HfO2 shows promise for emerging ferroelectric and resistive switching (RS) memory devices owing to its excellent electrical properties and compatibility with complementary metal oxide semiconductor technology based on mature fabrication processes such as atomic layer deposition. Oxygen vacancy (Vo), which is the most frequently observed intrinsic defect in HfO2-based films, determines the physical/electrical properties and device performance. Vo influences the polymorphism and the resulting ferroelectric properties of HfO2. Moreover, the switching speed and endurance of ferroelectric memories are strongly correlated to the Vo concentration and redistribution. They also strongly influence the device-to-device and cycle-to-cycle variability of integrated circuits based on ferroelectric memories. The concentration, migration, and agglomeration of Vo form the main mechanism behind the RS behavior observed in HfO2, suggesting that the device performance and reliability in terms of the operating voltage, switching speed, on/off ratio, analog conductance modulation, endurance, and retention are sensitive to Vo. Therefore, the mechanism of Vo formation and its effects on the chemical, physical, and electrical properties in ferroelectric and RS HfO2 should be understood. This study comprehensively reviews the literature on Vo in HfO2 from the formation and influencing mechanism to material properties and device performance. This review contributes to the synergetic advances of current knowledge and technology in emerging HfO2-based semiconductor devices. 

Role of oxygen vacancies in ferroelectric or resistive switching hafnium oxide

Although the HfO2-based ferroelectric thin films are compatible with the complementary metal-oxide-semiconductor (CMOS) technology and exhibit excellent scalability, they still suffer from low dielectric tunability, metastable ferroelectric orthorhombic phase and poor endurance. Here, we fabricated the HfO2/ZrO2/HfO2 sandwich structured thin films via atomic layer deposition process as well as the influence of voltage cycling, Hf/Zr ratios and annealing temperature on ferroelectricity and dielectric tunability of them was investigated systematically. At pristine states, the antiferroelectric-like behavior of polarization-voltage hysteresis loops become more and more obvious with the decrement of ZrO2 content in the middle layers. After waking-up, the large remnant polarization (2Pr, ∼43 μC/cm2) and high dielectric tunability (∼51 %) can be achieved under low operating voltage of 2.5 V. Noting that the 2Pr is still higher than 40 μC/cm2 and the dielectric tunability remains about 50 % after 109 cycles. In addition, the HfO2/ZrO2/HfO2 thin films also exhibited high energy storage properties with the total energy storage density of ∼62 J/cm3 and the recoverable energy storage density of ∼32 J/cm3. These results indicated that the strategy of sandwich structure design is beneficial to promoting the development of electronic devices towards miniaturization, integrated, and multifunction along with high performance and reliability. 

Achieving excellent ferroelectric and dielectric performance of HfO2/ZrO2/HfO2 thin films under low operating voltage

Despite its scalability and CMOS process compatibility, the limited endurance and sub-optimal stress response of ferroelectric Zr-substituted hafnia [(Hf,Zr)O2] have been one of the key impediments toward its integration into practical device and technology applications. Here, using electrical measurements complemented by photoluminescence spectroscopy, we investigate the underlying mechanisms behind this behavior in 10 nm thick W/Hf0.5Zr0.5O2/W capacitors. Analyzing the evolution of leakage current with stress cycles and the spectroscopic response of the stress-induced leakage current, we attribute the behavior to defect levels, which lie at 0.6 eV from the conduction band edge of the ferroelectric. Photoluminescence spectroscopy, in turn, further corroborates the defect level's position within the bandgap while enabling its attribution to the presence of oxygen vacancies. This work helps to identify oxygen vacancies as the key factor responsible for the degraded endurance and stress response in (Hf,Zr)O2 and subsequently motivates the exploration of methods to reduce the oxygen vacancy concentrations without destabilizing the ferroelectric orthorhombic phase.

Oxygen vacancy contributions to the electrical stress response and endurance of ferroelectric hafnium zirconium oxide thin films

The influence of biaxial stress on the maximum and remanent polarizations of 10 nm thick hafnium zirconium oxide thin films in metal–ferroelectric–metal capacitor structures has been quantified. In the as-prepared state with a nominal biaxial tensile strain of 0.20% and no applied extrinsic stress, remanent and maximum polarizations of 7.6 and 13.1 μC/cm2, respectively, were measured using a 2 MV/cm applied electric field. Reducing the intrinsic strain by 0.111% through the application of a compressive uniaxial stress results in a decrease in the remanent and maximum polarizations to 6.8 and 12.2 μC/cm2, respectively. The polarization dependence on strain is nearly linear between these values. The observed variation in polarization with strain is consistent with strain impacting ferroelastic switching whereby in-plane tension increases the fraction of the short polar axis orienting out-of-plane, hence increasing out-of-plane polarization. In contrast, reducing the in-plane strain through compression results in an increase in the fraction of the long non-polar axis orienting out-of-plane, thereby decreasing out-of-plane polarization. 

Applied in-plane strain effects on the polarization response of ferroelectric hafnium zirconium oxide thin films

The discovery of unconventional scale-free ferroelectricity in HfO2-based fluorite thin films has attracted great attention in recent years for their promising applications in low-power logic and nonvolatile memories. The ferroelectricity...

Recent progress on defect-engineered ferroelectric HfO2：A next step moves forward by multiscale structural optimization

Impact of Oxygen Content on Phase Constitution and Ferroelectric Behavior of Hafnium Oxide Thin Films Deposited by Reactive High-Power Impulse Magnetron Sputtering

The solid‐state synthesis of perovskite BiFeO3 has been a topic of interest for decades. Many studies have reported challenges in the synthesis of BiFeO3 from starting oxides of Bi2O3 and Fe2O3, mainly associated with the development of persistent secondary phases such as Bi25FeO39 (sillenite) and Bi2Fe4O9 (mullite). These secondary phases are thought to be a consequence of unreacted Fe‐rich and Bi‐rich regions, that is, incomplete interdiffusion. In the present work, in situ high‐temperature X‐ray diffraction is used to demonstrate that Bi2O3 first reacts with Fe2O3 to form sillenite Bi25FeO39, which then reacts with the remaining Fe2O3 to form BiFeO3. Therefore, the synthesis of perovskite BiFeO3 is shown to occur via a two‐step reaction sequence with Bi25FeO39 as an intermediate compound. Because Bi25FeO39 and the γ‐Bi2O3 phase are isostructural, it is difficult to discriminate them solely from X‐ray diffraction. Evidence is presented for the existence of the intermediate sillenite Bi25FeO39 using quenching experiments, comparisons between Bi2O3 behavior by itself and in the presence of Fe2O3, and crystal structure examination. With this new information, a proposed reaction pathway from the starting oxides to the product is presented.

Solid state synthesis of BiFeO3 occurs through the intermediate Bi25FeO39 compound

Corrigendum to ’Impact of oxygen content on phase constitution and ferroelectric behavior of hafnium oxide thin films deposited by reactive high-power impulse magnetron sputtering’ [Acta Materialia 239 (2022) 118220]

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Impact of Oxygen Content on Phase Constitution and Ferroelectric Behavior of Hafnium Oxide Thin Films Deposited by Reactive High-Power Impulse Magnetron Sputtering

Solid state synthesis of BiFeO3 occurs through the intermediate Bi25FeO39 compound

Corrigendum to ’Impact of oxygen content on phase constitution and ferroelectric behavior of hafnium oxide thin films deposited by reactive high-power impulse magnetron sputtering’ [Acta Materialia 239 (2022) 118220]