The composition, morphology, and structure of a gallium-containing carbon deposit that forms under certain conditions during hydride-organometallic vapor phase epitaxy (HOMVPE) of GaN were characterized by several techniques. The deposits are produced during pyrolysis of trimethyl gallium and results in gas-phase depletion of Ga, which reduces the growth rate and reproducibility of GaN growth. The morphology of the deposit depended on temperature, changing from granular to tubular-shape, and then to powder-like features with increasing temperature. The deposits were composed of metallic gallium cores surrounded by graphite skin layers. Interestingly, in the temperature range 560 to 660 degrees C the structure consisted of graphitic carbon-walled tubes filled to varying extents with Ga. The addition of H-2 to the carrier gas stream was found to be an effective method for reducing or eliminating the formation of the deposit. (c) 2005 The Electrochemical Society.