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Layered materials for electronics
Grace Huili Xing Electrical and Computer Engineering, Materials Science and Engineering Cornell University Electrical Engineering, University of Notre Dame Students/Postdocs: Vishwanath Suresh, Mingda Oscar Li, Rusen Yan, Shudong Xiao Faculty collaborators: Debdeep Jena, David Mueller (Cornell); Lei Liu, Tengfei Luo, Xinyu Liu, Jacek Furdyna, Sergei Rouvimov, Vladimir Protasenko, Susan Fullerton, Alan Seabaugh (UND); KJ Cho, Moon Kim, Bob Wallace (UT Dallas); Randall Feenstra (CMU); Andy Kummel (UCSD); Joshua Robinson (PSU); Libai Huang (Purdue) Partly funded by NSF, AFOSR, SRC/DARPA Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 1 Graphene: The “Devices” Landscape
Flexible electronics Graphene Conven2onal Devices Novel Devices BisFETs FETs RF LNAs Transparent electrodes GNR TFETs GNR FETs Bilayer FETs f-­‐mul2pliers, mixers SymFETs Sensors Klein-­‐FETs Veselago Lens Tunable Photodetectors THz source/detectors/modulators Passive circuit elements Plasmonics Courtesy of D. Jena
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 2 Layered Materials: The “Devices” Landscape
Flexible/Wearable Conven2onal Devices FETs EmiSers Memories RF amplifiers f-­‐mul2pliers, mixers Modulators Layered Materials insulators, semiconductors, metals, superconductors (0 eV – 6 eV) Detectors Novel Devices Vallytronics Nonlinear op2cs Thin-­‐TFETs Solar Cells Thermoelectronics Passive circuit elements Sensors/Actuators Collec2ve effects T-­‐FETs Phase change memories Phase engineering Phonon/Spin engineering Keep looking for more ideas Logic – Memory – Analog – Communication Devices - Sensors/Actuators - Flexible/Wearable
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 3 Outline Logic – Memory – Analog – Communication Devices
Sensors/Actuators - Flexible/Wearable
Passive elements – Active devices
•  Initial market applications should be Defect Resilient
•  Defect Control: within the layered materials (growth/
stacking), on and around layered materials
(passivation)
•  Contact Engineering in highly scaled devices
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 4 GaN RF technology in Xing-Jena group
Difference between InGaAs & GaN is due to gm G
G
n+ GaN Plasma oxide n+ GaN
InAl(Ga)N
AlN
GaN/SiC
Speed ling a
c
s
d
Lg, Ls
ling a
c
s
g
L
G Passiva5on
S
D
InAl(Ga)N
AlN
GaN/SiC
eng
Gate l
2009 ing l
a
c
s
)
th (Lg
Regrown contacts: reduce R in charging delay (RC) Dielectric Free passiva2on (DFP): reduce gate extension S
2011 Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) n+ GaN
InAl(Ga)N
n+ GaN
AlN
GaN/SiC
T gates, speed limited by Cfringing/gm: increase gm! n+ GaN
G
Plasma oxide
InAl(Ga)N
AlN
GaN/SiC
n+ GaN
DFP: O2 Plasma
G
D
InAl(Ga)N
AlN
GaN/SiC
2013 Year 5 Ohmimc contacts in 2D Crystals
Debdeep Jena, Kaustav Banerjee, Grace Huili Xing Nature Materials 13 1076 (2014) Goal: 2D crystals electronics Bo'lenecks & Solu1ons Transport: Tunneling vs mobility Contacts: High contact resistance Epitaxy: Control of defects Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 6 Material characterization: thermal property, band alignment Rusen Yan
simulation
measured
Andras Kis
Rusen Yan et al
ACS Nano, 2014
•  First measurement of monolayer MoS2 thermal
conductivity: ~100x smaller than that of graphene
•  Thermal conductivity theory by Prof. Tengfei Luo
•  First band alignment with graphene electrode
using internal photoemission (IPE)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) Rusen Yan et al, APL
2012 and 2013
Kun, Nano Lett. 2013
7 TFET 101 n- TFET (tunnel field effect transistor)
Thermal tail is small due to the bandgap filtering thus TFETs are not
fundamentally limited by the thermionic emission process in MOSFETs.
n- MOSFET
Thermal tail is responsible to the subthreshold current
EB
EB
A. Seabaugh and Q. Zhang, Proceedings of the IEEE, 98(12), 2095 (2010)
Anderson & Anderson, Fundamentals of Semiconductor
Devices, McGraw Hill (2005)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 8 Benchmarking switches 102
2012
ST: Spin-Torque
SpinFET
Energy (fJ)
100
10-1
Graphene pn junction
BISFET
CMOS high performance
Preferred
Corner
10-4
10-1
Graphene
nanoribbon
Tunnel-FETs
100
ST oscillator
All-spin
logic
ST Transfer/
Domain-Wall
ST
Majority Nanomagnet
Logic
gate
CMOS low power
Heterojunction
III-V
10-2
10-3
ST transfer triad
Spin-wave
101
TFETs outperform other
technologies in industry
benchmarks
101
102
Delay (ps)
103
104
Nikonov and Young, Proceedings of IEEE (2013)
Uniform methodology for benchmarking beyond-CMOS devices, 2012
arXiv:1302.0244 [pdf]
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 9 Steep transistors based on tunneling in compound semiconductors E ne rg y (e V )
1.0
MIND results (compiled by
Alan Seabaugh)
• 
• 
•  Tunneling from quasi-3D source to 2D
channel produces steep slope
•  Near broken gap alignment produces
steeper slope
G aS b
E1
0.0
InP InA s
-­‐1.0
G. Zhou, H. G. Xing et al, EDL 2011
D
WTe2
G
S
S
D
E ne rg y (e V )
• 
First demo of homojunction TFETs:
Datta et al, 2009 IEDM
First demos of Type-I TFETs:
Xing et al, 2011 EDL
First demos of Type-II TFETs:
Datta et al, 2011 APEX;
Xing et al, 2011 DRC
First demos of Type-III TFETs:
Wernersson et al, 2012 DRC
Xing et al, 2012 IEDM
0.5
E fp
-­‐0.5
III-V Tunnel FETs
• 
V G S = 0 V
O F F s ta te
0
1.0
0.5
E1
0.0
InP InA s
-­‐0.5
-­‐1.0
20
V G S = 0.2 V
G aS b
E fp
O N s ta te
MoS2
0
5
10
15
P os ition (nm)
20
G. Zhou, H. G. Xing et al,
IEDM 2012
M. Li, H. G. Xing et al. JAP 2014.
arXiv: 1312.2557
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 5
10
15
P os ition (nm)
10 III-­‐V and 2D-­‐semiconductor TFETs: scalability ITRS: TFET body thickness ~ 1-2 nm for 10 nm node
D. Jena, Proceedings of IEEE (2013)
•  Quantization renders sub-2 nm region
inaccessible for 3D semiconductor TFETs
•  Stacked TFET geometry offers unique
electrostatic control of broken gap
heterojunctions
Incomplete list compiled by H.G. Xing in 2013
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 11 New Device Proposal: Thin-­‐TFET •  Two-dimentional Heterojunction
Interlayer Tunnel FET (Thin-TFET)
•  Predicted performance is among
the best intrinsic switching
energy – delay product
Mingda Oscar Li
M. Li et al, DRC 2014, M. Li et al, J-­‐EDS 2014 M. Li et al, JAP 2014 Another embodiment: lateral heterojunccon V D S = -­‐0.4 V
10
1
V D S = -­‐0.3 V
10
0
V BG=0
V D S = -­‐0.2 V
10
-­‐1
V D S = -­‐0.1 V
A ve ra g e S S for V D S = -­‐0.2 V from -­‐3
10
-­‐2
10
-­‐3
-­‐0.4
10 to 10 µA /µm :
~ 21 m V /de c
-­‐0.3
-­‐0.2
V T G (V )
-­‐0.1
0.0
C u rren t D en s ity ( µA /µm )
60
V BG=0
V T G = -­‐0.4 V
V T G = -­‐0.3 V
40
V T G = -­‐0.2 V
10
2
3
10
Greg
Snider
C u rre n t D e n s ity ( µA /µm )
David
Esseni
20
V T G = -­‐0.1 V
0
-­‐0.4
V T G = 0 V
-­‐0.3
-­‐0.2
-­‐0.1
0.0
V D S (V )
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 12 Devices: MoS2/WSe2 Vertical P-­‐N Junctions 10-6
Pd
MoS 2/WSe 2 heterojunction
Schottky junction
-8
10
p-n junction
Vg (V)
MoS2
WSe2
-40
i
p
-20
n
p
0
n
i
20
n
n
40
n
n
-10
I d (A)
10
WSe2
MoS2
Vg
-40V
-20V
0V
20V
40V
-12
10
Ti/Au
-14
10
-16
10
-2
-1
0
1
2
Vd (V)
n-MoS2
(Shudong Xiao et al, DRC 2014)
MoS 2 FET
14
10
Vg= -20V, p-n junction
2.0
WSe2 F ET
Pd
Ti
⎛ dV ⎞
log⎜
⎟
⎝ dI ⎠
12
10
1.5
10
10
Vd (V)
R (Ω)
p-WSe2
8
n-MoS2
15 .24
1.0
14 .18
13 .12
10
12 .05
10 .99
n-WSe2 Pd
Vg= 40V, Schottky diode
9.9 28
0.5
6
10
8.8 65
Ti
Pd
7.8 03
6.7 40
-40
-20
0
Vg (V)
20
40
-40
-20
0
20
40
Vg (V)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 13 Dual gated vdW tunnel diodes and transistors Top gate
Mo
S2
S
ZrO
Top
gate
2
D
MoS2/WSe2
WS
e2
ZrO
2
Bottom gate
Bottom gate
20 nm
1.0
2.0
VGate-MoS = 3V
2
VNDR-peak (V)
I D (nA)
1.5
VGate-WSe =
-3.5 V
1.0
-3.3 V
-3.0 V
2
0.5
0.0
0.0
77K
0.3
0.8
η = 0.83
0.6
77K
0.4
0.6
VD (V)
0.9
1.2
-3.6
-3.4
-3.2
-3.0
-2.8
V Gate-WSe (V)
2
(UC Berkeley) Javey Group: T. Roy et al. ACS Nano 2015 9 (2), 2071-2079
The first observation of NDR in TMDC heterostructures (at 77 K)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 14 vdW solids Esaki Diodes First 2D crystal Esaki Diodes at RT (Rusen Yan and H. G. Xing et al., SubmiYed, arXiv:1504.02810) Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 15 Outline •  Introduction
•  Our attempt on Defect Control
–  MBE growth
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 16 Chemical Vapor Transport Growth (I) Xiaodong Xu et al, U. Washington, 2013
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 17 Scattering and Mobility limits in Monolayer MoS2 Intrinsic
mobility
accessible in
CLEAN,
SUSPENDED
layers Very low
impurity
densities:
intrinsic/remote
phonon
scattering
determine the
highest
attainable
mobilities.
Currently reported electron mobilices are limited by Ionized impurity scafering High-κ gate dielectrics can increase the
electron mobility only for samples
infected with very high impurity
densities
D. Jena, Silicon Nanoelectronics Workshop 2014
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 18 Chemical Vapor Transport Growth (II) MoO3
S
Jun Luo et al. Rice University (2013) and many other group
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 19 Challenges in growth of transition metal dichalcogenides (TMD) CVD, PVD, MBE?
Zoom View - Mo-Se Phase Diagram (1990 Brewer L.)
http://www1.asminternational.org.proxy.library.nd.edu/asm
MBE
Advantages:
-  in-situ growth
monitoring for layer
number control
-  clean growth
environment,
heterostructures
-  non-equilibrium growth
may offer paths for
stoichiometry control.
MoSe2
Phase diagram
Challenges: low metal
adatom mobility (hybrid
MBE growth is being
explored)
Vishwanath, H.G. Xing et al. J-2D, 024007 (2015)
ASM international 2006
Mo-Se Phase Diagram (1990 Brewer L.)
ASM Alloy Phase Diagrams Center, P. Villars, editor-in-chief; H. Okamoto and K. Cenzual, section editors;
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) ASM International,
Park,
OH,
USA,
20 2006-2013
http://www1.asminternational.org/AsmEnterprise/APD,
Materials
Calculated MoS2 phase diagram and defect formation energy Zoom View - Mo-S Phase Diagram (1990 Brewer L.)
http://www1.asminternational.org.proxy.library.nd.edu/asmenterpri...
KJ Cho, UT Dallas, 2014
lars, editor-in-chief; H. Okamoto and K. Cenzual, section editors;
nterprise/APD, ASM International, Materials Park, OH, USA, 2006-2013
Vishwanath, H. G. Xing et al. J-2D, 024007 (2015)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 21 MBE growth of layered materials Conventional
epitaxy: 3D on 3D
crystals
Van der Waals
epitaxy: 2D on 2D
crystals
Quasi Van der Waals epitaxy: 2D on
terminated 3D crystals
e.g. Bi2Se3 on GaAs
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 22 Pioneers of MBE of 2D crystals •  A. Koma Group [2]
Material
Group
Materials grown by Van der
Waals epitaxy
Quasi-1D
Se/Te
Te/Se/Ta
Quasi-2D
TX2(MoSe2, GaSe)/MoS2
•  W. Jaegermann Group Material Source
Materials grown
Metal organic Van der
Waals epitaxy
WS2/TX2 (HOPG,
MoTe2 (0001))
Van der Waals epitaxy
using Knudsen cell
TX2 (SnSe2,SnS2)/
TX2 (SnSe2,SnS2,
WSe2 MoS2, MoTe2,
GaSe)
TX2 (MoSe2)/SnS2
TX2/mica
Quasi-2D
on 3D
TX2(NbSe2, MoSe2)/SGaAs(111)
TX2 (MoSe2) /CaF2(111)
Organic
Phthalocyanine/TX2
[3]; [4]
ü  They demonstrated proof of concept MBE
growth of 2D crsytals.
ü  Limited characterization was done by them,
esp. using Transmission Electron
Microscopy (TEM).
[2] A.Koma et.al. Journal of Crystal Growth, Vol. 111, 1029—1032 (1991); [3] S. Tiefenbacher et.al. Surface
Science 318 (1994) L1161-L1164; [4] R. Schlaf et.al. J. Appl. Phys. 85, 2732- 2753(1999);
APS March Meeting 2014, Suresh Vishwanath (sv728@cornell.edu)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) Vishwanath et al. J-2D, 024007 (2015)
23 MBE MoSe2: giant bandgap renormalization (UC Berkeley/Stanford) Miguel Ugeda, Zhi-Xun Shen and Michael Crommie et al. Nat. Mat. (2014)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 24 MBE growth of MoSe2 on different substrates
MoSe2 on
HOPG
Cs-HRTEM
Plane-view HRTEM
Diffraction
MoSe2
HOPG
MoSe2 on
CaF2
MoSe2
CaF2
MoSe2 on
Sapphire
MoSe2
Sapphire
Vishwanath, H. G. Xing et al. J-2D, 024007 (2015)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 25 Optical property of MBE MoSe2 PL of monolayer MBE MoSe2
Absorption spectrum of MBE MoSe2
Sharp band edge absorption:
< 20 meV/dec!
(comparable to the best 3D
semiconductors)
Lorentzian fit of the PL peak gives the maximum at 1.565eV which is
consistent with the published data at around 1.57eV.
Vishwanath et al. J-2D, 024007 (2015)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 26 Low defect MBE WSe2 •  Very low defect density
found in MBE WSe2
•  Electronic bandgap in
monolayer WSe2 found
to be 2.27 eV
•  Type-I band alignment
found between
monolayer and bilayer
WSe2.
Vishwanath, Jun Park, Andy Kummel and Grace Xing et al, submitted (2015)
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) 27 Conclusions Contact Engineering
Defect resilient applications
Defect Control
High performance devices
Lemma of New Technology
(Herbert Kroemer, 1995)
SnSe
The principle applications of any
sufficiently new and innovative
technology always have been –
and will continue to be –
applications created by that
technology.
Grace Huili Xing (hxing@nd.edu, grace.xing@cornell.edu) Exfoliated MoS2
28