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

• Published in: Crystal growth & design Vol. 22; no. 6; pp. 3770 - 3777
• Main Authors: Nakamura, Yuta, Yamashita, Naoto, Kamataki, Kunihiro, Okumura, Takamasa, Koga, Kazunori, Shiratani, Masaharu, Itagaki, Naho
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.06.2022
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.2c00145

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 ξ 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 ξ, 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 ξ of 13.7 nm. This finding provides a useful tool for understanding the mechanism of the inverted SK mode.