Shigehisa Shibayama

Ferroelectric phase stabilization of HfO2 by nitrogen doping

We report that nitrogen (N) doping can drive the ferroelectricity of HfO2. It was found that N doping can cause the transition from a monoclinic phase to a highly symmetric phase. The role of N doping is discussed from the viewpoints of charge balance and bond-constraining effects. The former is responsible for the structural transformation from a paraelectric phase to a ferroelectric phase by forming an oxygen vacancy. In addition, Hf?N and N?O bonds with covalent characteristics have strong effects on HfO2 structural and electrical properties, and thus contribute to a marked HfO2 para-/ferroelectric transition.

Ferroelectricity of nondoped thin HfO2 films in TiN/HfO2/TiN stacks

We report on the impact of TiN interfaces on the ferroelectricity of nondoped HfO2. Ferroelectric properties of nondoped HfO2 in TiN/HfO2/TiN stacks are shown in capacitance–voltage and polarization–voltage characteristics. The Curie temperature is also estimated to be around 500 °C. The ferroelectricity of nondoped HfO2 clearly appears by thinning HfO2 film down to ~35 nm. We directly revealed in thermal treatments that the ferroelectric HfO2 film on TiN was maintained by covering the top surface of HfO2 with TiN, while it was followed by a phase transition to the paraelectric phase in the case of the open surface of HfO2. Thus, it is concluded that the ferroelectricity in nondoped HfO2 in this study was mainly driven by both of top and bottom TiN interfaces.

Kinetic pathway of the ferroelectric phase formation in doped HfO2 films

The dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants. Both differences and similarities are discussed among various dopants by focusing on two major factors, the oxygen vacancy (Vo) and the dopant ionic size. First, the doping concentration dependence of the remanent polarization in 27 (±2) nm HfO2 films is quantitatively estimated. Then, by comparing the polarization result with the structural transformation in doped HfO2, the pathway of the dopant-induced HfO2 phase transition is discussed among monoclinic, ferroelectric orthorhombic, tetragonal, and cubic phases. Finally, it is addressed that a dopant species independent phase transition route may exist in HfO2 owing to the same kinetic transition process, in which the ferroelectric phase seems to be at an intermediate state between tetragonal and monoclinic phases.

Evolution of ferroelectric HfO2 in ultrathin region down to 3 nm

The ferroelectric properties of ultrathin Y-doped HfO2 films were investigated. Ferroelectricity was demonstrated experimentally in 3 nm-thick Y-doped HfO2 via direct detection of displacement currents during polarization switching. The dependence on the HfO2 thickness within the 30 to 3 nm range revealed that the ferroelectric properties decrease rapidly below a critical thickness. In the ultrathin HfO2 region, methods such as higher Y doping or metal capping annealing were required to further stabilize the ferroelectric phase. These methods could be used to enhance the switchable polarization (Psw) to 35 μC/cm2 in 5 nm- and 10 μC/cm2 in 3 nm-thick Y-doped HfO2. This paper indicates that HfO2 ferroelectricity is scalable even in the ultrathin region.

General relationship for cation and anion doping effects on ferroelectric HfO 2 formation

This work discusses the general relationship for cation and anion doping effects on the HfO2 para-/ferroelectric transition, which will provide us a helpful instruction for precise HfO2 ferroelectricity design. In addition, ferroelectric N-doped HfO2 has been demonstrated as a gate dielectric film on an oxide semiconductor for ferroelectric field-effect transistors (FeFETs).

Study of wake-up and fatigue properties in doped and undoped ferroelectric HfO 2 in conjunction with piezo-response force microscopy analysis

This paper reports wake-up and fatigue effects in ferroelectric HfO2 with and without Y-doping by comparing macroscopic capacitor characteristics with nano-level piezo-response force microscopy (PFM) analysis. Even though initial characteristics are almost the same between with and without Y-doping, endurance characteristics are really different macroscopically, and PFM study microscopically supports the endurance characteristics as well. This fact suggests that the robustness of HfO2 ferroelectricity should be sensitive to the doping.