oral defence
TRANSCRIPT
Carrier Transport in Dirac-band Materials and Their Device Applications
Gaurav [email protected] [email protected]
PhD DefencePhD Defence
Computational Nanoelectronics and Nanodevices Lab (CNNL)Department of Electrical and Computer Engineering
National University of Singapore 1
29th October, 2015
Motivation
Bi2Se3 3D-TI Properties & Methodology
Carrier Transport Characterizing Resistance Contact Effects
Devices Interconnects Band-Alignment induced Resonance
Outline
2
3
What Device Community is trying to ACHIEVE ?
Present Computing Chips
Robust Transport&
Zero Power (Standby)
Motivation
Future Computing Chips
Large Static and Dynamic Power
Dissipation
4
Why ?
Safely put laptops on lap !!!
No need to repeatedly operate to Replace Implants
Save Lives & Money(Bioelectronics)
Battery Drainage requires operation to replace implant
DataCentre
275 Billion kWh consumed in 2011 by
Data Servers !!!
Save Energy
Motivation
http://www.nanowerk.com/news/id25227_2.jpg 5
How (Our Scope)?Motivation
Dirac-Band Materials
Graphene Heavier Group-IV
2D Group-IV Monolayers Bi2Se3 3D-Topological Insulator
Carrier Transport- Ballistic and Acoustic Phonons
(Chapter 6)Devices - Graphene
electro-optic transistor
(Chapter 3)
Devices - Spin-Separator and
Filter(Chapter 4 and 5)
Carrier Transport- Contact Effects
(Chapter 7)
Carrier Transport- Defects(Chapter 8)
Carrier Transport- Longitudinal & Vertical Transport
(Chapter 9)
Device - Interconnects(Chapter 8)
Devices – Resonance Devices(Chapter 9)
Large Fermi-Velocity Spin-Polarization
2D-TI (QSH Phase)Spins flow on edges
Real Space
Topological Insulator (TI)
6
Ordinary Insulator Topological Insulator
Change in Property (Physical/Electronic)
Trivial Phase
Topological Phase
(non-trivial inverted)
Band-Inversion
Dirac-Bands
A genre of Strong Spin-Orbit Coupling Materials
Motivation
https://www-ssrl.slac.stanford.edu/research/highlights_arc
hive/topological_insulator.htmlhttp://spectrum.ieee.org/image/1876231
3D-TI – Spins flow on Surface
Momentum Space
http://www.nature.com/am/journal/v3/n1/full/am201117a.html
Independent Channels for up and down spin in opposite direction on edges (2D-TI) or surfaces (3D-TI).
Perfect Transport
No Heat Dissipation in the Channel (strictly true only for 2D-TI edge transport)
Electron Transport in Topological Insulators
unless: Spin-Flip Mechanisms (Magnetic Impurities) Break Time-Reversal Symmetry (Magnetic-Field)
Back-scattering Prohibited
7
Excellent Material for Transport and Electronic Devices
Motivation
8
MotivationJ. Burton, Nature Vol. 466 (15 July 2010)
Topological Insulators
9
Objectives of this ResearchMotivation
1. Develop NEGF formalism based Quantum transport Simulator for extensively parallelized computing.
2. Investigate carrier transport through Dirac-Band materials.
3. Apply Dirac-band materials and investigate their feasibility for prospective device schemes.
Motivation
Bi2Se3 3D-TI Properties & Methodology
Carrier Transport Characterizing Resistance Contact Effects
Devices Interconnects Band-Alignment induced Resonance
Outline
10
1. Single Dirac Bands of high Fermi-velocity on each Surface.
Spin Texture (K-Space)
+kx-kx
-ky
+ky(b)
Top Surface
Bottom Surface
Bulk
(a)
Semiconducting
xy
zlayers
[1] Nature Nanotech. vol. 9, no. 3, pp. 218-224, 2014 11
3D-TI Properties & Device
VGS
Source Drain
VDS
xyz
Bi2Se3 TIInsulator
Phenomenological Gate
Bi2Se3 Bulk Bandgap ~ 0.3 eVLargest Bandgap among all proven 3D-TIs Greatest isolation between non-trivial and trivial bands.
Bi2Se3 3D-TI
2. Spin-Momentum Locking. (Opposite Spin Texture on Opposite Surfaces).3. Spin-Polarized Surface Transport. [1]
2 2x y1 2
2 2x y2 1
p 2 2x y 1 2
2 2x y 2 1
k /m d+k /m ivk -vk 0
d+k /m k /m 0 -ivk +vkH = , (1)
-ivk -vk 0 k /m d+k /m
0 ivk +vk d+k /m k /m
Each layer of Bi2Se3 described in pz orbital basis
v (2.5 eV-Å) is Fermi-velocity, m1 (0.125 eV-1Å-2)and m2 (-0.04 eV-1Å-2) are the orbital masses d (-0.22 eV) is introduced to generate a gaptz (0.35 eV) is hopping b/w adjacent layers tight-binding parameter
z
z
0 0 0 0t 0 0 0T= , (2)0 0 0 00 0 t 0
1 1†1 2 2
†2 3
Η Τ 0
Τ Η Τ
Η= , (3)Τ Η
.
0 .
Fitting parameters extracted from
ab-initio. (Dr. Hsin Lin )
k.p Model in kx-ky space
Tight-Binding Interaction along
z-axis (Nearest Neighbour)
Hamiltonian
12
Bi2Se3 3D-TI
Gupta et al., PRB 89, 245419 (2014)
Se
Bi
Quintuple Layer (QL) (0.943 nm)
Image Courtesy: Huang Wen
L. A. Wray, S. Y. Xu, Y. Q. Xia, D. Hsieh, A. V. Fedorov, Y. S. Hor, R. J. Cava, A. Bansil, H. Lin, and M. Z. Hasan, “A topological insulator surface under strong Coulomb, magnetic and
disorder perturbations,” Nature Physics, vol. 7, no. 1, pp. 32-37, Jan, 2011.
Our Blue Bands superimposed over Experimental DataValidating Model for Slab (1/2)
13
Bi2Se3 3D-TI
Image Courtesy:
Dr. Hsin Lin
Top Surface
Bottom Surface
Layer Number5 10 15 20
|Ψ(z)
|2 D
irac B
and
s
0.6
0.5
0.4
0.3
0.2
0.1
0
(b)
Bottom SurfaceTop Surface
0.2
0.1
0
-0.1
-0.2-0.02 0 0.02
kx ax
En
erg
y (e
V)
(a)
20 QL (ky=0)
Surface Band
ky
ay
kx ax
-0.05 0 0.05
0.04
0
-0.04
(c)
k y a
y
kx ax
-0.05 0 0.05
0.04
0
-0.04
(d)------------- Spin Texture – Conduction Band -------------
Validating Model for Slab (2/2)
14
Bi2Se3 3D-TI
2e fG = dE T(E) (- )
h E(6)
∂∂∫
DOS(E) fn = dE (- )
L E(7)
∂∂∫
Conductance (G)
Linear Free Charge Density (n)
(8)G .L
μ =n e
Mobility (μ)
-1
D0 0 S phGr = (E+iη)I-H -U -Σ -Σ -Σ (1)
Green’s Function (Gr)
†i[DOS(E)] = ([Gr - Gr ]) (5)
2π
Density of States (DOS(E))
†
S DT(E) = Trace(Γ Gr Γ Gr ) (4)Transmission (T(E))
acph[Σ ] = D [Gr] (2)Self-Energy of Phonons
Level Broadening †Γ = i ([Σ - Σ ]) (3)
Modeling Equations (NEGF)
15
Bi2Se3 3D-TI
Good Books for Learning Quantum Transport via NEGF:[1] S. Datta, Quantum Transport: Atom to Transistor, Cambridge Press (2005)[2] S. Datta, Lessons in Nanoelectronics, World Scientific Publishing (2012)
jeff j
1 1(9)
μ μ= ∑
Matthiessen's Rule
1. Ballistic Transport (CPU Cluster) transverse axis in mode space.
2. Acoustic Phonons (CPU Cluster) transverse axis in mode space.
3. Other Defects (GPU Cluster) (Random Distribution – Examined Over Different Concentrations) in Real Space
1. Charge Impurities2. Vacancies3. Edge Roughness
4. Mobility (Phonons) & Mobility (Other Defects Together) Effective Mobility (μeff)
Modeling Scheme
16
Bi2Se3 3D-TI
Tesla C2070 and M2090 in CUDA 5.0 supported by MAGMA 1.3 (Matrix Algebra on GPU and Multicore Architectures) and LAPACK
3.2.1 (Linear Algebra Package) librariesMax. Size Unit-Cell : 244 MB ;
Device Hamiltonian for RGF : 14 GB ; Data Section : 45 GB
Motivation
Bi2Se3 3D-TI Properties & Methodology
Carrier Transport Characterizing Resistance Contact Effects
Devices Interconnects Band-Alignment induced Resonance
Outline
17
Nano Lett. 10 (1), 2010
Many Experimentalists have observed
Metallic Trend
Insulating Trend
Few Experiments have shown
Nature Communications 3 (757), 2012
Yet, Everybody Claims to be
capturing Transport through Topological
Surface band !!!
At least Four Groups have observed
maxima
Reason never been discussed
PRL 106 (196801), 2011
Two Disputes: a) Resistance vs Temperature
18
Carrier Transport
Magnitude of dimensionless electron-phonon coupling constant λ :
Range 0.08 to 0.43
Weak Strong
Theory Thalmeier, PRB 83, 125314 (2011)
Giraud et al. PRB: 83, 245322 (2011) & 85, 035441 (2012)
ARPESPark et al., New Jour Phys 13, 013008 (2011); Pan et al. PRL 108,187001 (2012)
Hatch et al., PRB (R) 83, 241303 (2011)
Helium Scattering
Zhu et al., PRL 108,185501 (2012)
electron trajectory
spin
Two Disputes: b) Strength & Role of Acoustic Phonon
19
Carrier Transport
Temperature (K)50 150 250
Resi
sta
nce
(Ω
)
1600
1200
800
400
0
TI DS
VDS
0.01 eV
0.05 eV
Ef = 0.05 eV
μS μD
0.16 eV
Ef = 0.2 eV0.2 eVμS μD Temperature (K)
Resi
sta
nce
(x
Wid
th)
(ohm
s-μm
)
0.05 eV0.06 eV
0.1 eV0.2 eV
100 200
2000
1500
1000
500
50 150 250
0.025 eV0.050.06
0.10.2
Experiment* Our Simulation
*S.S. Hong, J.J. Cha, D.S.Kong, Y. Cui, Nat Commun, 3 (2012) 757
Explained all controversial Experimental Data (> 50 K) published on Resistance Measurements on 3D-TI [1]
[1] Gupta et al., PRB 89, 245419 (2014)
Provided microscopic picture of transport mechanism in 3D-TI accounting for number of physical and electronic parameters
Role of Acoustic Phonons in 3D-TI
20
xy
z
Carrier Transport
21
Thickness-dependent [1] & Thickness-independent [2]
Transport
[1] Kim et al., PRB 84 (073109), 2011 [2] Bansal et al., PRL, 109 (116804), 2012
15 20 25 30 35 40 Slab Thickness (QL)
103
102
Res
ista
nce
(oh
ms-
μm
)
0.0 eV
0.15 eV
0.075 eV
0.2 eV
(a)
Current Density (μA/μm)
Dra
in C
urr
ent
(μA
/μm
)50 150 250
40
30
20
10
(e)
Temperature (K)
En
ergy
(eV
)
2 4 6 8 10 12
0.1
0
-0.1
T = 300 K 0.5
0.3
0.1
(f)
En
ergy
(eV
)
(d)
2 4 6 8 10 12
0
-0.1
-0.2
VDS = 0.16 V 2
1
0-0.1 0 0.1
100
50
0
-50
-100
(b)
Dra
in C
urr
ent
(μA
/μm
)
VDS (Volts)
Dra
in C
urr
ent
(μA
/μm
)
(c)
2 4 6 8 10 12
10
6
2
0.040.06 0.080.02
Layer Number
Ballistic Transport: QL, VDS, TCarrier Transport
Gupta et al., Physica E, 74, 10-19 (2015)
Layer Number
Layer Number
VDS
1 10 100 1000
251 316 398 501
0.12
0.10
0.08
0.06
0.04
T = 50 K
30 100 900
0.1 1 10 1000.2
0.1
0
T = 250 K
Δ f
SD x
100
0
(a) (b)
Bottom Surface DOS (/eV/μm)22
Phonon Scattering Δ fSD (Top Horizontal scale in Magenta)
Solid Color Lines: Surface DOS) (Bottom horizontal scale in Black)
Dashed Color Lines: Transmission(Top horizontal scale in Magenta)
Ballistic Weak Acoustic (λ = 0.08)
Strong Acoustic (λ = 0.25)
DOS Spread α Strength of Phonon Scattering
En
ergy
(eV
)
DOS for Bulk Band
Band-Edge
Density of States : DOS
ε
En
ergy
(eV
)Transmission and ΔfSD
Ef = 0.1 eV Lx = 30 nm55
50
45
40
Res
ista
nce
(Ω
-μm
)
Temperature (K)50 100 150 200 250
Carrier Transport
23
FM/NM Contacts (1/2)
0 40 80 120 160 Temperature (K)
|V(M
)-V
(-M
)| (μ
V) 5
3
1
(c)Experiment [1]
0 50 100 150
0.3
0.2
0.1
Temperature (K)
SP
Dra
in E
nd
(d)Simulation [2]
2.5
1.5
0.5
2 4 6 8 10Layer Number
2 4 6 8 10Layer Number
0 K 300 K 300 K (Phonon)
(a) (b)
+y SM-y SM
Cu
rren
t (μ
A/μ
m) 2.5
1.5
0.5
[1] Nature Nanotech. vol. 9, no. 3, pp. 218-224, 2014[2] Gupta et al., Scientific Rep. 5 (9479), 2015
FM Source – Bi2Se3 Drain
Current on surface layers
As per spin-texture
Gate
Source DrainBi2Se3 TI
x
yz
θϕ
M TS-CB
BS-CB
Carrier Transport
“Contact” Magnetization Vector
SM DM
24
Gate
Source DrainBi2Se3 TI
x
yz
θϕ
M
Carrier Transport
Current on surface layers As per spin-texture ???
FM/NM Contacts (2/2)
5
0
0.5
0
Cu
rren
t (μ
A/μ
m)
(+y)FM-TI-NM
NM-TI-NM
2 4 6 8 10Layer Number
NM-TI-(+y)FM
SM
DM
S D
+y
TS-CB
BS-CB
S D-y
Cu
rre
nt
(nA
/μm
)
exTI Source
Transport Direction (nm)
0
-0.4
-0.8
20 40 60 80
(a)
25
(−y) 100 % DM FM Drain @ ky = 0
Contact Effects - Theory
1
0.6
0.2
(b)
20 40 60 80
f S =
1, f
D =
0f S
= 0
, fD =
1
Carrier Transport
TS-CB BS-CBLayer 10 Layer 1
S D-y
+=
Layer 10 Layer 9 Layer 8 Layer 7 Layer 6Layer 5 Layer 4 Layer 3 Layer 2 Layer 1
Current on surface layers is as per spin-texture, but so do contact dependent transmission and reflection
26
Transport Key-Points1. 3D-TI exhibits complex transport behaviour, where resistance
is a function of at least:(a) Fermi-level (b) Temperature (c) slab thickness (d) channel length (e) channel bias (f) electron-phonon (e-ph) coupling.
2. Observing insulating trend DOES NOT necessarily imply operation in the surface bands for a given 3D-TI. Just e-ph coupling may be weak.
3. FM source contacts may reduce bulk transport (effective contribution) by forcing current through the surface.
4. FM contacts may be used for generating negative surface current as a signature of 3D-TI.
5. FM contacts may result in seemingly contrasting observations w.r.t. expectations from surface spin-texture.
Carrier Transport
Motivation
Bi2Se3 3D-TI Properties & Methodology
Carrier Transport Characterizing Resistance Contact Effects
Devices Interconnects Band-Alignment induced Resonance
Outline
27
Figure from ITRS 2005 Interconnect Chapter
Present Status Cu Interconnects
1997: IBM announced transition to Cu interconnects from Al.
Less Resistive Speed ↑ More Durable and Scalable than Al
Present Challenges with Cu: High Resistivity Electro-migration Grain Boundary Issues Side Wall scatterings
Alternate Material ???
28
Devices
Effect of Defects on Mobility
1. Robustness to edge roughness.
2. Unlike De-facto samples [1], defect compensated samples [2] have roughly ballistic mobility.
[1] F. X. Xiu et al., Scientific Reports 2, Article number: 669 (2012) [2] S. S. Hong et al., Nature Communications 3, Article number: 757 (2012) 29
Devices
Ef = 0.100 eVEf = 0.175 eV Ef = 0.125 eVEf = 0.150 eV
Edge Roughness
2% Defects
10% Defects
250200150
250200150
100 200 300
Charge Impurities5 x 1018cm-3
2 x 1019cm-3
6 x 1019cm-3
200
130
80
120
70
5020 100 200 300
Vacancies250
150
200
100
100 30 100 200 300
5 x 1018cm-3
2 x 1019cm-3
6 x 1019cm-3
Mo
bili
ty (
cm
2 /V
/se
c)
Temperature (K)
10QL - 80 nm long - 60 nm wide
Effect of Phonons on Mobility
1. Phonons significantly degrade mobility with temperature.2. Ballistic Mobility increases with wire length.3. Defects (low conc.) hardly affect ballisticity of TI.4. Phonon scattering scales with wire length.
30
Devices
Wire Length (nm)50 100 150
Defects
Acoustic Phonons
300 K180
140
100
60
20
Mo
bili
ty (
cm
2 /V
/se
c)
Ef = 0.100 eVEf = 0.175 eV Ef = 0.125 eVEf = 0.150 eV
Temperature (K)0 100 200 300
200
150
100
50
AcousticPhonons
80 nm long
Net Mobility and Comparison1. 300K Bulk Cu Mobility ~ 30 cm2/V/s Comparable to Bi2Se3 TINano Letters 11 1925-7 (2011) observed 10 cm2V−1sec−1
at 245 K for 3.5 nm thick sample in which inter-surface scattering would be much stronger than 10-13 QL samples, while Thin Solid Films 534 659-65 (2013) observed 23 cm2V−1sec−1 at room temperature for 30 nm thick sample.
2. 300K Cu σ ~ 2.5-3.3 x 107 (Ω-m)-1
2K Bi2Se3 σ ~ 8.9-34.5 x 103 (Ω-m)-1
High electron density in CopperLow Density of States near Dirac-Point :3D-TI
Cu vs Bi2Se3 3D-TI
GNR vs SWCNT vs Bi2Se3 3D-TI1. GNR and SWCNT : small electron-phonon coupling BUT Low DOS
issue near Dirac-Point.2. GNR can be stacked (MLGNR-AsF5 doped) / SWCNT Bundled.3. 3D-TI not scalable like GNR or SWCNT+ phonon effect.
31
Devices
Gupta et al., Scientific Rep. 4, 6838 (2014)
Temperature (K)
140
100
60
2050 100 200 300
Mo
bili
ty (
cm
2 /V
/se
c)
0.100 eV
0.175 eV
Ef 0.075 eV
0.125 eV
0.150 eV
IEDM 2008 & 2010 (UCSB)
Inference for Interconnects1. ITRS 2011 probably assumed only surface transport and
overlooks “scattering” from defects and especially phonons.
2. Pros:1. High Mobility at low temperature.2. ‘Some’ immunity to impurities and edge roughness.
3. Cons:1. Room-Temperature Operation is an issue (phonon)2. Low Density of States near Dirac-point.
4. At least need: 3D-TI with much larger bulk-band gap and very weak e--ph coupling
5. Thin Bi2Se3 3D-TI may not be right material for this application.
32
Devices
IEDM 2013We successfully convinced Device Community to remove 3D-TI from interconnects chapter at least for now No Longer in ITRS 2013
33
Channel (Ch) Material : Graphene
Dielectric
Insulator (Tunnel)
Ch 2
Ch 1
VBG
xy
z
e- flow
VTG
symFET
[1, 2]
μn
μp
[1] Pei Zhao et al. IEEE Trans. in Elec. Devices, 60, 951-957 (2013)[2] R. M. Feenstra et al. J. Appl. Phys. 111, 043711 (2012)
qVDS = 2ΔE
ΔEqVDS
qVDS < 2ΔE
ΔE
Ch 2
Ch 1z
qVDS > 2ΔE
qVDS
μp
μn
2ΔE/q
VDS
orVTG
ID
Resonant State: High Transmission Very High Selectivity (Ratio) Independent of Temperature
Band-Alignment induced ResonanceDevices
Application
Analog Frequency Doubler
Resonant Devices can also be used for Analog
(a) Multipliers (b) Oscillators
[1] Pei Zhao et al. IEEE Trans. in Elec. Devices, 60, 951-957 (2013)
Freq. (id) = 2 x Freq. (vds or vgs)
Can 3D-TI provide Spin + Resonance Functionality for non-
volatile electronics ?(both need surface-bands)
ID
VDS or VGSInput:vds or vgs
Output: id
[1]
Bias-Point
34
Devices
Device Principlex
yz
μS
μD = μS – VDS
VGS < 0
VGS > 0Δ
VDS = 2 Δ
Top Gate
SourceDrain
Bi2Se3 TI
Bottom Gate
Dielectric
VDS
VTG
VBG = VTG= VGS
35
Devices
Potential Gradient
B
C
Design-A
Design-B & C
Source
Drain
Bottom Gate
VDS
VTG= VGS
VBG = 0
VGS = -Δ
VGS = -2 Δ
μS
μD = μS – VDS
VGS < -2Δ
Δ
Δ
VGS > -2Δ VDS = 2 Δ
Gupta et al., Scientific Rep. 4, 6220 (2014)
0.04
0.02
0
-0.02
-0.04-0.2 0 0.2
kya
−0 Δ
kya
−2 Δ
-0.2 0 0.2kya
−1 Δ
-0.2 0 0.2
Resonant Condition (0 K) – Design AEn
erg
y (e
V) 1.5
1
0.5
0
Δ = 0.04 eV
μS
μD
-Δ
Mode MismatchVGS = -0Δ
Δ μS
μD
Δ
-Δ
Mode MatchVGS = -1Δ
μS
μD
Δ
-Δ
Mode MismatchVGS = -2Δ
ModeFiltering Fermi-Velocity VB < CB
36
Devices
Transmission @
Results: Design-A
Resonance at :VGS = -Δ
Δ = 0.04 eV300K200K100K0KT :Δ : 0.04 eV0.06 eV0.025 eV
250
150
50
Gate Voltage (VGS / Δ)-2 -1.5 -1 -0.5 0
1.188
1.257
1.295
Cur
rent
(μA
/μm
)
IRI0
Ratio: IR
I0
Ratio is quite small Current at VGS = 0 is slightly more than at – 2Δ Local minima on either sides of resonance peak.
37
Devices
Gupta et al., Scientific Rep. 4, 6220 (2014)
Results show collective effect of: Band-Alignment induced Resonance. Mode-mismatch at Contacts Effect of gate potential on channel DOS.
Δ
200
160
120
-2 -1.5 -1 -0.5 0
Cur
rent
(μA
/μm
)
1.257
1.230
1.176
1.120
Gate Voltage (VGS / Δ)
T
T = 0 K
VGS
-0Δ -1Δ -2Δ
Ratio and asymmetry worsen with increase in temperature
Vertical Transport: Design B35
25
15
5
30
20
10
1
Sla
b L
aye
r
5 10 15 20 25 30 35
Current Density(μA/μm)
Transport Direction (nm)
38
Devices
Source
Drain
Bottom Gate
VDS
VTG= VGS
VBG = 0
Current flows both along longitudinal (x-axis) and vertical (z-axis) direction
Transport in Design-B (0K)En
erg
y (e
V)
kya
0.04
0.02
0
-0.02
-0.04
(b) (c)
−1 Δ −0 Δ
(d) (e) (f)
−2 Δ−4 Δ −3 Δ
0 0.5 1 1.5
-0.1 0.1kya kya kya kya
Resonance is visible, But: Ratio < 1
Loss in Device Gain
Large Asymmetry Signal Distortion
40
30
20
(Fo
r Δ =
0.0
2 e
V)
100
50
(Fo
r Δ =
0.0
4 e
V)
-4 -3 -2 -1 0 Gate Voltage (VGS / Δ)
(a) Current (μA/μm)
0.981
0.948
39
Devices
-0.1 0.1 -0.1 0.1 -0.1 0.1 -0.1 0.1
No mode mismatch b/w channel and contactsVDS = 2 Δ
Transport in Design-CEn
erg
y (e
V)
(b) (c) −1 Δ −0 Δ(d) (e) (f)
−3 Δ−4 Δ −2 Δ
0 0.5 1 1.5
0.04
0.02
0
-0.02
-0.04
40
Mode mismatch b/w channel and contacts
Combines worse of Design-A and B
Devices
No Resonance is observed. Minima not at -2Δ because of still some weak mode-matching for vertical transport.
kya-0.1 0.1
kya kya kya kya-0.1 0.1 -0.1 0.1 -0.1 0.1 -0.1 0.1
36
32
28
120
110
100
-4 -3 -2 -1 0Gate Voltage (VGS / Δ)
(a)
VDS = 2 Δ
(Fo
r Δ =
0.0
2 e
V)
(Fo
r Δ =
0.0
4 e
V)
Current (μA/μm)
Inference for Resonant Device
41
Devices
1. Therefore, the results show collective effect of: Band-Alignment induced Resonance. Mode-(mis)match at Contacts Effect of gate potential on channel DOS.
2. Larger band-gap 3D-TI may improve the ratio by providing larger energy-window for device operation.
3. Assumption of pure vertical transport in literature is inappropriate.
4. Order of performance: Design-A > Design-B > Design-C
5. Design-A performance may be slightly improved by limiting transverse modes [1].
6. In my opinion, none of the three designs is suitable especially for Room-Temperature.
[1] Gupta et al., Nanoscale 4, 6365-6373 (2012)
42
Major Results1. Quantum transport Simulator based on NEGF was developed for 3D-
TI and 2D Group-IV monolayers, that exploits heavy parallelism on GPU and CPU Clusters.
2. By investigating carrier transport through Bi2Se3 3D-TI, we: Provided fundamental insights in device transport and scattering
mechanisms. explained contrasting experimental data on resistance
characterization. explained spin-polarized current experiment via FM contacts and
current distribution across layers. suggested new transport-based methods of validating 3D-TI.
3. By investigating 3D-TI devices via Bi2Se3 as example, we: Appraised number of assumptions in existing TI literature Showed thin Bi2Se3 3D-TI may have limited promise for interconnects Showed although resonance can be obtained by band-alignment
operation, its magnitude needs significant enhancement for real applications.
43
Future Works
1. Reason for100o scattering angle threshold
Phys. Rev. Lett. 112, 136802 (2014)
Study evolution of decoherence in 3D-TI . Quantum mechanically simulate the scattering mechanism via Wigner Transport Formalism.
2. AlN substrate effectuating gapless Dirac band in 3QL Bi2Se3
ACS Nano, 2014, 8 (7), pp 6614–6619
Ab-initio simulations for heterostructure Transport properties investigation via NEGF
44
Publications (Journals)[1] Gaurav Gupta, Mansoor Bin Abdul Jalil, Bin Yu and Gengchiau Liang, "Performance evaluation of electro-optic effect based graphene transistors", Nanoscale 4, 6365-6373 (2012).
[2] Gaurav Gupta, Hsin Lin, Arun Bansil, Mansoor Bin Abdul Jalil, Cheng-Yi Huang, Wei-Feng Tsai and Gengchiau Liang, “Y-Shape Spin-Separator for two-dimensional Group-IV Nanoribbons”. Applied Physics Letters 104 (3), 032410 (2014).
[3] Gaurav Gupta, Hsin Lin, Arun Bansil, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Role of Acoustic Phonons in Bi2Se3 Topological Insulator Slabs: A Quantum Transport Investigation”. Physical Review B 89, 245419 (2014).
[4] Gaurav Gupta, Mansoor Bin Abdul Jalil, and Gengchiau Liang, “Effect of Band-Alignment Operation on Carrier Transport in Bi2Se3 Topological Insulator”, Scientific Report 4, 6220 (2014).
[5] Gaurav Gupta, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Evaluation of mobility in thin Bi2Se3 Topological Insulator for prospects of Local Electrical Interconnects”. Scientific Reports 4, 6838 (2014).
[6] Mohammad Abdullah Sadi*, Gaurav Gupta* and Gengchiau Liang, “Effect of phase transition on quantum transport in group-IV two-dimensional U-shape device”, Journal of Applied Physics 116, 153708 (2014). (*Authors contribute equally)
[7] Gaurav Gupta, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Contact Effects in thin 3D-Topological Insulators: How does the current flow ?”, Scientific Reports 5, 9479 (2015).
[8] Gaurav Gupta, Hsin Lin, Arun Bansil, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Carrier Transport in Bi2Se3 Topological Insulator Slab”. Physica E: Low-dimensional Systems and
Nanostructures 74, 10-19 (2015).
45
Publications (Conferences)[1] Gaurav Gupta, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Comparison of Electro-Optic Effect based Graphene Transistors,” 2012 International Conference on Solid State Devices and Materials (SSDM 2012) September 25-27, 2012, Kyoto International Conference Center, Kyoto, Japan.
[2] Gaurav Gupta, Argo Nurbawono, Minggang Zeng, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Theoretical study on Topological Insulator based Spintronic Tristable Multivibrator,” 2013 International Conference on Solid State Devices and Materials (SSDM 2013) September 24-27, 2013, Hilton Fukuoka Sea Hawk, Fukuoka, Japan.
[3] Gaurav Gupta, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Is Sub-10nm Thick 3D-Topological Insulator Good for the Local Electrical Interconnects?”, IEEE International Electron Devices Meeting (IEDM 2013) December 9-11, 2013, Washington DC, USA. (Travel Grant Award: $860)
[4] Gaurav Gupta, Mansoor Bin Abdul Jalil and Gengchiau Liang, “Band-Alignment Induced Current Modulation in Bi2Se3 Topological Insulator,” 2014 International Conference on Solid State Devices and Materials (SSDM 2014) September 8-11, 2014, Tsukuba International Congress Center, Tsukuba, Ibaraki, Japan. (Travel Grant Award: Yen 70000)
[5] Mohammad Abdullah Sadi*, Gaurav Gupta*, and Gengchiau Liang, “Effect of Phase Inversion on Quantum Transport in Group IV Two-Dimensional U-shape Device,” 2014 International Conference on Solid State Devices and Materials (SSDM 2014) September 8-11, 2014, Tsukuba International Congress Center, Tsukuba, Ibaraki, Japan. (*Authors contribute equally)
[6] Gaurav Gupta and Gengchiau Liang, “Quantum Transport in Two-Dimensional Group-IV monolayers and Topological Insulators”, World Congress of Smart Materials (WCSM 2015), March 23-25, 2015, Busan, Republic of Korea. (Invited Talk)
46
Publications (Others)
Book-ChaptersIn-Press
[1] Gaurav Gupta, Minggang Zeng, Argo Nurbawono, Wen Huang, and Gengchiau Liang, “Applications of Graphene Based Materials in Electronic Devices,” Chapter 19, Volume 6 (Applications and Industrialization), Graphene Science Handbook, CRC, in press.
[2] Wen Huang, Argo Nurbawono, Minggang Zeng, Gaurav Gupta, and Gengchiau Liang, “Electronic structure of graphene based materials and their carrier transport properties,” Chapter 26, Volume 2 (Nanostructure and Atomic Arrangement) of Graphene Science Handbook, CRC, in press.
Patents[1] Hsin Lin, Wei-Feng Tsai, Chen-Yi Huang, Horng-Tay Jeng, Tay-Rong Chang, Gaurav Gupta, Gengchiau Liang and Arun Bansil, “Transition Metal Dichalcogenides-Based Spintronic Devices”, US Provisional Application No.: 62/058,437, Priority Date: 1st October 2014.
47
Supervisors: Assoc. Prof. Gengchiau Liang Assoc. Prof. Mansoor Bin Abdul Jalil
Collaborators: Dr. Hsin Lin (NUS/Northeastern University) Prof. Arun Bansil (Northeastern University) Prof. Bin Yu (New York University)
Funding Agencies:MOE, ASTAR, NRF, NUS Research & PGF Scholarship
PhD Committee for helping me to improve the Thesis
Mentors, Friends and Colleagues
Mom, Dad and Sister
Acknowledgement
Thank You All for listening
48
Gaurav [email protected] [email protected]
Computational Nanoelectronics and Nanodevices Lab (CNNL)Department of Electrical and Computer Engineering
National University of Singapore