Surface properties of c-plane GaN grown by plasma-assisted molecular beam epitaxy

Two series of GaN van Hoof structures with different thicknesses of an undoped GaN cap layer were grown under metal-rich conditions by plasma-assisted molecular beam epitaxy. These were then investigated by contactless electroreflectance (CER) to study the Fermi-level position of the (0001) GaN surface after growth as well as after chemical treatment using Piranha solution. The first and second series of samples were grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respectively. A clear CER resonance followed by Franz–Keldysh oscillations (FKOs) of various periods was clearly observed for both sample series before and after chemical treatment. The Fermi-level position of the GaN surface was determined from the analysis of FKOs related to the built-in electric field in the undoped GaN layer. For the as-grown GaN surface, the Fermi level was found to be located 0.42 and 0.57 eV below the conduction band in samples grown on GaN/sapphire templates and high-pressure bulk GaN crystals, respectively. For the Piranha-etched GaN surfaces, the Fermi level was pinned at almost the same energy (0.49 and 0.48 eV) in each of the two sets of samples. This means that this cleaning procedure, which is commonly used before device processing, is able to saturate the surface states at a certain level of Fermi-level pinning.