Modeling of Thermally Aware Carbon Nanotube and Graphene Based Post CMOS VLSI Interconnect

This work studies various emerging reduced dimensional materials for very large-scale integration (VLSI) interconnects. The prime motivation of this work is to find an alternative to the existing Cu-based interconnect for post-CMOS technology nodes with an emphasis on thermal stability. Starting from the material modeling, this work includes material characterization, exploration of electronic properties, vibrational properties and to analyze performance as a VLSI interconnect. Using state of the art density functional theories (DFT) one-dimensional and two-dimensional materials were designed for exploring their electronic structures, transport properties and their circuit behaviors. Primarily carbon nanotube (CNT), graphene and graphene/copper based interconnects were studied in this work. Being reduced dimensional materials the charge carriers in CNT(1-D) and in graphene (2-D) are quantum mechanically confined as a result of this free electron approximation fails to explain their electronic properties. For same reason Drude theory of metals fails to explain electronic transport phenomena. In this work Landauer transport theories using non-equilibrium Green function (NEGF) formalism was used for carrier transport calculation. For phonon transport studies, phenomenological Fourier’s heat diffusion equation was used for longer interconnects. Semi-classical BTE and Landauer transport for phonons were used in cases of ballistic phonon transport regime. After obtaining self-consistent electronic and thermal transport coefficients, an equivalent circuit model is proposed to analyze interconnects’ electrical performances. For material studies, CNTs of different variants were analyzed and compared with existing copper based interconnects and were found to be auspicious contenders with integrational challenges. Although, Cu based interconnect is still outperforming other emerging materials in terms of the energy-delay product (1.72 fJ-ps), considering the electromigration resistance graphene Cu hybrid interconnect proposed in this dissertation performs better. Ten times more electromigration resistance is achievable with the cost of only 30% increase in energy-delay product. This unique property of this proposed interconnect also outperforms other studied alternative materials such as multiwalled CNT, single walled CNT and their bundles

Characterization of Carbon Nanostructures Based on Transmission Line Model.

PhDIn the past two decades carbon nanotubes and graphene have attracted a lot of research attention due to their exceptional electronic properties. The research focus on improving the synthesising techniques will eventually lead to their applications in terahertz wave, millimetre wave and microwave frequencies. In this thesis, a modelling technique based on the transmission line theory is proposed to calculate the 2-port S-parameters of vertically aligned CNT arrays with finite sizes and arbitrary cross sections. The process takes into account all the coupling in the array and gives the analytical solution of S-parameters. The simulation results from the proposed technique are compared with results obtained by effective single conductor model and shows a good matching for small arrays and an increasing difference with the increase of array sizes. From the S-parameters, the fundamental properties of CNT arrays such as input impedance and absorption are obtained and compared with measurement results in microwave frequencies. The dependence of these properties on ambient temperature and host medium are also presented to explore the tunability of CNT arrays. From the Fabry-Perot the wave propagating velocity is also calculated for arrays with different sizes and fitted with a power function. The S-parameters allows the extraction of the complex permittivity, permeability and conductivity of the CNT array. The extracted permittivity and absorption are compared with measurement results. The graphene nanoribbons are simulated in the same manner. The graphene sheet on top of a microstrip gap is simulated using transmission line model at microwave frequencies to show the impact of parasitics and contact resistances. Finally, a graphene based microwave absorber is proposed and modelled under both electric and magnetic bias. The absorber shows good broadband absorption rate and a potential for turning transparent and opaque to microwaves under both electric and magnetic bias.China Scholarship Council (CSC

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters

Two-Dimensional Electronic Materials and Devices: Opportunities and Challenges

The unprecedented growth of the Internet of Things (IoT) and the 4th Industrial Revolution (Industry 4.0) not only demands dimensional scaling of device technologies but also new types of applications beyond today’s electronics. Two-dimensional (2D) materials, a group of layered crystals (such as graphene and MoS2) with unique properties, have emerged as promising candidates for IoT and Industry 4.0 since they can, not only extend the scaling with unprecedented performance and energy efficiency but also exhibit high potential for novel electronic devices. However, such nanomaterials suffer from significant challenges in process integration, especially in the modules that involves the formation of interfaces between 2D materials and conventional bulk materials. Thus, realizing high-performance energy-efficient 2D electronic devices has been challenging. This dissertation focuses on understanding the fundamental issues in such 2D materials (such as contacts, interfaces and doping) and in identifying applications uniquely enabled by these materials.First, a comprehensive treatment of metal contacts to 2D semiconductors, which has been a huge hurdle for 2D electronic technologies, will be presented. As a pioneering study, new interface physics originating from the unique dimensionality and surface properties have been revealed [1]. Solutions to minimize contact resistance are described though techniques of interface hybridization [2] and seamless contacts [3], [4]. These techniques transform 2D semiconductors from solely scientifically-interesting materials into high-performance field-effect transistor (FET) technologies, such as MoS2 FETs with record-low contact resistances [5], [6] and WSe2 FETs with record-high drive current and mobility [7]. Beyond metal interfaces, dielectric interface is crucial for preserving the carrier mobility in 2D channels, for which a solution enabled by buffer layers has been proposed [8]. On the other hand, the vertical van der Waals interfaces between 2D and 3D semiconductors, which retain the advantages of pristine ultra-thin 2D films as well as maximized tunneling area/field, have been studied and exploited into a novel beyond-silicon transistor technology – the first 2D channel tunnel FET (TFET) [9], which beat the fundamental limitation in the switching behavior of transistors. Recent results from the engineering of such 2D-3D semiconductor interfaces by surface reduction/passivation are described, showing a significant boost of drive current. While conventional diffusion/ion implantation methods are infeasible for 2D materials, two efficient doping techniques that are specific for 2D materials – surface doping [10], [11] and intercalation doping [12] are presented. The theoretical study of surface doping using ab-initio methods helped develop a novel doping scheme that uniquely exploits the Lewis-base like pedigree of 2D semiconductors without disturbing the structural integrity of the 2D atomic layer configuration [13], as well as a novel electrocatalyst based on MoS2 that achieved record high hydrogen evolution reaction (HER) performance [14]. On the other hand, intercalation doping has been employed to demonstrate graphene based transparent electrodes with the best combination of transmittance and sheet resistance [12], and also the first graphene interconnects with excellent performance, reliability and energy-efficiency [15], [16]. Moreover, by uniquely exploiting the high kinetic inductance and conductivity of intercalation doped graphene, a fundamentally different on-chip inductor has been demonstrated [17], [18], with both small form-factors and high inductance values, that were once thought unachievable in tandem. This 2D technique provides an attractive solution to the longstanding scaling problem of analog/radio-frequency electronics and opens up an unconventional pathway for the development of future ultra-compact wireless communication systems. Finally, a novel dissipative quantum transport methodology based on Büttiker probes with band-to-band tunneling capability is developed for 2D FETs [19]. Subsequently, gate-induced-drain-leakage (GIDL), one of the main leakage mechanisms in FETs especially access transistors, is evaluated for the first time for 2D FETs. The results establish the advantages of certain 2D semiconductors in greatly reducing GIDL and thereby support use of such materials in future memory technologies.The dissertation concludes with a vision for how a smart life can be realized in the future by harnessing the capabilities of various 2D technologies in the era of IoT and Industry 4.0.[1] J. Kang, D. Sarkar, W. Liu, D. Jena, and K. Banerjee, “A computational study of metal-contacts to beyond-graphene 2D semiconductor materials,” in IEEE International Electron Devices Meeting, 2012, pp. 407–410.[2] J. Kang, W. Liu, D. Sarkar, D. Jena, and K. Banerjee, “Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors,” Phys. Rev. X, vol. 4, no. 3, p. 31005, Jul. 2014.[3] J. Kang, D. Sarkar, Y. Khatami, and K. Banerjee, “Proposal for all-graphene monolithic logic circuits,” Appl. Phys. Lett., vol. 103, no. 8, p. 83113, 2013.[4] A. Allain, J. Kang, K. Banerjee, and A. Kis, “Electrical contacts to two-dimensional semiconductors,” Nat. Mater., vol. 14, no. 12, pp. 1195–1205, 2015.[5] W. Liu et al., “High-performance few-layer-MoS2 field-effect-transistor with record low contact-resistance,” in IEEE International Electron Devices Meeting, 2013, pp. 499–502.[6] J. Kang, W. Liu, and K. Banerjee, “High-performance MoS2 transistors with low-resistance molybdenum contacts,” Appl. Phys. Lett., vol. 104, no. 9, p. 93106, Mar. 2014.[7] W. Liu, J. Kang, D. Sarkar, Y. Khatami, D. Jena, and K. Banerjee, “Role of metal contacts in designing high-performance monolayer n-type WSe2 field effect transistors.,” Nano Lett., vol. 13, no. 5, pp. 1983–90, May 2013.[8] J. Kang, W. Liu, and K. Banerjee, “Computational Study of Interfaces between 2D MoS2 and Surroundings,” in 45th IEEE Semiconductor Interface Specialists Conference, 2014.[9] D. Sarkar et al., “A subthermionic tunnel field-effect transistor with an atomically thin channel,” Nature, vol. 526, no. 7571, pp. 91–95, Sep. 2015.[10] Y. Khatami, W. Liu, J. Kang, and K. Banerjee, “Prospects of graphene electrodes in photovoltaics,” in Proceedings of SPIE, 2013, vol. 8824, p. 88240T–88240T–6.[11] D. Sarkar et al., “Functionalization of Transition Metal Dichalcogenides with Metallic Nanoparticles: Implications for Doping and Gas-Sensing,” Nano Lett., vol. 15, no. 5, pp. 2852–2862, May 2015.[12] W. Liu, J. Kang, and K. Banerjee, “Characterization of FeCl3 intercalation doped CVD few-layer graphene,” IEEE Electron Device Lett., vol. 37, no. 9, pp. 1246–1249, Sep. 2016.[13] S. Lei et al., “Surface functionalization of two-dimensional metal chalcogenides by Lewis acid–base chemistry,” Nat. Nanotechnol., vol. 11, no. 5, pp. 465–471, Feb. 2016.[14] J. Li, J. Kang, Q. Cai, W. Hong, C. Jian, and W. Liu, “Boosting Hydrogen Evolution Performance of MoS2 by Band Structure Engineering,” Adv. Mater. Interfaces, vol. 1700303, 2017.[15] J. Jiang et al., “Intercalation doped multilayer-graphene-nanoribbons for next-generation interconnects,” Nano Lett., vol. 17, no. 3, pp. 1482–1488, Mar. 2017.[16] J. Jiang, J. Kang, and K. Banerjee, “Characterization of Self - Heating and Current - Carrying Capacity of Intercalation Doped Graphene - Nanoribbon Interconnects,” in IEEE International Reliability Physics Symposium, 2017, p. 6B.1.1-6B.1.6.[17] X. Li et al., “Graphene inductors for high-frequency applications - design, fabrication, characterization, and study of skin effect,” in IEEE International Electron Devices Meeting, 2014, p. 5.4.1-5.4.4.[18] J. Kang et al., under review.[19] J. Kang et al., under review