Epitaxial Growth of Atomically Flat Single-Crystalline (ZnO)_x(InN)_1-x Films on O-Polar ZnO Substrates by Magnetron Sputtering

(ZnO)<i>_x</i>(InN)_1-<i>x</i> (called “ZION” hereinafter), pseudo-binary alloys of ZnO and InN, are promising materials for optoelectronic devices [1] since they have tunable band gaps across the entire visible spectrum and high exciton binding energies of 30–60 meV. Recently, we have succeeded in epitaxial growth of ZION films by radio-frequency (rf) magnetron sputtering on 1.6%-lattice mismatched ZnO substrates [2]. Especially, on O-polar surfaces, which provide longer migration length of Zn/In atoms than Zn-polar surfaces do, high-quality ZION films with the full-width at half maximum (FWHM) of (002) x-ray rocking curve (XRC) of 0.2° have been obtained. However, they all turned out to be polycrystalline and to have large root-mean-square (RMS) roughness around 7 nm. Here, aiming to improve the structural properties of ZION films, effects of the surface morphology of substrates on the growth of ZION films are investigated. The morphological data, obtained by atomic force microscopy (AFM), are analyzed statistically deriving the surface height distribution, RMS roughness, and skewness; the last one relates to the third moment of the height distribution and is a measure of the asymmetry of the distribution. The atomic structures of ZION films are observed using x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Finally, based on these results, we perform sputtering deposition of atomically flat single-crystalline ZION films by tailoring the surface morphology of the substrates.<br/>Two types of ZnO substrates with different surface morphologies were prepared; one was not annealed, and another was annealed at 1000°C in air for 10 hours. ZION films were deposited on the O-polar surface of the substrates by rf magnetron sputtering in Ar/N₂/O₂ atmosphere at 450°C. The chemical composition ratio of the films was (ZnO)_0.85(InN)_0.15, confirmed by x-ray fluorescence spectrometry. XRD analysis shows that all the films were epitaxially grown on the substrates.<br/>We found that both the RMS roughness, <i>R</i>_q, and the skewness, <i>R</i>_sk, of the ZnO substrates significantly influences on the structural properties of ZION films. The former clearly affects the surface roughness of the ZION films, <i>R</i>_q of which decreases from 7.1 to 2.7 nm with decreasing the <i>R</i>_q of the substrates from 2.3 to 0.2 nm. On the other hand, <i>R</i>_sk correlates to lattice relaxation process during the film growth. XRD measurements indicates that on the non-annealed ZnO substrates with large <i>R</i>_sk, ZION films lose coherency with the substrates and have relaxed lattice parameters. A possible reason is that existence of a small portion of spikes on the surface, which appears as tail components in the height distribution, limits the migration of adatoms and leads to secondary nucleation (<i>R</i>_q, the second moment of the height distribution, does not reflect such tiny tail components). The lattice relaxation occurs through the secondary nucleation. While, on the annealed ZnO substrates with small <i>R</i>_sk, ZION films grow coherently on the substrates, where secondary nucleation might be suppressed. This result is consistent with TEM results. The HRTEM images show that the ZION film has an atomically sharp film/substrate interface and grows fully coherently on the annealed ZnO substrate for more than 15 monolayers (ML).<br/>Based on these results, we finally deposited 30-ML ZION films on annealed O-polar ZnO substrates, which have small <i>R</i>_q of 0.2 nm and small <i>R</i>_sk of 0.75, and succeeded in the epitaxial growth of atomically flat single-crystalline ZION film with <i>R</i>_q of 0.73 nm. This research made an important contribution to the realization of device-grade ZION films.<br/>This work was supported by JSPS KAKENHI Grant Numbers JP21H01372, JP21K18731, NTT collaborative research, Toyota Riken Scholar, The Murata Science Foundation.<br/>[1] N. Itagaki, <i>et al</i>., Mater. Res. Express, 1, 36405 (2014).<br/>[2] R. Narishige <i>et al</i>., Jpn. J. Appl. Phys., 60, SAAB02 (2021).