Adsorption of O2on Ge(100): Atomic Geometry and Site-Specific Electronic Structure

Germanium is considered to be a potential semiconductor to replace silicon in future high-performance microelectronic devices. Yet, when compared to Si, very little is known about the surface chemistry of Ge surfaces. In this article, we report on the oxygen adsorption on Ge(100)-(2 × 1) surfaces and the related changes in the density of surface states. In particular, the adsorption geometry is examined both in reciprocal- and real-space using reflection high-energy electron diffraction and scanning tunneling microscopy, respectively. Our findings reveal that the insertion of oxygen atoms into the Ge backbonds is not favored under the investigated reaction conditions, i.e., at a moderate O2exposure. The O2-exposed surface exhibits a local (1 × 1) surface reconstruction, which implies that dimer bonds are destroyed upon oxygen adsorption. Surface states related to Ge dangling surface bonds present on the clean Ge(100) surface are found to be passivated. In addition, a high density of expelled substrate atoms is observed on the surface, leading to a drastic increase in surface roughness. These Ge adatoms induce distinctly different electronic properties compared to the well-ordered, oxidized surface regions. Our findings significantly contribute to the understanding of semiconductor-oxide interfaces, in particular with respect to the initial oxidation processes and the atomic and electronic structure of oxide complexes.