Comprehensive Study on the Kinetic Formation of the Orthorhombic Ferroelectric Phase in Epitaxial Y-Doped Ferroelectric HfO₂ Thin Films

Abstract

The crystal structure and ferroelectric properties of 12- to 18 nm-thick epitaxial YO1.5-HfO2 films with 5–9% YO1.5 on (111)­ITO//(111)­YSZ substrates are investigated to clarify the formation mechanism of the ferroelectric phase. The ferroelectric orthorhombic phase can be obtained by transformation from the higher symmetric tetragonal phase by surmounting a relatively low energy barrier. The orthorhombic phase is obtained for 6% and 7% YO1.5-doped HfO2 films by heat treatment at 1000 °C. Although the 5% YO1.5-doped HfO2 film heat-treated at 1000 °C is in a monoclinic phase, the orthorhombic phase was increased by heat treatment at 1200 °C because the high temperature promotes the phase transition from the monoclinic phase in as-deposited films to the tetragonal phase. The 8% and 9% YO1.5-doped HfO2 films have a tetragonal structure without the transition to the orthorhombic phase. Nevertheless, the 8% YO1.5-doped HfO2 film exhibits ferroelectricity by polarization-electric field hysteresis measurement. A microarea X-ray diffraction study reveals that the electric-field-induced phase transition can take place in an 8% YO1.5-doped HfO2 film. The comprehensive study of high-temperature X-ray diffraction measurements implies that the tetragonal phase in 8% YO1.5-doped HfO2 is a supercooled state. Therefore, external stimulation, such as application of an electric field, induces the transition from the tetragonal to the orthorhombic phase. The supercooled tetragonal phase can also be reduced by a slower cooling rate. These results reveal that the formed phase in YO1.5-doped HfO2 epitaxial film is not governed by the simple difference in the formation energy; rather, the kinetics is more important for obtaining the ferroelectric orthorhombic phase.

Substrate-dependent differences in ferroelectric behavior and phase diagram of Si-doped hafnium oxide

“…Less (100) and (110) signal is observed for the 10:1 sample. While this might be explainable by increased tetragonal phase, this would not match to the electrical results, as other groups have reported that super-cooled tetragonal phase does not result in AFE-like behavior but reduced P R and P S [20,21]. Moreover, the shape of the {200} -peak is slightly changed for the 10:1 sample, as indicated by the smaller TiN-related shoulder and reduced intensity of the (200) shoulder at smaller angles.…”

“…unlikely as an origin for AFE-like behavior, since the operating temperature is far below the Curie temperature [31,32] and recent investigations demonstrated the irreversible nature of this phase transition [20,21]. Internal bias fields and domain wall pinning on the other side would require diffusion/drift processes during wake-up, which are not observed [22].…”

“…Moreover, combined with results from other groups, which reported on a generalized doping dependence for different dopants [44], the here proposed phase diagram can be adapted for other doping elements. In addition, very high doping levels can result in super-cooled tetragonal phase [20,21], which can be converted to orthorhombic phase by electric field cycling and should result in P S increase. Depending on the present stress level, the orthorhombic phase will then either behave AFE-like or ferroelectric.…”

“…Since most thin films are of polycrystalline nature, the presence of small amounts of monoclinic grains in the microstructure cannot be excluded, as recently shown by transmission Kikuchi diffraction [19]. In addition, a tetragonal phase is present for temperatures above the Curie temperature or in a super-cooled state for very high doping concentrations [20,21].…”

Influence of Annealing Temperature on the Structural and Electrical Properties of Si-Doped Ferroelectric Hafnium Oxide

“…As the Curie temperature is in the range of 350–550 °C, HfO 2 films are expected to consist mainly of that orthorhombic phase, which has been recently confirmed, demonstrating that the antiferroelectric-like behavior is driven by ferroelastic switching (90°-domain wall motion). However, for very high doping concentrations, the supercooled tetragonal phase may be preserved. , …”