Using synchrotron x-ray scattering the evolution of the biaxial strain at the interface of the Pt layer (100 Angstrom) 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 [111]//GaN [0001] and Pt [1 (1) over bar0] HGaN [ 11 (2) over bar0]. Due to a lattice mismatch between Pt and GaN, a biaxial tensile strain (+0.9%) to the Pt layer and a compressive strain (-0.9%) to the GaN substrate were introduced in the as-deposited state. After annealing at 450 degreesC, 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. (C) 2003 American Vacuum Society.