Gallium nitride (GaN), a wide band gap semiconductor (Eg=3.4 eV) has unique physical properties which opened up the door to widespread applications in modern electronic devices. The semiconductor is used as a base for fabrication of visible and UV lasers, light-emitting diodes, high temperature and frequency detectors, and transistors. In all these devices the electronic properties of bare and thin-covered GaN surfaces have impacts on their efficiencies.
Herein surface sensitive techniques with an emphasis on ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) are used to understand physical and chemical properties of bare and film-covered GaN(0001) surfaces. Thin films of several elements: nickel, palladium, manganese, arsenic and antimony (Ni, Pd, Mn, As, Sb) are included. The properties of bare GaN surfaces  as well as various adsorbate/GaN phase boundary are reported [2-7]. Different types of thin films behavior under an influence of annealing in ultrahigh vacuum conditions are distinguish. Metal films form surface alloys with gallium (NiGa, PdGa) while semi-metals layers easy evaporate from GaN surface, however, the layer in direct contact with the substrate can react with it causing a change in the surface properties of GaN(0001).
 M. Grodzicki, P. Mazur, A. Ciszewski, Appl. Surf. Sci. 440, 547 (2018)
 M. Grodzicki, P. Mazur, et al., Appl. Phys. A 120, 1443 (2015)
 M. Grodzicki, P. Mazur, et al., Appl. Surf. Sci. 304, 24 (2014)
 M. Grodzicki, P. Mazur, J. Brona, A. Ciszewski, Appl. Surf. Sci. 481, 790 (2019)
 M. Grodzicki, P. Mazur, A. Sabik, Sur. Sci. 689, 121460 (2019)
 M. Grodzicki, J. Rousset, et al., Appl. Surf. Sci. 493, 384 (2019)
 M. Grodzicki, P. Mazur, A. Sabik, Appl. Surf. Sci. 512, 145643, (2020)