Environmentally-friendly substance expected to lead to commercially-viable devices in THz band--Innovative manufacturing method using graphene
Created by Tohoku University, NICT, Shin-Etsu Chemical and others

A research group that included Associate Professor Hirokazu Fukidome and others at Tohoku University's Research Institute of Electrical Communication, together with Shin-Etsu Chemical Co., Ltd., the High Energy Accelerator Research Organization, Institute of Materials Structure Science (KEK-IMSS), the Japan Synchrotron Radiation Research Institute (JASRI) and the National Institute of Information and Communications Technology (NICT), has created an innovative manufacturing method for environmentally-friendly, ultra-high-speed devices that uses graphene. This manufacturing method ensures the highest level of quality in the world, makes it possible to reduce costs to less than one-hundredth, and furthermore overcomes the shortcomings that have conventionally plagued graphene devices. Accordingly, it is a technology that will realize the commercial viability of devices that operate in the THz (terahertz) band that is essential to "Beyond 5G," and the graphene transistors that will be realized as a result of this technology, which will be capable of operating in the THz band, will contribute to an environmentally-friendly Beyond 5G. It will also be possible to divert the same technology to realize power/high-speed transistors for use in 5G and Beyond 5G that make use of silicon carbide (SiC) and gallium nitride (GaN), not just graphene.

In many cases existing devices for use with Beyond 5G make use of elements such as rare indium (In) and harmful arsenic (As). As such, there is a danger that these devices will not comply with the intent of the Sustainable Development Goals (SDGs) advocated by the United Nations. Consequently, in addition to these existing devices, research and development on THz band devices that use environmentally-friendly substances is becoming a pressing issue.

In that respect, graphene is an environmentally-friendly substance, and one which possesses excellent physical properties, including that electrons are able to travel throughout the entire substance at maximum speed. However, graphene devices have also posed challenges, among them, the high cost of substance production, and that even when made into devices they do not display the performance that is expected.

Up to now, the research group that arose out of this industry-government-academia collaborative research had created a graphene on silicone (GOS) technology for the first time in the world. This GOS technology promotes direct growth of graphene through the medium of a SiC thin film on a silicon (Si) substrate.

However, it was necessary to improve the quality of the graphene grown with the GOS technology and realizing THz band operation with transistors that employ GOS had proven difficult.
Consequently, it had been hoped that a technology could be created for growing high-quality graphene at low cost on a substrate such as Si or sapphire, and which could be used for devices.

With that in mind, the research group has now developed a new graphene manufacturing method that grows the graphene on a hybrid SiC substrate developed by Shin-Etsu Chemical.

The hybrid SiC substrate is transcribed into a substrate suited to use for devices, such as an Si or sapphire substrate, by injecting hydrogen ions (H+) into bulk SiC substrate to create incisions at a depth of around 1μm from the substrate surface, and causing a thin film of high-quality SiC monocrystal to peel off.

A major advantage of this hybrid SiC substrate process is that the high-priced bulk substrate can be used repeatedly. More than 100 sheets can be manufactured from one bulk substrate, it is possible to form large sheets of three inches or more, and the ingredients-related cost of the graphene manufacturing process can be significantly reduced to less than one-hundredth of conventional processes.

Furthermore, the research group made full use of angle-resolved photoemission spectroscopy devices and spectroscopic photoemission and low energy electron microscopes at synchrotron radiation facilities such as the Photon Factory and SPring-8 to reveal that the graphene that was obtained boasts world-class quality and properties. It succeeded in developing high-performance transistors using this graphene.

These transistors realize a high degree of modulation and current saturation simultaneously in  input gate voltage (Vg)-output drain current (gm), which had been difficult with conventional graphene transistors, and analysis of their operating characteristics shows that the transistors which the group developed will be able to operate in the THz band.