Emerging nonvolatile memory technologies are at the forefront of next-generation computing architectures, driven by the exponential growth of data and the pressing need for energy-efficient storage solutions. Among these, ferroelectric random-access memory (FeRAM) and resistive random-access memory (RRAM) stand out due to their low power consumption and seamless integration with CMOS processes. Of particular interest are binary oxides such as zirconium oxide (ZrO₂) and hafnium oxide (HfO₂), materials traditionally used as high-k dielectrics, now recognized for their versatile ferroelectric and resistive switching properties.
Recent breakthroughs have revealed surprising ferroelectric-like behavior in ultrathin amorphous binary oxide films, even those as thin as approximately 1 nm. This phenomenon challenges conventional understanding, as the absence of long-range order in amorphous materials makes direct observation of key defects like oxygen vacancies extremely difficult. Prior investigations have identified oxygen ion dynamics, especially oxygen vacancy mobility, as pivotal in governing polarization switching mechanisms in these materials.
To address the challenge of probing these elusive vacancies, researchers have turned to terahertz time-domain spectroscopy (THz-TDS)—a technique sensitive to low-energy excitations on the scale of millielectronvolts, precisely matching the energy of ion hopping and localized vibrations. By analyzing terahertz wave absorption and transmission, THz-TDS indirectly detects ionic oscillations and localized states tied to oxygen vacancies, offering unprecedented insights into ionic conductivity at the nanoscale.
In a groundbreaking collaborative study involving Xidian University, the Beijing Academy of Quantum Information Sciences, and Huazhong University of Science and Technology, THz-TDS was employed to directly probe the vacancy oscillation modes in amorphous ZrO₂ films with varied oxygen-vacancy concentrations. Complementing these measurements with infrared reflectivity and first-principles density functional theory (DFT) simulations, the researchers mapped the optical absorption and reflection features in both amorphous and crystalline films. Their findings, published in Opto-Electronic Advances, illuminate the critical role of oxygen vacancies in determining ionic conductivity and polarization phenomena.
Experimental data revealed additional terahertz absorption peaks between 1 and 2 THz, separate from phonon-related peaks around 11 THz. These low-frequency peaks were attributed to localized states induced by oxygen vacancies, a conclusion supported by theoretical calculations. Temperature-dependent studies further confirmed that these vacancy oscillations modulate ionic migration. DFT analysis demonstrated how the presence of oxygen vacancies shifts infrared absorption features, causing redshifts or generating new peaks, thereby affecting the films’ polarization response.
This refined understanding bridges key knowledge gaps in amorphous oxide ferroelectricity, establishing oxygen vacancy dynamics as a fundamental microscopic mechanism. From a practical standpoint, amorphous binary oxides synthesized via low-temperature atomic layer deposition offer seamless back-end-of-line compatibility, bypassing the thermal constraints of conventional crystallization. This paves the way for ultra-low-power, high-density memory devices and neuromorphic computing platforms at technology nodes beyond 5 nm.
Looking ahead, the research team aims to enhance THz spectroscopic methods to enable in-situ and operando probing under diverse stimuli like electric fields and mechanical stress. These advances will target precise control over vacancy concentration and distribution, and exploration of new oxide materials with superior ferroelectric-like functionalities, heralding a new paradigm for memory and computational device engineering.
Article Title: Vacancy oscillating mode in amorphous binary oxide film by terahertz time domain spectroscopy
News Publication Date: 7-Jun-2026
References: DOI: 10.29026/oea.2026.250217
Image Credits: Haiyun Liu from Beijing Academy of Quantum Information Sciences
Keywords
Ferroelectricity, Oxygen vacancies, Amorphous oxides, Terahertz time-domain spectroscopy, Ionic conductivity, Zirconium oxide, Memory devices, Density functional theory
