High-Pressure Synthesis of Ferroelectric Y-Doped HfO₂

The discovery of ferroelectric HfO₂-based films by Böscke et al. was a great surprise since these materials were used as high-k gate dielectrics in Si-based complementary metal-oxide-semiconductor (CMOS) devices. Only the paraelectric monoclinic (<i>P</i>2₁/<i>c</i>), tetragonal (<i>P</i>4₂/<i>nmc</i>), cubic (<i>Fm</i>-3<i>m</i>), orthorhombic I (<i>Pbca</i>), and orthorhombic II (<i>Pnma</i>) phases exist in the equilibrium pressure-temperature phase diagram of HfO₂. On the other hand, the origin of the ferroelectric phase is the metastable orthorhombic <i>Pca</i>2₁ phase which is usually stabilized by the film thickness effect due to surface energy. Therefore, the ferroelectricity was observed in thin HfO₂ films with a thickness below ~30 nm. This implies that the material properties include thin film-specific effects such as surface energy and strain effects, and the fundamental characteristics are unknown. To understand essential properties, the synthesis of ferroelectric bulk HfO₂ is indispensable. Recently, Mimura <i>et al</i>., fabricated 1 μm-thick ferroelectric 7%YO_1.5-93%HfO₂ films, which suggests that ferroelectric bulk HfO₂ could be synthesized using Y doping. After that, <i>Xu</i> et al., synthesized a 12%YO_1.5-HfO₂ single crystal with a <i>Pca</i>2₁ structure using a laser-diode-heated floating zone technique. In this study, we demonstrate the high-pressure synthesis of ferroelectric bulk YO_1.5-HfO₂ using high-pressure synthesis.<br/>Starting materials, Y₂O₃ and HfO₂ powders with compositions of <i>x</i>%YO_1.5-(1-<i>x</i>)%HfO₂ (<i>x</i> = 0, 3, 5, 7, 12) were mixed using an agate mortar. High-pressure synthesis was carried out by the following two routes. 1.) The mixed powders were pressed into pellets and calcined at 1500°C for 10 h in air. Then, the calcined pellets were ground into powder. The powders were allowed to treat in a TRY cubic multi-anvil-type high-pressure apparatus (NAMO 2001) and then quenched to room temperature. 2.) The mixed powders from starting materials were directly allowed to treat in a high-pressure apparatus. The pressure, temperature, and holding time were at 3-7.7 GPa, 800-1400°C, and for 30 min, respectively for each route. To identify the crystal phases, XRD 2<i>θ</i>-<i>θ</i> scanning (X’Pert³ Powder, PANalytical, λ = 0.154 nm) was performed.<br/>For route 1, the calcined samples before high-pressure synthesis showed the monoclinic and cubic phases in <i>x </i>= 3-12, suggesting consistency with the phase diagram of the Y₂O₃: HfO₂ system. After high-pressure synthesis, the orthorhombic phase <i>Pca</i>2₁ or <i>Pbca</i> was obtained. With an increase in the pressure and temperature, the yield of the phase increased, but the maximum was 28.9 wt% in 5% YO_1.5-95%HfO₂ treated at 7.7 GPa, 1300°C. The yield of the cubic phase did not change much before and after the high-pressure synthesis, suggesting that only the lower-density monoclinic phase transforms into the higher-density phase while the highest-density cubic phase does not at high pressure. For route 2, the orthorhombic phase was obtained with &gt; 70 wt%. These results suggest that the high-pressure synthesis method is one of the methods to stabilize bulk HfO₂-based ferroelectrics. The results of second harmonic generation (SHG) measurements to confirm the non-centrosymmetric ferroelectric phase will be discussed at the conference.