QuantiÞcation of Trap State Densities at High- k/III-V...

Roman Engel-Herbert*, Yoontae Hwang,and Susanne Stemmer

Materials Department, University of California, Santa Barbara

*now at Penn State University, Department of Materials Science and Engineering

Quantification of Trap State Densities at High-k/III-V Interfaces

International Symposium on Advanced Gate Stack TechnologySeptember 29, 2010

Albany, NY

Acknowledgements

• TEM/STEM

• Joel Cagnon, Nick Rudawski, James LeBeau

• Temperature-dependent CV and conductance

• Jeff Huang (Sematech)

• Funding

• SRC Nonclassical CMOS Research Center

Outline

• CV characteristics of high-k/In0.53Ga0.47As interfaces

• Methods to quantify Dit

• Conductance method

• CV-based method (Terman)

• Guidelines to determine Fermi level unpinning and quantify Dit

• Dit reduction using forming gas anneals

• Summary

Introduction

Y. C. Chang et.al., Appl. Phys. Lett. 92, 072901 (2008).

S. Koevshnikov et.al., Appl. Phys. Lett. 92, 222904 (2008).

HfO2 ZrO2

❖ Typical room temperature characteristics of high-k/n-In0.53Ga0.47As interfaces:

❖ “False inversion” due to response of midgap states

❖ Sometimes incorrectly attributed to “true inversion” or “weak inversion”

❖ Finite slope of 1 MHz curve at negative gate bias

❖ Large frequency dispersion in accumulation

Introduction

K. Martens, Ph.D. Thesis, KU Leuven (2009).

Simulations for GaAswith midgap Dit peak of 2.2×1013 cm-2eV-1

Experiments for HfO2/In0.53Ga0.47Aswith midgap Dit of ???

0.5

0.4

0.3

0.2

0.1 Cap

acita

nce

[µF/

cm2 ]

-4 -2 0 2 4 Gate Voltage [V]

100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

300 K

0.5

0.4

0.3

0.2

0.1 Cap

acita

nce

[µF/

cm2 ]


100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

200 K

600 K

300 K

CV measurements: Jeff Huang, Sematech

This talk focuses on In0.53Ga0.47As channels,where Dit can be probed at room temperature

Introduction

❖ Reported Dit values span several orders of magnitude, despite often very similar admittance characteristics

❖ Many different methods have been used

❖ Need to characterize Dit from MOSCAPs because transistor fabrication may introduce additional defects

Introduction

HAADF/STEM

No interface layer Films are crystalline

HfO2

In0.53Ga0.47As

As-decapping:Clean, well-defined

growth surface

In0.53Ga0.47As

n+-InP

RHEED of growth surface

Films are subjected to different post-deposition anneals in nitrogen and forming gas, respectively,

which results in very different Dit

➙ Systematic evaluation and comparison of analysis methods

HfO

O

O

O

Source: Hf[OC(CH3)3]4

✦ Hf bonded to four oxygens

✦ No excess oxidant

Reduction in mid-gap Dit by forming gas anneals

1.0

0.8

0.6

0.4

0.2

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]

43210-1-2-3Gate voltage [V]

f

1 kHz 10 kHz 100 kHz 1 MHz

18 nm HfO2

FG anneal1.5

1.0

0.5

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]

-2 -1 0 1 2Gate voltage [V]

f


9 nm HfO2

FG anneal

After forming gas annealNo forming gas anneal

❖ Large midgap-Dit response

❖ Fermi level effectively pinned at midgap

❖ Much reduced midgap-Dit response

❖ Very efficient Fermi level movement for 9 nm HfO2 film across midgap

Y. Hwang, R. Engel-Herbert, N. G. Rudawski, S. Stemmer, Effect of post-deposition anneals on the Fermi level response of HfO2/In0.53Ga0.47As gate

stacks, J. Appl. Phys. 108, 034111 (2010).

1.0

0.8

0.6

0.4

0.2

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]

43210-1-2-3Gate Voltage [V]

f


18 nm HfO2

N2 anneal

Conductance Method

Advantages

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]

210-1-2Gate voltage [V]

4

3

2

1

1

4

3

2

1

10

8

6

4

2

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]

420-2Gate voltage [V]

6

6

4

2 12

8

4

0 Gp/

Aω

q [1

012/e

Vcm

2 ]


❖ Efficient movement of conductance peak with gate bias provides evidence for Fermi level movement around midgap

❖ Conversely, if the conductance peak does not move, the Fermi level is effectively pinned at midgap

❖ In principle, the Dit at midgap can be obtained directly:

18 nm HfO2 9 nm HfO2

But...

Conductance Method

Issues

❖ Requires knowledge of Cox, which is not equal to Cacc (n-type)

❖ If Cox is overestimated, the peak height is reduced and thus the Dit is lower than the true value

❖ At high Dit and high EOT, errors become very large (see papers by IMEC group)

Lin et al., Appl. Phys. Lett. 94, 153508 (2009).

Conductance Method

Issues

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]


4

3

2

1

1

4

3

2

1

10

8

6

4

2

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]


6

6

4

2 12

8

4

0 Gp/

Aω

q [1

012/e

Vcm

2 ]


18 nm HfO2 9 nm HfO2

Cox < qDit Cox > qDit

Cannot get a reliable Dit from conductance method for this stack

✓ Dit estimate should be ok

Conductance Method

Issues

❖ Only midgap Dit can be determined

❖ At room temperature only midgap Dit responds

❖ In principle, lower temperatures should move the characteristic frequency of traps near the band edges into the measurable frequency range

❖ Trap response freezes out at low temperatures

❖ Reduced thermal broadening causes fewer traps to respond

Conductance Method

Issues

❖ Dit values near the band edges are likely too low

❖ Is the apparently sharply peaked midgap Dit real or an artifact from the conductance method?

No forming gas anneal

CV-Based Methods

Terman Method

❖ Requires calculation an ideal CV curve

❖ Ideal CV must take into account electronic structure of III-V semiconductor

❖ Nonparabolicity of Γ band

❖ Filling of X, L valleys

❖ Fermi level moves deep into the conduction band → Boltzmann statistics not appropriate to calculate charge in the semiconductor

!

n" # x( )[ ] =4

$

2$ kBT m"%

h2

&

' (

)

* +

3

2 ," (1+-," )(1+ 2-," )

exp ," .q# x( )kBT

+ /"

0

1 2 2

3

4 5 5 +1

d,"0

6

7 !

!

n " x( )[ ] = ni" x( )[ ]

i=#, X ,L

$ !

!

Qs "s( ) = #sE $ x = 0( )[ ]

= % Sign "s( ) 2 %q#s N D % N A + p $ x( )[ ] % n $ x( )[ ]{ }d$ x( )$ b

$ b +" s

&!

!

d 2" x( )dx2

= #$ x( )%s

= #q p " x( )[ ] # n " x( )[ ] + N D # N A{ }

%s

!

!

Cdos "s( ) = #dQs "s( )

d"s

!

R. Engel-Herbert, Y. Hwang, S. Stemmer, Appl. Phys. Lett.

97, 062905 (2010).

!

Dit "s( ) =Cox

q

d"s

dVg

#

$ % %

&

' ( (

)1

)1

#

$

% %

&

'

( ( )Cdos "s( ) !

Equations

Terman

CV-Based Methods

Terman Method

❖ The classical CV curve does not reveal the asymmetry due to the difference in the density of states of valence and conduction bands.

❖ The parabolic valley approximation underestimates the increase of semiconductor charge with band bending and thus the semiconductor capacitance, causing the asymmetry in the CV to be overestimated.

❖ At a gate bias of 1 V, Cdos is 25% lower for the parabolic case.

Calculated ideal CV and band bending for a 3 nm EOT

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Cap

acita

nce

[µF/

cm2 ]

-3 -2 -1 0 1 2 3 Gate Voltage [V]

HF curve

classic Γα=0

Γ Γα+X+L

Cox

-0.9

-0.6

-0.3

0.0

0.3

0.6

Ban

d be

ndin

g ψ

s [eV

]

-3 -2 -1 0 1 2 3 Gate voltage [V]

classic Γα=0

Γ Γα+X+L

R. Engel-Herbert, Y. Hwang, S. Stemmer, Appl. Phys. Lett. 97, 062905 (2010).

CV-Based Methods

Terman Method

R. Engel-Herbert, Y. Hwang, S. Stemmer, Appl. Phys. Lett. 97, 062905 (2010).Y. Hwang, R. Engel-Herbert, N. G. Rudawski, S. Stemmer, J. Appl. Phys. 108, 034111 (2010).

0.6

0.5

0.4

0.3

0.2

0.1

Cap

acita

nce

[µF/

cm2 ]


measured ideal

Cox

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Ban

d be

ndin

g ψ

s [e

V]


measured ideal

2.5

2.0

1.5

1.0

0.5

0.0

Cap

acita

nce

[µF/

cm2 ]

-2 -1 0 1 2 Gate voltage [V]

9 nm HfO2

FG anneal

Cox

-0.6

-0.4

-0.2

0.0

0.2

0.4

Ban

d be

ndin

g ψ

s [eV

]


FG (9 nm HfO2)


❖ Large CV stretch-out

❖ Minimum capacitance does not reach semiconductor capacitance calculated from doping

❖ Band bending is less then half of semiconductor band gap

➜ Fermi level is effectively pinned at midgap

❖ Minimum capacitance reaches the semiconductor capacitance calculated from doping

❖ Band bending exceeds half of semiconductor band gap

✓ Fermi level can be moved past midgap

18 nmN2 anneal

Comparison of Different Methods

❖ Both conductance method and Terman method agree qualitatively ❖ Provide a consistent picture of Fermi level (un)pinning and midgap Dit response

2.5

2.0

1.5

1.0

0.5

0.0

Cap

acita

nce

[µF/

cm2 ]


9 nm HfO2

FG anneal

Cox

0.6

0.5

0.4

0.3

0.2

0.1

Cap

acita

nce

[µF/

cm2 ]


measured ideal

Cox

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]


6

6

4

2 12

8

4

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

6.0

5.5

5.0

4.5

4.0

3.5Lo

g (f)

[H

z]


4

3

2

1

1

4

3

2

1

10

8

6

4

2

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

1.5

1.0

0.5

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]


f


9 nm HfO2

FG anneal

-0.6

-0.4

-0.2

0.0

0.2

0.4

Ban

d be

ndin

g ψ

s [eV

]


FG (9 nm HfO2)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Ban

d be

ndin

g ψ

s [e

V]


1.0

0.8

0.6

0.4

0.2

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]


f


18 nm HfO2

N2 annealPinned at midgapNot pinned at

midgap (FG anneal)

Pronounced midgap Dit reponse

Capacitance does not reach minimum

capacitance

Band bending less than half of the

band gap

Limited conductance peak

movement

Reduced midgap Dit reponse

Capacitance reaches minimum capacitance

Band bending more than half of the

band gap

Conductance peak movement


Criteria to Establish Fermi Level Unpinning


1. The dopant concentration and oxide capacitance should be determined independently, as they are needed both in conductance and CV-based methods.

2. To establish that the Fermi level is not effectively pinned at midgap, it should be shown that the high-frequency CV reaches the ideal depletion capacitance determined by the semiconductor doping

3. The room temperature parallel conductance should show a frequency-dependent shift of the conductance peak maximum with gate voltage. If the conductance peak maximum shifts to frequencies that are less than 2 kHz for n-MOSCAPs it is indicative that the Fermi level can be moved into the lower part of the band gap.

2.5

2.0

1.5

1.0

0.5

0.0

Cap

acita

nce

[µF/

cm2 ]


9 nm HfO2

FG anneal

Cox

0.6

0.5

0.4

0.3

0.2

0.1

Cap

acita

nce

[µF/

cm2 ]


measured ideal

Cox

pinned unpinned

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]


6

6

4

2 12

8

4

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

6.0

5.5

5.0

4.5

4.0

3.5

Log

(f) [

Hz]


4

3

2

1

1

4

3

2

1

10

8

6

4

2

0 Gp/

Aω

q [1

012/e

Vcm

2 ]

pinned unpinned

Criteria to Establish Fermi Level Unpinning


4. The room temperature conductance method gives reasonable estimates of the fast Dit around midgap, only if Cox > qDit .

5. The ideal CV curve must be calculated taking into account the low conduction band density of states and the nonparabolicity of the Γ valley.

6. If true inversion is claimed it should be shown that the experimentally achieved bend bending is sufficient to achieve inversion.

1011

2

46

1012

2

46

1013

2

Qs [

cm-2

]

-0.8 -0.4 0.0 0.4 ψs [eV]

classic Γα=0

Γ Γα+X+L

T=300K

ND=1x1017cm-3

A bend bending of more than -0.6 eV is needed for inversion

-0.6

-0.4

-0.2

0.0

0.2

0.4B

and

bend

ing ψ

s [eV

]


FG (9 nm HfO2)

FG annealed stack

1.5

1.0

0.5

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]


f


9 nm HfO2

FG anneal

No indication of inversion response at 1 kHz

Lower frequencies needed?

Dit From Different Methods


61012

2

46

1013

2

46

1014

2

4 D

it [cm

-2eV

-1]

0.8 0.6 0.4 0.2 0.0 -0.2 ΔE [eV]

ECBEVB

conductance method

Terman method

Near midgap: good agreement

Conductance method only measures fast Dit while Terman measures all Dit

Issues with Conductance Method

Issues with Terman Method

❖ The HF CV curve is not a true HF curve

❖ Causes errors near band edges

❖ Conductance method as developed by Nichollian and Brews assumes that Dit and capture cross-section are independent of energy, which works for Si, with its very uniform Dit distribution

❖ Does not work for III-V

❖ Conductance method needs to be revisited for III-V’s

Recommendation:

Use Terman method until issues with conductance method are fixed

Dit From Different Methods


Is the sharply peaked midgap Dit predicted by the conductance method real?

G. Brammertz et al., J. Electrochem Soc. 155, H945 (2008).

Ga2O3/GaAsAl2O3/InGaAs

1012

2

46

1013

2

46

1014

2

4

Dit

[eV

-1cm

-2]

-0.6 -0.4 -0.2 0.0 0.2

Trap level [eV]

ECB

1012

2

46

1013

2

46

1014

2

4

Dit

[eV

-1cm

-2]

-0.6 -0.4 -0.2 0.0 0.2

Trap level [eV]

nonparabolic Γ ECB


❖ For high-Dit stacks, a peak at midgap is indeed observed also in the Terman method

❖ Terman method reveals clear changes in Dit distribution with forming gas anneal (not just magnitude)

How to reduce the Dit of high-k/InGaAs stacks

Y. Hwang, R. Engel-Herbert, N. G. Rudawski, S. Stemmer, J.

Appl. Phys. 108, 034111 (2010).

Y. C. Chang et.al., Appl. Phys. Lett. 92, 072901 (2008).

1.0

0.8

0.6

0.4

0.2

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]


f


18 nm HfO2

N2 anneal

Most high-k/InGaAs MOSCAPs look like this.... Forming gas anneals significantly reduce mid-gap Dit

1.5

1.0

0.5

0.0

Cap

acita

nce

dens

ity [µ

F/cm

2 ]


f


9 nm HfO2

FG anneal

-0.6

-0.4

-0.2

0.0

0.2

0.4B

and

bend

ing ψ

s [eV

]


FG (9 nm HfO2)

E. Kim et al., Appl. Phys. Lett. 96, 012906

(2010)

Largest band bending ever reported, due to combination of lower

Dit and high Cox, though Dit is still high

Huge mid-gap Dit

Summary

• Developed a set of guidelines to establish Fermi level unpinning at high-k/InGaAs interfaces

• Dit quantification:

• Many issues with conductance methods, some poorly understood

• Terman method gives upper estimate for slow and fast Dit

• Many pitfalls in all methods: incorrect semiconductor doping, Cox, need correct band structure model,...

• Forming gas anneals reduce the midgap Dit significantly

QuantiÞcation of Trap State Densities at High- k/III-V...

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