Growth of Single-Crystalline ZnO Films on 18%-Lattice-Mismatched Sapphire Substrates Using Buffer Layers with Three-Dimensional Islands

Heteroepitaxial growth of single-crystalline zinc oxide (ZnO) films on a c-plane sapphire substrate is an important technology for electronics and optoelectronic devices. Recently, the inverted Stranski-Krastanov (SK) mode has been demonstrated, and it has realized the heteroepitaxial growth of ZnO films on a sapphire substrate by sputtering. In this mode, a 10 nm-thick buffer layer consisting of three-dimensional islands (3D buffer layers) initially forms and relaxes the strain, and then, a two-dimensional ZnO film (2D layer) grows involving small strain. To clarify the correlation between the structural properties of the 3D buffer layers and the 2D layer, we introduce a figure of merit (FOM) of ZnO films: the reciprocal of the product of the full width at half-maximum (FWHM) of the (002) and (101) planes of X-ray rocking curves (XRCs) and root-mean-square (RMS) roughness. We find that the FOM of the 2D layers correlates with the RMS roughness, the in-plane orientation, and the lateral correlation length ζ of the surfaces of the buffer layers. We observe a surprisingly high correlation coefficient of 0.97. Our results imply that on the buffer layers with larger ζ, adatoms more easily reach the thermodynamically favored lattice positions. Thus, high-quality single-crystalline ZnO films, where the (002) plane XRC-FWHM and the RMS roughness are 0.05° and 1.5 nm, respectively, are grown on the buffer layers with a large ζ of 13.7 nm. This finding provides a useful tool for understanding the mechanism of the inverted SK mode.