Abstract
High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing an off-axis 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 cm2/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.
Highlights
With buffer layer Without buffer layer 0.1 20 30 40 50 60 70 80 90 100 x when using nitrogen-mediated crystallization (NMC) buffer layers, we studied the PL spectra from ZnO films, which provides an insight into the nature of the crystal defects
3.3 x-ray diffraction (XRD) Results a variation of sample position x brings a significant change in the crystal growth mode as well as the surface roughness of ZnO, we clarified that fluctuation of the crystal orientation in both in-plane and out-of-plane directions is independent of sample position x when NMC buffer layers are utilized
ZnO films with high out-of-plane alignment are obtained by using NMC buffer layers, independent of the sputtering configuration
Summary
ZnO is a remarkable multifunctional material with a distinctive property set and a huge range of existing and emerging applications, such as varistors, transparent conducting electrodes, thin-film transistor, and gas sensors.[1,2,3,4,5,6,7,8,9,10] ZnO has recently been recognized as a candidate for high-performance ultraviolet light-emitting diodes (LED) and laser diodes (LD) that take advantage of its high exciton binding energy of 60 meV,[11,12,13] which is much larger than that of the commercial LED material GaN (25 meV). On the 2-D layers, subsequent growth of three-dimensional (3-D) columnar grains with poor alignment occurs so that the strain energy stored in the film is released (Stranski-Krastanov mode).[23] In contrast, our NMC buffer layers possess a high surface concentration of discrete ZnO islands on the sapphire substrates even at the very early stage of crystal growth because nitrogen atoms introduced into the deposition atmosphere disturb the growth of ZnO islands Since such buffer layers have good in-plane alignment, ZnO crystals that originate from the grains of the buffer layers are expected to be grown laterally and to be coalesced if secondary nucleation is suppressed. Aiming at realizing high-quality ZnO films with a low defect density, the effects of off-axis configuration together with NMC buffer layers are investigated through the analysis of the properties of ZnO films fabricated at various substrate/target configurations
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