Structural characterization of two-step growth of epitaxial ZnO films on sapphire substrates at low temperatures

We have investigated two-step growth of high-quality epitaxial ZnO films, where the first layer-the buffer layer (nucleation layer template)-is grown at a low temperature (230-290 degrees C) to induce a smooth (two-dimensional) growth. This is followed by growth at a moderate temperature similar to 430 degrees C to form high-quality smooth ZnO layers for device structures. It was possible to reduce the growth temperature to 250-290 degrees C and obtain a smooth epitaxial template layer on sapphire (0001) substrates with surface roughness less than 1 nm. After the high-temperature growth, the film surface undulations (roughness) increased to about 2 nm, but it is still quite smooth. The calculation of c and a lattice parameters by high-resolution x-ray diffraction shows that the a lattice parameter is fully relaxed at the growth temperatures but the c lattice parameter is dependent on the defect concentration in the growing film. A decoupling between a and c lattice parameters of the films is observed, which leads to abnormal Poisson's ratios ranging from 0.08 to 0.54. The decoupling of the lattice parameters is analysed based on growth characteristics and the presence of strain and defects in the grown films. We present our detailed studies on the nature of epitaxy, defects and interfaces by using comprehensive x-ray diffraction and high-resolution TEM studies.