Abstract
Regarding crystalline film growth on large lattice-mismatched substrates, there are two primary modes by which thin films grow on a crystal surface or interface. They are Volmer-Weber (VW: island formation) mode and Stranski-Krastanov (SK: layer-plus-island) mode. Since both growth modes end up in the formation of three-dimensional (3D) islands, fabrication of single crystalline films on lattice-mismatched substrates has been challenging. Here, we demonstrate another growth mode, where a buffer layer consisting of 3D islands initially forms and a relaxed two-dimensional (2D) layer subsequently grows on the buffer layer. This 3D-2D mode transition has been realized using impurities. We observed the 3D-2D mode transition for the case of ZnO film growth on 18%-lattice-mismatched sapphire substrates. First, nano-sized 3D islands grow with the help of nitrogen impurities. Then, the islands coalesce to form a 2D layer after cessation of the nitrogen supply, whereupon an increase in the surface energy may provide a driving force for the coalescence. Finally, the films grow in 2D mode, forming atomically flat terraces. We believe that our findings will offer new opportunities for high-quality film growth of a wide variety of materials that have no lattice-matched substrates.
Highlights
Regarding crystalline film growth on large lattice-mismatched substrates, there are two primary modes by which thin films grow on a crystal surface or interface
Regarding the heteroepitaxy of large lattice-mismatched systems, there are two primary modes by which thin films grow on a crystal surface or interface
They are i) Volmer-Weber (VW: island formation) mode[28] and ii) Stranski-Krastanov (SK: layer-plus-island) mode (Fig. 1a,b)[29,30]. Since both growth modes end up in the formation of three-dimensional (3D) islands, fabrication of single crystalline films on lattice-mismatched substrates has been challenging[28,29,30]. We demonstrate another growth mode, where a relaxed buffer layer consisting of 3D islands initially forms and relaxed two-dimensional (2D) layers subsequently grow on the buffer layer (Fig. 1c)
Summary
Regarding crystalline film growth on large lattice-mismatched substrates, there are two primary modes by which thin films grow on a crystal surface or interface. This transition from the 3D to 2D growth mode occurs only when the lattice planes of initially formed 3D islands in the buffer layer are well aligned with those of the substrates; otherwise, coalescence of islands does not occur. We experimentally demonstrate the 3D-2D mode transition for the case of ZnO film growth on 18%-lattice-mismatched sapphire substrates, where nitrogen atoms are employed as impurities.
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