Defects and their origin in thin films of (001) alkaline earth oxides

MgO is used as an optical isolation layer for waveguides epitaxially grown on silicon. The crystalline perfection of MgO is a critical issue because it serves as a substrate for the single crystal, perovskite guiding layer. Imperfections in the MgO will result in imperfections in the guiding layer and lead to large optical losses for the planar waveguide structure. We show that the most common defect to form in thin films of MgO are twin boundaries between {111}-type planes. The highest density of twins is observed when (001) MgO is grown directly on silicon/MgO interlayers containing barium. Twinning is shown to accommodate the large size of barium impurities incorporated in the MgO films through the formation of internal grain boundaries and open surfaces other than the growing (001) of MgO.