Epitaxial Growth of Single-Crystalline ZnO Films on Sapphire Substrates via Inverted Stranski-Krastanov Mode by Low-Power Magnetron Sputtering

ZnO is a wide band gap semiconductor (3.37 eV) with a high exciton binding energy of 60 meV. Due to these properties, there has been considerable interest in recent years in the development of single crystalline ZnO films for optoelectronic devices. However, the large lattice mismatch of 18% between ZnO and the most used substrate, sapphire, often results in polycrystalline structures with poor crystal quality. To solve the problem, we have developed a new crystal growth method using “inverted Stranski-Krastanov (SK) mode” [1]. In this mode, strain-relaxed nano-sized three-dimensional (3D) islands (buffer layers) are initially grown by using impurities that reduce the surface energy. Then the islands coalesce to form a two-dimensional (2D) layer after desorption of impurities, and eventually, the film grow in 2D mode and form a single crystal. This mode has enabled us to grow single crystalline films on large lattice mismatched substrates. Here we demonstrate inverted SK growth of ZnO films on sapphire substrates by magnetron sputtering using N atoms as impurities. We investigate the effects of supplied radio frequency (RF) power, a key parameter determining the flux of sputtered particles to the substrates, on the crystal growth behavior especially during the growth of 2D layers.<br/>All films were fabricated by RF magnetron sputtering using ZnO targets with the purity of 99.99 %. First,10-nm-thick ZnO buffer layers were deposited on c-plane sapphire substrates in Ar/N₂ atmosphere at 780°C. The flow rates of Ar and N₂ were fixed at 24 and 1 sccm, respectively. The total gas pressure was 0.35 Pa. Then, 500-nm-thick ZnO films were deposited on the buffer layers in Ar/O₂ atmosphere at 800°C. The flow rates of Ar and O₂ were fixed at 45 and 5 sccm, respectively. The total gas pressure was 0.70 Pa. The crystal quality was examined by x-ray diffraction (XRD) technique which used a Cu Kα radiation. The surface morphologies were observed using atomic force microscope (AFM) under tapping mode.<br/>We found that low-power sputtering facilitates 2D growth of ZnO films on the buffer layers. The 500-nm-thick ZnO films fabricated at low power of 40W is a single crystal with an atomically flat surface (root-mean-square (RMS) roughness: 0.58 nm), where the full width at half maximum (FWHM) of the (10-11) and (0002) x-ray rocking curves (XRC) are significantly small of 0.27° and 0.05°, respectively. On the other hand, the films fabricated at 50 and 70 W are polycrystalline and have rough surfaces with the RMS roughness of 0.61–0.81 nm. This difference might be attributed to the difference in the flux of sputtered particles to the substrates. At low RF power, low particle flux allows adatoms to migrate on the terraces of the films and to reach the steps before colliding with other adatoms, whereas at high RF power, high flux leads to secondary nucleation and thus to 3D growth. In addition, we observed that (10-11) and (0002) XRC FWHM increases to 0.31° and 0.10°, respectively, when RF power is further decreased to 20 W. This might be due to the too-low kinetic energy of sputtered particles that limits the migration of adatoms.<br/>We conclude that a combination of low-power sputtering and inverted SK growth is a promising method to produce high-quality single-crystalline films on large lattice mismatched substrates.<br/>This work was supported by JSPS KAKENHI Grant Numbers JP21H01372, 21K18731, NTT collaborative research, Toyota Riken Scholar, The Murata Science Foundation.<br/>[1] N. Itagaki et al., Sci. Rep. <b>10</b>, 4669 (2020).