The chemical bonding and electronic properties of wet, chemically treated p-GaN surfaces were studied using synchrotron radiation photoemission spectroscopy. Chlorine-based chemical bonding was identified on the conventional HCl-treated p-GaN surface, which is associated with a shift of the surface Fermi level toward the conduction band edge by ∼0.9 eV with respect to the thermally cleaned surface. Compared to the HCl-treated surface, the surface Fermi level on the KOH-treated surface lies about ∼1.0 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-type materials than the HCl-treated surface. The smaller surface barrier height to p-GaN after KOH treatment can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the surface Fermi level position within the band gap for thin metal overlayers of Au, Al, Ni, Ti, Pt, and Pd on n-GaN and p-GaN. Nonequilibrium effects were taken into account by measuring the Fermi edge of the metal overlayer. There are two different behaviors observed for the six metals studied. For Au, Ti, and Pt, the surface Fermi level lies about 0.5-eV higher in the gap for n-type than for p-type GaN. For Ni, Al, and Pd, the surface Fermi level position is independent of doping, but varies from one metal to the other. Results for Ni, Pd, and Al fit a modified Schottky-Mott theory, while Au, Ti, and Pt demonstrate a more complex behavior. Atomic force microscopy was used along with photoemission to investigate the growth mode of each metal on the GaN surface.

The effects of surface chemical treatments and metal deposition on the InN surface are studied via synchrotron-based photoemission spectroscopy. Changes in the In 4d core level as well as the valence band spectra are reported. The surface Fermi level position, E F , relative to the valence band maximum was determined for both Au and Ti Schottky barriers. E F lies at an energy of 0.7 eV above the valence band maximum for Au deposited on annealed InN and 1.2 eV above the valence band maximum for Ti deposited on Ar-sputtered InN. These results that the surface Fermi level lays at or above the conduction band maximum when a value of InN band gap of 0.7-0.9 eV is assumed.

The surface chemistry and electronic properties of n-GaN surfaces were studied via x-ray photoemission spectroscopy before and after wet chemical treatments. Shifts of the surface Fermi level were measured with the change in position of the Ga 3d core level peak. HCl treatment of n-GaN led to a 0.9 eV shift of the surface Fermi level toward the conduction band minimum, while KOH treatment led to a 0.3 eV shift of the surface Fermi level toward the valance band maximum. These shifts lead to a reduction in the surface barrier for HCl-treated n-GaN and for KOH-treated p-GaN, potentially improving contact resistance. The changes in surface chemistry indicate that a N (or Ga) deficiency with HCl(KOH) treatment alters the surface state density through the formation of donor (acceptor)-like states.

Using synchrotron x-ray scattering the evolution of the biaxial strain at the interface of the Pt layer ͑100 Å͒ with p-type GaN was investigated as a function of annealing temperature. Furthermore, the effects of the biaxial strain on the change of ohmic contact resistivity were interpreted. The Pt layer grew epitaxially on GaN with the relationships of Pt ͓111͔//GaN ͓0001͔ and Pt ͓11 0͔ //GaN ͓112 0͔. Due to a lattice mismatch between Pt and GaN, a biaxial tensile strain ͑ϩ0.9%͒ to the Pt layer and a compressive strain ͑Ϫ0.9%͒ to the GaN substrate were introduced in the as-deposited state. After annealing at 450°C, the strains were fully relaxed and the position of the surface Fermi level moved 0.21 eV toward the valence band maximum. Furthermore, the contact resistivity decreased by 1 order of magnitude. The results provide evidence that the change of the interfacial strain causes the movement of the surface Fermi level position in the band gap of GaN, leading to a change in the magnitude of contact resistivity.