Reliability issues and role of defects in high-k dielectric HfO₂ devices

The ability to shrink Si-based transistors is reaching the spatial scale of sub-0.1 μm, close to fundamental limits. For an oxide thickness as low as 2 nm, quantum effects start to become important, and an acceptable reliability is not achievable. High-k dielectrics provide high capacitance, which is compatible with SiO₂-based devices with larger physical thickness, so that they are expected to prevent the direct tunneling of electrons through thin oxides. Especially, HfO₂ and its related alloys have received much attention due to their high dielectric constant, large band gap, and relatively low leakage current. Despite extensive studies, there remain several problems to solve, such as unacceptably high flat-band voltages in the use of a p+ poly-Si electrode and threshold voltage instability during device operation. Here, we introduce reliability issues commonly raised in high-k dielectrics, particularly Hf-based oxides. Based on electronic structure calculations for defects, such as point defects, hydrogen, and Si impurities, in HfO₂, we discuss the stability and the influence of defects on device performance under various growth conditions, the film morphology on Si substrates, the origin of the interfacial defects causing the threshold voltage instability, and the Fermi level pinning in the poly-Si/HfO₂ gate.