Symposium NM1 : Emerging Non-Graphene 2D Materials

Abstract not available.

Two-dimensional layered materials have many useful properties that may be useful for future high-performance, energy efficient computing systems technology. Semiconducting 2D layered materials such as MoS₂ and black phosphorous (BP) have shown promise as transistor channel materials. I will present our results on making low-resistance electrical contacts to these materials [1, 2]. Graphene, a semi-metal, has very useful properties that makes it particularly suited for extending the use of copper as interconnect wires. These useful properties include serving as a highly effective copper diffusion barrier (thus replacing TaN as the copper liner), and providing enhanced electromigration immunity for a graphene/Cu composite with graphene cladding the copper wire. We will present recent experimental results on these two very promising applications of graphene to complement copper wires [3, 4]. We have also developed a system-level modeling framework that allows us to benchmark the speed performance and energy efficiency of a microprocessor, including contributions from parasitics and wiring interconnects using industry-standard place-and-route optimization tools coupled with compact device models. We will present results that compare 5-nm FinFET CMOS with MoS₂ and BP transistors, as well as various wiring options including multilayer graphene and graphene cladded Cu wires [5].

Acknowledgements: Supported in part by member companies of the Stanford Initiative for Nanoscale Materials and Processes (INMP), member companies of the Stanford SystemX Alliance, the National Science Foundation (EFRI 2-DARE: Energy Efficient Electronics with Atomic Layers (E3AL), Award Abstract #1542883), and also in part by Systems on Nanoscale Information fabriCs (SONIC) and Function Accelerated NanoMaterials Engineering (FAME) Center, two of the six SRC STARnet Centers, sponsored by MARCO and DARPA.

References:

[1] S. Lee, A. Tang, S. Aloni, H.-S. P. Wong, “Statistical Study on the Schottky Barrier Reduction of Tunneling Contacts to CVD Synthesized MoS2,” Nano Lett., 16 (1), pp 276–281 (2016).

[2] L. Li, M. Engel, D. B. Farmer, S.-J. Han, H.-S. P. Wong, “High Performance p-Type Black Phosphorus Transistor with Scandium Contact,” ACS Nano, 10 (4), pp 4672–4677 (2016).

[3] L. Li, X. Chen, C.-H. Wang, J. Cao, S. Lee, A. Tang, C. Ahn, S. S. Roy, M. S. Arnold, and H.-S. P. Wong, “Vertical and Lateral Copper Transport through Graphene Layers,” ACS Nano, 2015, 9 (8), pp 8361–8367.

[4] L. Li, Z. Zhu, T. Wang, J.A. Currivan-Incorvia, A. Yoon, and H.-S. P. Wong, “BEOL Compatible Graphene/Cu with Improved Electromigration Lifetime for Future Interconnects,” IEEE International Electron Devices Meeting (IEDM), paper 9.5, 2016.

[5] C.-S. Lee, B. Cline, S. Sinha, G. Yeric, and H.-S. P. Wong, “32-bit Processor Core at 5-nm Technology: Analysis of Transistor and Interconnect Impact on VLSI System Performance,” IEEE International Electron Devices Meeting (IEDM), paper 28.3, 2016.

Two-dimensional van der Waals materials have shown novel fundamental properties and promise for wide applications. Here, we reported, for the first time, an experimental demonstration of the in-situ characterization and control of the anisotropic thermal conductivity of black phosphorus. We develop a novel platform based on ultrafast optical spectroscopy and electrochemical control to investigate the interactions between ions and the 2D lattice. We characterize the anisotropic thermal properties of black phosphorus and discover a strong tenability over thermal conductivity by ionic intercalation. A dramatic reduction of thermal conductivity by up to 6 times from that of the pristine crystal have been observed. This study provides a unique approach to explore the fundamental energy transport involving lattices and ions in the layered structures, and may open up new opportunities in controlling energy transport based on novel operation mechanisms and the rational design of nanostructures.

Atomically thin ReS₂ and NbSe₂ crystals are new types of 2D materials that have different crystal structures and electronic properties from common transition metal dichalcogenides (TMDs), e.g. MoS₂, MoSe₂, WS₂, and WSe₂. We measured ultralow frequency Raman response of ReS₂ and NbSe₂ atomic layers. ReS₂ has unique distorted 1T structure. We found that the two shear phonon modes in bilayer ReS₂ are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other TMD materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of interlayer modes and determine the stacking order of bilayer ReS₂. Few-layer ReS₂ exhibits rich Raman peaks at frequencies below 50 cm^-1, where a panoply of interlayer shear and breathing modes are observed. Atomically thin NbSe₂ is a metallic layered TMD with novel charge-density-wave (CDW) and superconductive phases. We observed both the interlayer breathing modes and shear modes at frequencies below 40 cm^-1 for samples of 2 to 15 layers. Their frequencies, Raman activities, and environmental instability depend systematically on the layer number. We find that, although NbSe₂ has different stacking order from MoS₂, they share the same crystal symmetry groups and exhibit similar Raman selection rules for interlayer phonons. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, with no observable response to the CDW formation in NbSe₂. Our results reveal that the interlayer phonons can serve as an effective probe of the interface properties and interlayer interactions in these 2D atomic layers.