal stripe with (0001) facet on the top and (1-101) facet on the sides. It was found that the ridge growth on the facets were sensitive to the growth pressure, in agreement with numerical results. At a low pressure of 100 Torr, we got a uniform thickness of AlGaN alloy, but the alloy composition was not uniform. Exponential variation of the composition gave the effective diffusion length of Ga on the order of 0.7 μm which was independent of the growth pressure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)