Origin of Fermi level pinning in high-k gate stack structures studied by operando hard x-ray photoelectron spectroscopy

Abstract

To clarify the Fermi level pinning observed in Hf based gate structures, we employed two types of Hf based gate structures and investigated their bias-dependent potential distribution in the structures using operando hard x-ray photoelectron spectroscopy. In the case of the Pt/HfSiO2/SiO2/p-Si(100) structure, the Si substrate and SiO2 peaks in Si 1 s were shifted when a bias voltage was applied to the structure, indicating that Si substrate was mostly biased. Thus the Pt/HfSiO2/SiO2/p-Si(100) structure had an ideal metal oxide semiconductor structure. On the other hand, applying a bias voltage in the case of the Pt/HfO2/SiO2/p-Si(100) structure shifted HfO2 in Hf 3d, SiO2, and the substrate in Si 1 s, suggesting that a potential drop occurred at the Pt/HfO2 interface. Thus the Fermi level pinning was occurred at the Pt/HfO2 interface. In order to clarify the Fermi level pinning, angle-resolve O1 s photoelectron spectroscopy (PES) was performed at the Pt/HfO2 interface. Analysis of the O1 s PES revealed the formation of a SiO2 layer at the Pt/HfO2 interface, which could be the origin of the Fermi level pinning observed in Hf based gate structures.