Formation of metastable MgO structures on type-III oxide surfaces: Effect of periodic out-of-plane electric dipole moment of substrates

Metastable structures of MgO overlayer on type-III oxide surfaces were achieved by molecular beam epitaxy. Periodic out-of-plane dipole moment of substrates was revealed to play a dominant role in the formation of metastable oxide structures. Under near-equilibrium epitaxial conditions, this dipole moment was found to drive the over-grown MgO to inherit the out-of-plane substrate lattice, resulting in some new structures with different stacking sequences and bonding configurations from those of thermodynamically stable phase. As typical cases, by adopting the substrate dipole moment along the growth direction, MgO overlayers with metastable wurtzite (WZ) and stacking-fault (SF) structures were formed on α-Al2O3 (0 0 0 1) and MgAl2O4 (1 1 1) surfaces, respectively.