First Principles Study of Metal-Oxide Interfaces
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First Principles Study of Metal-Oxide Interfaces
Hong Zhu
Department of Chemical, Materials & Biomolecular EngineeringInstitute of Materials Science, University of Connecticut
Major Advisor: Prof. Rampi RamprasadAssociate Advisor: Prof. Harold Brody Associate Advisor: Prof. Rainer Hebert
Ph. D. Dissertation Proposal
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Gate electrode (poly-Si)
Insulating layer (SiO2)
Si substrate (p-type)
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- - - -
I
Vt
Motivation: MOS devices
source drain
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Motivation: MOS miniaturization
Metal
sourcesource
draindrain
Si substrate Si substrate Si substrate
SiO2 SiO2
Poly-SiPoly-Si
Si substrate
HfO2source drain
metal
Silicon substrate
1.1nm SiO2
Poly Si electrode
Intel 65nm production: 2005• SiO2 only 3-4 atomic layers thick.
Metal
Intel 45nm production: 2007• metal gate & HfO2 based high k.
• Stability (1000 0C)• Effective work function
(EWF)
a b
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WF EWF
Metal
Evac,m
EF
HfO2
Di,net
Motivation: Effective work function (EWF)
VBM
CBM
EWF=WF+4πDi,net/ANMOS: ~4 eV PMOS: ~5.1 eV
Evac,o
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Motivation: Metal gate issuesMetal
•EWF thermal instability•Large Vt
•Interesting EWF shifts due to interfacial doping layer.
After high T annealing
1. H.Y. Yu, et al, IEEE Electron Device Lett. 25, 337 (2004).2. W. S. Hwang et al, IEEE T ELECTRON DEV 55, 2469 (2008)3. S. B. Samavedam et al, Electron Devices Meeting (2003).
4. W. S. Yang, et al, Surface and coating tech. 131, 79 (2000).5. H.-C. Wen, (2006), unpublished Ph.D.thesis, The University of Texas at Austin.
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Outline Objectives
• EWF thermal instabilities• Critical factors of dopants on EWF shifts• Feasibilities of various approaches to tune EWFs
Completed research• WFs of Pt, TiCxN1-x and TaCxN1-x
• Interface phase diagrams for Pt-HfO2
• EWF as a function of interfacial structure, T and PO2
Summary Future research
• Tuning Di,net (or EWF) by adding doping layer (DL) or molecular nanolayer (MNL) at metal-HfO2 interfaces
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Vacuum work functions
Ti/Ta
C/N
The stacking sequence
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Vacuum work function
0.00 0.25 0.50 0.75 1.003
4
7
8
9(111) C
xN
1-x/Ti-terminated
(110) TiN/TiC0.5
N0.5
/TiC-terminated(001) TiN/TiC
0.5N
0.5/TiC-terminated
x
5.3
5.4
5.5
5.6
5.7
5.8
5.9
(111)(110)
Wor
k fu
nctio
n (e
V)
Surface orientation
(001)
Pure TiN Pure TiC(a) Pt (b) TiCxN1-x
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Vacuum work function
0.00 0.25 0.50 0.75 1.003
4
7
8
9(111) C
xN
1-x/Ti-terminated
(110) TiN/TiC0.5
N0.5
/TiC-terminated(001) TiN/TiC
0.5N
0.5/TiC-terminated
x
5.3
5.4
5.5
5.6
5.7
5.8
5.9
(111)(110)
Wor
k fu
nctio
n (e
V)
Surface orientation
(001)
Surface orientation controls the work function significantly.
Pure TiN Pure TiC(a) Pt (b) TiCxN1-x
exp. (111) Pt: 5.8~5.9 eV exp. (001) TiN: 2.9 eV
1. Y. Saito et al, Appl. Surf. Sci. 146, 177 (1999).2. R. Martin, Electronic Structure: Basic Theory and Practical Methods (Cambridge University Press, New York,
2004).
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Oxidized interface
(ɵO=2ML)
Abrupt interface
(ɵO=1ML)
Hf
O
Pt
Clean interface
(ɵO=0ML)
Interface phase diagram
DFT energies
at 0 K
Thermodynamics
First principles thermodynamics
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400 600 800 1000 1200 1400
Clean
10-10
10-15
10-20
10-25
PO
2 in a
tm
O=0
O=0
.25O=0
.5
O=2
O=0.75
1010
105
100
10-5
Temperature (K)
achievable processing condition
Oxidized
Interface phase diagram
1. W.-B. Zhang, and B.-Y. Tang, Appl. Phys. Lett. 94, 091901 (2009).2. W.-X. Li, C. Stampfl, and M. Scheffler, Phys. Rev. B 68, 165412 (2003).
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Interface phase diagram: Si-HfO2
: SiO2 : decomposed interface: critical condition for SiO2 to decompose1. S.J. Wang, et al, Appl. Phys. Lett. 82, 2047 (2003)2. M. Copel, et al, Appl. Phys. Lett. 85, 458 (2004).3. N. Miyata, et al, Appl. Phys. Lett. 82, 3880 (2003).
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EWF of Pt-HfO2 as a function of θO
Di,net (or EWF) is dependent on θO.
EWF=WF+4πDi,net/A
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Thermal instability of EWF
Averaged EWF:
Ac
hie
va
ble
rang
e
2
0
2
0θθθ
2
0θθ
)/exp(
)/exp(EWFEWFEWF
O
OO
O
O
O
O
O
kT
kTp
The reported experimental values: 4.6-5.51 eV
The interfacial O coverage variation with T and P is responsible for EWF instability.
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Outline Objectives
• EWF thermal instabilities• Critical factors of dopants on EWF shifts• Feasibilities of various approaches to tune EWFs
Completed research• Vacuum WFs of Pt, TiCxN1-x and TaCxN1-x
• Interface phase diagrams for Pt-HfO2
• EWF as a function of interfacial structure, T and PO2
Summary Future research
• Tuning Di,net (or EWF) by adding doping layer (DL) or molecular nanolayer (MNL) at metal-HfO2 interfaces
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• Methodology:1. Study vacuum WF for multicomponent system
2. Create the interface phase diagrams
3. Compute EWF & investigate (T,P) dependence of EWF
• Issues addressed1. Identify factors that control the vacuum WFs of
several metals
2. Investigate the structure for two important interfaces
3. Illustrate the importance of O point defects on EWFs.
Summary
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Outline Objectives
• EWF thermal instabilities• Critical factors of dopants on EWF shifts• Feasibilities of various approaches to tune EWFs
Completed research• Vacuum WFs of Pt, TiCxN1-x and TaCxN1-x
• Interface phase diagrams for Pt-HfO2
• EWF as a function of interfacial structure, T and PO2
Summary Future research
• Tuning Di,net (or EWF) by adding doping layer (DL) or molecular nanolayer (MNL) at metal-HfO2 interfaces
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Future research
DL
HfO2
Metal(Pt, TiN)
Doping Layer (DL): Al2O3, La2O3, Y2O3
1. H.Y. Yu, et al, IEEE Electron Device Lett. 25, 337 (2004).2. W. S. Hwang et al, IEEE T ELECTRON DEV 55, 2469 (2008)3. H.-C. Wen, (2006), unpublished Ph.D.thesis, The University of Texas at Austin.
After high T annealing
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Cu/MNL/SiO2
Cu/pre-anneal MNL/SiO2
Cu/SiO2
MNL
Future research
HfO2
MNL
Pt
CHH
S
(CH3S)2 (10-6 torr)
At high T AuAu
WF for clean Au surface
Molecular nanolayer (MNL):-CH2- molecular chain terminated with S, Hf 1. D. D. Gandhi, et al, nature 477, 299 (2007).
2. V. De Renzi et al, Phys. Rev. Lett. 95, 046804 (2005)
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Group members:Ning, Tang, Tom, Ghanshyam, Satyesh, Chenchen and Yenny
Committee members:Profs. Rampi Ramprasad, Harold Brody and Rainer HebertProfs. Leon Shaw and Prof. Pamir Alpay
Computational resources:IMS computation clusters; SGI supercomputer in SoE; NSF Teragrid computers
Funding:NSF & ACS-PRF
Acknowledgment
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Thank you for your attention!
Questions
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• Density Functional Theory (DFT)– Many nuclei-many electron problem one electron problem
– Supercell approach– Phase, structure and electronic information
• Computational times– Interface calculation 21 days in one AMD 2.0 GHz processor
(Supercell of ~120 atoms).
• Computational accuracy
rrrVm iiieff
2
2
2
Computational Methods DFT Exp.
(a) (b)(b)
Lattice constant or bond length (angstrom) Theory (eV)
Exp
erim
ent
(eV
)
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Back up data
Interfacial structure plays a major role in determing EWF.The location of Ef and interface defect states affect the charge transfer and the EWF.
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The stacking sequence of TiCxN1-x
<001>
TiC0.5N0.5
TiN
TiN
TiC0.5N0.5
.
.
.
.
.
.
.
.
.
TiC0.5N0.5
TiC0.5N0.5
TiN
TiN
.
.
.
.
.
.
.
.
.
Application and Results: TiCxN1-x WF tunability
Slab A Slab BTiC
0.5N
0.5-terminated TiN-terminated
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-13
-12
-11
-10
-9
-8
N (e
V)
-11 -10 -9-13
-12
-11
-10
-9
-8
N (e
V)
C (eV)
-11 -10 -9
-13
-12
-11
-10
-9
-8
N (e
V)C (eV)
-11 -10 -9
-13
-12
-11
-10
-9
-8
C (eV)
Shifting with T and PN2
(001) TiN-terminated
(001) TiN-terminated
(001)
TiC0.5
N0.5
-terminated
-terminated(001) TiC
-terminated(111) Ti
-terminated(001) TiC0.5
N0.5
-terminated(111) Ti-terminated
(001) TiC0.5
N0.5
-terminated(111) Ti(111) Ti-terminated
(111)N-terminated
-11 -10 -9C (eV)
-terminated
(001) TiC(111) Ti-terminated
μN (
eV)
The (001) surfaces of TiCxN1-x are the most stable surfaces.
(a) TiN (b) TiC0.25N0.75 (c) TiC0.5N0.5
(d)TiC0.75N0.25 (e) TiC
TiCxN1-x surface stability
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Back up data
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Band diagram across Si/HfO2/ideal metal
gate stack: (a) NMOS, (b) PMOS. Threshold voltage (Vt) is ~ 0. WF
and EWF are EF with
respect to metal vacuum level and oxide vacuum level, respectively.
Importance of EWF
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• The voltage separating the accumulation and depletion regime is referred to as the flatband voltage, VFB.
• The flatband voltage is obtained when the applied gate voltage equals the workfunction difference between the gate metal and the semiconductor.
• If there is a fixed charge in the oxide and/or at the oxide-silicon interface, the expression for the flatband voltage must be modified accordingly.
MOS capacitor- flat band
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High k issues• High leakage currents and low dielectric
constant due to crystallization– As-deposited: amorphous (preferred)– Crystallizes at 400~500 °C
– Doped HfO2 with alloying elements.• Si, Y, La, F, N• Increase crystallization temperatures• Stabilize higher k phases
amorphous cubic tetragonal monoclinic k ~ 30 k ~ 29 k ~ 35 k ~ 16
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Continuous θO
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: SiO2
: critical condition for SiO2 to decompose:decomposed interface
1. S. J. Wang, et al., Appl. Phys. Lett. 85, 2047 (2003)2. M. Copel, et al., Appl. Phys. Lett. 85, 458 (2004).3. N. Miyata, et al., Appl. Phys. Lett. 82, 3880 (2003).
Si-ѲO-m-HfO2: Interface phase diagram
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