Microstructure of ZnO films synthesized on MgAl₂O₄ from low-temperature aqueous solution: growth and post-annealing

The microstructure of ZnO films synthesized from low-temperature (90 °C) aqueous solution on (111) MgAl₂O₄ single crystal substrates was characterized by X-ray diffraction, high-resolution scanning electron microscopy, conventional and high-resolution transmission electron microscopy. To examine the thermally activated microstructural evolution of the ZnO, both as-deposited and annealed films were characterized. The ZnO films were confirmed to have a ZnO\( [10\bar{1}0](0001)\left\| {{\text{MgAl}}_{ 2} {\text{O}}_{4} [011](1\bar{1}1)} \right. \) orientation relationship, with Zn polarity normal to the surface. Despite their highly oriented nature, the ZnO films have a columnar grain structure with low-angle (<2.5°) grain boundaries. In addition to lattice dislocations forming low-angle grain boundaries, threading dislocations were observed, emanating from the interface with the substrate. In annealed films, thermally generated voids were observed and appeared to preferentially form at grain boundaries and dislocations. Based on these characterization results, mechanisms are proposed for film growth and microstructural evolution. Finally, the diffusion coefficient of vacancies via dislocations at grain boundaries in the produced ZnO films was estimated.