Effects of substrate surface polarity on heteroepitaxial growth of pseudobinary ZnO–InN alloy films on ZnO substrates

(ZnO)_ X (InN)_1- X films have been epitaxially grown on 0.9%-lattice-mismatched ZnO substrates at 450 °C by sputtering. Films fabricated on O-polar substrates exhibit higher crystal quality and smoother surface. The full width at half-maximum of (0002) rocking curve and the root-mean-square roughness ( R _q) of a 30-nm-thick film on O-polar surface are 0.21° and 2.71 nm, respectively, whereas those on Zn-polar one are 0.32° and 4.30 nm, respectively. R _q on O-polar surface further decreases to 0.73 nm as the thickness decreases to 10 nm, where we successfully obtained atomically flat single-crystalline films having atomically sharp interface with the substrates. High-resolution transmission electron microscopy revealed the Stranski–Krastanov (layer plus island) growth for O-polar case and just 3D islanding mode growth for Zn-polar one. All the results indicate the much longer migration length of adatoms on O-polar surface during the film growth, enabling adatoms to reach their thermodynamically favored positions even at low substrate temperature. Graphical abstract