Annealing Effect of Transparent Ohmic Contacts to n-ZnO Epitaxial Films

ZnO epitaxial films were grown on sapphire substrates by molecular beam epitaxy. The diffraction pattern shows an extremely strong ZnO (0002) peak at 34.43DG with a full width half-maximum of 0.19DG. Low resistivity and highly transparent indium tin oxide (ITO), RuOz(1 < or = x < or = 2), and TiW ohmic contact were achieved by radio frequency sputtering and annealing treatment. The transmittance of 450DGC-annealed ITO, 650DGC-annealed Ru, and 200DGC-annealed TiW were measured to be 94, 68, and 61%, respectively, with a wavelength of 400 nm. Moreover, the resistivities of 450DGC-annealed ITO, 650DGC-annealed Ru, and 200DGCannealed TiW were measured to be 9.2 X 10-5, 4.1 X 10-5, and 4.7 X 10-3 Omega cm, respectively. Finally, the specific contact resistance of 450DGC-annealed ITO, 650DGC-annealed Ru, and 200DGC-annealed TiW on ZnO films was estimated to be 2.15 X 10-4, 2.72 X 10-4, and 2.56 X 10-4 Omega cm2, rrespectively, by circular transmission line model method.