NCN 1 Neophytos Neophytou Advisory Committee Chairs: Mark Lundstrom, Gerhard Klimeck Members:...

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Neophytos Neophytou

Advisory Committee Chairs: Mark Lundstrom, Gerhard Klimeck Members: Ashraful Alam, Ahmed Sameh

Network for Computational Nanotechnology Purdue UniversityWest Lafayette, Indiana USA

Quantum and atomistic effects in nano-electronic devices

Ph.D. Thesis Defense, May 22nd, 2008

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Introduction – Device trend

Robert Chau (Intel), 2004Robert Chau (Intel), 2004

Device Challenges:1) Atoms are countable2) Strain3) Material /potential variations on nanoscale4) Crystal orientation5) III-V, Ge, InGaAs

Electronic structure features:1) Strong quantization2) Band coupling3) Non-parabolicities4) Quantum mechanics

Design Challenges1) Low dimensionality2) Parameter fluctuations3) Scalable – last for 2 generations

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Si MOSFET alternatives? CNTs

Issues: Chirality Metallic vs. Semiconducting Alignment

CNT (Delft group-1998)

Top gate+High-k(Javey et. al.) BTBT(IBM) Oscillator (IBM)

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Si MOSFET alternatives? NWs

Singapore group (IEDM 2006)

Samuelson groupEDL 2006

Still based on Si, so might have easier integration Gate all around for better electrostatics

Scattering in 1D – surface roughness? Bandstructure effects?

Samsung (APL 2008)D=8nmL=22nm

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Si MOSFET alternatives? III-Vs

Kim et. al. 200660nm InGaAs

Kim et. al. 200740nm InAs

Freescale IEDM 2007

Intel EDL 2008

High mobility, high speed, close to the ballistic limit, but low DOS (DOS Bottleneck) Lower VD, low dissipation - tunneling/leakage? Large series resistance

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Open questions

How will these devices perform at the scaling limit? What parameters control their performance? Low dimensionality: Sensitivity to defects and fluctuations?

Are they advantageous to the Si MOSFET?

Modeling tools used here: Quantum transport – NEGF (CNT, III-V HEMT) 3D (CNTs) Atomistic bandstructure (NWs)

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Why atomistic is needed –motivation

Valley splitting Band coupling

Valence band anisotropy Warped bands

m* valid m* NOT valid

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Motivation for TB

NN sp3d5s*-SO

The bulk bandstructure(from Anisur Rahman’s thesis)

[100] [110] [111]

Based on Localized Atomic OrbitalsSuitable for: Structure deformations, strain Material variations, heterostructures Surface truncation Potential variations: treated easily

Needs a large set of fitting parameters Computationally expensive

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Outline

1) 1D channel sensitivity to atomistic defects

2) Bandstructure effects in nanowires:

Self consistent model for NWs

Electron transport

Hole transport

3) III-V HEMT devices

4) Conclusions – Future work

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Defects in 1D channels

vacancy

Neophytou APL 2006, APL 2007, JCE 2007

Insulator: 4nm HfO2 (k=16)S D

22.5 nmDoping: ND = 109 /m

25 nmIntrinsic

Insulator: 4nm HfO2 (k=16)S D

22.5 nmDoping: ND = 109 /m

25 nmIntrinsic

1) NEGF2) 3D electrostatics3) Atomistic TB

CNTFET on nanoHUB.org

ID: ~27% reduction

Dangling bonds in NWs

ID: ~30% reduction

charged impurity

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Outline

Electron transport

Hole transport

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The self-consistent model for NWs

Bandstructure

(states)

EF1 - qVDS

EC(x)Semiclassical

Ballistic Transport

EF1 - qVDS

EC(x)Semiclassical

Ballistic Transport

Transport(state filling - charge)

CHANNEL

Poisson

Charge Potential

Simple model but provides physical insight

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Why need a SC model? ~0.5nm

Neophytou SISPAD 2007

Charge Variations

Ec changes

Ev changes

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Numerical issues of the SC model

Hamiltonian size – (dep. on wire size and orientation)3nm x 3nm with SO: 4k x 4k - 9k x 9k12m x 12nm with SO: 55k x 55k

eigenvalue problem150 k-points, 60 eigenvalues

Parallelized per bias point: Vd is constant Vg is varied

Timing (per bias point):3nm device: a few hours 12nm devices: 1 – 2 days

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Outline

Electron transport

Hole transport

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NMOS [100], [110], [111] wire comparison

Same capacitance/ charge in all wires [110], [100], then [111] on performance

OX Stot

Neophytou TED 2008

CS: - CQ (30%)-potential/charge variations

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Masses change with quantization

mlmtmtC

mtmlmtB

mtmtmlA

mz *my *mx *

mlmtmtC

mtmlmtB

mtmtmlA

mz *my *mx *

NW mass is controlled by

quantization of the 6 ellipsoids

[100], [111] wire masses increase [110] mass decreases

Neophytou TED 2008

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Non-parabolicity and anisotropy in the dispersion

Neophytou TED 2008

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Outline

Electron transport

Hole transport

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Ek for holes in 6nm wires

Corner effects – electrostatics Directionality in the charge - bandstructure

High gate bias

Neophytou TED 2008

(1-10)

Energy surfaces

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Anisotropy implications on the device performance

Kobayashi et. al.JAP 103, 2008

[110] side variations, do not affect the device – VT, Ion [100] side variations, affect the device

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Outline

Electron transport

Hole transport

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Motivation: del Alamo group HEMTs

Source Drain

InGaAs/InAlAs

InAlAs(11 nm)

InGaAs(MQW)

InAlAsBuffer

InP(6 nm)

Typical IDS vs. VDS

Reference: Dae-Hyun Kim et al. IEDM 2006

• how close to ballistic limit? • role of mobility• degradation of gm at high VG

Typical Gm vs. VGS

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Approach

Simulation:

• 2D Poisson in the cross section • NEGF in the channel and upper buffer layer (ballistic)• Include Rs to fit low VDS conductance to experiment• Bulk material masses (In0.7Ga0.3As)• Adjust ΦB to achieve the experimental VT

• Parallelization: One VG set per CPU (constant VD) 2 hours per bias point – 20 hours per I-V

δ-dop. n++

L_side2.1e12/cm2 10e12/cm2

L_side2.1e12/cm210e12/cm2

60nm40nm 60nm

15nmInGaAs

InAlAs

500nmInAlaAs

n+ n++δ-dop. n++

L_side2.1e12/cm2 10e12/cm2

L_side2.1e12/cm210e12/cm2

60nm40nm 60nm

15nmInGaAs

InAlAs

500nmInAlaAs

n+ n++

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Series resistance and “Ballistic” mobility

170 - m( )

~ 170 - 450 cm /V-s

DS B ins G T

I W C V V

(Depending on the Tins)

ballistic simulation

measured(Tins = 3 nm, L = 60 nm)

ballistic + Rs = 400 -m

“ballistic mobility:”

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LG = 60 nm vs. Tins

Tins=3nm Tins=11nmTins=7nm

1) Except for high VG, all results can be explained as a ballistic FET with series R

2) Series resistance increases as Tins decreases

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Source limits

2) Barrier collapses

Gm rolls off in the ballistic model too.

1) OFF state

3) Gate loses control

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Charge

CS degrades Cins by 2.5 x

Q=Cins(VG-VT)

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Velocity

1) Non-parabolicity degrades the velocity by ~10%

Velocity is low:

Due to quantum mechanical reflections and tunneling

v ~ 2.7

v~ 3.6

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Outline

Self consistent model

Electron transport

Hole transport

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Conclusions (1)

1) 1D channel are sensitivity to single atomistic defects:

Vacancy, charged impurities, dangling bonds

2) Transport in nanowires:

Non-parabolicity, anisotropy causes mass variations, charge distribution variations

EMA cannot be used in general

NMOS: [110], [100] perform better, [111] worse

PMOS: [111], [110] perform better, [100] worse

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Conclusions (2)

3) III-V HEMT devices:

Ballistic channel + RSD

Low charge and velocity, low “apparent” mobility

Importance of the source design

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General concluding comments

Low dimensional devices:

Importance of CS, CQ, that degrade COX

But variations in parameters that influence C do not affect the device

Velocity is important

But, low mass tunnels more so the velocity can be reduced

Device is mostly controlled by external parameters rather than the channel (RSD, parasitics)

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Future work

Identifying the ultimate MOSFET: Perform appropriate comparisons between Si MOSFETs and UTB, NW devices at the scaling limit. Power, speed, Ion, Ion/Ioff, leakage, parasitics Device to circuit level Optimal strain and wafer/transport orientation, material, Is it different at each technology node? Which device for which application – identify appropriate use

Modeling for nanoscale devices: Contacts in low dimensional devices: DD + Ballistic NEGF, dephasing mechanisms (equilibrium or not?) Schottky barriers Inexpensive treatment and use of complex bandstructures, and distortions. (zone unfolding). NEGF + TB – real or mode space for PMOS in NWs and UTB

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AcknowledgementsProf. Mark LundstromProf. Gerhard KlimeckProf. Ashraful AlamProf. Ahmed Sameh

Jin Guo, Siyu Koswatta, M.P. Anantram (CNT)Diego Kienle, Eric Polizzi, Shaikh Ahmed (CNTFET)Anisur Rahman, Jing Wang, Mathieu Luisier (Bandstructure)Abhijeet Paul (generalized poisson for SC model)Titash Rakshit (HEMT)

Yang Liu (UTB work)Gengchiau Liang, Dmitri Nikonov (Graphene)

All others in EE350

NCN for the computational resourcesCheryl Haines

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BACKUP

Neophytou APL 2006

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Explanations for the Ek - transport

[100] subbands [110] subbands

Quantization surfaces-Structural

-Electrostatic

Neophytou TED 2008

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Anisotropy implications on the device performance

Neophytou, in preparation

[1-10]

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Figure 1 – The different quantizations of the different surfaces

(110) surface(100) surface

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Figure 2 – Current surface for variation of the dimensions

[1-10]

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Tins / gate length dependence

- Low DOS degrades charge

- At high VG the measured current deviates from the ballistic limit.

- As LG decreases, ID approaches the ballistic limit

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3nm wire dispersions in different orientations

3nm-[100] 3nm-[110] 3nm-[111]

Mass at Γ: 0.27 (0.19) Degeneracy : 4 Excited states shift down

Mass at Γ: 0.16 (0.19) Degeneracy : 2 Valley Splitting

Mass: 0.46 (0.43) Degeneracy: 6

OFF-Γ

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Approach - Bandstructure

In0.7Ga0.3As – undistortedm*=0.048m0 (matches DOS up to 0.2eV) (account for non-parabolicity)L valleys are too high

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Transconductance degradation

1) Cannot be explained by series resistance

2) Possibly scattering at high VG (not loss of confinement or upper valleys

3) Is there a ballistic mechanism that can

explain this?

NCN 1 Neophytos Neophytou Advisory Committee Chairs: Mark Lundstrom, Gerhard Klimeck Members:...

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