Scaling of the performance of carbon nanotube transistors 1 Institute of Applied Physics, University...
28
Scaling of the performance of carbon nanotube transistors 1 Institute of Applied Physics, University of Hamburg, Germany 2 Novel Device Group, Intel Corporation, Hillsboro, OR 3 IBM Research Division, TJ Watson Research Center, Yorktown Heights, NY • Why carbon nanotube transistors? • Evidence for Schottky barriers • Carbon nanotube Schottky barrier transistors • Gas adsorption versus doping • Scaling of transistor performance • New device designs & capabilities • Conclusions S. Heinze 1 , M. Radosavljević 2 , J. Tersoff 3 , and Ph. Avouris 3
-
Upload
isabella-ford -
Category
Documents
-
view
216 -
download
2
Transcript of Scaling of the performance of carbon nanotube transistors 1 Institute of Applied Physics, University...
Scaling of the performance ofcarbon nanotube transistors
1 Institute of Applied Physics, University of Hamburg, Germany2 Novel Device Group, Intel Corporation, Hillsboro, OR
3 IBM Research Division, TJ Watson Research Center, Yorktown Heights, NY
• Why carbon nanotube transistors?• Evidence for Schottky barriers• Carbon nanotube Schottky barrier transistors• Gas adsorption versus doping• Scaling of transistor performance• New device designs & capabilities• Conclusions
S. Heinze1, M. Radosavljević2, J. Tersoff3, and Ph. Avouris3
Carbon nanotube field-effect transistors comparable with Si MOS-FETs
Nanotube FETs with top gates: Nanotube FETs with top gates: • turn-on gate voltage is about 1Vturn-on gate voltage is about 1V
S. J. Wind et al., Appl. Phys. Lett. 80, 3817 (2002).
260nm CNFET
26015~ 0.52100
1501302321
50nm SOI MOSFET
501.5~ -0.2650
970650
Gate length (nm)Gate oxide thickness (nm)Vt (V)ION (A/m)(Vds = Vgs-Vt ~ -1V)IOFF (nA/m)Subthreshold slope (mV/dec)Transconductance (S/m)
(all p-type)
• favorable device characteristicsfavorable device characteristics
Vtip = -2V
map transport current as a function of moving, charged AFM tip
current increase when gating the source junction barrier thinning.
(a)
(b)
M. Freitag et al., Appl. Phys. Lett. 79, 3326 (2001).
Evidence for Schottky barriers:scanned gate microscopy at contacts
Evidence for Schottky barriers:ambipolar conduction in SWNTs
R. Martel et al., PRL 87, 256805 (2001).
Bottom gate CNFETs with Ti contacts annealed;conversion from p-type to ambipolar conductance
Evidence for Schottky barriers: Influence of the contacts for CNFETs
NT
Vg
Vs = 0
Vd
-500
-400
-300
-200
-100
0-1.5-1.0-0.50.0
Drain Voltage [V]
Vg=-1.5V to 0V0.5V steps
Cur
rent
[nA
]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
Cur
rent
[A]
-2.0 -1.0 0.0 1.0
Gate Voltage [V]
Vd=-0.9V to -0.5V0.2V steps
Vd
Vs = 0
L=300nm
tox=5nm
Switching S & D changes:– Slope by factor of 2– ON-state by factor of 5
not due to bulk, it is a contact effect
M.Radosavljević et al.
dNT=1.4nm Eg~0.6eVTypical SBs for NTs ~ 0.3eV
Conventional vs. Schottky barrier FET
Schottky Barrier Transistor
ambipolar Characteristic
Conventional Transistor
p-type Characteristic
Transmission through Schottky barrier
Landauer-Büttikerformula for current:
WKB approximation + single NT band:
1086420Distance from Contact (nm)
Conduction Band
1.00.80.60.40.20.0Transmission (E)
0.3
0.2
0.1
0.0
-0.1
E (
eV)
T(E)*[F(E)-F(E-eVd)] (arb.units)
Self-consistent SB-transistor modelfor needle-like contact
• Cylindrical gate at RGate
• Metal electrode of NT diameter• Analytic electrostatic kernel G• Test of approximations for
Solution by self-consistency cycle
V ( )) )(4
(V gate
RG zz z dzz
( )
V( )() ) )( (cE z
ef F E g Ez e z dE
Charge on thenanotube:
Electrostaticpotential:
NTMetal
Gate
Needle-like contact:conductance vs. gate voltage NT
Metal
Gate
10-9
10-8
10-7
10-6
10-5
10-4
Con
duct
ance
(S
)
-1.0 -0.5 0.0 0.5 1.0
Gate Voltage (V)
50
40
30
20
10
0C
onductance (S
)
holetunneling
electrontunneling
Ideal sharp Metal-NT Contact turn-on voltage ~ Eg/2
Carbon nanotube transistors with planar gates
Calculated NT-potential
• Solve a 2D boundary value problem Vext(x)• Local approximation for potential from NT charge
Electrostatic Potential
10-9
10-8
10-7
10-6
10-5
10-4
Co
nd
uct
ance
(S
)
-20 -10 0 10 20
Top Gate Voltage (V)
Conductance Modulation
Influence of the contact geometry
NTMetal
Gate
PRL 89, 106801 (2002)
Scaled Characteristics
Gas adsorption vs. doping:Experimental observations
V. Derycke et al., APL 80, 2773 (2002).
Doping with Potassium Gas Adsorption (O2)
Increase of Potassium
4
3
2
1
0C
urr
ent
(nA
)
-15 -10 -5 0 5 10 15
Gate Voltage (V)
In Air
Annealed in Vacuum
Increase of O2
Uniform doping:Experiment vs. SB model
Doping with Potassium Needle-Contact Model
NTMetal
Gate
Increase of Potassium
CalculatedDoping Characteristics
n-doped at510-4 e/atom
NTMetal
Gate
Uniform doping of nanotube
n-doped at110-3 e/atom
CalculatedDoping Characteristics
Uniform doping of nanotubeNT
Metal
Gate
Gas adsorption: Experiment vs. SB model
Gas Adsorption (O2) Needle-Contact Model
4
3
2
1
0
Cu
rren
t (n
A)
-15 -10 -5 0 5 10 15
Gate Voltage (V)
In Air
Annealed in Vacuum
Increase of O2
NTMetal
Gate
Gas adsorption:Change in metal workfunction
Metal workfunctionincreased by 0.2eV
Calculated Gas Adsorption Characteristics
NTMetal
Gate
How does the performance of Schottky barrier CNFETs scale?
J. Appenzeller et al., PRL 89, 126801 (2002).
10-13
10-12
10-11
10-10
10-9
10-8
Cu
rren
t (
A)
-1.5 -1.0 -0.5 0.0 0.5 1.0
Gate Voltage (V)
110 mV/dectox=2nm
130 mV/dectox=2nm
170 mV/dectox=5nm
280 mV/dectox=20nm
ultra-thin oxide CNFETs:
Why is the thermal limit of 60 mV/decade not reached?
Scaling law with oxide thickness?
Device geometry
Turn-on vs oxide thickness for bottom gate SB-CNFETs
Vscale ~ sqrt(tox)
Analytic model for thin sheet contact
Source
Gate
Gate
z
Gate
tox
tox
Potential near the Edge:
Analytic model applied to bottom gate SB-CNFETs
Single, empirical factor for bottom gate devices
Scaling of turn-on performance of CNFETs with oxide thickness
Largest improvementsby optimization of thecontact geometry
Analytic Model
PRB 68, 235418 (2003)
120
80
40
0
He
igh
t (n
m)
4003002001000
Length (nm)
Source Drain
Top Electrode
Bottom Gate
Nanotube 0.9
0.4
0.3
0.2
tox
Scaling of drain voltage for ultra-thin oxide CNFETs?
-1.2
-0.9
-0.6
-0.3
0.0
0.3E
ner
gy
(eV
)
4003002001000Position along Nanotube (nm)
So
urc
e
Dra
in
tox=30nm tox=2nm
Vdrain=+0.8V, Vgate=+0.4V
Minimal Current (OFF-current)
rises with lower oxide thickness0.3
0.2
0.1
0.0
-0.1
En
erg
y (e
V)
20151050-5
Distance from Contact (nm)
• independent barriers – one controlled by Vg, the other by Vd–Vg
• identical (and minimal) hole/electron current at Vg = Vd–Vg Vd = 2Vg
120
80
40
0
Hei
gh
t (n
m)
4003002001000
Length (nm)
Source Drain=0.5V
Top Electrode
Bottom Gate=1V 0.9
Nanotube
0.4
0.3 0.2
0.1
Ultra-thin oxide: turn-on voltage ~ Vd
Effect of drain voltage for ultra-thin oxide CNFET
exponential increase of OFF current with Vd
Bottom-gate: tox=2nm
Scaling of drain voltage: model vs. experiment
APL 83, 2435 (2003)
tox=2nm
OFF state problem for transistor light emission device
Infrared light emission from a SWNT: J. Misewich et al., Science 300, 783 (2003).
Asymmetric device design to solve OFF state problem
Symmetric CNFET (tox=2nm) unfavorable OFF state
APL 83, 5038 (2003)
Asymmetric CNFET low OFF current & p- and n-type device for Vd<0 and Vd>0
Conclusions
• Transistor action in CNFETs due to Schottky barriers ambipolar transfer characteristic (I vs Vg)• Nanoscale features of contacts are essential• Gas adsorption modifies band line-up at the contact• Scaling in turn-on regime with sqrt(tox)• Scaling of drain voltage at ultra-thin oxides necessary• New device physics: light emission device• New device designs may be favorable
CN Transistors competetive with Si MOSFETs, however:
