Critical differences in the surface electronic structure of Ge(001) and Si(001): Ab initio theory and angle-resolved photoemission spectroscopy

Even with renewed interest in Ge as a competitor to Si in field-effect transistors, several key features of the surface electronic structure of Ge(001) have remained controversial. Notably, the origin of strong Fermi-level pinning in Ge has been heavily debated. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) and first-principles hybrid density functional theory calculations, we compare and unambiguously establish the critical differences between the electronic structure of the Si and Ge (001) surfaces. We explicitly show that the surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge. It means that the evanescent states near the Ge surface play an essential role in the strong Fermi-level pinning. Additionally, we identify the origin of a number of highly debated ARPES features for Ge(001) and Si(001).