"Future mobile phones can be flexible, and high-definition flexible displays can make mobile phones roll up like newspapers." Zhang Guangyu, deputy director and researcher of the Songshan Lake Materials Laboratory, talked about the team’s latest research and looked forward to the development of future mobile phones to DeepTech trend.
On September 21, the team`s paper "Large-scale flexible and transparent electronics based on monolayer molybdenum disulfide field-effect transistors" was published in the Journal of Electronics Natureelectronics. The Songshan Lake Materials Laboratory has a strong academic background, led by the Institute of Physics, Chinese Academy of Sciences, and jointly established by the Dongguan Municipal Government, the Institute of Physics of the Chinese Academy of Sciences and the Institute of High Energy Physics.
In the experiment, a four-inch high-quality, highly oriented single-layer molybdenum disulfide film obtained by epitaxial growth was used, combined with traditional micromachining technology, and by optimizing the insulating layer and contact resistance, a large-area flexible and transparent molybdenum disulfide field effect transistor and Various logic devices.
These devices show excellent characteristics: the transistor device density can reach 1518/cm2, and the yield rate is as high as 97%, which is the highest index among the reported results; a single device also shows better electrical properties and flexibility.
This work was jointly completed by the Institute of Physics of the Chinese Academy of Sciences and the Songshan Lake Materials Laboratory, and was funded by the National Natural Science Foundation of China, the National Key Research and Development Program, the B-type pilot project of the Chinese Academy of Sciences, and the Youth Promotion Association of the Chinese Academy of Sciences.
The main material used in this study is a two-dimensional semiconductor material-monolayer molybdenum disulfide (MoS2). At present, driven by the development of miniaturization and flexibility of semiconductor devices, two-dimensional semiconductor materials represented by molybdenum disulfide have shown unique advantages. They not only have excellent optical, electrical and mechanical properties, but also have ultra-thin Transparent physical properties, the thinnest can be only one atomic layer thickness, very suitable for preparing lighter, thinner, faster and more sensitive electronic devices.
In 2015, the International Semiconductor Alliance pointed out in the Technology Roadmap (ITRS) that two-dimensional semiconductors are key materials for next-generation semiconductor devices.
However, the current research on monolayer molybdenum disulfide based on device applications still faces two major problems:
1. In terms of material preparation, it is difficult to obtain high-quality large-scale molybdenum disulfide wafers;
2. In terms of device technology, it is difficult to achieve high-density, high-performance, and large-area uniform device processing. This is also a common problem that new semiconductor materials must overcome from the laboratory to the market.
If the above problems are solved, the application of molybdenum disulfide in the flexible electronics industry will also be faster.
At present, single-layer molybdenum disulfide wafers are facing problems such as small grain size and random orientation. The existence of a large number of grain boundaries (interfaces between grains with the same structure but different orientations) leads to poor electrical quality of the material . Moreover, the uneven distribution of grain boundaries will result in poor device uniformity, so high-end electronic devices cannot be integrated.
In order to solve the above problems, Zhang Guangyu’s team used a self-designed and built multi-source chemical vapor deposition system, using vertical growth and multi-point nucleation methods to epitaxially prepare a four-inch high-quality continuous single-layer molybdenum disulfide on a sapphire substrate Wafer.
Because it is epitaxial growth, a single crystal substrate is first required. In the field of conventional semiconductors, when using single crystal sapphire substrate to epitaxial gallium arsenide and gallium nitride, the epitaxial material needs to increase the buffer layer adapted to the substrate lattice, thereby reducing the crystallinity between the material and the substrate. Lattice mismatch.
However, two-dimensional materials such as molybdenum disulfide have no dangling bonds on the surface, and the interaction with the substrate surface is relatively weak, and the lattice matching degree is not high. It belongs to van der Waals epitaxy, so it can be directly on the four-inch sapphire substrate. Realize the epitaxial growth of a single layer of molybdenum disulfide.
The epitaxial high-quality film is spliced by large grains with high orientation (0° and 60°). There are only twin boundaries in the film. A high-resolution transmission electron microscope has observed nearly perfect 4|4E type grain boundaries, and The average grain size is greater than 100 μm, which greatly improves the crystal quality of the wafer. This work was published in the recent Nano Letters.
Thanks to the unique multi-source design, the prepared wafers have remarkable uniformity. Talking about the multi-source design, Zhang Guangyu gave a vivid example: Just like taking a watering can to spray water on the wall, the first-generation equipment only has one nozzle, and the spray area is relatively small at this time;
The second generation device uses three nozzles to spray together, so that the spray area can be expanded three times; the third generation device uses six sources to spray together, in which case the spray area is larger and more uniform.
Although molybdenum disulfide has so many advantages, molybdenum disulfide is currently unable to replace silicon. Zhang Guangyu told DeepTech that in the semiconductor industry, silicon materials have been explored and studied for nearly 70 to 80 years, and related devices have been developed for 50 to 60 years.
Silicon has been very mature in terms of material preparation and device processing, so it has naturally become the mainstream of the semiconductor field. Some other semiconductor materials have better electronic properties than silicon, but they still face problems in materials and processing techniques, so they cannot replace silicon. In the same way, molybdenum disulfide is not to replace silicon, but to make up for the shortcomings of silicon and give play to the advantages of the material itself.
Zhang Guangyu said that there are certain bottlenecks in the development of silicon materials. Moore's theorem has hit the ceiling. The next step is to realize devices below 3 nanometers. However, if the size of silicon electronic devices is reduced to less than ten nanometers, its performance will be greatly reduced. However, two-dimensional semiconductor materials such as molybdenum disulfide are very thin and can solve the principle obstacles, thus showing great advantages.
Independent design of multi-source chemical vapor deposition equipment
The main equipment used in this research is a four-inch multi-source chemical vapor deposition system independently designed by the team. Zhang Guangyu said that except for the stove and accessories which are commercial products, the rest are all designed and built by themselves.
The team has also made a patent layout in terms of equipment, and has now authorized Dongguan Zhuoju Technology Co., Ltd. to work together to promote the industrialization of domestic high-performance chemical vapor deposition equipment.
This equipment is mainly used for the epitaxial growth of TMDs materials. It has been built to the third generation. The difference between different generations is that it can be compatible with the growth of large samples. Take the microwave oven as an example. Originally, it can hold one bowl, but when it becomes larger, it can hold ten bowls, which expands the system of the above equipment.
In addition to the above-mentioned unique growth technology. This achievement also benefited from the three major device processing technologies: the use of compatible micro-processing technology to make devices layer by layer; the use of a unique physical adsorption and chemical reaction combined atomic layer deposition method; the use of gold/titanium/gold multilayer structure as Contact electrode.
Talking about these three processes, Zhang Guangyu said that in the device processing process, the team uses traditional micromachining processes to fabricate devices layer by layer, achieving cleanness and compatibility between device layers and ensuring a large area of device arrays. Uniformity. This use of standard micromachining methods in semiconductor technology is conducive to quickly entering the market.
On the other hand, an atomic layer deposition method combining physical adsorption and chemical reaction is used to improve the quality of the device insulation layer. Due to the lack of dangling bonds on the surface of two-dimensional materials, traditional atomic layer deposition methods (chemisorption) cannot deposit high-dielectric insulating materials (aluminum oxide or hafnium oxide, etc.) on two-dimensional materials or gold surfaces.
The improved atomic layer deposition method firstly uses excessive source pulses to make it physically adsorbed on the surface of the material, and then uses multiple pulses of water to oxidize the source layer by layer to aluminum oxide or hafnium oxide, so that the insulating layer is placed on the gold bottom electrode and the second electrode. High-density direct deposition on the three-dimensional material, thus ensuring the high quality of the insulating gate of the device.
In the optimization of the contact electrode, the team adopted a three-layer gold/titanium/gold structure, adding a layer of titanium in the middle of the structure, which not only reduced the contact resistance, but also solved the mechanical performance problem, so the electrode became very stable. According to Zhang Guangyu, no matter how "toss" on the flexible device, the electrode will not fall off.
Reach the highest international electronics quality
Mentioning that this research has reached "the highest electronic quality molybdenum disulfide in the world", Zhang Guangyu told DeepTech that there is currently no other method in the world to find out the key indicators of material structure, such as grain size, orientation, and grain boundary density right.
The large-area flexible molybdenum disulfide electronic device prepared by the team has high density, high yield and high performance. It is worth noting that the dark current of the device is very small (below hundred femtoampere), so it can be used to prepare low-power devices.
For example, even if a computer is in standby mode, its CPU does not seem to be working, but it is still consuming power and has dark current. Therefore, when the dark current of the device is very small, the static power consumption can be reduced.
The team's device research on molybdenum disulfide is currently focused on three directions: low-power devices, high-performance devices and flexible devices. It is reported that the performance of some current semiconductor devices has reached the limit, which is determined by the characteristics of the material itself.
At this stage, the team is most concerned about flexible devices. Such devices do not need to achieve extreme performance or ultra-low power consumption. They only need a certain degree of flexibility. Under bending, kneading, pulling, and pressing, their performance will not be improved. Too much change.
Zhang Guangyu said that this research can promote the application of two-dimensional semiconductor materials in flexible displays and smart wearable devices. In the short term, the product that the team will focus on is a flexible display.
Zhang Guangyu believes that in the next 10-20 years, flexible wearable devices have great market prospects. Take contact lenses as an example. In the future, contact lenses may be flexible devices that integrate a screen and control panel. At the same time, such contact lenses are also more intelligent and can be used to browse information.
At present, many international teams have been conducting research on molybdenum disulfide wafers and devices, but there is still a gap between the quality of their wafers and the team's level. According to Zhang Guangyu, after the publication of this paper, many domestic and foreign colleagues believed that the material of our team was "the best molybdenum disulfide in the world", and they came to seek sample cooperation.
The reason why it is still limited to the laboratory stage is that the product is still in an experimental period, and it will take a long time to truly enter the industry. At present, South Korea's Samsung and a Taiwanese chip manufacturer are also advancing research in related directions. In the future, under the collective research of scientific research and industry, breakthroughs should be made.
The long-term plan of Zhang Guangyu's team is to make flexible displays, and the long-term plan is to make high-end logic devices. In the near future, perhaps Chinese users will be the first to use products that include the team`s technology.