Off-axis sputter deposition of ZnO films on c-sapphire substrates by utilizing nitrogen-mediated crystallization method

Abstract

Abstract. High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing an offaxis sputtering configuration together with buffer layers prepared via nitrogen-mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and, thus, to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two-dimensional growth of subsequently grown ZnO films, being particularly enhanced by employing an off-axis sputtering configuration in which the substrate is positioned out of the high-energy particles, such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and a high electron mobility of 88 cm 2 ∕V · s are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high-quality epitaxial ZnO films. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Improvement of m‐plane ZnO films formed on buffer layers on sapphire substrates by mist chemical vapor deposition

“…b, two peaks spaced by 180° were observed for both ZnO and sapphire, since the crystal lattices of m‐plane ZnO and m‐plane sapphire are both rectangular with two‐fold symmetry. The c ‐axes of ZnO and that of sapphire perpendicularly crossed each other on the sapphire substrate, namely ZnO [0001] || sapphire and ZnO || sapphire [0001]. In this φ scan measurement, the ZnO (10–11) and sapphire (11–20) planes were used as the monitoring planes, where Bragg conditions were set to be satisfied.…”

“…and , it was shown that the thickness and crystallographic properties were homogeneous across the substrate, and that the ZnO film was epitaxially grown on the sapphire substrate . In the direction lateral to the sapphire substrate, four small rectangular ZnO lattices are formed on one large sapphire rectangular lattice in the sapphire [0001] direction, indicating domain‐matching epitaxy , where the lattice mismatch in the ZnO [0001] direction is 9.7%, and that in the ZnO direction is 0.1%.…”

“…However, in the case of ZnO, it is not easy to form amorphous layer, and polycrystal ZnO layers are formed even at low temperature. On the contrary, Itagaki et al reported that high‐quality ZnO films were formed by sputtering deposition on ZnO buffer layers with small‐grain polycrystal . This is probably because small‐grain polycrystal of ZnO worked as the appropriate nucleation centers since the AlN low‐temperature buffers for GaN epilayers have similarly many nucleation centers.…”

“…In this study, we improved the crystal quality of the uniform ZnO epitaxial films by introducing ZnO buffer layers , since there are fewer reports on formation of the buffer layers by mist‐CVD. In addition, we focused on the formation of nonpolar m‐plane (10–10) ZnO films on sapphire substrates, since high emission efficiency is expected from quantum‐well layers.…”

Effects of morphology of buffer layers on ZnO/sapphire heteroepitaxial growth by RF magnetron sputtering

“…However, because of the low crystallization temperature of ZnO, it is difficult to obtain amorphous like layers by just lowering the deposition temperature. Recently, we have developed a new type of buffer layers fabricated by "nitrogen mediated crystallization (NMC) method", where nucleation and crystal growth is controlled by not lowering deposition temperature, but introducing impurity atoms that inhibit crystal growth [10][11][12][13]. By utilizing the buffer layers fabricated by NMC method (NMC-ZnO buffer layers), we have succeeded in fabrication of atomically-flat ZnO films on lattice mismatched sapphire substrates.…”