Strain as a Global Factor in Stabilizing the Ferroelectric Properties of ZrO₂

Abstract

Since the discovery of ferroelectricity in doped HfO2 and ZrO2 thin films over a decade ago, fluorite‐structured ferroelectric thin films have attracted much research attention due to their excellent scalability and complementary metal‐oxide semiconductor compatibility compared to conventional perovskite ferroelectric materials. Although various factors influencing the formation of the ferroelectric properties are identified, a clear understanding of the causes of the phase formation have been difficult to determine. In this work, ZrO2 films deposited by atomic layer deposition and chemical solution deposition have resulted in films with completely different structural properties. Regardless of these differences, a general relationship between strain and phase formation is established, leading to a more unified understanding of ferroelectric phase formation in undoped ZrO2 films, which can be applied to other fluorite‐structured films.

Pure ZrO2 Ferroelectric Thin Film for Nonvolatile Memory and Neural Network Computing

“…After the wake up, an increase in the o-phase fraction can be observed, which can account for the increased ferroelectricity shown in Figure b. In Figure g, the o/t phases are preferred over the m/r phases, which is related to the in-plane tensile strain induced by the low thermal expansion coefficient of RuO 2 electrodes. , In the wake-up process, other phases transit to the o phase as a result of the electric field stimuli and V O migration. ,− , …”

“…The ZrO 2 layer is ∼8 nm thick and shows clear top and bottom interfaces with the electrodes. The grains of ZrO 2 span the whole film thickness in the out-of-plane direction, meaning that the grain sizes are restricted in the out-of-plane direction by the film thickness. , Panels b and c (e and f) of Figure show the magnified views of the o and t phases in Figure a (d). The distinguished phases fit well with the theoretical structures denoted by the green spheres in the graphs. ,− Details for the phase identification are discussed in section 4 of the Supporting Information.…”

“…In Figure 2g, the o/t phases are preferred over the m/r phases, which is related to the in-plane tensile strain induced by the low thermal expansion coefficient of RuO 2 electrodes. 23,37 In the wake-up process, other phases transit to the o phase as a result of the electric field stimuli and V O migration. 30,[46][47][48]52 The cross-sectional EDS mappings of the device in the pristine and woken-up states are shown in panels h and i of Figure 2, respectively.…”

“…18 The ferroelectricity of the ZrO 2 thin film was experimentally revealed in a Pt/ZrO 2 (6.1− 19.6 nm)/Pt structure. 11 Further investigations revealed that the ferroelectricity of ZrO 2 films can be influenced by many factors, such as doping, 19−21 film thickness, 22,23 epitaxial strain, 24 interfacial layers, 25 electrode material, 26 and oxygen source dose time. 27 It was found that the macroscopic ferroelectric responses can be ascribed to the formation of the orthorhombic or rhombohedral (r) phase in the ZrO 2 films.…”

“…To obtain a ferroelectric ZrO 2 film, one method is to introduce an in-plane tensile strain to stabilize the ferroelectric o phase . It has been reported that the in-plane tensile strain after rapid thermal processing (RTP) can be introduced by utilizing an electrode material with a lower thermal expansion coefficient (TEC) than that of the ferroelectric layer (the TEC of ZrO 2 is ∼10.5 × 10 –6 K –1 ). , For example, by utilizing a low-TEC tungsten electrode (∼4.5 × 10 –6 K –1 ), a remnant polarization ( P r ) of ∼21.5 μC/cm 2 can be achieved in the W/ZrO 2 (10 nm)/W structure, but the leakage current density is high (∼5 × 10 –7 A/cm 2 at 1 MV/cm) as a result of the oxygen vacancy (V O ) generated during the interfacial oxidation of W. ,− In comparison to ZrO 2 , RuO 2 also has a low TEC of ∼6.4 × 10 –6 K –1 and may contribute to stabilizing the ferroelectric o phase in ZrO 2 .…”