Development of Twin Boundaries in Ag(111) Thin Films Grown on Atomically Flat Cu(111)

Growing thin films without grain boundaries (GBs) is challenging. Furthermore, growing twin boundary (TB)-free thin films has been considered improbable owing to the inevitable formation of TBs during the initial growth of thin films. Initially, about 10¹² TBs develop on a 2-inch substrate. Then, the twin domains merge into large ones that share the same crystallographic orientation as the film continues to grow. The presence of large numbers of TBs and GBs occurring in a typical film growth process complicates their differentiation and increases the difficulty in analyzing the behavior of TBs as the film thickness increases. A single-crystal Ag thin film was successfully grown in this study without GBs, allowing the examination of how the number of TBs changes with increasing film thickness. The use of a Cu(111) substrate with an atomically flat surface facilitates the growth of the Ag single-crystal thin film. Our investigation of Ag films ranging from 5 nm to 40 nm in thickness revealed that a region free from TBs can be achieved once the film reaches 80 nm in thickness.