Engineering Strategies in Emerging Fluorite-Structured Ferroelectrics

Abstract

Ferroelectricity has been increasingly applied to develop fluorite-structured oxides since its discovery in 2011. Unlike conventional ferroelectric materials, these materials are compatible with complementary metal oxide semiconductor technology with established fabrication techniques. Early stage research has focused on enhancing the ferroelectricity in solid solution films for nonvolatile data storage. Recently, engineering strategies, such as those that entail the use of morphotropic phase boundaries to achieve high permittivity and develop artificial structures such as nanolaminates/superlattices, have been suggested to enhance ferroelectricity/antiferroelectricity; such strategies have the potential to provide new pathways for advanced nanoelectronics. Furthermore, the dielectric constant, remanent polarization, and processing window applied in these strategies yield more desirable outcomes than the solid solutions. Such strategies are also believed to be applicable to the design of cell capacitors for dynamic and ferroelectric random-access memory technology. However, a comprehensive review of these ferroelectricity-related engineering strategies has not yet been reported. Thus, recently developed engineering strategies for fluorite-structured ferroelectrics are reviewed in this paper.

A review on morphotropic phase boundary in fluorite-structure hafnia towards DRAM technology

“…Conventional Al 2 O 3 , ZrO 2 , HfO 2 , and nano-laminate structures (ZAZ, HZH, and AZO) have been adopted to suppress the leakage current. However, the formation of the MPB in nano-laminate thin films between heterogeneous ferroelectrics has been reported [ 50 , 51 ]. The lamination method for producing a thin film was developed to prevent crystal growth into undesired phases, which was adopted for the formation of MPB.…”

Novel Approach to High κ (∼59) and Low EOT (∼3.8 Å) near the Morphotrophic Phase Boundary with AFE/FE (ZrO2/HZO) Bilayer Heterostructures and High-Pressure Annealing

“…In this context, the o-phase relative free energy is lower than that of the t-phase region, and hence, temporary propagation drives toward the t-phase region. 33 Figure 6c shows EOT vs film thickness for the single layer (HZO and ZrO 2 ) and bilayer capacitors (ZrO 2 /HZO). The inset figure clearly shows that the lowest EOT region (3.6∼4.1 Å) is found for the bilayer device.…”

“…As a result, the κ is maximized at near the MPB, and hence, there is a reduced EOT in the vicinity of the MPB. 1,17,33 One of the approaches to reduce EOT is thinning of high-κ layer. However, although thinning of the HZO films can improve the capacitance, it results in a raised leakage current density, leading to reliability issues.…”

“…33−37 Additionally, the dielectric constant was improved by optimizing several parameters such as the doping ratio, ozone dosage, temperature, and strain in the ferroelectric hafnia material system. 33,38 Recently presented work on Zr doped hafnia has given an indication to obtain a high κ and reduced EOT near the MPB. Also, HfO 2 −ZrO 2 superlattice structures (ultrathin film of 2 nm) reported the lowest EOT of 6.5 Å for FET applications.…”

“…The MPB is a mixed crystalline phase, which exists between the phases of AFE (tetragonal phase, t-phase) and FE (orthorhombic phase, o-phase) in a hafnia ferroelectric system. As a result, the κ is maximized at near the MPB, and hence, there is a reduced EOT in the vicinity of the MPB. ,, One of the approaches to reduce EOT is thinning of high-κ layer. However, although thinning of the HZO films can improve the capacitance, it results in a raised leakage current density, leading to reliability issues.…”