Sputter epitaxy of ZnO-InN alloy films on sapphire substrates realized via 3D buffer layers and kinetically driven growth control

We demonstrate epitaxial growth of high-quality (ZnO)_0.9(InN)_0.1 films (ZION films) on sapphire substrates using three-dimensional (3D) ZnO island buffer layers. These buffer layers play a crucial role in overcoming the 19% lattice mismatch between (ZnO)_0.9(InN)_0.1 and sapphire, providing lattice-relaxed nucleation sites with a well-aligned orientation and reducing the interfacial free energy between the film and underlying layer. The resulting (ZnO)_0.9(InN)_0.1 films exhibit excellent crystal quality, with a remarkably narrow full width at half maximum of 0.04° in the x-ray rocking curve of the 0002 reflection, surpassing films grown on lattice-matched, yet costly, bulk ZnO substrates. We find that superior crystal quality is achieved with a smaller substrate off-cut angle (θ_off) and a higher buffer deposition rate (DR_buffer), A small θ_off (≤0.4°) promotes the formation of nanoscale 3D islands by maintaining crystallographic terrace widths wider than the adatom migration length, thereby suppressing strained step-flow growth and enabling effective strain relaxation at the island surfaces. Meanwhile, a high DR_buffer (≥0.28 nm/s) increases the flux of sputtered atoms relative to their diffusivity on the substrate surface, facilitating the nucleation of finer 3D islands and further enhancing lattice strain relaxation. These results present a complementary perspective to conventional epitaxial growth approaches, which typically favor lower deposition rates and larger θ_off to enhance step-flow growth. This 3D-ZnO buffer layer technique offers a kinetically driven pathway for high-quality epitaxy on lattice-mismatched substrates, not only advancing ZION-based optoelectronic and energy-harvesting applications but also providing a scalable method for other materials that lack cost-effective lattice-matched substrates.