Two-dimensional (2D) semiconductors, such as MoS2, enable the unprecedented possibilities to solve the bottleneck of transistor scaling and to build novel logic circuits with faster speed, lower power consumption, flexibility and transparency, benefiting from their ultra-thin thickness, dangling-bond-free flat surface and excellent gate controllability. In 2015, International Technology Roadmap for Semiconductors (ITRS) clearly pointed out that 2D semiconductors are key materials for next-generation high-performance devices. In 2021, Intel listed 2D MoS2-based transistor technology as one of the three breakthrough technologies for the next decade.
Tremendous efforts have been devoted to exploring the scaled-up potentials of monolayer MoS2, including both wafter-scale synthesis of high-quality materials and large-area devices. In terms of a further improvement of the electronic quality of the large-scale monolayer MoS2, structural imperfections should be eliminated as much as possible; however, there is not much space left for monolayer MoS2 after ten years of synthesis optimizations in this field. Another key direction is to switch to multilayer MoS2, e.g., bilayers and trilayers, since they have intrinsically higher electronic quality than monolayers and thus are conducive to higher-performance devices and logic circuit. However, due to the fundamental limitation of thermodynamics, it is still a great challenge to realize wafer-scale multilayer MoS2 with high-quality and large-scale uniformity.
In a study published in National Science Review, a research team from China has now overcome the fundamental limitations of thermodynamics by exploiting the proximity effect of substrate and achieved, for the first time, the growth of high-quality multilayer MoS2 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to 6.
‘To achieve the layer-by-layer epitaxy of multilayer MoS2, we independently design an oxygen-assisted 4-inch multi-source CVD system. The breakthrough of controllable epitaxy of high-quality multilayer MoS2 wafers would lay a solid material foundation for large-scale high-performance 2D electronic devices, and is expected to play a paramount role in sub-10 nm ultra-short channel devices, flexible displays and smart wearable devices,’ says Dr. Qinqin Wang, the first author of the work.
Compared to monolayers, thicker-layer MoS2 field-effect transistors show significant improvements in device performances. For long-channel devices, the average field-effect mobility at room temperature can increase from ~80 cm2·V-1·s-1 for monolayers to ~110/145 cm2·V-1·s-1 for bilayer/trilayer devices, improved by 37.5%/81.3%. For trilayer MoS2 field-effect transistors, the highest room temperature mobility can reach up to 234.7 cm2·V-1·s-1, setting a new mobility record for devices based on 2D transition-metal sulfide semiconductors. For devices with channel-length of 100 nm, the current density (Vds=1 V) is increased from 0.4 mA·μm-1 for monolayer to 0.64/0.81 mA·μm-1 for bilayer/trilayer, showing an enhancement factor of 60%/102.5%. Remarkably, for 40 nm short-channel devices, a record-high on-current densities of 1.70/1.22/0.94 mA/μm at Vds=2/1/0.65 V, as well as a high on/off ratio exceeding 107, are achieved in trilayer MoS2 field-effect transistors.
‘Considering that, in well-developed thin-film transistors (TFTs), field-effect mobility is 10-40 cm2·V-1·s-1 for indium–gallium–zinc-oxide TFTs and 50-100 cm2·V-1·s-1 for low-temperature polycrystalline silicon TFTs, the competitive average field-effect mobility, e.g., larger than 100 cm2·V-1·s-1, achieved in this work strongly uncover a great potential of these multilayer MoS2films for high-performance TFT applications,’ says Dr. Jian Tang, one of the lead authors of the work.
‘The on-current density of trilayer MoS2 devices with a channel length of 40 nm can reach a record-high value of 1.70 mA/μm at Vds=2 V, outperforming the previous state-of-the-art MoS2 transistors. Such high on-current density also exceeds the target of high-performance logic transistors from the International Roadmap for Devices and Systems (IRDS) 2028, and hence moves a step closer to practical applications of 2D MoS2 in electronics and logic circuits at sub-5 nm nodes,’ adds Professor Guangyu Zhang, who leads the group behind the study.