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We have carried out thermodynamics studies of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique and found that the superconductor MgB2 phase is thermodynamically stable only under fairly high Mg pressures at elevated temperatures. This has lead us to the investigation of chemical vapor deposition in which the pressure during the film deposition can be high. Although the initial effort on metal-organic chemical vapor deposition (MOCVD) was not successful due to carbon contamination, a unique hybrid physical-chemical vapor deposition (HPCVD) technique has successfully produced high quality in situ MgB2 films. The epitaxially-grown MgB2 films show high transition temperature and low resistivity comparable to the best bulk samples, and their surfaces are smooth. In this paper, the details of the technique and the results of the HPCVD films are presented.