Role of insoluble atoms in the formation of a three-dimensional buffer layer in inverted Stranski–Krastanov mode

The inverted Stranski–Krastanov (SK) mode is useful for heteroepitaxial growth of single-crystalline ZnO films on 18% lattice-mismatched sapphire substrates. We studied the role of nitrogen atoms during fabrication of a three-dimensional island-shaped buffer layer. We found an unprecedented maximum in the substrate temperature dependence of the density of the crystal grains, which facilitated the growth of flat ZnO layers. To reveal the mechanism of the aforementioned maximum, we measured the absolute N atom density in Ar/N₂ sputtering plasma [N]_plasma by vacuum-ultraviolet absorption spectroscopy. At [N]_plasma = 2.2 × 10¹⁰ cm⁻³, we fabricated a ZnO film with a pit-free surface, attributable to the large surface reaction probability and small incorporation ratio of N atoms into the ZnO films. To describe these results, we applied an Ising model. The analytical calculations provide insights for inverted SK mode and clearly reveal the critical effects of the flux densities. Graphical abstract: [Figure not available: see fulltext.]