Electronic properties and band alignments of Hf-based gate dielectrics on silicon

Ultra thin Hf-based dielectrics are being considered for possibly replacing SiO$_2$ gate oxide in silicon based metal- oxide-semiconductor (MOS) transistors. In this work we investigated the electronic structure and band alignments of the Hf-based gate dielectrics on silicon, which dictate the device performance of transistors in the sub-90nm devices. The electronic structure of HfO$_2$/Si interface showed dangling bond states at the interface due to the reduced coordination of Hf caused by the intrinsic bond constraints at the HfO$_2$/Si interface. However, our calculations indicate that these dangling bonds could be passivated by hydrogen or oxygen, which can appropriately change the coordination numbers at the interface, thereby removing the dangling bond states. We also considered the interface of HfSiO$_4$/Si and found that there are no dangling bond states at the interface, making HfSiO$_4$ a promising interfacial layer to improve the interface quality. Band offsets at the interfaces with Si were calculated using density functional theory, and it showed that the band offsets vary depending upon the interface stoichiometry. Band alignments were also determined experimentally using XPS and were in excellent agreement with the theoretical results. Incorporation of nitrogen into the HfO$_2$ network resulted in notable changes in the valence band structure of the material and the corresponding band alignments with silicon and is found to depend on the nitrogen concentration in the bulk of HfO$_2$ as well as at the interface.