PPT thesis defense_nikhil

Hexagonal Boron Nitride: Ubiquitous Layered Dielectric for Two-Dimensional

ElectronicsNikhil Jain

Thesis Committee Members:Prof. Bin Yu (Research Advisor)

Prof. Carl Ventrice Jr.Prof. Vincent LaBellaProf. Ernest Levine

Prof. Sergey Rumyantsev (RPI)

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Introduction to 2D materials

Graphene/h-BN heterostructures

h-BN as an ubiquitous dielectric

Substrate

Gate dielectric

Passivation layer

Intercalation layer

Conclusions and future

directions

Outline of Presentation

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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A Paradigm Shift

New Material Platform “Ubiquitous” Electronics

Ultra-thin materials Self-limited processing Ultimate scalability Hetero-integration Flexible, soft, transparent Open, connected “Things”

Silicon PlatformMicro/Nano Electronics

Bulk materials Low scalability Stiff, hard, brittle Externally powered Packed, isolated “chips”

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What are 2D Layered Materials?

(Courtesy: Y. Cui, Stanford Univ.)

Materials where individual layers of covalently bonded atoms/molecules are held together by van der Waals

forces

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Graphene

Molybdenum Disulfide

2D Semi-Metal

3-atom-thick monolayer

Gallium Selenide4-atom-thick monolayer

5-atom-thick monolayer

Bismuth Selenide

Hexagonal boron nitride

2D Insulator

2D Semiconductors

Classification of 2D Materials

based on electronic structure

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2004

Extraction of graphene by Andre Geim and Konstantin Novoselov using scotch tape method

1937

R. E. Peierls and L. D. Landau suggest that strictly 2D crystals could not exist

1962

Hanns-Peter Boehm coins the terms graphene

1980s

Theoretical studies on graphene confirm massless Dirac equation & anomalous Hall effect

2005

Geim and Novoselov exhibit free-standing 2D crystals of boron nitride, several transition metal dichalcogenides, and complex oxides

2D Materials: Brief History

1947

Wallace calculates the band structure of single-layer graphite

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2D Materials – Extraction Methods

The crystalline quality and correspondingly the electronic properties rely on the method used to extract the 2D material

nanosheet under study.Micromechanical exfoliation Liquid-phase or

chemical exfoliation

Chemical vapor deposition

K. S. Novoselov et al, Phys. Scr., 2012

Image Source: http://www.azonano.com

Image Source: http://emps.exeter.ac.uk/

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Why Graphene?

The electrons in the pz orbital hybridize to give Π and Π*

bands

Momentum confined to two dimensions

Zero-gap semiconductor

Two sets of 3 Dirac points

Fermi energy at Dirac Point

Cone like linear dispersion relation within 1eV of Dirac point

Zero effective mass of charge carriers in the region

Fermi velocity, vF ≈ 106 m/s

Dirac Points

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D. R. Cooper et al, International Scholarly Research Notices 2012

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Intrinsic advantages Superior electrical

conduction (µ ~ 20,000 cm2/Vs: 20X of silicon

Excellent thermal conduction

(~5.3x103 W/m-K: 10X of copper) High mechanical

strength (Young’s modulus: 0.5 TPa)

3-5% light absorption (monolayer)

Graphene: Key Properties

TEM

Optical Image Lattice Structure

AFM

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Electrical Analysis

Charge carrier density, n = ε0ε: Permittivity of SiO2

e: Electron charge t: SiO2 thickness

Resistivity, Mobility, µ = Alternately, field-effect mobility is

given by:

µ =

C = (Gate Capacitance)

In this work, the term mobility refers to field-effect mobility.

At Vg = 0, n should vanish but minimum

conductivity is introduced by thermally generated

carriers and electrostatic spatial inhomogeneity.

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Graphene: One-atom-thick sheet with no “bulk”, but all surfaces

Behavior is extremely sensitive to its interface with neighboring materials like:

Supporting substrate Top surface (ambient environment)

The “Real Significance”

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Carrier mobility ~ 200,000 cm2/V.s for suspended graphene.

– Actual values: 1000 ~ 3000 cm2/V.s on SiO2 substrate

Graphene/Dielectric Interface

Graphene electrical conduction is largely impacted by interface with dielectrics.

Images Courtesy: Enrico Rossi, CMTC, University of Maryland

Spatial inhomogeneity increases ON current and scattering sites decrease the OFF current.

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Joule-heating Induced Breakdown

Carrier scattering mechanisms increase resistivity in graphene. Impurity and defect scattering – Interface effect Longitudinal acoustic (LA) phonon scattering – Intrinsic

effect Surface polar phonon (SPP) scattering – Substrate effect

Voltage

Current

TemperatureJoule Heating

I2R

Resistivity

Causes Breakdo

wn

LA and SPP scattering increases with temperature.

Images Courtesy: H.-S. P. Wong, Stanford University 14

Graphene Breakdown

creates a gap

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h-BN: An Ideal 2D Dielectric

Hexagonal Boron Nitride High crystal quality (negligible defect

density) Atomically smooth surface Free of surface state High-energy surface polar phonons Thermal conductivity: ~20 W/m-K (20X of

SiO2)

Image Courtesy: C Casiraghi

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Problem Statement

While 2D material-based heterostructures can be immensely useful for next generation electronics, 2D materials are extremely sensitive to their immediate environment.

SiO2 and other dielectrics currently used in the fab make a highly invasive interface with 2D materials.

Pristine properties of graphene can be seen in suspended orientations but it is not feasible to make chips using structures suspended in vacuum.

Can h-BN fulfill the role of an ideal dielectric neighbor to graphene for the purpose of making on-chip

components?

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Research Goals

Develop effective processes to prepare 2D material-based functional heterostructures

Demonstrate prototypes of applications: field-effect transistors (FETs) and on-chip Interconnects using graphene/h-BN heterostructures

Study the role of h-BN as a non-invasive dielectric neighbor for graphene

Explore basic physical/electrical behavior of interest from the performance and reliability standpoint 17

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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2D Based New 3D SolidsRational Stacking-By-Design

A. K. Geim, Nature, 2013

Selective assembly of 2D materials can lead to innovative device design

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Heterostructure Formation

2D heterostructures: building elements in future electronics

ACVD over

Bex

ACVD

stacked

over

BCVD

ACVD grown

over BCVD/ex

In situ CVD

growth of

A/B

• Subscript “Ex” signifies exfoliated material• Subscript CVD signifies material growth by chemical vapor deposition

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CVD Graphene Growth

Step 1: Ramp up to 1000C with Ar (80 sccm) + H2 (5 sccm)Step 2: Anneal the Cu strip at 1000C (Same gas flow)Step 3: Graphene growth in CH4 (30 sccm) + H2 (5 sscm)Step 4: Cool down in Ar (80 sccm) + H2 (5 sccm)

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Layer by Layer (LbL) fabrication is efficiently used for emerging 2D layered structures.

Large-area assembly using CVD grown graphene monolayer is possible.

CVD graphene growth

Monolayer transferring

Multilayer stacking

Assembly of CVD Graphene

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** CAB – Cellulose Acetate Butyrate

Assembly of Exfoliated h-BN

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Summary

Facile processes to make 2D heterostructures have been developed.

CVD growth of graphene and transfer to any target substrate has been demonstrated.

Assembly of exfoliated materials to target substrate has been demonstrated with multiple methods. Necessary as long as CVD growth methods for other

materials are still being developed.

Layer-by-layer stacking of nanosheets to create ternary (or thicker) heterostructures has been shown. With controlled precision on where the third layer is

assembled.

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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Hexagonal Boron Nitride Single-crystalline Atomically smooth surface Free of surface state High-energy surface phonons Thermal conductivity: ~20

W/m-K (20X of SiO2)

Silicon Dioxide Amorphous Surface roughness Rich in trapped charges Low-energy surface phonons Thermal conductivity: ~1.04

W/m-K

Graphene

h-BN

(lattice mismatch ~ 1.6%)

h-BN: Substrate for Graphene

Image Courtesy: Jarillo-Herrero Group, Quantum Nanoelectronics, MIT

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Graphene On h-BN (GOBON)

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Electrical Performance of GOBON

Conductivity and mobility improvement is observed in GOBON when compared with graphene (CVD or exfoliated) on SiO2.

Resistivity (at VG = 0V) drops by approximately 19x in GOBON as compared with that on SiO2.

At the carrier density of 1×1012 cm-2, carrier mobility in GOBON is improved by about 17x compared with CVD graphene on SiO2.N. Jain et al, IEEE Electron

Device Letters, 33 (7), 2012

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Reliability Enhancement in GOBON

Due to improved thermal conductivity of h-BN, the permissible current and voltage before permanent breakdown in graphene are enhanced.

PBD = JBD (VBD – JBDRC)

~ 7X increased power density @ breakdown

Thermal conductivity: ~20 W/m-K): ~20 times that in SiO2 (1.04 W/m-K)

Prevent Joule heat built up in graphene

where,JBD = Current density at breakdownVBD = Voltage at breakdownRC = Contact resistance

N. Jain et al, IEEE Electron Device Letters, 33 (7), 2012

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Electrical Annealing Effect

Electrical annealing shifts the Dirac point in graphene on SiO2, but this change is avoided in GOBON due to less interfacial trap charges

G/h-BN

G/SiO2

T. Yu, Applied Physics Letters 2011, 98, 243105.

N. Jain et al, IEEE Electron Device Letters, 33 (7), 2012

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Summary

h-BN has been shown to be an excellent substrate for graphene.

Graphene resistivity on h-BN is found to be 19 times lower than on SiO2 (the current standard substrate).

There is a 17-fold improvement in graphene mobility when placed on h-BN compared with SiO2.

Improved heat dissipation through h-BN results in higher values of current density and power density required to cause Joule heating-induced breakdown in graphene.

The Dirac point in GOBON structures is stable under the effect of electrical annealing.

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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h-BN as Gate Dielectric

h-BN could also serve as gate dielectric

k = 3.9EG = 5.97 eVself-terminating surfacechemically inert

Key questions:What is the dielectric behavior? 33

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Titanium Nitride (TiN) filled trenches are created in a Si/SiO2 wafer to act as a gate for GOBON FET

Buried Gate Structures: Fabrication

* This process is done in the

fab 34

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GOBON FET with h-BN as Gate Insulator

* FET fabrication process is same as shown in previous section.

G/h-BN/TiN

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Performance of GOBON FETs

Carrier mobility of CVD graphene on h-BN (on TiN) is 1.4X higher than mechanically exfoliated graphene on SiO2 at effective electric field of 2x105

V-cm-1

N. Jain et al. Carbon, 54, 396–402 (2013)

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Dielectric Strength of h-BN

No dielectric breakdown up to very high electric field (15 MV/cm) Transition from insulating to leakage occurs at a voltage that is

directly proportional to h-BN multilayer thicknessN. Jain et al. Carbon, 54, 396–402 (2013)

h-BN is a robust dielectric which resists dielectric breakdown at high electric fields.

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Summary

h-BN has been shown to be a robust gate dielectric for FETs made with graphene.

Graphene mobility is enhanced in GOBON FETs compared with graphene FETs with SiO2 as gate dielectric.

As a gate dielectric, h-BN does not undergo dielectric breakdown even under very high electric field of 15MV/cm.

h-BN undergoes a reversible transition to a leaky dielectric at high fields, which is dependent on layer thickness.

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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Need for graphene encapsulation

Whatever be the substrate, environmental adsorbents reduce graphene conduction

Adsorbent sites act as charge traps

Encapsulation with traditional capping materials degrades graphene quality

h-BN as a passivating layer conforms to graphene surface 40

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Fully Encapsulated Graphene

* CAB – Cellulose Acetate Butyrate

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Passivation Effect of Top h-BN

Insensitive to environmental (ambient) impact R-V characteristics show no variation in air and in vacuum for

encapsulated device No variation in contact resistance between ambient and

vacuum N. Jain et al, Nanotechnology, 24, 355202 (2013)

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67% increase in breakdown power density compared to uncovered GOBON devices due to increased heat dissipation through both graphene surfaces

No reduction in carrier mobility

Electrical Behavior

N. Jain et al, Nanotechnology, 24, 355202 (2013)

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Summary

h-BN has been shown to be an effective passivation layer for graphene devices.

When passivated with h-BN, graphene performance becomes insensitive to the measurement conditions (ambient or vacuum).

Graphene – Metal contact performance is improved.

Higher current density and power density are needed to cause breakdown in encapsulated graphene devices.

The improvement is achieved without a compromise on carrier mobility. 44

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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Cu CNT Graphene

Max current density (A/cm2) ~106 > 1x108 > 1x108

Melting Point (K) 1356 3800 (graphite) 3800 (graphite)

Tensile Strength (GPa) 0.22 22.2 23.5

Thermal Conductivity (×103 W/m-K) 0.385

1.75Hone, et al.

Phys. Rev. B 1999

3 - 5Balandin, et al. Nano Let., 2008

Temp. Coefficient of Resistance (10-3 /K) 4

< 1.1Kane, et al.

Europhys. Lett.,1998

-1.47Shao et al.

Appl Phys. Lett., 2008

Mean Free Path@ room-T (nm) 40

> 1000McEuen, et al.

Trans. Nano., 2002

~ 1000Bolotin, et al.

Phys. Rev. Let. 2008

x102

x10

x25

x102

Graphene as a Conductor

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Towards “3-D Graphene”

At small critical dimensions (width < 100 nm), ρGraphene < ρCu

Small cross section in monolayer graphene limits conduction.

Multilayer graphene has less sheet resistance than monolayer graphene.

Onset of inter-layer scattering of charge carriers in multi-layer graphene doesn’t allow the sheet resistance to scale down as expected

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Double-Layer Graphene (DLG): Fabrication

DLG structure with h-BN between two monolayer graphene sheets with direct metal contact with both graphene layers

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Massless Dirac Fermions in DLG

DFT simulation of the dispersion relation of the DLG structure indicates that carriers are massless Dirac

fermions

* DFT analysis was performed by our collaborators at University of Washington.

Band splitting in BLG

Π and Π* bands divide in four bands due to interlayer scattering

Degeneracy is restored in DLG

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Raman Spectra of Graphene

Single 2D peak in monolayer graphene

Due to coupling between layers, two or four peaks exist in 2D

band (>2 layers)

1400 1600 1800 2000 2200 2400 2600 2800 3000

2D band

Norm

aliz

ed in

tensi

ty

Wavenumber (cm-1)

1layer 2layer 3layer 4layers 5layer Graphite

G band

More layer number - Intensity ratio of G/2D

increased

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Freitag, M. Nat Phys, 2011, 7, 596–597

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Raman Spectral Analysis for Scattering Measurement

2D peak in the Raman spectrum of bilayer graphene is composed of four components arising from the band split at Dirac point.

Reduced height of the overall 2D peak

Increase in IG/I2D

Increase in FWHM2D

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Raman Spectral Analysis for Scattering Measurement

Addition of graphene layers results in increase in IG/I2D and FWHM2D.

For stacked turbostratic graphene, addition of each layer results in lesser increase than in exfoliated graphene, indicating reduced scattering in stacked graphene

Similar effect is seen in FWHM2D

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Raman Spectral Analysis

IG/I2D and FWHM2D in DLG is similar to monolayer graphene (much lower than stacked or exfoliated

BLG)

Introduction of h-BN as an intercalation layer in double-layer graphene reduces interlayer carrier

scattering. 53

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Electrical Characterization

Reduced interlayer scattering allows higher current in DLG.

Current and conductivity in DLG ~ MLG > BLG

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Performance Enhancement

Mobility and breakdown current density in DLG show enhancement.

Carrier Mobility in DLG > MLG

JBD in DLG > 2x JBD in BLG

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Reliability Improvement

Under extreme electrical stress, DLG resists breakdown more than MLG and BLG.

At an elevated temperature (150C) under the effect of a constant voltage (10V), the DLG sample withstands a current density of ~ 475 mA/cm2

The mean time to failure (MTTF) for DLG is ~ 75 and ~4000 times higher than that for BLG and MLG systems

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Summary

h-BN has been shown to be an interposer layer that prevents interlayer scattering from degrading the performance of double-layer

graphene.

Increase in the IG/I2D ratio and FWHM2D have been shown as indicators of interlayer scattering.

Random-stacked (turbostratic) graphene shows lower interlayer scattering than Bernal-stacked graphene.

As an intercalation layer, h-BN removes interlayer scattering resulting in ideal current scaling due to layer stacking.

Higher carrier mobility and resistance to breakdown at extreme electrical stressing conditions are also observed in DLG.

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Substrate

Gate dielectric

Passivation layer

Intercalation layer


directions


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Conclusions

h-BN has been explored as a multi-function dielectric for future 2D material enabled electronics.

Facile assembly/fabrication processes for 2D heterostructures have been demonstrated.

h-BN serves as excellent supporting substrate, largely preserving “pristine” graphene electronic transport.

h-BN is demonstrated as a highly robust gate dielectric (medium-k value).

Fully encapsulated 2D heterostructure (h-BN/graphene/h-BN) provides passivation and enhancement of maximum power density in graphene without compromising electrical conduction.

As an intercalation layer between graphene layers, h-BN reduces interlayer scattering and restores mobility to ‘monolayer-like’ value while also making the structures more robust to stress.

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Future Directions

(1) Direct all-CVD growth process GOBON: Graphene growth on exfoliated h-BN BNOG: h-BN growth on CVD/exfoliated graphene

(2) Study of 2D heterostructure properties

(3) On-chip device, interconnect, circuit demonstration

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Future Directions

CVD growth of h-BN on copper

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Superlattice-like structures of graphene/h-BN

Future Directions

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Acknowledgments

Lab Members (Present and Past): Dr. Bhaskar Nagabhirava Dr. Tianhua Yu Dr. Tanesh Bansal Dr. Mariyappan Shanmugam Dr. Fan Yang Robin Jacobs-Gedrim Eui Sang Song Thibault Sohier Christopher Durcan

Our Collaborator: Prof. M. P. Anantram (Univ. of Washington,

Seattle)

CNSE CSR Team: Dr. Vidya Kaushik Dr. Prasanna Khare Megha Rao

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Journal Publications

1. N. Jain, M. Murphy, R. B. Jacobs-Gedrim, M. Shanmugam, F. Yang, E. S. Song, and B. Yu, “Electrical Conduction and Reliability in Dual-Layered Graphene Heterostructure Interconnects,” IEEE Electro Device Letters, vol. 35, no. 12, 1311-1313 (2014).

2. R. B. Jacobs-Gedrim, M. Shanmugam, N. Jain, C. A. Durcan, M. T. Murphy, T. M. Murray, R. J. Matyi, R. L. Moore, and B. Yu, “Extraordinary photoresponse in two-dimensional In2Se3 nanosheets,” ACS Nano, 8, 1, 514-521 (2014).

3. N. Jain, C. A. Durcan, R. B. Jacobs-Gedrim, Y. Xu, and B. Yu, “Graphene interconnects fully encapsulated in layered insulator hexagonal boron nitride,” Nanotechnology, 24, 355202 (2013).

4. N. Jain, T. Bansal, C. A. Durcan, Y. Xu, and B. Yu, “Monolayer Graphene/Hexagonal Boron Nitride Heterostructure,” Carbon, 54, 396–402 (2013).

5. T. Bansal, C. A. Durcan, N. Jain, R. B. Jacobs-Gedrim, Y. Xu, and B. Yu, “Synthesis of Few-to-Monolayer Graphene on Rutile Titanium Dioxide,” Carbon, 55, 168-175 (2013).

6. M. Shanmugam, N. Jain, R. B. Jacobs-Gedrim, Y. Xu, and B. Yu, “Layered insulator hexagonal boron nitride for surface passivation in quantum dot solar cell,” Applied Physics Letters, 103, 243904 (2013).

7. R. B. Jacobs-Gedrim, C. A. Durcan, N. Jain, and B. Yu, “Chemical Assembly and Electrical Characteristics of Surface-Rich Topological Insulator Bi2Se3 Nanoplates and Nanoribbons,” Applied Physics Letters, 101, 143103 (2012).

8. E. Kim, N. Jain, R. Jacobs-Gedrim, Y. Xu, and B. Yu, “Exploring Carrier Transport Phenomena in CVD-Assembled Graphene FET on Hexagonal Boron Nitride,” Nanotechnology, 23, 125706 (2012).

9. N. Jain, T. Bansal, C. Durcan, and B. Yu, “Graphene-Based Interconnects on Hexagonal Boron Nitride (h-BN) Substrate,” IEEE Electro Device Letters, vol. 33, no. 7, 925-927 (2012).

ARTICLES UNDER REVIEW 10. N. Jain, R. Jacobs-Gedrim, Y. Xu, and B. Yu, “Resistive Switching in Ultra-Thin Two-Dimensional van der Waals

Dielectric” Nature Communications (2015). 11. N. Jain, R. B. Jacobs-Gedrim, M. Murphy, M. Shanmugam, F. Yang, Y. Xu, and B. Yu, “Electrical Conduction in Two-

Dimensional Graphene/Hexagonal Boron Nitride/Graphene Heterostructure,” Nano Letters (2015).12. R. Jacobs-Gedrim, M. Murphy, N. Jain, F. Yang, M. Shanmugam, E. Song, Y. Kandel, P. Hesamaddin, D. B. Janes, and

B. Yu, “Reversible Crystalline-Amorphous Phase Transition in Chalcogenide Nanosheets”, Nature Materials (2015).

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Thank You for Your Attention

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Significance of Environment

Open graphene is subject to severe degradation over time due to the effect of adsorption of ambient molecules like N2, H2O and O2

Graphene/metal contact I-V behaviorTime-dependent contact resistance shift

Demand: Graphene covered with an insulator which protects its pristine electrical behavior

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Metal Contacts Graphene at 1-D

Edge

Fabrication made simpler with only one patterning step for the G/h-BN/G stack and one metallization

step

L Wang et al, Science 342, 614 (2013)

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2D Band Curve Fitting Results

Bilayer Graphene Trilayer Graphene

2600 2650 2700 2750 2800

P1: 2656

P2: 2688

P3: 2707

P4: 2722

Wavenumber (cm-1)

2600 2650 2700 2750 2800

P1: 2694

P2: 2719

Wavenubmer (cm-1)

2600 2650 2700 2750 2800

P1: 2696

P2: 2722

Wavenumber (cm-1)

Four Layer Graphene

2600 2650 2700 2750 2800

P1: 2695

P2: 2725

Wavenumber (cm-1)

Five Layer Graphene

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Raman Spectra of s-MLG

More layer number:

• 2D band blue shift• Intensity ratio of G/2D increased.

• Less coupling between layers, only one peak exists in 2D band (2~5 layers)

1400 1600 1800 2000 2200 2400 2600 2800 3000

Wavenumber (cm-1)

as -- -- -- -- --

2D band

G band

1400 1600 1800 2000 2200 2400 2600 2800 3000

2D band

Norm

aliz

ed in

tensi

ty

Wavenumber (cm-1)

1layer 2layer 3layer 4layers 5layer Graphite

G band

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Lifetime Reliability Study

Sustained current in graphene can lead to degradation and eventual failure of the wire

Comparison of stacked BLG and G-BN-G heterostructure can provide information about improvement in graphene interconnect reliability by incorporation of h-BN between graphene layers

Mean Time to fail (MTTF) in G-BN-G heterostructure will be higher than MLG and stacked BLG at same current density

X Chen et al, IEEE EDL 2012

PPT thesis defense_nikhil

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