Unraveling Ferroelectric Polarization and Ionic Contributions to Electroresistance in Epitaxial Hf_0.5Zr_0.5O₂ Tunnel Junctions

Abstract

Tunnel devices based on ferroelectric Hf 0.5 Zr 0.5 O 2 (HZO) barriers hold great promises for emerging data storage and computing technologies. The resistance state of the device can be changed by a suitable writing voltage. However, the microscopic mechanisms leading to the resistance change are an intricate interplay between ferroelectric polarization controlled barrier properties and defect-related transport mechanisms. Here is shown the fundamental role of the microstructure of HZO films setting the balance between those contributions. The oxide film presents coherent or incoherent grain boundaries, associated to the existence of monoclinic and orthorhombic phases in HZO films, which are dictated by the mismatch with the substrates for epitaxial growth. These grain boundaries are the toggle that allows to obtain either large (up to ≈ 450 %) and fully reversible genuine polarization controlled electroresistance when only the orthorhombic phase is present or an irreversible and extremely large (≈ 10 3 -10 5 %) electroresistance when both phases coexist.are the resistances after polarizing the junction with writing voltages V W + or V W and R min (V W +,-) is the minimum resistance among these states. Accordingly, binary high (OFF) and low resistance (ON) states can be written in a ferroelectric memory cell and read by probing its resistance. It has also been shown that by performing minor polarization loops, ferroelectric tunnel devices can store information in different resistive states, mimicking the functioning of a memristive element. [4,5] This approach has been successfully achieved by using ferroelectric perovskites such as BaTiO 3 , [6][7][8][9] Pb(Zr 0.2

Reversible oxygen migration and phase transitions in hafnia-based ferroelectric devices

“…This observation suggested the two mechanisms are independent. Sulzbach et al (24) also reported a similar divergence in the TER as a function of the applied voltage in HZO layers before breakdown. However, the electric polarization in hafnia has been theoretically proposed to originate from oxygen vacancies through electrostrictive effects, strongly suggesting the extrinsic nature of the polarization switching (29).…”

Epitaxial Ferroelectric HfO2 Films: Growth, Properties, and Devices

“…[121][122][123] Therefore, filamentary ionic conduction can also play an important role in the found irreversible large electroresistance. Sulzbach et al 76,87 have followed two strategies to overcome this effect. The first is based in the control of the amount of orthorhombic phase by epitaxial stress.…”

Ferroelectric Properties and Polarization Fatigue of La:HfO2 Thin‐Film Capacitors

“…Similar phenomena have also been observed in (111) o ‐orientated Hf 0.5 Zr 0.5 O 2 epitaxial thin films, in which the o‐phase is favored on substrates with lattice constants larger than those of STO. [ 19,36 ] The formation of the metastable o‐phase may be ascribed to epitaxial strains. Recent theoretical calculations show that compressive strains are beneficial to the formation of the o‐phase in {100}‐orientated HfO 2 , [ 6,37–39 ] whereas for the {111} orientation, the o‐phase can be stabilized in a wide range of strains, even tensile strains.…”