Hydride vapor phase epitaxy (HVPE) is a promising technique and being actively developed for growth of GaN substrates which are demanded for fabrication of e.g. light emitting diodes and laser diodes in the short wavelength range. The main reaction providing GaN growth in HVPE is that of ammonia and gaseous GaCl, with hydrogen and nitrogen being the usual carrier gases. We have studied the impact of carrier gas composition using mixtures of H₂ and N₂ on the growth process. Experiments were carried out in AIXTRON vertical HVPE reactor using 3.5 µm GaN substrates grown on sapphire templates by MOVPE (metal-organic vapor phase epitaxy). In-situ reflectometry, Nomarski microscopy, photoluminescence and X-ray diffraction were used to characterize the grown layers. After readjusting NH₃ and GaCl partial pressures according to the simulated gas-flow dynamics, a H₂ fraction in the range of 0-60 % was found to be favorable regarding the layer quality. Under these conditions it was possible to grow 100 µm thick GaN films without cracks and at a high growth rate of about 240-300 µm/h. X-ray diffraction measurements of 002 and 302 rocking curves on 100 µm thick films revealed dislocations densities of 10⁸ cm^-2. 70% of H₂ resulted in cracks and damaged film surface, possibly due to enhanced GaN decomposition, which would also explain the decreased growth rate of only around 150 µm/h. Broadened X-ray rocking curves indicated a higher dislocation density of about 7∙10⁹ cm^-2. In general, H₂ is not only a carrier gas, but also a reaction product in GaN growth and thus participates in the growth process. The role of H₂ in the surface kinetics and its impact on layer quality and optical properties will thus be discussed. 

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1. Growth of thick Al_xGa_1-xN layers on sapphire by hydride vapor phase epitaxy