Growth of single crystalline films on lattice-mismatched substrates through 3D to 2D mode transition

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 highquality film growth of a wide variety of materials that have no lattice-matched substrates.

Growth of Single-Crystalline ZnO Films on 18%-Lattice-Mismatched Sapphire Substrates Using Buffer Layers with Three-Dimensional Islands

“…The temperatures of the substrate ( T buffer ) were set to 760–830 °C. Two ZnO ceramic targets, the size of which was 2 inches and the purity is more than 99.99%, were used to obtain enough fluxes of sputtered particles to the substrates . The targets were sputtered prior to the deposition of buffer layers for 15 min for cleaning the surface of the targets.…”

“…Both modes result in three-dimensional (3D) islands and polycrystalline films with a rough surface, so the fabrication of single-crystalline films on a lattice-mismatched substrate requires a new mode of crystal growth. The possibility of the new mode has been predicted in 2004, and recently, we have demonstrated the new mode: the inverted SK mode . In this mode, a 3D island buffer layer involving dense grain boundaries initially forms and relaxes the strain due to the lattice-constant mismatch; then, a small strained two-dimensional (2D) layer easily grows on the buffer layer .…”

“…This increase drives the coalescence of crystal grains deposited on the buffer layer and forms 2D layer, and finally, a single crystalline film grows. Through the inverted SK mode, we have successfully grown single-crystalline ZnO films on 18%-lattice-mismatched sapphire substrates . The essence of this mode transition from 3D to 2D is controlling kinetics of insoluble impurities on the surface.…”

Recent Advances in Theoretical Development of Thermal Atomic Layer Deposition: A Review

“…On the other hand, in the Frank–van der Merwe growth mode, also known as bi-dimensional growth, the surface adhesive force is stronger than the intra atom cohesive force, leading to layer-by-layer growth on the substrate. That is, the atoms completely cover the surface, producing a complete monolayer before the subsequent layer is formed on top [ 59 ]. If both the Volmer–Weber and Frank–van der Merwe growth modes are combined, the growth mode is called Stranski–Krastanov.…”

“…After forming a rather thick film, the growth mode switches to island growth [ 58 ]. Where the transition happens is affected by the chemical and physical properties of both film and substrate materials [ 57 , 58 , 59 ]. Figure 3 presents a schematic of the three growth modes.…”