Graphene and new 2D materials:
Opportunities for High Frequencies
applications
April 21th, 2015
H. Happy, E. Pallecchi, B. Plaçais, D. Jiménez, R.
Sordan, D. Neumaier
Graphene Flagship – WP4 HF electronic [email protected]
Outline
•  Graphene FET: State of the art
•  GFET optimization
•  Gate contact
•  Current saturation exploring velocity saturation
•  Ballistic devices
•  Graphene flagship WP4 –High frequency
electronic: Some achievements
•  Summary and outlook
[email protected]
2
Progress in fabrication process
GFET on SiC substrate
Devices on Substrate
Optical Microscope Image
Full Device SEM Image
FIB cross section of the active part of device
3
Study of parametric variation of performances
8 Devices (Number) 7 30
25
20
5 4 3 2 1 10
0 ft_intr
14 21 28 34 41 48 ft_DUT
5
/ (GHz) fmax
0
-5
-10
-15
1
10
frequency (GHz)
M.S. Khenissa et al.; EuMIC 2014 –
Roma – Italy,
doi: 10.1109/EuMIC.2014.6997796
(IEMN)
100
Devices (Number) Gains (dB)
15
6 8 7 6 5 4 3 2 1 0 9 12 15 18 21 24 27 fmax (GHz) 4
GFET – State of the art
Figures of merit: ft, fmax
ft,: Current gain cut-off frequency
fmax Maximum oscillation frequency (over this frequency, there is no power gain)
F. Schwierz, Proceedings of the IEEE, vol. 101, no 7, 2013
5
How these figures of merit (FOM) are defined ?
Conventional GFET structure
RF probes for measurements
Top view of dual-gated graphene FET
Device under test (DUT)
How to extract performances of GFET ?
(De-embedding procedure)
6
Extraction of extrinsic/intrinsic performance
G
Rg
VGS
RGD
RGS
CGS
CGD
gm
VGS
1/ gd
Rd
D
CDS
50
Gains (dB)
40
H21
30
20
Rs
H21
S
fmax
10
ft = 30 GHz
ft
fmax = 20 GHz
0
-10
Ft_probes = 12.5 GHz
ft_Probes_plan
-20
0.1
Frequency (GHz)
1
10
100
•  Pads structures are capacitive
•  fmax remains constant with capacitive transform
7
How to define figures of merit (FOM)
Small signal equivalent circuit of GFET without access lines
cdsp
cgdp
cgsp
Source
Rs
Rg
G
Cgs
R gs
cgdp
Gate
Cgd
gm VGS
R gd
Drain
Rd
Rg
RGD
RGS
cgsp
VGS
CGS
CGD
1/gd
Rd
CDS
gm
VGS
D
cdsp
Rs
C ds
1/ gd
S
Ft < Ft < Ft = Fc = Gm/(2 πCgs)
Fmax = Fmax < Fmax_intr
17 < 30 < 80 (GHz)
23
= 23
< 55 GHz
8
Some drawbacks of GFET
o  Contact and access resistance
o  No pinch-off
o  Lack of current saturation
Compare to the typical HEMT devices
Av ≈
gm
gd
Low Voltage gain
High value of access resistance
Main objective: Fmax # 100 GHz
9
Outline
•  Graphene FET for High Frequency (HF):
State of the art
•  GFET optimization
•  Gate contact
•  Current saturation exploring velocity saturation
•  Ballistic devices
•  Graphene flagship WP4 –High frequency
electronic: Some achievements
•  Summary and outlook
[email protected]
10
Metal-Graphene Contact Resistance - Modeling
Components for contact resistance
Device
Electrostatics
Specific contact resistivity (3Dà2D)
Lateral resistance
11
Metal-Graphene Contact Resistance
Graphene etched below pure Au contacts
Rc < 90 Ωµm
1400
1200
1000
R (Ω)
• 
800
600
R at V0 (Dirac point)
400
500 nm
Au (100 nm)
Rc W = 86 Ω µm
200
0
Rc W = 87 Ω µm
0
200 400 600 800 1000 1200 1400
L (µm)
R. Sordan (WP4 Flagship)
12
Velocity saturation - Requirements
Low contact resistance
High mobilty graphene layer and low doping (phonon
saturation)
50 Ωµm < Rc < 150 Ωµm
Ft extr = 20 GHz
Fmax = 13 GHz
(Wg = 2x50 µm, Lg = 1 µm).
O. Habibpour et al.; EuMW 2014 – Roma – Italy (WP4 Flagship)
13
Velocity saturation – impact of 2D materials
G-FET
GoBN
Y. Wu et al. / Nano
Letters 12 (2012) 3062
L=0.6 µm
I Meric et al. / IEDM (2011)
I Meric et al. IEEE, Vol. 101, No. 7,
(2013)
"↓$%& ~34 ()* 14
Towards tunable contact and ballistic GFET
Artist view of device
SEM picture (L=200 nm)
contact gate
Drain Drain
16 n
Source
SEM
m hB
N
Channel
gate
Source
Gated Pd-contacts, 20 nm thick W-gates
Contact
- - - - - - -
contact
junctions
+ + + + + +
Contacted graphene
Free graphene
Contact gate (2)
Channel gate (1)
h-BN(20nm)
SiO2
Contact
- - - - - - -
+ + + + + +
Contacted graphene
Contact gate (2)
Contact gate (2)
Vg2
Si++
Q. Wilmart et al. (unpublished) WP4 Flagship
15
Towards tunable contact and ballistic GFET
Vcont
G2
Drain
S
D
SEM
Sourc
e
Differential resistance
Q. Wilmart thesis
Vds
Drain
100µm
G1
~
Vch
Sourc
e
Pulsed contact gating
RF-gain switching
High impact of BN – good dielectric compared to Al2O3
16
Towards tunable contact and ballistic GFET
L=500nm, W=1.5 µm
Velocity saturation !
Vg=-3...0V
Vg=0...3V
17
Outline
•  Graphene FET for High Frequency (HF):
State of the art
•  GFET optimization
•  Gate contact
•  Current saturation exploring velocity saturation
•  Ballistic devices
•  Graphene flagship WP4 –High frequency
electronic: Some achievements
•  Summary and outlook
[email protected]
18
From devices to circuits
Graphene-based
Inverters
Av = - 11.3
High voltage gain Av = 11,3 with L = 2 µm
19
From devices to circuits – Ring oscillator
VDD
VDD
VDD
The highest oscillation frequency
1
2
3
4
was fo = 4.3 GHz at the gate
length L = 0.9 µm
L = 0.8 µm and W = 5 µm
E. Guerriero et al., ACS Nano 7, 5588 (2013) WP4 Flagship
20
Summary and outlook
•  Progress in graphene processing is made
•  H-BN show a great impact on the GFET performances
•  Looking for large scale material
•  Expertise already transfer also to WP8 (Flexible electronics)
Recent results using CVD
Graphene from Graphenea
C. Sire et al, Nano Le0. 12, 1184 (2012) (CEA – IEMN -‐ Northwestern U.)
fmax= 7 GHz and ft= 20 GHz
on flex substrate
21
22
23
WP4 – HF electronics
•  Nanostructuration of GFET channel (graphene
nanoribbonds)
SEM image of nanoribbons and
nanomesh obtained by e-beam
lithography
Nanoribbonds width:
from 10 nm to 50 nm
24
Nanostructuration of GFET channel
GFET on SiC with Hydrogen intercalation
Mobility: 2300 cm2/V.s
@ Vds = 300 mV
U_ext, U_intr, H21_DUT,
H21_ext, H21_intr (dB)
60
50
40
30
20
10
Ft_probes = 17 GHz
0
-10
0.01
Lg = 100 nm
0.1 Frequency
1 (GHz) 10
100
fmax = 30 GHz
fmax = 70 GHz
ft = 50 GHz
ft = 140 GHz
WP4 Flagship (IEMN)
(Without access
resistance)
25
De-embedding structures
Measurement plan
Metal
Graphene FET
[Ymeasured]
Pad
Oxide
“open” structure
Extrinsic
performance
« Intrinsic » performance
[Yextr] = [Ymeasured] [Ypad]
[Yintr] = [Ymeasured] - [Yopen]
Only parasitic capacitances are removed
26