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Dilute nitride GaP(N,As)/Si(100) tandem absorber structures are considered for efficient direct photoelectrochemical water splitting. Both Si and GaP have already been used for hydrogen evolution; their monolithic integration promises sufficient voltage for bias-free photoelectrolysis and efficient use of the solar spectrum. The preparation of the III-V/Si(100) heterointerface thereby is the crucial step towards low-defect absorber structures. We study the growth of GaP/Si(100) quasi-substrates, as the template for the integration of lattice-matched dilute nitride layers, in situ during metalorganic vapor phase epitaxy. Reflection anisotropy spectroscopy provides precise control over the epitaxial surface structure during preparation enables in situ quantification of anti-phase disorder and allows optical access to the heterointerface. [ABSTRACT FROM AUTHOR]