Transcript Slide 1

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
[email protected]
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www.mrs.org.sg
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Outline
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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
G
S
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CMOS Scaling
D
Technology node (nm)
S
200
200
150
150
100
100
50
50
0
2000
2010
2005
Junction depth (nm)
G
0
2015
Year
ITRS roadmap shows the expected reduction in device dimensions
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Why High-k oxides ?
SiO2
HK Oxide
Gate
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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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Choice of High K Oxide
10
SiO 2
Band Gap (eV)
9
Al2O3
8
MgO
CaO
7
ZrO2
ZrSiO4
HfO 2
HfSiO 4 Y2 O3
La 2O 3
SrO
Si 3 N4
6
5
4
Ta 2O 5
BaO
TiO 2
3
2
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0
10
20
30
K
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60
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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
Power law shift!
Oxygen vacancy?
Negative-U traps?
Time evolution of threshold voltage Vth under static and dynamic stresses of
different frequencies, for (a) n-MOSFET, and (b) p-MOSFET. The Vth evolution
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 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
Fm3m
Cubic
P42/nmc
Tetragonal
P21/c
Monoclinic
J. Kang, E.-C. Lee and K. J. Chang, PRB, 68, 054106 (2003)
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Cubic HfO2
12
HfO2
8
Hf s
Density of States(a.u)
Energy (eV)
4
0
-4
-8
Hf p
Hf d
Os
-12
-16
Op
W
L

X
W K
Vasp
Cutoff energy = 495 eV
GGA
Eg = 3.68 eV (direct)
(Exp gap ~ 5.8 eV)
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-20
-15
-10
-5
0
5
10
15
20
Energy(eV)
Valence band = O 2p
Conduction band = Hf d
Peacock and Robertson, JAP (2002)
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Computational Details
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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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Supercell
Total Energy
Charge State Energy (eV)
V-VV0
V+
V++
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13.73
7.02
0.00
-6.20
-13.35
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Energetics

0

0
2V  V  V
2V  V  V
V

V


Excothermic (0.32 eV)

Excothermic (0.94 eV)
 2V
0
Excothermic (0.38 eV)
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
(a)
Positive bias for n-MOSFET
Electrons are injected to HK
V0  V- (meta-stable)  V--
Vg < 0
(b)
Negative bias for p-MOSFET
Holes 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 Vth
Experimental and simulation results for n-MOSFET
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Formation Energy
12
Formation Energy (eV)
11
10
9
V++
8
V+
V0
7
V-
6
V--
5
4
3
2
0
1
3
2
4
Fermi Energy (eV)
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
AC plane
BC plane
V-2
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Relaxation of NN Hf atoms
2
1
V
V

(b)
(a)
C2v Mode
Breathing Mode
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Relaxation of NN Hf Atoms
Charge Breathing Mode
State
 (Å)
C2v Mode
1 (Å)
2 (Å)
V--
0.14
0.11
-0.006
V-
0.07
0.06
0.002
V0
0.03
̶
̶
V+
-0.08
̶
̶
V++
-0.16
̶
̶
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Effect of Lanthanum
140
Vth shift (mV)
NMOSFET, Room Temp.
120 Stress Voltage: Vth+1.5 V
X. P. Wang et al. VLSI2006
HfO2, EOT~1.45 nm,
100
HfLaO
15% La, EOT~1.4 nm
50% La, EOT~1.3 nm
80
60
40
20
0
1
10
100
Stress Time (s)
1000
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
The formation energies of oxygen vacancies at varies
sites in monoclinic HfO2 and pyrochlore HfLaO,
calculated by ab initio total energy calculations.
V3 site in HfO2
Formation energy
E(eV)
6.51
Site density
D( nm-3)
28.6
V4 site in HfO2
6.39
28.6
Oxygen vacancy site
Td site in Hf2La2O7
C2v site in Hf2La2O7
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7.23
6.51
6.3
38.0
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V3
Td
C2V
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Summary
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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
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Currently polycrystalline silicon (poly-Si) gate electrode is used.
Problems:
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high gate resistance
boron penetration
Fermi level pinning
poor compatibility with high- gate dielectrics
increase of EOT due to gate depletion
G
S
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Need metal gate!
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Eliminates the gate depletion problem
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Eliminates boron penetration problem
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Reduces the gate sheet resistance
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Generally more compatible with alternative gate dielectric or high-permittivity (highk) gate dielectric materials than poly-Si.
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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
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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.
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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?
Ni-m-ZrO2
Ni
Transition Metal
Monolayer/half-monolayer
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-mZrO2 interfaces,
(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)
The interface is formed
using c-ZrO2(001) and
fcc Ni(001) surfaces.
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Computational Details
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DFT, planewave, pseudopotential method (vasp)
Ultrasoft pseudopotential & GGA
Cut off energy: 350 eV
K points: 8x8x1
In plane lattice constants constrained to that of cZrO2
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-PtZrO2 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
n
EF
Eg Oxide
Eg  Si
p
Ni
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m Oxide
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p-type Schottky Barrier Height
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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
(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
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where Eg is the energy gap of the dielectric
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The experimental band gap of 5.80 eV was used.
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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
Au
Pt
Ni
Ru
Mo
Al
V
Zr
Ti
W
Ni
V
Al
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
1
1
5.77
5.6
4.40
4.5
3.9
3.23
3.6
3.64
3.45
4.40
4.40
3.6
3.23
5.1
5.65
5.15
4.71
4.6
4.28
4.3
4.05
4.33
4.55
5.15
4.3
4.28
0.16
0.16
0.37
0.27
0.51
1.06
0.69
1.01
0.80
0.15
0.24
0.44
0.63
1.20
1.98
3.06
3.06
3.44
3.64
3.73
3.86
3.87
4.02
2.19
3.17
4.00
4.60
3.82
2.74
2.74
2.36
2.16
2.07
1.94
1.93
1.78
3.61
2.63
1.80
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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?
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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.
Bulk Ni
Ni
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m
Bulk ZrO2
O
Ionic m-O bonds
Charged metal layer and its image
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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)
YSZ
S-YSZ
Intensity (arb. unit)
(a)
YSZ
S-YSZ
(b)
Method
Structure
p(eV)
n (eV)
DFT-GGA
O-t
Zr-t
O-v
2.13
3.80
2.92
3.67
2.00
2.88
O-rich
O-deficient
2.60
3.36
3.20
2.44
2.2
3.2
2.60 eV
(c)
P
Ni-YSZ
Ni-S-YSZ
(d)
0.76 eV
Ni-YSZ
Ni-S-YSZ
n
-4
-2
0
XPS
IPEa
2
4
6
8
10
12
14
178 180 182 184 186 188 190 192
Afanas'ev et al. JAP 91, 3079 (2002).
Binding Energy (eV)
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Interface bonding dependent SBH:
experimental evidence (in-situ XPS)
Structure
Ni-ZrO2
Ni-Al-ZrO2
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Coverage
0.5
1.0
0.5-1.0
0.5
1.0
~0.5
Method
Fp (eV)
Fn (eV)
DFT-GGA
DFT-GGA
XPS
DFT-GGA
DFT-GGA
XPS
3.02
2.17
2.60
3.62
4.98
3.76
2.76
3.63
3.20
2.18
1.82
2.04
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Summary
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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
Y Y Sun
S J Wang
A Huan
M F Li
Aug 29 - Sept 1, 2006
Physics Department, NUS
Physics Department, NUS
Institute of Materials Research & Engineering
Institute of Materials Research & Engineering
Dept of Electrical & Computer Engineering, NUS
Institute of Microelectronics
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