  We study the adsorption behavior of Ga on (0001) GaN surfaces combining
experimental specular reflection high-energy electron diffraction with
theoretical investigations in the framework of a kinetic model for adsorption
and ab initio calculations of energy parameters. The measurement of a Ga/GaN
adsorption isotherm allows the quantification of the equilibrium Ga surface
coverage as a function of the impinging Ga flux. The temperature dependence is
discussed within an {\em ab initio} based growth model for adsorption taking
into account the nucleation of Ga clusters.
