The band gap energy of ordered and strained In_(x)Ga_(1-x)P as a function of ternary composition was studied. Epitaxial growth using a metalorganic vapor phase epitaxy technique at a reactor pressure of 20 mbar and T_(g)=580℃ allowed us to prepare a set of samples with nearly constant ordering parameter η. Optical measurements were performed at room temperature using a rotating polarizer ellipsometer with a spectral energy range 1.4-5.1 eV. Comparing the experimental data with the theory, we have shown that the band gap energy E_(g) dependence on composition closely follows the prediction of Wei and Zunger S. Wei and A. Zunger, Phys. Rev. B 49, 14337 (1994). This prediction is more valid as the commonly used parabolic interpolation of E_(g) between InP and GaP values.
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