www.c2s2.org
1 2 3
MSD Annual Review May 1-2, 2012
Task 6.1: Atomistic Simulation of Graphene Transistors
1S. Kim, 2M. Luisier, 3T. B. Boykin, 1J. Geng, 1J. Fonseca, 1G. Klimeck1Purdue University, 2ETH Zürich, 3University of Alabama in
Huntsville
NEMO5- https://engineering.purdue.edu/gek
cogrp/software-projects/nemo5/
S D
G
GS D
Bulk
Graphene
Efield
Excellent high field transport/mobility
Problem: No bandgap
http://en.wikipedia.org/wiki/Graphene
Akturk, JAP2008Shishir, JPCM2009
Wang, PRL2008
Experimental realization
Confinement
Betti, ITED2011
Simulation (pz-TB)
• Experimental realization of GNRFETs• Edge roughness is very important at small width (w<2.5 nm)
Why Graphene?
Graphene Nanoribbon
Graphene Nanoribbon Transistors
pz vs p/d Tight-binding Model
Bandgap Id-Vgs Characteristics
p/d better match with DFTpz-wrong bandgap
pz-wrong off-current
OMEN: Boykin, et al., JAP2011
Edge Roughness and Hydrogen Passivation Roughness
• Atomistic study of edge roughness and hydrogen passivation roughness
• Reproducing experimentally possible situation
inv
1
eff
1
qNdL
dR
densityelectron :
length scattering:
inv
eff
N
L
d
d
I
VR
Edge roughness
Hydrogen passivation roughness
S D
Vd
Id
S D
Vd
Id
CH
Mobility vs Experiment
• Edge roughness limited mobility much smaller than hydrogen passivation limited mobllity
Experiment: Wang, PRL2008
Hydro. Pass.
Edge roughness
n~ 0.95x1013/cm2
2
Bandstructure Effects
Hydrogen Passivation Roughness Edge Roughness
Second subband
First subband
Ef Ef
AGNR-13 AGNR-12
P=50 %
CH
CH
DIBL suppressed
Graphene Nanomesh
Experiment: Liang et al., NanoLett 2010
NEMO5 Simulation Structure
33 nm
138x138 uc33 nm
Bandgap vs. Neckwidth
Graphene
Bandgap BandgapZero bandgap
w = 26 nm w = 19 nm w = 11 nm
Bandgap
Flat bands are ignored in bandgap calculation
(crieterion: dE/dk<0.53 eV )
w
Bandgap Comparison with Experiment
• Trend of experimental data captured
• Overestimation of bangap at a small neck width < 10 nm
Effects of Edge States
• Bandgap uncertainty due to edge roughness
• Electron Localization
Eg
Edge states
D=24 nm
w=9.7 nm
Γ Γ
Edge states criterion: dE/dk<0.53 eV
o Conclusion
• Importance of p/d model in graphene modeling
• Significant effects of edge roughness on electron mobility via bandstructure modification in GNRs
• Relatively less effective hydrogen passivation effects in GNRs
• GNM bandgap prediction through NEMO5 simulation
o Future Work
• GNR width dependent mobility and ON/OFF-current
• GNM effects of edge roughness, different shape of holes
• GNM transmission/mobility calculation
• GNR, GNM self-consistent transport simulation
A
A
Conclusion / Future Work
pz vs p/d Tight-binding Model
• p/d model necessary to reproduce the asymmetry at Dirac point
Graphene Bandstructure
OMEN- https://engineering.purdue.edu/gek
cogrp/software-projects/omen/
OMEN: Boykin, et al., JAP2011
