Undoped and Mg-doped GaN thick films with a columnar structure have been grown on gold-covered fused silica substrates in a three-zone horizontal quartz-tube reactor. The undoped films were grown by a two-step chemical vapor deposition method using gallium and ammonium chloride as starting reagents and ammonia as carrier gas. The Mg-doped films were deposited by the same method, but with an initial Ga-Mg alloying step. These GaN thick films have a wurtzite structure and exhibit strong room-temperature luminescence with the characteristic GaN band-edge, and the Mg-related emissions for the doped films. Their optoelectronic properties are comparable to GaN films grown epitaxially with other more elaborate techniques. This growth technique can be used for inexpensive, large-area electroluminescent devices. (C) 2008 Elsevier B.V. All rights reserved.