Electronics with 2D Crystals: Scaling extender, or ... · 2D crystal semiconductors extend vertical...

1 Debdeep Jena ([email protected]), University of Notre Dame

Electronics with 2D Crystals: Scaling extender, or harbinger of new functions?

1st Workshop on Data Abundant Systems Technology

Stanford, April 2014

Debdeep Jena ([email protected])

Electrical Engineering, University of Notre Dame


Outline

• Charge-based electronics Conventional Neo

• 2D Crystals Electrostatics Scaling Bonds/Interfaces/Heterostructures Dielectrics Inversion Transport Effective masses, conventional transport Tunability Contacts

• ‘Neo’ electronics possibilities enabled by 2D crystals

• Challenges moving forward


Outline






The Transistor: Up against ‘fundamental’ limits

• The transistor is an electronic switch: Digital Electronics • It is also an amplifier: it has gain • Gain @ high speed: RF electronics • Switching @ high voltages: Power electronics

S D G


Charge-based electronics wins for digital electronics

10-3

10-2

10-1

100

101

102

10-4

2012

103

CMOS low power

Delay (ps)

Energ

y (

fJ)

CMOS high performance

104 100 101 102 10-1

BISFET

Graphene

nanoribbon

III-

V

Heterojunction

Graphene pn junction

Spin-wave

Spin

FE

T

Preferred

Corner

Nanomagnet

Logic

All-spin

logic

ST oscillator

ST transfer

triad

ST

Majority

gate

ST: Spin-Torque

ST Transfer/

Domain-Wall

Charge- vs spin-based LOGIC

(industry benchmarks)

Energy vs. delay of inverters with fanout 4 with current-controlled switching, Vdd=0.01 V

Physics hiding ‘under the hood’


Electronic switches today

FETs Bipolars

TFETs Other candidates

22nm FinFET

Conventional

Neo

• Scaling • Surfaces/Interfaces • Power consumption • Efficiency/cost

• Scaling • Surfaces/Interfaces • Power consumption • Efficiency/cost • Complementary logic

• Scaling • Surfaces/Interfaces • Performance • Complementary logic

BisFETs, MottFETs, etc…

• Realistic demos • Scaling • Surfaces/Interfaces • Performance • Complementary logic

Need breakthroughs in… Need breakthroughs in…

Need breakthroughs in… Need breakthroughs in…

Silicon CMOS: “The reports of my death are greatly exaggerated”


Bandstructure of traditional semiconductors

• 3D semiconductors: sp3 orbitals. • Conduction by carriers near band edges. • Conduction band (electrons): s-like spherical, isotropic • Valence band (holes): p-like highly anisotropic

sp3 hybridization

Real-space picture of Electron Orbitals


Outline






2D Crystals: Graphene, Semiconductors, and more…

Graphene Family: Graphene: symmetry zero bandgap Boron Nitride: broken symmetry 5.2 eV bandgap MX2 Family: Semiconducting Metallic, Charge-density wave Magnetic, Superconducting Common characteristics: No out-of-plane chemical bonds Thinnest materials known

Graphene, BN MoS2, MX2 family


Outline






As 3D Crystal semiconductors become small…


2D Crystals offer a NEW electronic phase space

2D crystals


Scaling and electrostatics with 2D crystals

2D crystals

2D crystal semiconductors extend vertical scaling. Excellent electrostatics in 2D geometries. Double-gates natural (this is what SOI always wanted to be!). Lateral scaling: Controllable by doping/bandgaps.


Outline






Bonds/Interfaces/Heterostructures

• Chemical bonds are made of s, p, and d-orbitals. • No out-of plane bonds No dangling bonds Low chance of interface traps. • Interfaces are ‘pristine’, no strain as in 3D heteroepitaxial materials. • Heterostructures are formed by ‘stacking’ or ‘van-der-Waal’ epitaxy. • Band-offsets are ‘pristine’ and ‘easily’ measured.


Outline






Dielectrics for 2D Crystals: HfO2

• SS ~ 74 mV/decade. • On/Off~108. • Conventional ALD seems to work, but hysteresis may be present. • Nature Nano (Kis group, 2011).


TFT switches today & the case for layered materials

• Traditional TFT materials (amorphous Si, organics, oxides) have either • Low mobilities (< 1 cm2/V.s) or • Very high subthreshold slopes (~1 V/decade) due to defects

• Layered materials offer a unique solution

log(ID)

VGVDD

MOSFET

off on

SS mobility

IEDM 2012


Multilayer MoS2 Thin-Film Transistor (TFT)

Nat. Comm. 2012

• SS ~ 70 mV/decade. • On/Off~107. • Robust current saturation: first time in a 2D layered crystal! • Current saturation is very important in transistors for gain & fan-out


Dielectrics for 2D Crystals: 2D crystal BN!

• 2D BN breakdown field ~8 MV/cm! • No dangling bonds, much cleaner. • 2D dielectrics for 2D crystals seems feasible. • Columbia group.


Outline






Carrier inversion in MoS2: Observed

• Switching from n-channel to p-channel achieved with ALD dielectric. • Contacts to one type of carriers inversion is slow. • Very essential for complementary logic with 2D crystal semiconductors! • Notre Dame.


First MoS2 ‘circuits’…

• First rudimentary logic circuits using MoS2

• EPFL (2012) and MIT (2012).


Outline






Effective masses, Mobilities

• In-plane effective masses are ‘large’ DOS is high, mobility is low (~few 100’s cm2/V.s) • Electron and hole effective masses are ‘similar’ • Guo group (Florida).


Effective masses, Mobilities of MoS2

TMD semiconductors to date (expt)

• Experimental mobilities are VERY LOW! • What are the fundamental limits of transport and mobility of TMD semiconductors?


Current drives in the ballistic limit: Projected

• What is lost in transport is gained back in electrostatics: high current drives in scaled limits. • Guo group (Florida).

pMOS nMOS


Outline






Scattering and Mobility limits in Monolayer MoS2

T=300 K

ns=1013 cm-2

m ~4200cm2 Vs

N I 1011cm-2( )

Phonon

scattering

Charged

impurity

scattering

109

1010

1011

1012

1013

101

102

103

104

23.5

ZrO2/HfO

2

13.5

SiO2/HfO

2

10.8AlN/Al

2O

3

2.5

5.1BN/BN

Mo

bil

ity

(c

m2/V

s)

Impurity density (cm-2)

Air/Air

SiO2/Air

1

e

Currently reported electron mobilities are

limited by Ionized impurity scattering

Very low impurity

densities:

intrinsic/remote

phonon scattering

determine the highest

attainable mobilities.

Intrinsic mobility

accessible in

CLEAN,

SUSPENDED layers

High-κ gate dielectrics

can increase the electron

mobility only for samples

infected with very high

impurity densities


Outline






Quasi-2D properties in a Wide-Bandgap ‘3D’ Crystal

• Ga2O3 devices demonstrated in 2012/2013 • There are crystals ‘between’ 2D and 3D … quasi-2D? • A wholly unexplored arena for new high temperature & high-voltage logic devices


Quasi-2D high-voltage transistors: on-chip power conditioning

Nanomembrane high-voltage transistors with Ga2O3


MBE growth of extreme-bandgap oxides

Initial MBE growths of Ga2O3 at Notre Dame

2e-8 5e-8 8e-8 1.1e-7

4.9 eV 4.9 eV No growth No growth

MBE Ga2O3


Outline






2D Crystal Device Roadmap

We are

here today


Materials challenge: TMD/layered semiconductors

Molecular Beam Epitaxy of 2D Crystal Heterostructures For precise DOPING & Heterostructures (Xing, Furdyna, Jena)


2D Crystal Electronics Benchmarking


Herbert Kroemer’s message


Surprises in d-orbitals: Superconductivity

Case in point: Superconductivity in

MoS2 FETs: d-orbital effects show up!!


The ‘Golden Pavilion’ temple in Kyoto

Electronics with 2D Crystals: Scaling extender, or ... · 2D crystal semiconductors extend vertical...

Documents

Transcript of Electronics with 2D Crystals: Scaling extender, or ... · 2D crystal semiconductors extend vertical...