(at.%) alloy thin films are prepared on Cu underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The substrate temperature is varied from 100 to 500 . The film growth behavior and the detailed resulting film structure are respectively investigated by in situ reflection high-energy electron diffraction and by a combination of out-of-plane and in-plane X-ray diffractions. With increasing the temperature up to 300 , formation of ordered phase is recognized. The films deposited above 400 consist of only epitaxial crystal, whereas the films deposited below the temperature include amorphous phase and/or a hexagonal ordered phase other than . When the temperature is increased from 300 to 500 , the long-range order degree of film increases from 0.60 to 0.97. phase formation is enhanced with increasing the temperature.