This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers have captured in real time the "birth moment" when the performance of a next-generation memory material is determined. The key lies in controlling oxygen vacancies, microscopic defects created when oxygen atoms are missing from their normal positions.
A research team led by Professor Min Hyuk Park of the Department of Materials Science and Engineering at Seoul National University College of Engineering, in collaboration with Professor Yunseok Kim of Sungkyunkwan University, Professor Younghwan Lee of Chonnam National University and Dr. Tae-Yeol Jeon of the Pohang Accelerator Laboratory, tracked in real time the entire process by which a ferroelectric phase capable of storing information forms during the heat treatment of hafnia-zirconia (HZO), a material attracting attention for next-generation nonvolatile memory. The researchers found that the concentration of oxygen vacancies changes both the crystallization temperature and the final crystalline phase, resulting in substantial differences in memory storage performance.
The work is published in the journal Advanced Functional Materials. Ferroelectrics are materials capable of retaining their electrical state even after power is turned off. This makes them key candidates for nonvolatile memory that preserves information without power and for low-power semiconductor devices.
Hafnia-based ferroelectrics, in particular, are highly compatible with conventional silicon semiconductor fabrication processes and can exhibit ferroelectricity even at thicknesses of only a few nanometers, making them an active area of research for next-generation high-density memory. The challenge is that HZO does not possess ferroelectric properties from the outset. The orthorhombic phase responsible for ferroelectricity in HZO has an asymmetric atomic arrangement and is a metastable crystalline phase that, while not the most thermodynamically stable state, can be maintained under specific processing conditions.
Rather than focusing solely on the final structure after heat treatment, the researchers examined the process by which crystals first emerge from an amorphous thin film and how different crystalline phases, including the ferroelectric phase, are subsequently selected. Such changes have been difficult to distinguish accurately using conventional analyses performed only before and after heat treatment. The key variable in the study was oxygen vacancies.
An oxygen vacancy is an atomic-scale defect formed when an oxygen atom is missing from its normal position in HZO. The researchers fabricated 10-nm-thick HZO films with different oxygen-vacancy concentrations by varying the amount of ozone supplied during atomic layer deposition, then compared how their crystallization pathways changed according to oxygen-vacancy concentration. The researchers then used synchrotron radiation at the Pohang Accelerator Laboratory to perform real-time grazing-incidence wide-angle X-ray scattering (GIWAXS), continuously tracking the entire process as the initially amorphous films transformed into crystals during heating and stabilized into their final structures during cooling.
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