We report on selective-area metal-organic vapor phase epitaxy and structural characterizations of magnetic MnAs nanodisk structures after the AlGaAs buffer layer growth on Si (1 1 1) substrates using multiple types of mask materials and designs, which are mask patterns (I) and (II) of SiO2 and SiON removing (I) the mask materials outside 100 x 100 mu m(2) square regions with periodical circular openings and (II) only the periodical circular openings within the 100 x 100 mu m(2) square regions. The results of structural characterizations show that two key issues, which are the unintentional growth of MnSi alloy in Si substrates and MnAs alloy with approximately 17% of Si atoms on nanodisks, are typically observed in the case of Si substrates with mask pattern (I) of SiO2. On the other hand, these two key issues are not observed in the case of Si substrates with mask pattern (II) of SiON. These results suggest that mask pattern (II) of SiON are effective to prevent the unintentional reactions of Mn atoms with Si substrates for growing MnAs nanodisks with a high degree of uniformity.