The GaN decomposition versus the annealing ambient (H 2 , N 2 + H 2 ) have been investigated in atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor. A comparison between the low temperature GaN (LT-GaN) decomposition in a vertical reactor and a horizontal one was discussed in order to explain the failure of high temperature GaN (HT-GaN) growth on Al 2 O 3 using 2D buffer layer under a mixture of H 2 and N 2 in our vertical reactor. Decomposition rates versus temperature and ambient gas were measured using real time laser reflectometry. The surfaces of these films were analyzed using a scanning electron microscopy (SEM). Hydrogen seems to considerably affect the LT-GaN surface morphology by a mass transport, diffusion on the substrate surface and by a decomposition process even in the presence of ammonia and nitrogen. The high decomposition rates of the LT-GaN measured in comparison to that determined for the HT-GaN one is attributed to the amorphous-like structure of the layer and the instability of Ga-N bonds. This favors the appearance of a mirror-like gallium surface in the beginning of this layer decomposition. The presence of liquid gallium catalyzes the decomposition process. HT-GaN SEM analysis revealed that the surface of the "totally" decomposed GaN surface is dominated by gallium droplets. These were removed. The region around and under these droplets is capped by a porous like GaN film. The coalesced gallium droplets may cover some areas of partially decomposed GaN and preserve them from H 2 attack and total decomposition.

Undoped GaN layers were grown on SiN-treated (0 0 0 1) sapphire substrates by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). The thermal decomposition of GaN layers was studied by in situ laser reflectometry and scanning electron microscopy (SEM) analysis. The thermal annealing process was performed in the MOCVD vertical reactor in the temperature range 900-1200 • C in N 2 atmosphere. It was observed that annealing at temperature above 1100 • C causes the decomposition of GaN layers. An activation energy of 3.2 eV was deduced for the GaN thermal decomposition in N 2 atmosphere. The GaN decomposed surface films were analyzed by SEM. These observations were correlated with the in situ laser reflectometry measurements.

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An investigation of MOVPE GaN growth on sapphire (0001) using Si/N treatment is reported. The growth was monitored in situ He-Ne laser reflectometry. By fitting the reflectivity signal, we derived expressions for the refractive index variation of both sapphire substrate and GaN as a function of the temperature. The morphological and optical property evolutions of the layer at all the growth stages were studied by atomic force microscopy (AFM) and transmittance measurements. A good agreement between AFM images and laser reflectometry signal is obtained. During the island growth and coalescence as clearly revealed by AFM images of the GaN layers, the transmittance signal evolves from featureless characteristic to a signal with regular oscillations.

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