Elucidating the formation mechanism of "germanene," a candidate qubit material

A research group including Assistant Principal Researcher Tomo-o Terasawa of the Research Group for Surface and Interface Science at the Advanced Science Research Center, Nuclear Science Research Institute, Japan Atomic Energy Agency (JAEA) in collaboration with the Institute of Industrial Science at the University of Tokyo and the Japan Fine Ceramics Center, announced on April 23 that they have elucidated the growth mechanism of "germanene," a sheet-like candidate material for quantum bits (qubits). By analyzing the formation process using proprietary technology, they revealed that an increase in the amount of germanium on the surface of a silver thin film during a two-step process of heating and cooling from room temperature is the key to sheet formation. This discovery is expected to contribute to the search for qubit materials. The results were published on March 10 in Chemistry of Materials, a journal issued by the American Chemical Society.

Germanene is a single-layer material in which germanium atoms are arranged in a honeycomb lattice pattern, similar to graphene, and theoretical predictions suggest it is a promising candidate material for realizing qubits. It was developed by Associate Professor Junji Yuhara and his colleagues at Nagoya University. It is formed into a sheet-like shape on top of a silver thin film after creating the silver thin film on a germanium substrate, followed by heating and cooling processes.

In nature, germanium exists most stably as a three-dimensional particulate crystal, and it had previously remained unknown why sheet-like germanene forms instead. Furthermore, germanene reacts easily with air, making the analysis itself highly challenging.

Therefore, this study aimed to clarify the mechanism behind the formation of sheet-like germanene by analyzing the previously unknown process of its creation.

First, the researchers fabricated a 150-nm-thick silver thin film on a germanium substrate, heated it to 500℃ in a vacuum, and analyzed the process of germanene formation during cooling to room temperature using Raman spectroscopy. For this analysis, they utilized a uniquely developed Raman spectroscopy apparatus capable of operating under high-temperature and vacuum conditions. This method allows for the investigation of the bonding states of germanium.

The analysis revealed that while the germanium that precipitated onto the silver thin film at 300℃ consisted of particles, the peak corresponding to this structure disappeared at 500℃, suggesting that the atoms had transitioned into a mobile, fluid state. Subsequently, as the system was cooled from the high temperature of 500℃, a peak indicating the presence of germanene appeared at 300℃.

To further investigate the behavior of the germanium atoms, the team conducted an analysis using X-ray photoelectron spectroscopy at a beamline constructed by JAEA at SPring-8. They observed the quantity of germanium atoms on the silver thin film while heating and cooling the sample. Their method enables the evaluation of the amount and the chemical state of atoms.

The results demonstrated that the amount of surface germanium increased during heating from room temperature up to 300℃, increased further at 500℃, and accumulated even more when cooled back down to 300℃. The researchers examined this mechanism based on the temperature-dependent reaction of germanium. The migration distance of germanium atoms varies with temperature. Estimating the migration distance during a 10-minute temperature hold, the atoms can migrate extensively over a distance of approximately 490 nm at 500℃, which exceeds the thickness of the silver thin film (150 nm).

On the other hand, when cooled from that state down to 300℃, the migration distance decreases to approximately 7.8 nm, making it impossible for the atoms to surpass the thickness of the silver thin film.

It was found that while cooling from 500℃ to 300℃, because a large amount of germanium dissolves into the silver thin film and the migration distance shortens, a high concentration of germanium accumulates at the surface. This promotes lateral bonding between the germanium atoms, resulting in the selective formation of germanene. Moving forward, these findings will enable the stable fabrication of germanene based on design principles.

Terasawa said: "As germanene undergoes further research as a candidate qubit material in the future, we believe these results are crucial as a technology that supports this entire research field from beneath. Furthermore, we expect that similar theoretical frameworks can be widely applied to materials development. Looking ahead, we plan to continue our research into technologies for isolating and extracting only the sheet-like germanene formed on the silver thin film."

Journal Information
Publication: Chemistry of Materials
Title: In Situ Study of Growth Mechanism of Germanene Segregated through Ag(111) Thin Films by Raman and X-ray Photoelectron Spectroscopy
DOI: 10.1021/acs.chemmater.5c03462