Synthesis of 2D materials
For the mass production of large-area graphene, chemical vapor deposition (CVD) is the most popular method, because it enables low-cost growth of the large area, high-quality graphene with good electrical properties. However, there have been difficulties in handling the CVD graphene due to its extremely thin nature when it is applied to the industrial process and several technical issues need to be addressed for the real applications of graphene in electronics and optoelectronics. In 2014, my group has successfully synthesized multilayer graphene films on an insulating substrate through laser-annealing of SiC. We reported a solid-phase synthesis of doped graphene by means of silicon carbide substrate including a dopant source driven by nanosecond-pulsed laser irradiation. This method provides in situ direct growth of doped graphene on an insulating SiC substrate without an additional transfer step.
Recently, new semiconducting 2D materials known as transition metal dichalcogenides (TMDCs) appeared, as represented by MoS2, MoSe2, WS2 and WSe2. Unlike graphene field-effect transistors (FETs) with less than ~10 of on/off ratio, the transistors made of these TMDCs can attain high current on/off ratio, which fulfills the current need for the commercial transistors, and the high mobility of TMDCs make the low power consumption of possible. In 2014, we have demonstrated the synthesis of large area MoSe2 via chemical vapor deposition on arbitrary substrates such as SiO2 and Sapphire for the first time. In addition, we have also developed a facile liquid-phase exfoliation method to improve the exfoliation efficiency for single-layer MoS2 or MoSe2 sheets.
Our group synthesizes two-dimensional materials such as graphene, TMDCs, and h-BN using various methods such as CVD, MOCVD, laser annealing, liquid phase exfoliation, and ALD. Based on this, we research the application to memory, transistors, optoelectornics, chemical, display, amd recently in neuromorphic electronics.
Process Technonlogy for 2D Materials
We are so confident that graphene and 2D materials will play a key role in the development of next-generation nanoscale devices or large-area displays. For the mass production of large-area graphene, CVD is the most popular method. However, handling the CVD-grown 2D materials is difficult due to their extremely thin nature when they are going through the industrial process.
Another important subject of the investigation has been the interface engineering between 2D materials and other parts of devices, like the dielectrics, substrates, and channel or source/drain doping process. Integration of high-quality gate dielectrics on graphene is an important technical challenge because the quality of the interface between the dielectrics and graphene channel affects the electrical characteristics of graphene devices, such as operating voltage, scaling capability, and device reliability. To address these issues, we proposed a new approach for integration of high-k dielectrics on graphene using a functionalized graphene monolayer as an ultra-thin seed layer on top of the graphene channel and novel methods for graphene transfer on various substrates with high degrees of freedom. Graphene transistors with top gate structure using these methods show significantly enhanced device performances in terms of carrier mobility and device reliability. In addition, we demonstrate high-performance graphene transistors on various substrates including flexible substrates. It is expected that our group’s approaches will pave the way for the realization of high-performance electronic devices based on graphene and 2D materials.
We successfully demonstrated a low-thermal budget mass-production-suitable synthesis of heteroatom-doped rGO through intense pulse light irradiation in ambient air within <10 ms and probed photothermally induced B-doping process by milisecond-scale transient temperature profiliing. Intense pulsed light-assisted optical engineering was concluded to be a facile and general strategy for carbon matrix doping with heteroatoms.
2D materials based Logic & Memory Transistor
Low-power, nonvolatile memory is an essential electronic component to store and process the unprecedented data flood arising from the oncoming Internet of Things era. Molybdenum disulfide (MoS2) is a 2D material that is increasingly regarded as a promising semiconductor material in electronic device applications because of its unique physical characteristics. However, dielectric formation of an ultrathin low-k tunneling on the dangling bond-free surface of MoS2 is a challenging task. MoS2-based low-power non-volatile charge storage memory devices are reported with a poly(1,3,5-trime-thyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) tunneling dielectric layer formed via a solvent-free initiated chemical vapor deposition (iCVD) process. By leveraging the inherent flexibility of both MoS2and polymer dielectric films, this research presents an important milestone in the development of low-power flexible nonvolatile memory devices.
Our group demonstrate a vertical-tunneling FET based on a MoS2-WSe2, MoS2-MoTe2, MoS2-Si heterojunction. The resulting p-n heterojunction shows a staggered band alignment in which the quantum mechanical band-to-band tunneling probability is enhanced. The device functions in both tunneling transistor and conventional transistor modes, depending on whether the p-n junction is forward or reverse biased, and exhibits a minimun subthreshold swing. Also we research a high-performance and hystresis-free transistor based on two-dimensional (2D) semiconductros featuring a van der Waals heterostructure, MoS2 channel, GaS two-dimensional gate insulator. This transistor based on van der Waals heterostructure exhibits considerable potential in digital logic applications with low-power integrated circuits.
Photo-Detector
With the advent of the 4th Industrial Revolution, technologies such as AI and smart sensors have been attracting attention. Representative examples include future automobiles, optical medical devices, and digital holograms. The global market is expected to grow at an average annual rate of 8% to about $800 billion in 22 years. In order to achieve the development of such optical fusion technology, basic optical technologies including lasers and photodetectors must be advanced together.
A photodetector that receives light and converts it into an electrical signal requires the following four typical performances. Silicon-based photodetectors that are currently widely used have the disadvantage of low reactivity in the case of PIN diode, and the disadvantage of high reactivity but very high operating voltage in the case of avalanche photodiode. Therefore, many attempts have been made to make photodetectors not only from silicon but also from new materials. Two-dimensional material-based photodetectors have been studied using a single material such as graphene or a two-dimensional semiconductor material, MoS2. Various materials as well as various mechanisms have been used. Representatively, the photovoltaic mechanism using electron-hole separation of the pn junction, the photoconductive mechanism that increases the photocurrent by moving the electron-hole pairs generated by the introduction of the light absorption layer into the channel region, and the two-dimensional In order to compensate for the low absorption rate of the material, a method using an external structure, a surface plasmon, has been studied.
Although various materials and mechanisms have been used, two-dimensional material-based photodetectors pose two challenges. First, it is difficult to improve the photoreactivity and response speed at the same time. Since the photoreactivity and response speed are related to the photocarrier lifetime, there is a trade-off relationship in which the response speed becomes slower as the reactivity increases. Second, making a photodetector that covers all of the broadband light is one of the challenges. Because semiconductors have a bandgap, they cannot absorb light with energy lower than the bandgap. Although many studies have been conducted to extend the detection band with graphene, the photoreactivity was not high due to the intrinsically low absorption of graphene.