This work discusses the influence of different cleaning procedures on p-gallium nitride (GaN) grown on sapphire by metal–organic chemical vapor deposition. The cleaned p-GaN surface was transferred into an ultrahigh vacuum chamber and studied by an X-ray photoelectron spectrometer, revealing that a cleaning with a so-called “piranha” procedure results in low carbon and oxygen concentrations on the p-GaN surface. Contrary, a cleaning that solely uses ethanol represents a simple cleaning but leads to an increase of carbon and oxygen contaminations on the surface. Afterward, the cleaned p-GaN samples underwent a subsequential vacuum thermal cleaning at various temperatures to achieve an atomically clean surface. X-ray photoelectron spectroscopy (XPS) measurements revealed residual oxygen and carbon on the p-GaN surface. Thus, a thermal treatment under a vacuum did not entirely remove these organic contaminations, although the thermal cleaning reduced their peak intensities. The complete removal of carbon and oxygen contaminants was only achieved by argon ion sputtering, which is accompanied by a strong depletion of the nitrogen on the p-GaN surface. The treatments cause a large number of surface defects preventing the formation of a negative electron surface when the p-GaN is activated with a thin layer of cesium.