| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2018) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeM |
| Session: | Nanostructural and Surface Morphological Evolution: Experiment and Theory |
| Presenter: | Naho Itagaki, Kyushu University, Japan |
| Authors: | N. Itagaki, Kyushu University, Japan D. Yamashita, Kyushu University, Japan K. Kamataki, Kyushu University, Japan K. Koga, Kyushu University, Japan M. Shiratani, Kyushu University, Japan |
| Correspondent: | Click to Email |
Success in semiconductor devices has been limited thus far because of lattice-mismatch problems between growth layers and substrate. Here, we report on a new method of single crystal growth beyond lattice matching condition, sputter epitaxy via “inverse Stranski-Krastanov (SK) growth mode”. Regarding heteroepitaxy on large lattice mismatched substrates, there are two primary modes by which thin films grow: 1) Volmer–Weber (VW: island formation) mode and 2) Stranski–Krastanov (SK: layer-plus-island) mode. In both modes, crystal growth ends up in formation of three-dimensional (3D) island (Fig. S1), making fabrication of single crystalline films challenging. On the other hand, in “inverse” SK mode, 3D islands are initially formed, and subsequent growth of 2D layers occurs on the 3D islands (Fig. S1 (c)). This 3D-2D transition, which is just opposite to the 2D-3D transition in SK mode, is caused by introducing impurity atoms into the growth atmosphere during the initial stage of crystal growth. In this study, we demonstrate the inverse SK growth of ZnO films on 18%-lattice mismatched sapphire substrates, where nitrogen is employed as impurity.
First, 3D island layers of 10 nm thickness were deposited by N2/Ar sputtering. Next, 2D layers of 1 μm thickness were deposited on the 3D island layers by O2/Ar sputtering.
We have successfully grown ZnO single crystals on sapphire substrates via inverse SK mode. XRD and AFM analyses revealed that the 3D island layers consist of nm-sized islands with high crystal quality (Fig. S2 (a)). Since the strain induced by the lattice mismatch is relaxed through the island formation, smaller islands have lower density of misfit dislocation, resulting in high crystal quality. In a conventional method, however, such small islands hardly grow because of the cost of additional surface energy due to the increased surface area. Therefore, we consider that the role of nitrogen is to reduce the surface energy through the adsorption on the island surface, taking the advantages of its active but ZnO-insoluble natures. In fact, our calculation indicates that nitrogen addition leads to 4-nm-sized island growth owing to the lowered surface energy (Fig. S3). Furthermore, we found that after cessation of N2 supply, crystal grains that grow originating from the 3D islands rapidly coalesce to form 2D layer, and eventually grow in a layer-by-layer (2D growth) fashion (Fig. S2 (b)).
We believe that our findings on this growth mode will offer new opportunities for designing materials with unprecedented properties.
This work was supported in part by JSPS KAKENHI 18H01206 and NTT collaborative research.