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First Principles Studies on High-k Oxides and Their Interfaces with Silicon and Metal Gate. Feng Yuan Ping ( 冯元平 ) Department of Physics National University of Singapore phyfyp@nus.edu.sg. www.mrs.org.sg. G. S. D. Outline. Introduction Oxygen vacancy in HfO 2 and La 2 Hf 2 O 7

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first principles studies on high k oxides and their interfaces with silicon and metal gate

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

Feng Yuan Ping (冯元平)

Department of Physics

National University of Singapore

phyfyp@nus.edu.sg

outline

G

S

D

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

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cmos scaling

G

S

D

CMOS Scaling

ITRS roadmap shows the expected reduction in device dimensions

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why high k oxides

SiO2

HK Oxide

Why High-k oxides ?

Gate

CB Si

  • 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

Rober Chau, Intel

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growth of zro 2 on si interface
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
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
Dynamic Charge Trapping

Power law shift!

Oxygen vacancy?

Negative-U traps?

Time evolution of threshold voltage Vthunder static and dynamic stresses of

different frequencies, for (a) n-MOSFET, and (b) p-MOSFET. The Vthevolution

has 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.

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hydrogen in hfo 2
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 hfo 2
Bulk HfO2

P21/c

Monoclinic

Fm3m

Cubic

P42/nmc

Tetragonal

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

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cubic hfo 2

W L  X W K

Cubic HfO2

Vasp

Cutoff energy = 495 eV

GGA

Eg = 3.68 eV (direct)

(Exp gap ~ 5.8 eV)

Valence band = O 2p

Conduction band = Hf d

Peacock and Robertson, JAP (2002)

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computational details
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

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energetics

Excothermic (0.32 eV)

Excothermic (0.94 eV)

Excothermic (0.38 eV)

Energetics

Negative-U Property!

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charge trapping mechanism

p+Poly-Si gate

electron

HK

Si sub.

electron

hole

n+Poly-Si gate

HK

Si sub.

Vg > 0

Vg < 0

(a)

(b)

Charge Trapping Mechanism

Negative bias for p-MOSFET

Holes are injected to HK

V0 V+ (meta-stable)  V++

Positive bias for n-MOSFET

Electrons are injected to HK

V0 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 v th
Frequency Dependence of Vth

Experimental and simulation results for n-MOSFET

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formation energy
Formation Energy

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 structures1

BC plane

Band Structures

AC plane

V-2

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relaxation of nn hf atoms
Relaxation of NN Hf atoms

2

1

V

V

(b)

(a)

C2v Mode

Breathing Mode

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effect of lanthanum
Effect of Lanthanum

X. P. Wang et al. VLSI2006

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

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effect of la

Td

V4

C2V

V3

Effect of La

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

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summary
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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gate material

G

S

D

Gate Material
  • 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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issues
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 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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tuning of workfunction

Transition Metal Monolayer/half-monolayer

Tuning of Workfunction?

Ni-m-ZrO2

Ni

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 zro 2
Bulk ZrO2

Very small lattice mismatch (<2%)

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models
Models

Supercells for the Ni-m-ZrO2 interfaces,

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

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

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computational details1
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
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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p type schottky barrier height
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 DEb 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, DV is the lineup of Vcore through the interface.
  • DEb 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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v core
Vcore

Average electrostatic potential at the cores (Vcore) 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
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
Results

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sbh tunability
SBH Tunability

Range of tuning: 2.8 eV!

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n type schottky barrier height1
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
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.

CCP2006

mechanism

Bulk Ni

Bulk ZrO2

Ni m O

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

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gap states
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

Method

Structure

p(eV)

n (eV)

DFT-GGA

XPS

IPEa

O-t

Zr-t

O-v

O-rich

O-deficient

2.13

3.80

2.92

2.60

3.36

2.2

3.67

2.00

2.88

3.20

2.44

3.2

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

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

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summary1
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
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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slide51

Thank you!

CCP2006