Defects responsible for the Fermi level pinning in n+ poly-Si/HfO2 gate stacks

Abstract

Based on density functional calculations, we propose a defect model that can explain flat band voltage shifts, especially in n+ poly-Si/HfO2 gate stacks. For two interface structures, with Si electrodes on top of crystalline and amorphous HfO2, we find the formation of O-vacancies at the interface, which exhibit weak Si–Si dimer bonds and low formation energies, very different from those in the oxide. Due to weak dimer bonds, charge trap levels lie near the Si conduction band edge, leading to the Fermi level pinning and flat band voltage shifts in n+ poly-Si gate electrodes.