Analysis of Performance Instabilities of Hafnia‐Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

Abstract

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high‐endurance memory applications like nonvolatile random‐access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field‐induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake‐up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.

Origin of Ferroelectric Phase in Undoped HfO2 Films Deposited by Sputtering

“…Both peaks of the PVD samples are in between the peak positions determined by Sowinska et al for ALD as‐deposited HfO 2 films and after forming a low resistive state with high amount of oxygen vacancies . From thermally stimulated depolarization currents (TSDC), it is known that the amount of oxygen vacancies is in a range from 0.1 to 0.25% after annealing . This low‐concentration difference correlates with the small peak position shift in Figure The strong shift in the peak position of the amorphous films stems from the fact that in the virgin state, almost no oxygen vacancies are present and, after electroforming, a high amount of oxygen vacancies concentrate at the TiN/HfO 2 interface .…”

A Comprehensive Study on the Effect of TiN Top and Bottom Electrodes on Atomic Layer Deposited Ferroelectric Hf0.5Zr0.5O2 Thin Films

“…This phenomenon, known as oxygen scavenging, can occur further during the RTA process. On the other hand, the oxygen vacancies remaining in the ferroelectric HZO film may degrade the reliability of the fabricated device [ 14 , 22 , 23 ]. Therefore, in order to obtain excellent reliability while maintaining a large ferroelectric polarization, an increase in oxygen vacancies at the interface is not necessarily desirable.…”

Epitaxial Integration on Si(001) of Ferroelectric Hf0.5Zr0.5O2 Capacitors with High Retention and Endurance

“…2,23 It can be appreciated that the HZO(111) peak is slightly shifted towards lower angles in comparison with the corresponding position (marked with a vertical dashed line) of polycrystalline films. 5,[24][25] The low intensity peaks close to HZO(111) are Laue reflections (a simulation is presented in Figure S2). Epitaxy of HZO and buffer layers was confirmed by XRD -scans ( Figure 1c).…”

“…Moreover, oxygen vacancies form close to the interfaces with the commonly used TiN electrodes. It is normally accepted [5][6][7] that domain boundaries and oxygen vacancies have a prominent role on undesired properties as nonswitchable ferroelectric domains, dielectric breakdown, electrical leakage, and wake-up effects. Thus, the polycrystalline nature of the HfO2 films reported so far, challenges investigating the ultimate thickness limit of ferroelectricity in HfO2 films.…”