The transparency of Cd(0001) films grown on Si(111)-7×7: Imaging the interface at atomic scale

Abstract

The interfacial structure plays a crucial role in controlling the growth of epitaxial film. However, it is still a formidable task to observe the interface structure at the atomic scale, because the detected signals from the interface become very weak due to the strong attenuation and decoherence when the incident wave passes through the films. With the advent of scanning tunneling microscopy (STM), significant progress has been made in observing the interface structure of thin films. Many studies have demonstrated the typical Si(111)-7×7 superstructure below the metal films in the past. Although the 7×7 superstructures are visible, the 12 Si adatoms in the unit cells can not be resolved. Hence, imaging the 7×7 superstructures below thin films at the atomic scale has not been achieved. In this work, we have grown the Cd epitaxial films with high quality on Si(111)-7×7 substrate, and realized the imaging of interface structure of Cd(0001) at atomic scale by using low temperature scanning tunneling microscopy. Under low bias voltage, we can clearly observe the atomic image of Si(111)-7×7, indicating the Si (111)-7×7 substrate remains intact during the growth process of Cd films. Moreover, due to the quantum size effect, the Cd films with even or odd monolayers exhibit distinct lateral resolution as well as surface roughness, even at the same bias voltage. With the bias variation, such kind of diversities can be reversed. We attribute the highly transparency of Cd(0001) films to the anisotropic effective mass of the electrons in Cd films, i.e. the transverse motion is much faster than the in-plane motion.