Tokyo Institute of Technology, RIKEN, and Okayama University announced on April 25, 2016 that they have developed MOS transistors using a new two-dimensional material, hafnium disulfide (HfS2). It is confirmed that this transistor has good saturation characteristics and current control characteristics, and the switching ratio reaches 104, which is a good electronic component material.
　　MOS transistors are the basic components of LSIs, and reducing the size of components is very important to improve performance. In recent years, the size of MOS transistors has been reduced to a channel length of less than 10nm. In the past, there were atomic-sized irregularities on the surface of semiconductor materials, and when extremely thin films were made, the current transport ability dropped sharply, which inevitably led to a reduction in the driving ability. On the other hand, two-dimensional materials have atomic-level flatness and thickness (less than 1 nm), and are expected to achieve high mobility in extremely thin film states.
　　HfS2 belongs to the two-dimensional crystal group of transition metal dichalcogenides. It is predicted by theoretical calculation that the electron mobility of its single atomic layer (about 0.6nm thick) is 1800cm2/Vs and the band gap is 1.2eV. These values exceed the physical properties of silicon, a representative semiconductor material. The most famous two-dimensional material, graphene, has an extremely high mobility, which is expected to reach 100,000 cm2/Vs. However, because it has no band gap, it faces the issue of reducing power consumption when used as an LSI element.
　　In the experiments, the researchers transferred HfS2 flakes several atomic layers thick, made by mechanical lift-off, onto a substrate (a video of the method is available online on Low-Dimensional Materials). The thickness of the flakes observed using an atomic force microscope is about 2 to 10 atomic layers. A metal electrode was formed on the sheet, and a MOS transistor structure with the semiconductor substrate on the back as the gate electrode was fabricated. Good on-off ratios were observed for the fabricated devices, and in the electric double-layer transistor structure using the electrolyte as the gate electrode, the drive current was increased to about 1000 times or more than when operating on the back gate.
       According to the technical engineer of Juna's low-dimensional materials online, the crystal structure of hafnium diselenide HfSe2 (Hafnium Selenide) is hexagonal, the size can reach 8mm, and the purity can reach more than 99.995%. In the future, Giant Nano researchers will use solid gate insulating films to achieve performance comparable to electrolyte electrodes by properly protecting the HfS surface while improving the contact with the electrodes, and fabricating ultra-low power devices. In addition, when HfS2 is bonded with other two-dimensional materials, it may show obvious quantum effects, and its application range is expected to be expanded to two-dimensional channel transistors. The research results have been published in the March 1 issue of the journal Scientific Reports.