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ZnO films with an excellent c-axis orientation, transparancy, and surface flatness have been prepared by high deposition-rate rf planar magnetron sputtering. The optical waveguide loss for the TE0 mode of He-Ne 6328 A line is as low as 2.0 dB/cm in a 4.2 μm thick film without postsputtering treatment. Single-crystalline ZnO films have been grown epitaxial onto sapphire substrates in a ZnO-H2-H2O-O2 system. In this growth, water vapor and intermediately-sputter-deposited very-thin ZnO epitaxial layer are very important to grow ZnO films with an optically flat surface. The optical waveguide loss for the TE0 mode (6328 A) is as low as 0.7 dB/cm in a 2.57 μm thick film without postgrowth treatment.