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GRAND Perspectives of graphene electronics Heinrich Kurz Advanced Microelectronic Center Aachen, AMO GmbH Institute of Semiconductor Electronics at RWTH Aachen University [email protected] www.amo.de Evolution vs. Revolution of Moore Organic Computing SETs Spintronics Beyond CMOS Quantenrechner More than Moore performance diversification - MEMS - sensors - RF - System-on-Chip - power electronics - polymer 22 nm More Moore 32 nm DESIGN 45 nm 65 nm - ballistic transport - photonics - C-interconnects -THz-transistor - non-silicon -graphene -parallel processing 90 nm 2 Hype Cycles - Gartner Case Study Nanotechnology Valid for any highly competitive technology field Positive Hype JT H3 N2B NNI NTD Negative Hype MA+ Take-Off MA SINE ST M 3 The essence of Graphene Single-layer Graphene Bilayer Graphene H Hamiltonian (Energy), m* effective mass, p momentum vF Fermi-velocity, c speed of light, σ Pauli matrices from A.K. Geim, Science 324,1530 (2009) 4 Topgate Passivation Dielectric Graphene Contacts Dielectric (SiO2) Backgate (Si) Interface SiO2 – Graphene Interface traps Surface termination Bonding Surface roughness water → mobility degradation, doping Carbon and CMOS: 4 - layer problem Passivation • Exclusion of atmospheric influences Dielectric • Same as for bottom SiO2 • Dielectric constant k • Leakage current • Electrotransport through dielectric Contacts • Contact resistance • Metall induced doping • Fermi level shift Graphene • Relaxed, unrelaxed, strain • p,n puddles • Edge orientation/termination • Unintentional hydrogenation (e.g. HSQ) • Influence of oxygen during etching 5 GRAND: Overall objectives Explore the potential of graphene for ICT Fabrication and simulation of switches (RF, FET, TFET, Sensors) and interconnects at the nanoscale. Can graphene fulfil its promise of taking CMOS to the “Beyond CMOS” era? 6 GRAND partners Simulation Device fabrication Coordinator Graphene based FETs and interconnects Simulation Graphene Synthesis Functionalization Characterization 7 GRAND: Overall objectives Advantages of graphene based devices • • • • High carrier mobility (>10.000 cm²/Vs at RT) High current carrying capability (>108 A/cm²) Ultimately thin, ultimate incarnation of the surfaces p- and n-type behavior nearly symmetric Challenges for realizing graphene based FETs Introducing a band gap (and preserve carrier mobility) (target: Ion/Ioff>104) Wafer-scale synthesis of graphene 8 Graphene FETs Three concepts for introducing a band gap: • Graphene nanoribbons • Bilayer Graphene with I electric field • Doping of graphene (replacing C-atoms) Experimentally realized yet 9 Graphene nanoribbon FETs 10 Graphene nanoribbons Simulation (TB) of perfect 3.3 nm GNR FETs Na = 28 W = 3.3 nm (Eg = 0.41 eV) doxid = 2 nm ION/IOFF > 104 can be achieved with W = 3.3 nm 11 R. Grassi et al., J. Comp. Elect. 8, 441 (2009) Graphene nanoribbons GNR fabricated by lithography 305K 9K 100M CBR Back-Gate W ~ 20nm R () Source 10M UCF 1M Drain 100k -30 -20 -10 0 10 20 30 Vg(V) Resistance increases with decreasing T: Energy gap! 12 AMO unpublished Graphene nanoribbons Pro: • On/off ratios >104 achievable for w < 4 nm Contra: • Mobility limited to values < 1.000 cm²/Vs • Edge-roughness dominates transport in devices • Width must be controlled with atomic precision 13 Bilayer Graphene FETs 14 Bilayer FET Symmetry breaking by vertical E-field introduces gap in bilayer graphene: E-field Two gates required to vary EG and EF 15 Bilayer FET Consequences for band gap in bilayer graphene: Always a conducting path; band gap not visible in transport. 16 Scanning gate microscopy Allows local mapping of charge neutrality point 17 M.R. Connolly et al., APL 96, 113501 (2010) Scanning gate microscopy Variation of the CNP in a graphene flake CNP at gm/G=0 18 M.R. Connolly et al., APL 96, 113501 (2010) Bilayer FET Possible solutions: Patterning to w<~200 nm Realized by CMOS compatible processes Reducing inhomogenities e.g. by functionalization 19 Bilayer FET Bilayer graphene FET w = 50 nm, l = 200 nm 20 B.N. Szafranek et al., APL 96, 112103 (2010) Bilayer FET Transfer-Characteristics Wolpertinger 1.2 Channel: w = 50 nm l = 200 nm 25 nm SiOx Dmax / ε0 = 1.6 V/nm at UBG = 40V and UTG = -4.5V B.N. Szafranek et al., APL 96, 112103 (2010) 21 Bilayer FET Characteristic parameters at RT Wolpertinger 1.2 Bilayer FET 7 nm GNR1 6 nm GNR1 On/off ratio 80 12 ~150 EG* 50 meV ??? ??? Slope 2 V/dec 40 V/dec ~10 V/dec Mobility 1.000 cm²/Vs 1.000 cm²/Vs 120 cm²/Vs [1] Jiao et al., Nature 458, 877 (April 2009) * EG determined by RDP – D relation 22 Bilayer FET Bilayer graphene TFET (TB-Simulations) tBG = tTG = 3 nm (SiO2) Emax = 1.3 V/nm Small slope switches possible with bilayer graphene 23 G. Fiori et al., IEEE Device Lett. 30, 1096 (2009) Bilayer FET Pro: • On/off ratios >104 possible with TFETs • Mobility of ~1.000 cm²/Vs already achieved Mobility > 5.000cm²/Vs possible Contra: • Device fabrication is more complex 24 Functionalization Route for realizing bilayer tunnel FETs: • Further reduction of charge inhomogenities • Advanced dielectric deposition • Controlled doping Functionalization by Tyndall 25 Functionalization Self assembled monolayer for functionalization of graphene 26 Tyndall to be published Functionalization Functionalization reduces inhomogenities without annealing 5000 4000 RXX ( 295 K N2 atmo Graphene Monolayer Not annealed! Diaminodecan functinalized not functionalized 3000 2000 1000 0 -30 -20 -10 0 10 20 30 VBG (V) Confirmation by scanning gate microscopy needed. 27 Tyndall to be published Summary Routes for realizing graphene based FETs explored theoretically and experimentally: • Graphene nanoribbon FETs show promising on/off ratios for w<4 nm. Draw backs: Low mobility and currently not realizable. • Bilayer tunneling FETs are a promising route for lowpower application. Advantages: Experimentally already realizable and high mobility. 28 Thanks to C.G. Smith M.R. Connolly D. Neumaier B.N. Szafranek D. Schall M. Baus Bologna G. Baccarani A. Gnudi E. Sangiorgi S. Reggiani S. Roche T. Poiroux F.Triozon Pisa M. Macucci G. Iannaccone G. Fiori Udine L. Selmi P. Palestri D. Esseni M. Bresciani A. Quinn B. Long M. Manning G. Visemberga 29