Study of the growth of thin Mg films on wurtzite GaN surfaces

The growth, structure and electronic properties of thin (less than or equal to 20 Angstrom thick) Mg films on clean, n-type wurtzite GaN have been studied using Auger, photoemission and electron-energy-loss spectroscopies and low-energy electron diffraction. Epitaxial growth occurs near 300 K leading to a (1 x 1)-ordered metal layer. Site exchange between Mg and Ga releases free Ga, which remains near the interface, and incorporates Mg into the GaN near the interface. Deposition of less than one monolayer of Mg moves the Fermi level similar to 0.60 eV farther down into the gap, from the initial position of 2.60 eV above the valence band maximum, where it remains essentially fixed with increasing Mg coverage. Annealing leads to desorption of metallic Mg which, for a thin film, is essentially complete at similar to 520 K. There is no indication of extensive intermixing of either Ga or N with the Mg. A residue of Ga and Mg adatoms or small non-metallic clusters, remaining after desorption of the metal, persists to at least 620 K. Desorption of metallic Mg has little or no effect on the Fermi-level position which, therefore, is determined mainly by trapping of electrons by accepters that result from Mg incorporation into the surface, rather than by Schottky-barrier formation. (C) 1998 Published by Elsevier Science B.V. All rights reserved.