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

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

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

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 H.-C. Wen, (2006), unpublished Ph.D.thesis, The University of Texas at Austin.

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

Vacuum work functions

Ti/Ta

C/N

The stacking sequence

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

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).

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

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).

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).

EWF of Pt-HfO2 as a function of θO

Di,net (or EWF) is dependent on θO.

EWF=WF+4πDi,net/A

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.

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

• 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

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

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

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)

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

Thank you for your attention!

Questions

• 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

)

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.

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

-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

Back up data

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

• 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

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

http://www.intel.com/technology/itj/2008/v12i2/1-transistors/figures/Table_1_lg.gif',%20'Table%201

Continuous θO

: 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