Feng Yuan Ping (冯元平 )Department of Physics

National University of Singaporephyfyp@nus.edu.sg

First Principles Studies on High-k Oxides and Their Interfaces with Silicon and Metal Gate

Aug 29 - Sept 1, 2006 CCP2006 2

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www.mrs.org.sgwww.mrs.org.sg

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Outline

Introduction Oxygen vacancy in HfO2 and La2Hf2O7

Tuning of metal work function at metal gate and high-k oxide interface

Properties of high-k oxide and Si interface Conclusion

S D

G

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ITRS roadmap shows the expected reduction in device dimensions

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G

CMOS Scaling

0

50

100

150

200

0

50

100

150

200

2000 2005 2010 2015

Tech

nolo

gy n

ode

(nm

)

Junc

tion

dept

h (n

m)

Year

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1.2 nm (5 atomic layers) physical SiO2 in production of 90 nm logic technology node; 0.8 nm physical SiO2 in research of transistors with 15 nm physical Lg

Gate leakage is increasing with reducing physical SiO2 thickness. SiO2 layers <1.6 nm have high leakage current due to direct tunneling. Not insulating

SiO2 running out of atoms for further scaling. Will eventually need high-K

Why High-k oxides ?

SiO2

HK Oxide

GateCB Si

Rober Chau, Intel

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Choice of High K Oxide

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Growth of ZrO2 on Si Interface

Wang et al. APL 78, 1604 (2001)Wang & Ong, APL 80, 2541 (2002)

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Problems with High K oxides

Among other problems, oxide has too many charge traps, and the threshold voltage (Vth) shifts from CMOS standards.

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Dynamic Charge Trapping

Time evolution of threshold voltage Vth under static and dynamic stresses ofdifferent frequencies, for (a) n-MOSFET, and (b) p-MOSFET. The Vth evolutionhas a power law dependence on stress time. C. Shen, H. Y.Yu, X. P. Wang, M. F. Li, Y.-C. Yeo, D. S. H. Chan, K. L. Bera, and D. L.Kwong, International Reliability Physics Symposium Proceedings 2004, 601.

Power law shift!

Negative-U traps?Oxygen vacancy?

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Hydrogen in HfO2

Formation energies for (a) interstitial H and H2 molecules, and (b) the VO-H complex.

J. Kang et al., APL, 84, 3894 (2004).

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Bulk HfO2

J. Kang, E.-C. Lee and K. J. Chang, PRB, 68, 054106 (2003)

Fm3mCubic

P42/nmcTetragonal

P21/cMonoclinic

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Cubic HfO2

VaspCutoff energy = 495 eVGGAEg = 3.68 eV (direct)(Exp gap ~ 5.8 eV)

-16

-12

-8

-4

0

4

8

12

Ene

rgy

(eV

)

W L X W K-20 -15 -10 -5 0 5 10 15 20

O p

O s

Hf d

Hf p

Hf s

HfO2

Den

sity

of

Sta

tes(

a.u)

Energy(eV)

Valence band = O 2p Conduction band = Hf d

Peacock and Robertson, JAP (2002)

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Computational Details

DFT, planewave, pseudopotential method (vasp) 2s and 2p electrons of O, 5d and 6s electrons of

Hf are treated as valence electrons. Cut off energy: 495 eV 80 atom supercell (3x3x3 primitive cells) Uniform background charge for charged vacancy

Supercell

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Total Energy

Charge State Energy (eV)

V-- 13.73

V- 7.02

V0 0.00

V+ -6.20

V++ -13.35

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Energetics

VVV 02 Excothermic (0.32 eV)

VVV 02 Excothermic (0.94 eV)

02VVV Excothermic (0.38 eV)

Negative-U Property!

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electron

electron

(a)

Vg > 0

HK Si sub.n+Poly-Si gate hole

(b)

Vg < 0

HK Si sub.p+Poly-Si gate

Charge Trapping Mechanism

Positive bias for n-MOSFETElectrons are injected to HKV0 V- (meta-stable) V--

Negative bias for p-MOSFETHoles are injected to HKV0 V+ (meta-stable) V++

In both cases, when the gate bias is removed, no charges are injected to HK, all charges in the O traps will be de-trapped,the gate dielectric remains neutral

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Frequency Dependence of Vth

Experimental and simulation results for n-MOSFET

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Formation Energy

2

3

4

5

6

7

8

9

10

11

12

0 1 2 3 4

Fermi Energy (eV)

Fo

rmat

ion

En

erg

y (e

V)

V++

V+

V0

V-

V--

A. S. Foster, et al. PRB 65, 174117 (2002)Formation energy for neutral vacancy: 9.36 eV (O3) & 9.34 eV (O4)Present calculation: 9.33 eV (relative to O atom)

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Band Structures

V0

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Band Structures

V-2

AC plane

BC plane

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(a) (b)

1

2

Breathing Mode C2v Mode

Relaxation of NN Hf atoms

V V

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Relaxation of NN Hf Atoms

Charge State

Breathing Mode C2v Mode

(Å) 1 (Å) 2 (Å)

V-- 0.14 0.11 -0.006

V- 0.07 0.06 0.002

V0 0.03 ̶J ̶J

V+ -0.08 ̶J ̶J

V++ -0.16 ̶J ̶J

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Effect of Lanthanum

1 10 100 10000

20

40

60

80

100

120

140

HfO2, EOT~1.45 nm,

HfLaO15% La, EOT~1.4 nm50% La, EOT~1.3 nm

NMOSFET, Room Temp.Stress Voltage: V

th+1.5 V

Vth s

hift

(mV

)

Stress Time (s)

Charge trapping induced Vth shift under constant voltage stress for HfO2, HfLaO with 15% and 50% La gate dielectric NMOSFETs.

X. P. Wang et al. VLSI2006

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Effect of La

Oxygen vacancy site Formation energy

E(eV) Site density

D( nm-3) V3 site in HfO2 6.51 28.6

V4 site in HfO2 6.39 28.6

Td site in Hf2La2O7 7.23 6.3 C2v site in Hf2La2O7 6.51 38.0

The formation energies of oxygen vacancies at varies sites in monoclinic HfO2 and pyrochlore HfLaO, calculated by ab initio total energy calculations.

V3

V4

TdC2V

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Summary

Oxygen vacancy in HfO2 has negative-U property. It is energetically favors trapping two electrons or two holes.

Oxygen vacancy is a main source of charge trapping in HfO2 and the origin for frequency dependence of dynamic charge trapping in HfO2 MOS transistors.

Large lattice relaxation for charged vacancies, due to strong electron-lattice interaction.

Oxygen vacancy has higher formation energy at Td site in La2Hf2O7.

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Currently polycrystalline silicon (poly-Si) gate electrode is used. Problems:

high gate resistance boron penetration Fermi level pinning poor compatibility with high- gate dielectrics increase of EOT due to gate depletion

Need metal gate! Eliminates the gate depletion problem

Eliminates boron penetration problem

Reduces the gate sheet resistance

Generally more compatible with alternative gate dielectric or high-permittivity (high-

k) gate dielectric materials than poly-Si.

The urgent need for alternative gate dielectrics to suppress excessive transistor

gate leakage and power consumption could speed up the introduction of metal

gates in complementary metal oxide semiconductor (CMOS) transistors.

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Gate Material

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Issues

The integration of metal gate with high- gate dielectric requires the metal effective work functions to be within ±0.1 eV of the Si valence- and conduction-band edges for positive- (PMOS) and negative-channel metal-oxide-semiconductor (NMOS) devices, respectively.

However, to find two metals with suitable work functions and to integrate them with current semiconductor technology remains a challenge.

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Work Function of Metals

Work function of several elemental metals in vacuum, on a scale ranging from the positions of the conduction band to the valence band of silicon.

Metal work functions are generally dependent on the crystal orientation and on the underlying gate dielectric.

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Can we tune the metal workfunction?

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Tuning of Workfunction?

ZrO2

NiTransition Metal Monolayer/half-monolayer

Ni-m-ZrO2

m = Au, Pt, Ni, Ru, Mo, Al, V, Zr and W (for half monolayer)m = Ni, V, and Al (for one monolayer)

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Bulk ZrO2

Very small lattice mismatch (<2%)

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Models

Supercells for the Ni-m-ZrO2 interfaces,

The interface is formed using c-ZrO2(001) and fcc Ni(001) surfaces.

(a) with one monolayer metal m (m=Ni, V, and Al).

(b) with half monolayer metal m (m=Au, Pt, Ni, Ru, Mo, Al, V, Zr and W)

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Computational Details

DFT, planewave, pseudopotential method (vasp) Ultrasoft pseudopotential & GGA Cut off energy: 350 eV K points: 8x8x1 In plane lattice constants constrained to that of c-

ZrO2 Electronic energy was minimized using a fairly

robust mixture of the blocked Davidson and RMM-DIIS algorithm. Conjugate gradient method for ionic relaxation

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Density of States

Spin resolved and atomic site-projected density of states (PDOS) for (a) Ni-Pt-ZrO2 interface and (b) Ni-Al-ZrO2 interface, with half monolayer of metal insertion. The PDOS for the Ni in the bulk region (Ni-bulk), interface metal m (Pt or Al), interface oxygen (O-Int.), and oxygen in the bulk region (O-bulk) are shown.

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Schottky Barrier Heights

Ni m Oxide Si

SigE OxidegEFE

n

p

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p-type Schottky Barrier Height p-type SBH is obtained using the “bulk plus lineup”

procedure, using the average electrostatic potential at the core (Vcore) of ions in the “bulk” region as reference energy

Eb the difference between the Fermi energy of Ni and the energy of the valence band maximum (VBM) of the oxide, each measured relative to Vcore of the corresponding “bulk” ions, V is the lineup of Vcore through the interface.

Eb is adjusted by quasiparticle and spin-orbital corrections (0.29 eV for Ni, +1.23 eV to the valence-band maximum of ZrO2, overall correction of 0.94 eV).

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Vcore

Average electrostatic potential at the cores (Vc

ore) of Ni (filled dark circle) and Zr (open circle) as a function of the distance from the interface for Ni-m-ZrO2 interfaces (m= Au, Ru, Ti) with half monolayer metal insertion. Breaks were introduced in the vertical axis (Vcore) between - 41 eV and -36 eV.

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n-type Schottky Barrier Height

where Eg is the energy gap of the dielectric

The experimental band gap of 5.80 eV was used.

The SBH can also be estimated directly from the difference between the Fermi energy and the energy corresponding to the top of the valence band given in the PDOS of oxygen in the bulk region. Results obtained using the two methods are in good agreement (within 0.1~ 0.2 eV).

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Results

m θ χ WF Qm p-SBH n-SBH

AuPtNiRu MoAlVZrTiWNiVAl

0.50.50.50.50.50.50.50.50.50.5111

5.775.6

4.404.53.9

3.233.6

3.643.454.404.403.6

3.23

5.1 5.65 5.154.71 4.6

4.28 4.3

4.05 4.33 4.55 5.154.3

4.28

0.160.160.370.270.511.060.691.010.800.150.240.440.63

1.201.983.063.063.443.643.733.863.874.022.193.174.00

4.603.822.742.742.362.162.071.941.931.783.612.631.80

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SBH Tunability

Range of tuning: 2.8 eV!

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n-type Schottky Barrier Height

n-SBHs of Ni-m-ZrO2 interfaces are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares (Al and W were not included).

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Workfunction of Ni(001) with m

Work functions of Ni(001) with half monolayer of metal m coverage are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares.

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Mechanism?

Contribution from the tails of the metallic wave functions which tunnel into the oxide band gaps or metal induced gap sates can be ruled out, due to short delay length (~0.9Å) which is nearly independent of the interlayer metal.

Interface dipole can contribute significantly to band alignment between the metal and oxide.

Ionic m-O bonds Charged metal layer and its image

Bulk Ni Bulk ZrO2

Ni m O

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Gap States

Penetration of electronic density of the gap states into the ZrO2 of Ni-m-ZrO2 interfaces. Position of the surface oxygen is set to z = 0 Å.

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Interface bonding dependent SBH: experimental evidence (in-situ XPS)

-4 -2 0 2 4 6 8 10 12 14 178 180 182 184 186 188 190 192

(b)

Ni-YSZ Ni-S-YSZ

YSZ S-YSZ

0.76 eV

Ni-YSZ Ni-S-YSZ

(d)

Inte

nsi

ty (

arb

. u

nit

)

Binding Energy (eV)

(c)

(a)

2.60 eV

YSZ S-YSZ

P

n

Method Structure p(eV) n (eV)

 DFT-GGA

  

XPS 

IPEa

O-tZr-tO-v

 O-rich

O-deficient

2.133.802.92

 2.603.36

 2.2

3.672.002.88

 3.202.44

 3.2

Afanas'ev et al. JAP 91, 3079 (2002).

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Interface bonding dependent SBH: experimental evidence (in-situ XPS)

Structure Coverage Method Fp (eV) Fn (eV)Ni-ZrO2 0.5 DFT-GGA 3.02 2.76

1.0 DFT-GGA 2.17 3.630.5-1.0 XPS 2.60 3.20

Ni-Al-ZrO2 0.5 DFT-GGA 3.62 2.181.0 DFT-GGA 4.98 1.82

~0.5 XPS 3.76 2.04

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Summary

A scheme for tuning the Schottky barrier height or workfunction of metal gate – high-k dielectric interface was proposed and has been experimentally confirmed.

By including a monolayer or half monolayer of transition metal between the metal gate and high-k dielectric, a tunability as wide as 2.8 eV can be achieved.

There exists a linear correlationship between the Schottky barrier heights / workfunction and the electronegativity

Preliminary experimental results with m=Al agree with prediction.

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Acknowledgement

Y F Dong Physics Department, NUS

Y Y Sun Physics Department, NUS

S J Wang Institute of Materials Research & Engineering

A Huan Institute of Materials Research & Engineering

M F Li Dept of Electrical & Computer Engineering, NUS

Institute of Microelectronics

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