Metal-aluminium gallium nitride Schottky contacts formation.
X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of various surface cleaning procedures on Al[x]Ga[1]-[x]N surfaces for x = 0.20 and 0.30. Results show that wet chemical etch in a HF solution followed by a 600°C in-situ annealing under ultra-high vacuum (UHV) is very effective in removing oxygen from the surface. Downward band bending of 0.87 eV and 0.99 eV also occurs between the solvents-treated and the annealed Al[x]Ga[1]-[x]N surfaces for x = 0.20 and 0.30, respectively.Increasing in-situ temperature annealing in increments of 100°C up to 600°C shows a re-ordering at the surface and subsurface with Ga and A1 moving deeper in the surface, whereas N goes to the topsurface. In addition, the Fermi level movement observed when increasing the temperature could be interpreted by the change in surface stoichiometry or by a creation of vacancies due to the ex-situ surface treatment which may, in turn, be activated/deactivated by temperature annealing. Atomic hydrogen clean (AHC) followed by 400°C in-situ UHV annealing is also found effective in removing O and C from Al[x]Ga[1]-[x]N surface (x = 0.20).The formation of Ag/Al[x]Ga[1]-[x]N (x = 0.20) and Ni/Al[x]Ga[1]-[x]N (x = 0.30) interfaces, where the substrate was subjected to HF etch followed by 600°C in-situ UHV anneal, has been studied by a combination of XPS, atomic force microscope (AFM), scanning tunneling microscope (STM) and current-voltage (I-V) measurements.XPS results suggest a layer-by-layer followed by islanding growth mode of Ag and Ni on Al[x]Ga[1]-[x]N. This is confirmed by the presence of metal islands at the metal-covered surfaces using AFM and in-situ STM. XPS investigation shows a more abrupt, well-defined Ag/Al[x]Ga[1]-[x]N interface compared to Ni/Al[x]Ga[1]-[x]N. Ag deposition on Al[x]Ga[1]-[x]N substrates causes upward band bending of 0.30 eV and 0.40 eV between the "clean" surface and the last metal deposition, for x = 0.20 and 0.30, respectively, while Ni induces downward band bending of 0.3 eV for x = 0.20. I-V measurements of Ag/Al[x]Ga[1]-[x]N (x = 0.30), where the substrate was cleaned using N[+] bombardment followed by 600°C annealing, yield a Schottky barrier height of 0.82 eV with ideality factor n = 1.21.XPS and I-V results on Ag/Al[x]Ga[1]-[x]N and Ni/Al[x]Ga[1]-[x]N are compared and discussed in terms of current models of Schottky barrier formation.
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Sheffield Hallam University (United Kingdom).