Realization of high efficiency III-V-on-Si solar cells requires preparation of an oxygen free Si substrate surface, which allows for subsequent antiphase domain free III-V epitaxy. For GaAsP/Si tandem cells, the impact of As on established Si processing in metal organic vapor phase epitaxy (MOVPE) is essential. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered As -modified Si(100) surfaces, and its impact on subsequently grown GaP epilayers. We apply optical in situ spectroscopy during MOVPE and show that process routes strongly affect the formation of double-layer steps on Si(100). We demonstrate that exposure to As, together with HF pre-treatment, enables processing of suitable Si(100) 2 degrees and 6 degrees offcut surfaces at temperatures below 820 degrees C. Subsequently grown GaP epilayers exhibit smooth surfaces free of antiphase disorder and distinct Laue oscillations in X-ray diffraction. Such low temperature processing, which is controllable in situ, is promising for further metamorphic GaAsP integration.