Transparent RuO_x contacts on n-ZnO

100 nm thick Ru films were deposited onto n-ZnO epitaxial layers by radio frequency sputtering. It was found that highly transparent Ru Ox was formed after O2 annealing. With an incident wavelength of 460 nm, it was found that transmittances of as-grown, 500°C -annealed, 600°C -annealed, and 700°C -annealed Ru films were 56.8, 73.5, 79.6, and 86.8%, respectively. It was also found that as-deposited Ru formed Schottky contact on n-ZnO. However, good ohmic contacts were formed between the annealed Ru films and the underneath ZnO. With 650°C annealing, we achieved a specific contact resistance of only 2.72× 10-4 cm2. Such a low specific contact resistance should be attributed to the formation of Ru Ox and the dissociation of oxygen atoms in ZnO during annealing.