Photoluminescence (PL) intensity from SiGe mixed crystals was investigated as a function of the growth temperature and/or atomic-hydrogen irradiation rate during molecular beam epitaxy (MBE). As a result, a clear one-to-one correspondence between PL intensity and the surface segregation length of Ge was obtained. This indicated that formation of Ge-Ge pairs during MBE is a key factor in determining PL intensity. Theoretical calculations showed that localized Si-Ge bonds cause s-like symmetry in the conduction band bottom and p-like symmetry in the valence band top. However, pairing of Ge-Ge atoms changes the local symmetry of the Si-Ge bonds and destroys the s-like symmetry in the conduction band. Such calculations explain the experimental results. This new knowledge triggered the development of "Ge-segregation-controlled MBE," which successfully enhanced the PL intensity. Consequently, a preliminary optoelectronic device, operating at 77 K with a light emitting diode and detector on the same wafer, was fabricated.