Recently the growth of high quality III-nitrides attracted a great deal of interest because of its usefulness for modern optoelectronics as well as high temperature and power devices. The growth of GaN by molecular beam epitaxy (MBE) using Ga and NH₃ is a sequence of elementary steps: adsorption-desorption, dissociation of NH₃, recombination of NH₂ radicals, incorporation of Ga and N atoms into lattice sites. This means that without the knowledge of the parameters of the microscopic surface processes it is not clear how to choose the optimum growth conditions.
In the present work the kinetics of GaN growth under Ga-rich conditions was investigated by reflection high energy electron diffraction (RHEED). Specular beam (SB) intensity evolution with time during successive supplying of the growth components onto a GaN surface was monitored. The strictly determined amount of Ga was deposited onto GaN surface without NH₃ flux first, then the surface was exposed to NH₃. The transformation time (τ) of the adsorbed Ga into GaN was measured as a function of the NH₃ flux and substrate temperature.
An increase of the growth rate with increasing substrate temperature was demonstrated. It was revealed also that growth rate (V~1/τ) depended on NH₃ pressure as V~Pⁿ, where n increased with increasing substrate temperature from 0.5 (below 760 degrees C) to 1 (above 800 degrees C).
In order to explain the experimental data a kinetic model was developed in the frame of the mean field kinetic equations:

NH₃^gas <=> NH₃^ads (adsorption and desorption)
NH₃^ads => NH₂^chem+H (dissociation)
NH₂^chem+NH₂^chem => N₂^gas+2H₂^gas (recombination)
NH₂^chem+Ga^ads => Ga-N+H₂^gas (incorporation into lattice site)

An analytical expression for the GaN growth rate was developed as a function of the elementary kinetic constants and NH₃ pressure. This kinetic model is in good agreement with the experimental data.

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