The impact of carrier gas on the GaN layers properties grown by atmospheric pressure metal-organic vapor-phase epitaxy (AP-MOVPE) on (001) and (11n) GaAs substrates were investigated. The Arrhenius plots of growth rate deduced from laser reflectometry measurements give an activation energy of E_a1=0.045 eV when the H₂ was used as the carrier gas. In the case of using N₂ as the carrier gas, the results give E_a2=0.081 eV as a value of activation energy, which is approximately 2 times greater than E_a1. Scanning electron microscopy results show that when N₂ is used, the resulting material quality is low, but the use of H₂ is successful to prevent the cracking of GaN layers and results in improvement of crystalline properties. From the X-ray diffraction result, we conclude that both (001) and (113) GaAs substrate orientations as well as the use of H₂ as the carrier gas favors the GaN growth with cubic structure, whereas the GaN hexagonal structure is favored for growth on (112) and (111) GaAs substrates orientations with N₂. Cathodoluminescence measurements show that a mechanism of phase transformation occurs when the growth temperature rise from 800 to 900^oC. Keywords: cubic GaN, hexagonal GaN, (hkl) GaAs, carrier gas.

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