image: [Left] The schematic shows the multi-functions of the 2D-WS2 bottom interfacial layer for interface stability and the vertically well-ordered domain structures of HZO, which leads to excellent ferroelectric properties. [Right] P-V hysteresis loops of ferroelectric capacitors without (red) and with (blue) 2D-WS2 bottom interfacial layer at pristine state. A significant increase in 2Pr is observed in the bottom interface engineering.
Credit: Korea Institute of Materials Science (KIMS)
The Korea Institute of Materials Science (KIMS) announced that Dr. Yong-Hun Kim from the Energy & Environment Materials Research Division and Dr. Kyung Song from the Material Characterization Center, in collaboration with Professor Hyun-Sang Hwang's team from POSTECH, have successfully developed a groundbreaking heterojunction technology. This technology integrates tungsten disulfide (WS₂), a two-dimensional (2D) material, with hafnium zirconium oxide (HZO), a ferroelectric material, achieving both interfacial stability and superior crystallinity. The results have been accepted by the International Electron Devices Meeting 2024 (IEDM 2024), one of the top three global semiconductor conferences, often referred to as the “Olympics of Semiconductors.”
Tungsten disulfide (WS₂) is a 2D material composed of tungsten and sulfur, characterized by an ultrathin atomic-layer structure. Despite its nanometer-scale thickness, it retains unique properties, making it highly suitable for applications in semiconductors and energy storage devices. Ferroelectric materials, such as HZO, exhibit spontaneous polarization even in the absence of an external electric field and are integral to non-volatile memory devices that retain information without power. Among ferroelectric materials, HZO is particularly well-known for its robust ferroelectric properties.
This study addressed the challenge of interfacial stability, a critical limitation in ferroelectric device technology, and improved the uniformity of device characteristics by controlling the crystallinity of HZO. Interfacial stability refers to the ability to minimize physical and chemical changes at the boundary between two materials, thereby maintaining the intrinsic properties of the materials over time. In semiconductor devices, higher interfacial stability directly contributes to preserving performance without degradation, which is closely tied to the overall reliability of the semiconductor.
Conventional atomic layer deposition (ALD)-based HZO deposition faces several challenges. One major issue is that during HZO deposition on the underlying electrode, exposure to high temperatures and oxygen induces unwanted chemical reactions. This results in the formation of a thin oxide layer that degrades the ferroelectric properties of HZO. Additionally, high-temperature annealing, which is essential to activate HZO's ferroelectric properties, causes oxygen atoms on the HZO thin film surface to migrate toward the electrode. This not only creates defects such as oxygen vacancies on the HZO surface but also leads to variability in ferroelectric performance across operational cycles.
To overcome these issues, it was essential to ensure the stability of the interface between the electrode and HZO. The research team addressed this by inserting a tungsten disulfide (WS₂) layer to maintain the interfacial stability between the underlying electrode and HZO. This approach minimized unwanted chemical reactions, protected the surface of the underlying electrode, and controlled the migration of oxygen atoms on the HZO thin film surface. As a result, the team successfully achieved stable interfacial properties, maximizing the ferroelectric performance of HZO.
Another major hurdle in commercializing HZO-based ferroelectrics is the disordered orientation of nanoscale domains, which leads to performance inconsistencies between devices and challenges in ensuring reliability. To address this issue, the research team leveraged the similarity between the lattice constant of tungsten disulfide (WS₂) and the interplanar spacing of specific HZO planes. This similarity allowed for excellent crystallinity when the materials were combined. By controlling the directional alignment of the material domains and maximizing their orientation uniformity, the team was able to significantly enhance the reliability of the devices.
The heterojunction technology integrating two-dimensional tungsten disulfide and HZO is considered a major technological breakthrough for developing next-generation ferroelectric-based non-volatile memory devices. Tungsten disulfide protects the underlying electrode in high-temperature environments while also promoting domain alignment, resulting in significant improvements in the performance of ferroelectric devices.
Dr. Yong-Hun Kim, the lead researcher at KIMS, stated, “We have resolved the critical challenge of controlling disordered domains, which was the biggest obstacle to commercializing HZO-based non-volatile memory devices, thereby achieving non-volatile memory characteristics with high reliability and durability.” The research team is continuing to optimize device performance under low-temperature processes (below 400°C) suitable for application in semiconductor back-end-of-line (BEOL) manufacturing.
This research was funded by the Ministry of Science and ICT, the fundamental research program of the Korea Institute of Materials Science, the Global TOP Strategy Research Group Program funded by the National Research Council of Science and Technology, and the Leading Researcher Program of the National Research Foundation of Korea. The results will be presented by Seung-Kwon Hwang, a doctoral student researcher at KIMS and the first author of the paper, at the International Electron Devices Meeting (IEDM) 2024 in San Francisco, USA, on December 11.
-----------------------------------------------------------------------
###
About Korea Institute of Materials Science(KIMS)
KIMS is a non-profit government-funded research institute under the Ministry of Science and ICT of the Republic of Korea. As the only institute specializing in comprehensive materials technologies in Korea, KIMS has contributed to Korean industry by carrying out a wide range of activities related to materials science including R&D, inspection, testing&evaluation, and technology support.