Impact of the crystal structure of HfO_2 on the transport properties of model HfO_2/Si/HfO_2 silicon-on-insulator field-effect transistors: A combined DFT-scattering theory approach

摘要

Motivated by the polycrystalline structure of the high permittivity dielectric HfO_2 in contact with Si, we report calculations of carrier transport in nanometer-thin atomistic silicon-on-insulator field-effect transistor models. To qualitatively understand the impact of different crystalline phases of the dielectric on the transport characteristics of the channel, we have investigated two polymorphs of HfO_2 interfacing with Si, namely, the well-known tetragonal (t-HfO_2) and the theoretically proposed anatase (a-HfO_2) phases. For the transport calculations we have employed tetragonal-(t-HfO_2/Si/t-HfO_2) and anatase-based (a-HfO_2/Si/a-HfO_2) NSOI films. Our calculations reveal that transport is more efficient for the anatase polymorph since its good lattice match to Si does not create interface states in the Si band gap. The tetragonal polymorph creates scattering states in the Si band valence-band edge through the presence of stretched Si-Si bonds at the interface, resulting in degraded transport characteristics. Our study suggests that different bonding arrangements along the channel length create regions of increased carrier scattering even in the absence of other scattering processes such as phonons, trapped charges, or interface roughness.