THEORETICAL STUDY OF CARBON-BASED MATERIALS AND THEIR APPLICATIONS IN NANOELECTRONICS KAI-TAK LAM NATIONAL UNIVERSITY OF SINGAPORE 2011 THEORETICAL STUDY OF CARBON-BASED MATERIALS AND THEIR APPLICATIONS IN NANOELECTRONICS KAI-TAK LAM A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILIOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements I would like to take this opportunity to thank my Ph.D supervisor, Assistant Professor Liang Gengchiau for his guidance and support throughout my graduate study at NUS I am immensely indebt to his willingness to share his knowledge and I greatly appreciate the intellectual freedom given in pursuing my research interests Without his careful supervision, constructive feedback and constant encouragement, much of the works in this thesis would not have been possible I am also grateful to Associate Professor Ganesh S Samudra and Assistant Professor Yeo Yee Chia for their advices and insightful discussions during my course of study In addition, I would like to express my gratitude toward Dr Chin Sai Kong from the Institute of High Performance Computing for his help and guidance during our collaborations Next, I would like to thank my fellow course mates and colleagues for their assistance and friendship during my Ph.D candidature They have made my graduate studies an enjoyable and memorable experience in my life In particular, thanks go out to Dr S Bala Kumar, Dr Da Haixia, Huang Wen, Qian You and many others for their valuable inputs and discussions I would also like to thank the Outreach group of the Department of Electrical and Computer Engineering for the opportunities in promoting science and engineering to the secondary and pre-university students and for the exposure to the works of fellow researchers in NUS Lastly, I would like to extend my heartfelt appreciation to my parents for their unwavering faith in me and to my sister, Jessica Lam Ching Yee for her constant encouragement Without their understanding and support, my graduate career would not have been possible i Table of content Acknowledgements i Table of content ii Abstract vii List of Figures ix List of Symbols xix Chapter Introduction 1.1 Why carbon? 1.2 Objectives of research 1.3 Thesis organisation 1.4 References Chapter Methodology 11 2.1 Density functional theory 11 2.2 pz-orbital tight binding method 13 2.3 Dirac tight binding method 17 2.4 Non-equilibrium Green’s function formalism 19 2.4.1 Ballistic limits 19 2.4.2 Phonon scattering 21 2.5 Summary 24 2.6 References 25 Chapter Material properties of graphene-based materials 27 3.1 Electronic structure of monolayer graphene nanoribbon 27 ii 3.1.1 Armchair edges 27 3.1.2 Zigzag edges 29 3.1.3 Dopant effect 30 3.2 Electronic structure of bilayer graphene nanoribbon 31 3.2.1 Armchair edges 32 3.2.2 Zigzag edges with dopants 33 3.2.3 Effect of changing interlayer distance on energy band gap 34 3.3 Stability and electronic structure of two dimensional Cx(BN)y compound 35 3.3.1 Introduction 35 3.3.2 Simulation model 37 3.3.3 Results and discussions 39 3.4 Summary 42 3.5 References 44 Chapter Schottky barrier field-effect transistors 47 4.1 Introduction 47 4.2 Simulation setup 49 4.3 Results and discussions 50 4.3.1 AGNR SBFETs 50 4.3.2 ZGNR SBFETs 51 4.3.3 Device performance comparison 52 4.3.4 Bilayer devices 53 iii 4.4 Summary 54 4.5 References 55 Chapter Bilayer graphene nanoribbon nanoelectromechanical system 57 5.1 Introduction 57 5.2 Operating principle 60 5.3 Parallel plate actuator floating gate design 63 5.4 Design evaluation 64 5.4.1 Capacitive parallel plate actuator 65 5.4.2 Electrostatic repulsive force actuator 68 5.5 Summary 73 5.6 References 74 Chapter Resonant tunnelling diode 78 6.1 Shape effects in graphene nanoribbon resonant tunneling diodes 79 6.1.1 Introduction 79 6.1.2 Simulation approaches 82 6.1.2 Results and discussion 83 6.2 Influence of edge roughness on graphene nanoribbon resonant tunnelling diodes 91 6.2.1 Introduction 91 6.2.2 Simulation approaches 93 6.2.3 Results and discussions 94 6.3 Summary 100 iv 6.4 References 102 Chapter Tunnelling field-effect transistor 106 7.1 Tunneling FET with heterojunction channel 106 7.1.1 Introduction 106 7.1.2 Simulation approachs 108 7.1.3 Results and discussions 109 7.2 Electrostatics of ultimately-thin body tunneling FET 113 7.2.1 Introduction 113 7.2.2 Simulation approaches 115 7.2.3 Results and discussions 115 7.3 Device performance of GNR MOSFET and tunneling FET with phonon scattering 120 7.3.1 Introduction 120 7.3.2 Simulation approaches 121 7.3.3 Results and discussions 122 7.4 Summary 126 7.5 References 127 Chapter Suggestions for future work 132 8.1 Bilayer graphene nanoribbon 132 8.2 GNR Schottky barrier field-effect transistors 133 8.3 High frequency applications 135 8.4 References 136 v Appendix I Appendix A: Derivation of Dirac equation for 2D graphene I Appendix B: Dirac Hamiltonian for GNR III Appendix C: List of publications IV vi Abstract Continual scaling down of silicon device, which is the main driving force in device performance enhancement, is not sustainable as we approach the physical limits of silicon and it is foreseen that new materials and novel device structures will be required for future device improvements In this regards, research in carbon electronics has been intensified since 2004 due to the physical realization of thermodynamically stable planar graphene Two-dimensional monolayer graphene sheets have unique electrical and physical properties which can be exploited in new device structures However, due to its semi-metallic nature, much focus has been given to converting graphene based materials into semi-conducting material, such as applying a perpendicular electric field to a bilayer graphene and impurity adsorption on the graphene surface A more commonly studied method involves cutting twodimensional graphene sheets into one-dimensional narrow ribbons, i.e graphene nanoribbons (GNRs), where the quantum confinement introduced by the physical edges generate an energy bandgap that is closely related to the width and edge configurations of the ribbon Such semi-conducting GNRs can be relatively easy to integrate into existing device structures and the unique electronic properties can be used in new device applications Both experimental and theoretical studies have been carried out extensively on integrating GNRs into existing device technologies such as metal-oxidesemiconductor field-effect transistors In addition, bilayer GNRs, which combine the unique electrical properties of GNRs and bilayer graphene, show great potential as versatile materials which can enable new device designs that take advantage of tuneable energy bandgap such as nanoelectromechanical devices Recent development vii in obtaining GNRs by unzipping carbon nanotubes has made the prospect of fabricating GNR-based electronic devices in large quantities more promising and hence, detailed understanding of the device physics of GNR-based devices are much needed This thesis, therefore, summarizes the investigation of the electronic structures of GNRs, both monolayer and bilayer, and materials with graphene-like atomic structure such as boron-nitride-carbon (B-N-C) compound In addition, potential devices that can be implemented with these materials are also studied in details Using various methods for the calculation of the electronic structure of the material, such as density functional theory, π-orbital tight-binding model and the Dirac equation model and utilizing the general non-equilibrium Green’s function approach to simulate the electron transport for device evaluations, with the inclusion of acoustic and optical phonon scattering, the performance of various devices such as Schottky Barrier fieldeffect transistors (FET), nanoelectromechanical switches, resonant tunnelling dioides and the effects of heterojunction, fringing field, and phonon scattering on tunneling FET based on GNRs are evaluated This exploration on the device physics and performance of carbon electronics serves to enhance the knowledge for post-silicon device investigations viii [25] X Li, X Wang, L Zhang, S Lee, H Dai, “Chemically derived, ultrasmooth graphene nanoribbon semiconductors,” Science 319 (5867), 1229 (February 2008) [26] L Jiao, L Zhang, X Wang, G Diankov, and H Dai, “Narrow graphene nanoribbons from carbon nanotubes,” Nature 458, 877 (April 2009) [27] J Cai, P Ruffieux, R Jaafar, M Bieri, T Braun, S Blankenburg, M Muoth, A.P Seitsonen, M Saleh, X Feng, K Müllen, and R Fasel, “Atomically precise bottom-up fabrication of graphene nanoribbons,” Nature 466, 470 (July 2010) [28] S O Koswatta, M S Lundstrom, and D E Nikonov, “Performance comparison between p-i-n tunneling transistors and conventional MOSFETs,” IEEE Trans Elec Dev 56 (3), 456 (March 2009) [29] Z Ren, “Nanoscale MOSFETs: Physics, Simulation and Design,” (2006) [Online: http://nanohub.org/resources/1917] [30] P Shemella and S K Nayak, “Electronic structure and band-gap modulation of graphene via substrate surface chemistry,” Appl Phys Lett 94 (3), 032101 (January 2009) [31] C Shen, L.T Yang, E.-H Toh, C.-H Heng, G.S Samudra, and Y.-C Yeo, “A new robust non-local algorithm for band-to-band tunneling simulation and its application to Tunnel-FET,” VLSI-TSA ’09, 113 (April 2009) [32] S.M Sze and K.K Ng, Physics of Semiconductor, pp 340, Hoboken, NJ: John Wiley & Sons, Inc (2007) [33] To isolate the fringing field effect, the quantum confinement effect due to the extremely small tbody was not accounted for in this simulation [34] C Anghel, P Chilagani, A Amara, and A Vladimirescu, “Tunnel field effect transistor with increased ON current, low-k spacer and high-k dielectric,” Appl Phys Lett 96 (12), 122104 (March 2010) [35] G Liang, N Neophytou, D E Nikonov, M S Lundstrom, “Performance Projections for Ballistic Graphene Nanoribbon Field-Effect Transistors,” IEEE Trans Electron Device 54 (4), 677 (April 2007) 130 [36] G Fiori and G Iannaccone, “Simulation of graphene nanoribbon field-effect transistors,” IEEE Electron Device Lett 28 (8), 760 (August 2007) [37] K.-T Lam, Y Yang, G S Samudra, Y.-C Yeo and G Liang, “Electrostatics of ultimately thin-body tunnelling FET using graphene nanoribbon,” IEEE Electron Device Lett 32 (4), 431 (April 2011) [38] Y Ouyang, X Wang, H Dai and J Guo, “Carrier scattering in graphene nanoribbon field-effect transistors,” Appl Phys Lett 92 (24), 243124 (June 2008) [39] X Wang, Y Ouyang, X Li, H Wang, J Guo, and H Dai, “Room-Temperature All-Semiconducting Sub-10-nm Graphene Nanoribbon Field-Effect Transistors,” Phys Rev Lett 100 (20), 206803 (May 2008) [40] M P Levendorf, C S Ruiz-Vargas, S Garg, and J Park, “Transfer-Free Batch Fabrication of Single Layer Graphene Transistos,” Nano Lett (12), 4479 (October 2009) 131 Chapter Suggestions for future work Finally, there are a number of interesting topics that can be extended from the works presented here The following sections introduce some possible directions of these investigations 8.1 Bilayer graphene nanoribbon In Chapter 3, the material properties of monolayer and bilayer GNR are investigated based on edge configurations, effect of dopants, ribbon widths and for the bilayer GNR, the effect of interlayer distance It is interesting to further study the bilayer GNR structure with misalignment, including the cases for monolayers of different widths and different stacking order In recent articles [1], [2], GNRs can be obtained via the ‘unzipping’ of carbon nanotube (CNT) along the length of the tube by chemical means Both single- and double-walled CNT are used in the experiment and hence monolayer and bilayer GNR can be obtained selectively However, as the double-walled CNT consists of concentric CNTs with different diameters, the bilayer GNR obtain is different from the structure we have studied earlier, i.e the monolayers would be misaligned There can be two types of misalignments: (i) the monolayers are of different widths, and (ii) the two layers are rotated by an angle θ Since the electrical properties of graphene materials are very sensitive to the geometry of the structure, it is of great interest to know about how these misalignments would affect the electrical properties of bilayer GNR and subsequently the device performances of such bilayer GNR devices Furthermore, in Chapter 5, the dependency of electrical property on the interlayer distance of bilayer GNR is exploited for the operation of a NEM switch and 132 various designs are discussed In a similar approach, the misalignment of the top and bottom layers of bilayer GNR can also be utlizied to enable different forms of NEMS devices Instead of varying the interlayer distance, a displacement sensor can be fabricated making use of the change in the degree of misalignment between the two layers, which leads to a change in the conductance of the channel material (a) (b) (d) (c) Fig 8-1 (a) Narrow width GNR can be obtained by ‘unzipping’ CNT Depending on the chirality of the tubes, different configurations of bilayer GNR, such as (b) one with different width AGNRs and (c) one with AGNR and ZGNR can be fabricated (d) The electrical properties of the material may be changed as the top layer is shifted with respects to the bottom layer, which can be utilized for NEMS applications 8.2 GNR Schottky barrier field-effect transistors In Chapter 4, a comparison of SBFETs with AGNR and ZGNR as the channel material is made based on the density functional theory and it was observed that the AGNR SBFETs provide a larger ION/IOFF ratio with a much lower leakage current and a similar ION than the ZGNR counterpart Further examination of AGNR SBFET reveals two additional geometrical shapes which result in a bending of the ribbon at 90 and 120 degrees, as shown in Fig 8-2(b) and (c), respectively These AGNR SBFETs have the same contacts (7-ZGNR) and channel (10-AGNR) but with different connecting junctions Therefore, it would be interesting to investigate how these connecting junctions affect the device performances of the AGNR SBFETs 133 Further modifications to these connecting junctions such as trimming of the corners and addition of impurities can also be incorporated in the study In addition, recent fabrication technique of obtaining GNRs from the ‘unzipping’ of carbon nanotube presents the possibility of realising CNT-GNR heterojunctions As the atomic structures of such heterojunctions are not yet fully studied and, unlike GNR heterojunctions, are highly complex due to the curvatures of the CNT, atomistic investigations are required to acertain the stable atomic structures and their effects on the electronic properties After which, the transport properties of such junctions can also be investigated for possible device implementations Fig 8-2 Possible atomic structures of AGNR SBFETs where the ZGNR and AGNR form (a) 30, (b) 90 and (c) 120 degrees with each other The transmission spectrum of these devices would be depenent on the connecting junctions (orange regions) (d) Connection between CNT and GNR with a connecting region which would have an interesting effect on the transport properties of such heterojunction 134 8.3 High frequency applications In Chapter 7, the digital performances of GNR tunnelling FETs are explored and it was shown that GNR TFETs can both provide a relatively high ON-state current and a small SS, even in the presence of phonon scattering We have also shown that heterjunction in the channel near the source-channel interface helps in further increasing the ON-state current and recent studies using a combination of semi-metallic carbon nanotube and GNR as channel indicate similar effects [3], [4] While our investigation focuses on the ballistic limits of the GNR HJ TFETs, the effect of phonon scattering is highly relevant due to the present of quantum states in the HJ region which are sensitive to phonon-electron interactions Concurrently, recent studies [5], [6] suggested that carbon-based materials would be suitable for high frequency applications As such, it would be interesting to investigate the high frequency response of GNR TFETs, as well as other device structures Given the current steady state simulator, the transconductance and drain conductance can be obtained, which can be used to extract the operation frequency and gain of the device The effect of phonon scattering can also be investigated according to different operating conditions Lastly, to accurately capture the transient performance of the devices, timedependent NEGF simulations can be implemented This can verify the operation frequency and the intrinsic delay of the device for both high frequency and digital applications Coupled with phonon scattering calculations, the device physics of the effect of electron-phonon interaction on the speed of device operation can also be investigated 135 8.4 References [1] D V Kosynkin, A L Higginbotham, A Sinitskii, J R Lomeda, A Dimiev, B Katherine Price, and J M Tour, “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons,” Nature 458, 872 (April 2009) [2] L Jiao, L Zhang, X Wang, G Diankov, and H Dai, “Narrow graphene nanoribbons from carbon nanotubes,” Nature 458, 877 (April 2009) [3] Y Yoon and S Salahuddin, “Barrier-free tunnelling in a carbon heterojunction transistor,” Appl Phys Lett 97, 033102 (July 2010) [4] L Leem, A Srivastava, S Li, B Magyari-Köpe, G Iannaccone, J S Harris and G Fiori, “Multi-scale simulations of partially unzipped CNT hetero-junction tunnelling field effect transistor,” IEDM Technical Digest 2010, 32.5 (December 2010) [5] Y.-M Lin, H.-Y Chiu, K A Jenkins, D B Farmer, P Avouris, A ValdesGarcia, “Dual-gate graphene FETs with fT of 50 GHz,” IEEE Electron Device Lett 31 (1), 68 (Jan 2010) [6] Y.-M Lin, A Valdes-Garcia, S.-J Han, D B Farmer, I Meric, Y Sun, Y Wu, C Dimitrakopoulos, A Grill, P Avouris, K A Jenkins, “Wafer-scale graphene integrated circuit,” Science 332 (6035), 1294 (June 2011) 136 Appendix Appendix A: Derivation of Dirac equation for 2D graphene     3  From E  E0  t  cos  k x a  cos  k y a   cos  kya  ,     2      let  k x , k y    k x   x , k y   y  Using cos  A  B   cos A cos B  sin A sin B and small angle approximation (ignore from 3rd order)  3a  9a 2  3a   3a  Since kx=0, cos   k x   x    cos   x      x    x 2      a          cos   k y   y    cos  a k y  cos  a  y   sin  a k y  sin  a  y                       1a   a  a  a   y   cos   1      k y     y  sin  k y        2           4  3a 2   a  3a 2 3a y     1     y   16  y   y  , k y   3a  forK1  2     forK Next using double angle formula for cosine,  a  a    cos  k y     cos     k y      cos a 3k y ,               cos a  k y   y   1  a 3 y    cos  a        3k y  a 3 y sin a 3k y  3a 2  3a 2 3a 4   1  y   a 3 y   y   y  , ky   2 2 3a      forK1  forK I Arranging all the terms together,  9a 2   3a 2 3a  3a 2 3a 1 1 E  E0  t 1  x   y  y   3 2 y  y   2 2    16   3a 2 3a  3a 2 3a 9a 2  1  E0  t  y  y   x     y  y   16 2  16    E0  t 3a 2 9a 2 3a 2  y  3a y   x    y  3a y  4 9a 2 at 3a 2 9a 2 6a 2  E0  t  y  E0  t y  x   x   y2   E0  32 4 4  x   y2  II Appendix B: Dirac Hamiltonian for GNR From Eq (2.3.4):     V ivF   k y     A  x  H  , assuming t D  ivF and          B V  ivF   k y    x    Solving Eq (2.2.1): H  E     V tD   k y     A  x    A     E       B  B V t D   k y      x      EA  VA  t D   k y  B  VA  t D k y B  tD B x  x  B  Bn 1 t t  B  n 1  EA  VA  t D k y B  D Bn 1  D Bn 1 2l0 2l0 2l0 x Similarly,     EB  VB  t D   k y  A  VB  t D k y A  t D A x  x  A  An 1  t t  EB  VB  t D k y A  D An 1  D An 1 A  n 1 x 2l0 2l0 2l0 Hence,  A  t 0   An 1   V E n  D     Bn  2l0 1   Bn 1   t D k y t D k y   An  t D  V   Bn  2l0   0   An 1  1   B     n 1  III Appendix C: List of publications Journal publications [1] K T Lam and G Liang, “An ab initio study on energy gap of bilayer graphene nanoribbons with armchair edges,” Appl Phys Lett 92, 223106 (2008) [2] H Teong, K T Lam , S B Khalid and G Liang, “Shape effects in graphene nanoribbon resonant tunneling diodes: A computational study,” J Appl Phys 105, 084317 (2009) [3] H Teong, K T Lam and G Liang, “A computational study on the device performance of graphene nanoribbon resonant tunneling diodes (GNR RTDs),” Jpn J Appl Phys 48, 04C156 (2009) [4] K T Lam, C Lee and G Liang, “Bilayer graphene nanoribbon nanoelectromechanical system device: A computational study,” Appl Phys Lett 95, 143107 (2009) [5] K T Lam, S K Chin, D W Seah, S Bala Kumar, and G Liang, “Effect of ribbon width and doping concentration on device performance of graphene nanoribbon tunneling field-effect transistors,” Jpn J Appl Phys 49, 04DJ10 (2009) [6] G Liang, S B Khalid, and K T Lam, “Influence of edge roughness on graphene nanoribbon resonant tunneling diodes,” J Phys D: Appl Phys 43, 215101 (2010) [7] Y J Shin, J H Kwon, G Kalon, K T Lam, C S Bhatia, G Liang and H Yang, “Ambipolar bistable switching effect of graphene,” Appl Phys Lett 97, 262105 (2010) [8] K T Lam, D W Seah, S K Chin, S Bala Kumar, G Samudra, Y C Yeo, and G Liang, “Graphene nanoribbon tunneling FET with heterojunction channel,” IEEE Electron Device Letters 31, 555 (2010) [9] S K Chin, D Seah, K.-T Lam, G S Samudra, and G Liang, “Device physics and characteristics of graphene nanoribbon tunnelling FETs,” IEEE Trans Electron Devices 57, 3144 (2010) IV [10] K T Lam, Y Yang, G S Samudra, Y.-C Yeo and G Liang, “Electrostatics of ultimately thin-body tunneling FET using graphene nanoribbon,” IEEE Electron Device Lett 32, 431 (2011) [11] K T Lam, Y Lu, Y P Feng and G Liang, “Stability and electronic structure of two dimensional Cx(BN)y compound,” Appl Phys Lett 98, 022101 (2011) [12] K.-T Lam, M S Leo, C Lee and G Liang, “Design evaluation of graphene nanoribbon nanoelectromechanical devices,” J Appl Phys 110, 024302 (2011) V Conference publications [13] K T Lam and G Liang, A first-principles study on edge doping of armchair graphene nanoribbon, 2nd IEEE International Nanoelectronic Conference 2008, INEC 2008, pp 109-111 (2008) (Oral presentation) [14] K T Lam and G Liang, Electronic Properties of Edge-doped Armchair Graphene Nanoribbon: an ab initio Approach, MRS Spring Meeting 2008 (Poster) [15] K T Lam and G Liang, An Ab Initio Investigation of Energy Bandgap of Monolayer and Bilayer Graphene Nanoribbon Based on Different Basis Sets, 8th IEEE Conference on Nanotechnology 2008, NANO ’08, pp 409-411 (2008) (Poster) [16] G Liang, H Teong, K T Lam, N Neophytou and D E Nikonov, Graphene Nanoribbon Transistors and Resonant Tunneling Diodes, 2008 International Conference on Solid-State Devices and Materials, SSDM 2008 (Oral presentation) [17] K T Lam and G Liang, A First Principle Study of Bilayer Graphene Nanoribbon Devices, The 2008 Asian Conference on Nanoscience and Nanotechnology, AsiaNANO2008 (Poster) [18] G Liang, H Teong, K T Lam, Possible Electronic Device Applications of Graphene Nanoribbons, The 2008 Asian Conference on Nanoscience and Nanotechnology, AsiaNANO2008 (Oral presentation) [19] G Liang, H Teong, K T Lam, Computational study of Graphene Nanoribbon Resonant Tunneling Diodes, 13th International Workshop on Computational Electronics, IWCE 2009 (Oral presentation) [20] Z Y Leong, K T Lam and G Liang, Device Performance of Graphene Nanoribbon Field Effect Transistors with Edge Roughness Effects: A Computational Study, 13th International Workshop on Computational Electronics, IWCE 2009 (Oral presentation) [21] K T Lam and G Liang, Computational Study on the Performance of Monolayer and Bilayer Graphene Nanoribbon Devices, 13th International Workshop on Computational Electronics, IWCE 2009 (Oral presentation) VI [22] K T Lam and G Liang, Computational Study of Nanoelectromechanical Device Using Bilayer Graphene Nanoribbon, International Conference on Materials for Advanced Technologies, ICMAT 2009 (Oral presentation) [23] K T Lam, Y Z Peck, C Lee and G Liang, Graphene Nanoribbon SchottkyBarrier Field Effect Transistor and its Application as a Nanoelectromechanical Device, 9th International Conference on Nanotechnology, IEEE Nano 2009 (Oral presentation) [24] S B Khalid, K T Lam and G Liang, Computational Study of Edge Roughness Effect on the Device Performance of Graphene Nanoribbon Resonant Tunneling Diodes, 2009 International Conference on Solid State Devices and Material, SSDM 2009 (Oral presentation) [25] K T Lam, S Bala Kumar, S K Chin, D W Seah and G Liang, Performance Evaluation of Graphene Nanoribbon Tunneling Field Effect Transistors, 2009 International Conference on Solid State Devices and Material, SSDM 2009 (Oral presentation) [26] K T Lam and G Liang, A Computational Evaluation of the Designs of a Novel Nanoelectromechanical Switch Based on Bilayer Graphene Nanoribbon, 2009 International Electron Devices Meeting, IEDM 2009 (Oral presentation) [27] K T Lam, H Da, S K Chin, G S Samudra, Y C Yeo and G Liang, A computational study on the device performance of graphene nanoribbon heterojunction tunneling FETs based on bandgap engineering, 2010 Device Research Conference, DRC 2010 (Oral presentation) [28] H Da, K.-T Lam, S K Chin, G S Samudra, Y.-C Yeo and G Liang, Performance evaluation of graphene nanoribbon heterojunction tunneling field effect transistors with various source/drain doping concentrations and heterojunction structure, 2010 International Conference on Solid State Devices and Material, SSDM 2010 (Oral presentation) [29] H Da, K T Lam, S K Chin, G S Samudra and G Liang, Source/Drain doping influence on heterojunction graphene nanoribbon tunneling field effect transistors, 4th IEEE International NanoElectronics Conference, INEC 2011 (Oral presentation) [30] K T Lam, Y Z Peck, Z H Lim and G Liang, Performance comparison of armchair-edged and nitrogen-doped zigzag-edged graphene nanoribbon Schottky barrier field-effect transistors, 4th IEEE International NanoElectronics Conference, INEC 2011 (Oral presentation) VII [31] G Liang, S.-K Chin, K.-T Lam and D W Seah, Quantum transport simulations of graphene based devices using Dirac Hamiltonian calibrated with π-oribtal tight binding approach, 4th IEEE International NanoElectronics Conference, INEC 2011 (Oral presentation) [32] K.-T Lam, S.-K Chin and G Liang, Device performance of graphene nanoribbon MOSFET and Tunneling FET with phonon scattering: A computational study, 2011 International Conference on Solid State Devices and Material, SSDM 2011 (Oral presentation) [33] V P Sreenivas, K T Lam and G Liang, RF performance of graphene nanoribbon MOSFET vs TFET, 2011 International Conference on Solid State Devices and Material, SSDM 2011 (Oral presentation) VIII .. .THEORETICAL STUDY OF CARBON- BASED MATERIALS AND THEIR APPLICATIONS IN NANOELECTRONICS KAI-TAK LAM A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILIOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER... vii in obtaining GNRs by unzipping carbon nanotubes has made the prospect of fabricating GNR -based electronic devices in large quantities more promising and hence, detailed understanding of the... Outreach group of the Department of Electrical and Computer Engineering for the opportunities in promoting science and engineering to the secondary and pre-university students and for the exposure