The crystallinity and refractive index of microcrystalline silicon oxide (μc-SiOx:H) n-layers and their dependence on the pressure and radio frequency power during the deposition process is correlated with plasma properties derived from in situ diagnostics. From process gas depletion measurements, the oxygen content of the layers was calculated. High crystallinities were observed for increased pressures and decreased powers, indicating clear differences to trends previously shown for microcrystalline silicon (μc-Si:H) material, which are explained by the varying oxygen incorporation. Amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with μc-SiOx:H intermediate reflector layers deposited at optimized pressures showed greatly improved series resistances. © 2013 American Institute of Physics.