Threading dislocations in epitaxial GeSn film directly grown on (001) Si substrates

Abstract

High-quality epitaxial GeSn films grown directly on Si substrates are highly desirable for integrated photonics applications, as eliminating the intermediate buffer layer simplifies device fabrication. A ∼1 μm thick, epitaxial Ge0.95Sn0.05 film was grown directly on a (001) Si substrate by remote plasma-enhanced chemical vapor deposition using a two-step process: an ultra-thin GeSn initiation layer was first deposited at 421 °C for 1 min, followed by the main film deposition at 330 °C for 85 min. Here, we analyze the detailed microstructure of the GeSn film using atomic force microscopy, Raman spectroscopy, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The Ge0.95Sn0.05 film exhibits a hill and valley-like surface morphology and a sharp interface with the substrate. It consists of a ∼150 nm bottom epilayer containing characteristic twin structures, and an ∼850 nm upper epilayer composed of dense, vertically oriented columnar structures with lateral dimensions ranging from approximately 200–300 nm. These columns are bounded by vertically aligned interfaces composed of straight threading dislocations that extend from near the substrate interface through the film to the surface. The Burgers vectors of the threading dislocations were identified as 1/2[110] and/or 1/2[−110]. The columnar boundaries, rich in strain, are responsible for the development of the observed rugged, hill and valley-like surface topography. The formation of the threading dislocations is likely attributed to local fluctuations in Sn content. The observed microstructure offers valuable insight into the growth mechanism, which can be leveraged to optimize the process for enhanced film quality suitable for device applications.