Two-dimensional semiconductor materials, represented by transition metal dichalcogenides (TMDCs), have the characteristics of extreme thickness, high mobility, and back-end heterogeneous integration. They are expected to continue Moore's law and realize integrated circuits with three-dimensional architecture. and industry attention. After nearly a decade of development, two-dimensional electronics has made great progress, but there are still challenges in the preparation of large-area single crystals, key device processes, and compatibility with mainstream semiconductor technologies.
The research group of Prof. Xinran Wang from the School of Electronic Science and Engineering of Nanjing University focused on the above problems and researched breakthroughs in the key technologies of two-dimensional semiconductor single crystal fabrication and hetero-integration, which provided new ideas for the development of integrated circuits in the post-Moore era. Relevant research results have been published in Nature Nanotechnology recently.
Building "atomic terraces" down-to-earth, breaking through two-dimensional semiconductor single crystal epitaxy
Semiconductor single crystal materials are the cornerstone of the microelectronics industry. Compared with the mainstream 12-inch monocrystalline silicon wafers, the preparation of two-dimensional semiconductors is still in the small-scale and polycrystalline stage. The development of large-area, high-quality monocrystalline thin films is the first step towards two-dimensional integrated circuits. . However, during the growth of two-dimensional materials, millions of microscopic chips are randomly generated, and it is only possible to obtain a monolithic single-crystal material by controlling all chips to maintain a strictly consistent arrangement direction.
Sapphire is a widely used substrate in the semiconductor industry and has outstanding advantages in mass production, low cost and process compatibility. The collaborating team proposed a scheme to artificially construct atomic-scale "terraces" by changing the direction of the atomic steps on the sapphire surface. The directional growth of TMDCs was achieved by the directional induced nucleation mechanism of "atomic terraces".
Based on this principle, the team achieved the epitaxial growth of a 2-inch MoS2 single crystal film for the first time in the world. Thanks to the improvement of material quality, the mobility of field effect transistors based on MoS2 single crystal is as high as 102.6 cm2/Vs, and the current density reaches 450 μA/μm, which is one of the highest comprehensive performances reported internationally. At the same time, the technology has good universality and is suitable for the preparation of single crystals of other materials such as MoSe2. This work has laid a material foundation for the application of TMDC in the field of integrated circuits.
Looking up at the stars, two-dimensional semiconductors bring light to future display technology
The breakthrough of large-area single-crystal materials makes it possible for two-dimensional semiconductors to be applied. In the second work, based on years of accumulation of third-generation semiconductor research, combined with the latest two-dimensional semiconductor single crystal solution, the cooperative team of the School of Electronics proposed a monolithic integrated ultra-high-resolution Micro LED display based on MoS2 thin film transistor driver circuit. Technical solutions.
Micro LED refers to a technology that uses micron-scale LEDs as light-emitting pixel units and assembles them with driving modules to form a high-density display array. Compared with the current mainstream display technologies such as LCD and OLED, Micro LED has cross-generational advantages in terms of brightness, resolution, energy consumption, service life, response speed and thermal stability, and is an internationally recognized next-generation display technology.
However, the industrialization of Micro LED still faces many challenges. First, it is difficult to match the driving requirements of high-density display units in small sizes. Secondly, the mass transfer technology popular in the industry is difficult to meet the development needs of high-resolution displays in terms of cost and yield. Especially for ultra-high-resolution applications such as AR/VR, not only the resolution is required to exceed 3000PPI, but also the display pixels need to have a faster response frequency.
The cooperative team aimed at the field of high-resolution micro-display, and proposed a technical solution for the 3D monolithic integration of MoS2 thin-film transistor driver circuit and GaN-based Micro LED display chip. The team developed a non-"massive transfer" low-temperature monolithic heterogeneous integration technology, using a nearly non-destructive large-size two-dimensional semiconductor TFT manufacturing process, to achieve a high-brightness, high-resolution microdisplay of 1270 PPI, which can meet the needs of future microdisplays. Display, vehicle display, visible light communication and other cross-field applications.
Among them, compared with the traditional two-dimensional semiconductor device process, the new process developed by the team improves the performance of thin film transistors by more than 200 percent , reduces the difference by 67 percent , and the maximum driving current exceeds 200 μA/μm, which is better than IGZO, LTPS and other commercial materials. It shows the huge application potential of two-dimensional semiconductor materials in the display driving industry. This work is the first in the world to integrate two emerging technologies of high-performance two-dimensional semiconductor TFT and Micro LED, which provides a new technical route for the future development of Micro LED display technology.
The above works are respectively named "Epitaxial growth of wafer-scale molybdenum disulfide semiconductor single crystals on sapphire" (corresponding authors are Prof. Wang Xinran and Prof. Wang Jinlan of Southeast University) and "Three dimensional monolithic Micro LED display driven by atomically-thin transistor matrix" (corresponding authors). It was published online in Nature Nanotechnology recently.
This series of work has been supported by projects such as Jiangsu Province's Frontier Leading Technology Basic Research Project, the National Natural Science Foundation of China, and the National Key RD Program. Changchun Institute of Optics and Mechanics, Chinese Academy of Sciences, Tianma Microelectronics Co., Ltd., Nanjing Huanxuan Semiconductor Co., Ltd., etc.
