High resolution ion beam analysis

Torgny Gustafsson

Review articles and Books

• L. C. Feldman, J. W. Mayer and S. T. Picraux, Materials Analysis by Ion Channeling, Academic Press (1982)

• I. Stensgaard, Surface studies with high-energy ion beams. Reports on Progress in Physics 55 989-1033 (1992).

• H. Niehus, W. Heiland and E. TagIauer, Low-energy ion scattering at surfaces. Surface Science Reports 17 (1993) 213-303.

• J.F. van der Veen, Ion beam crystallography of surfaces and interfaces. Surface Science Reports 5 199-288 (1985).

• R. Hellborg, H. J. Whitlow and Y. Zhang, Ion Beams in Nanoscience and Technology, Springer (2009)

Ion backscattering for materials analysis: 3 energy regimes

• Low energy ion scattering (~10 keV or less): LEIS

– Hard to quantify, very surface specific

• Medium energy (~ 50 – 200 keV) MEIS

– Quantitative, somewhat elaborate equipment, surface specific

• High energy (1 – 2 MeV) RBS

– Quantitative, simpler equipment

Advantages of ion beams*

• Penetrating (can access buried interfaces!)

• Mass specific

• Known interaction law (cross sections are known)

• Excellent depth resolution

*Mostly high and medium energy beams

Elementary considerations in ion backscattering (1)

k = the kinematic factor

Relation between incoming and backscattered ion energies

Cross section for backscattering

Elementary considerations in ion backscattering (2)

Depth profiling

Structural work using Medium Energy Ion Scattering (MEIS)

8

MEIS lab at Rutgers

ALD XPS

XPS

IR

NRP

MEIS

Electrostatic ion detector

9 Angle

Ener

gy

~20°

• Depth resolution of ~3 Å near surface • Angular resolution 0.2° • Mass-sensitive: E = E(M, θ) • Quantitative (cross sections are known)

Al

18O

16O

Surface structure and dynamics

N

i i

i

calc

i

Y

YY

NR

1

2

exp

exp1

100

-5 0 5 10 15

-15

-10

-5

0

50.6000

0.8200

1.040

1.260

1.480

1.700

1.920

2.140

2.360

2.580

2.800

3.020

3.240

3.400(a)

d

12 (

%)

d23

(%)

blocking cone

shadow cone

Monte Carlo simulation in the

binary-encounter approximation

Input:

• individual atomic positions

• anisotropic rms vibration amplitude

Output:

Δd

12

Δd

23

50 60 70 80 90 1001101201302

4

6

8

10

[211] [110] [011][011][011][101]

II IIII

Yie

ld (

ML)

Scattering angle (deg)

An (oversimplified) picture of the origin of the oscillatory relaxation

Reconstruction of the (110) surface of Au:

Possible structural models consistent with a (1x2) symmetry

Conclusions:

Missing row structure

Large first layer inwards contraction

Buckling in the lower layers, results in charge density smoothing

Another research example: Surface structure of TiC(001)

Y. Kido et al, PRB 61 1748 (2000)

Sub-nm Depth Resolution from a Materials Perspective:

• Who needs monolayer resolution?

• In this talk, three techniques:

– Elastic Recoil Detection Analysis (ERDA)

– Nuclear Resonance Profiling (NRP)

– Medium Energy Ion Scattering (MEIS)

• Research examples

Gate oxide

Barrier?

Gate

Source Drain Silicon

CMOS Gate Structure

>10nm

The SiO2/Si system: interface

of choice in microelectronics for

40+ years

Moore’s law says that the

gateoxide thickness will soon be

too small (~ 1 nm) due to large

leakage currents from quantum

mechanical tunneling

Motivation for a lot of work in this field today

SiO2 needs to be replaced by a higher dielectric constant material (“high-k”)

Advantages of ion beams

• Penetrating (can access buried interfaces!)

• Mass specific

• Known interaction law (cross sections are known)

• Excellent depth resolution

Medium Energy Ion Scattering (MEIS) - a low-energy, high resolution version of conventional Rutherford Backscattering (RBS)

A comparison between RBS and MEIS

RBS MEIS

Ion energy ~ 2 MeV ~ 100 keV

Detector resolution ~ 15 keV ~0.15 keV

Depth resolution ~ 100 Å ~ 3 Å

2 basic advantages vs. RBS: Often better dE/dx, superior detection equipment

Energy Dependence of dE/dx for Protons in Silicon

Maximum of ~ 14 eV/Å at ~ 100 keV!

This helps, but the greater advantage is the use of better ion detection equipment!

Energy (MeV)

Ion detection equipment

Magnetic

spectrometers

Electrostatic

spectrometers

Kyoto university

(Kimura)

Kobelco

FOM – IBM

(Tromp, van der

Veen, Saris ..)

High Voltage

Engineering

From the Ion beam analysis laboratory at Kyoto university; note the magnetic spectrometer.

The Kyoto – Kobelco very compact MEIS facility

Footprint:

~ 2.1 x 1.5 m

3D-MEIS •Pulsed ion beam

•Scattered (and/or recoiled) particles are detected

periodic atomic structure sample

New development: 3D-MEIS

S. Shimoda and T. Kobayashi

incident beam •Ion : He+ •Energy : 100 keV •Repetition : 500 kHz

x

TOF

wide solid angle

•2D blocking pattern •flight times of scattered (and/or recoiled) particles

3D detector position-sensitive and time-resolving MCP detector

6

4

2

0

-2

-4

42403836343230282624222018Scattered angle 2(°)

[110][331]

f-1 f-2 f-3 f-4

35.3°

Si

22.0°

Angle

fro

m t

he S

i(1

1̄0)

pla

ne (

°)

1 = 67.2° 1 = 62.2° 1 = 57.2° 1 = 52.2°

6

4

2

0

-2

-4

42403836343230282624222018Scattered angle 2(°)

f-1 f-2 f-3 f-4

b-1 b-2 b-3

22.3° 28.8° 39.3°

Er

Angle

fro

m t

he S

i(1

1̄0)

pla

ne (

°)

[011̄1][022̄3][011̄2]

1 = 67.2° 1 = 62.2° 1 = 57.2° 1 = 52.2°

Structural analysis of an Er-silicide on Si(111) substrate using 3D-MEIS

Fig. 3 Structural model of the ErSi2

Cross section cut by the ErSi2(2110) plane

__ Fig.1 3D-MEIS images of the intensities of He particles scattered (a) from Er atoms in the Er-silicide film and (b) from Si atoms in the Si substrate.

Fig. 2 TOF spectra obtained from the data detected in regions indicated by A and B in Fig.1.

(2110) __

A

B

(a)

(b)

S. Shimoda and T. Kobayashi

c

a2a1

B

A

70

60

50

40

30

20

10

0

Inte

nsi

ty (

counts

)

360340320300280260

TOF (ns)

A

B

Er

Si

Er Si

b-1

b-2

b-3

c=0.403nm

b-1 : [0112]

b-2 : [0223]

b-3 : [0111]

_

_

3a=0.665nm

K. Kimura et al. (Kyoto) NIM B99, 472 (1995)

Early High Resolution Work

Sb on Si(100) with caps of varying thickness; some Sb segregates to the surface

Recent Very High Resolution Work

Carstanjen et al. (Stuttgart) (to be published)

Note use of N+ and N2+ ions and charge exchange effects

The Stuttgart high resolution ion analyzer

Determining interface strain using monolayer resolution ion scattering and blocking (Moon et al.)

Ozone oxide, no strain

Thermally grown oxide, significant strain

ZrO

2 (ZrO2)x(SiO2)y

Si(100)

100 keV p+ Backscattered proton

energy spectrum

depth

profile

• Sensitivity: 10+12 atoms/cm2 (Hf, Zr) 10+14 atoms/cm2 (C, N)

• Accuracy for determining total amounts: 5% absolute (Hf, Zr, O), 2% relative 10% absolute (C, N)

• Depth resolution: (need density) 3 Å near surface 8 Å at depth of 40 Å

?

Spectra and information content

Depth resolution for 100 keV protons (resolution of the spectrometer 150 eV)

Stopping power SiO2 12 eV/Å; Si3N4 20 eV/Å; Ta2O5 18 eV/Å

"Near surface" depth resolution 3-5 Å; worse for deeper layers due to energy straggling

Depth resolution and concentration profiling

Layer model:

raw spectrum

5

4

3 2 1

layer numbers

Energy, keV3 Å

H+

SUBSTRATE

Layer 1

Layer 2

Layer 3

….

Layer n

• Areas under each peak

corresponds to the concentration

of the element in a 3Å slab

• Peak shapes and positions come

from energy loss, energy

straggling and instrumental

resolution.

• The sum of the contributions of

the different layers describes the

depth profile.

1. O18 uptake at the surface!

2. Growth at the interface

3. O16 loss at the surface

4. O16 movement at the interface!

Oxygen Isotope Experiments: SiO2 growth mode

Gusev, Lu, Gustafsson, Garfunkel, PRB 52, 1759 (1995)

Schematic model

SiO2

Transition zone,

SiOx

Si (crystalline)

Surface exchange

Growth

Deal and Grove

75 80 85 90 95

0

5

10

15

20 (3)(2)(1)

(1) as deposited film

(2) vacuum anneal at 600 oC

(3) vacuum anneal at 800 oC

no change in

O or Si profiles

minor rearrangement

of La upon annealing

high-K La-Si-O film

yie

ld (

a.u

.)

proton energy (keV)

Annealing up to 800 °C in vacuum shows no significant change in

MEIS spectra.

Surface remains flat by AFM.

Work on high-k films: MEIS spectra of La2SiO5

before and after vacuum anneal

75 80 85 90 95

0

5

10

15

20

25high-K La-Si-O film

(3)

(2)

(1)

(3)

(2)(1)

SiO

surfsurf intint

La

surfint

(1) as deposited film

(2) anneal in air at 400 oC

(3) anneal in air at 800 oC

La diffusionSiO2 growth at 800

oC

yie

ld (

a.u

.)

proton energy (keV)

stoichiometry and thickness

consistent with other analyses

400°C anneal leads to minor

broadening of the La, O and Si

distributions

800°C anneal shows significant

SiO2 growth at interface

La diffusion towards the Si

substrate

La2SiO5 before and after in-air anneal

MOx

Si

MOx

Si

SiO2+MOx + O2

Higher temperature annealing

75 80 85 90 95

0

5

10

15

20

high-K La-Si-O film

(1) as deposited film

(2) vacuum anneal at 845 oC

(3) vacuum anneal at 860 oC

(3)

(2)

(1)

La diffusion

surface Si

loss of O at 860 oC

yie

ld (

a.u

.)

proton energy (keV)

The film disintegrates!

ZrO2 film re-oxidized in 18O2

76 78 80 82 84 86 88 90 92 94 960

100

200

300

400

500

x518

O

16O

Si

Zr

as-deposited film

18

O2 reox. (1 Torr, 500

oC, 5 min)

Yie

ld (

a.u

.)

Proton Energy (keV)

No change in Zr profile

Surface flat by AFM Deeper O and Si

Significant interfacial SiO2 growth for

ZrO2, less for Al2O3

Dramatic oxygen exchange: 18O incorporation and 16O removal

SiO2 growth rate faster than for O2 on Si

Growth faster under ZrO2 than Al2O3

76 78 80 82 84 86 88

0

1

2

3

4

5

6

700 oC

600 oC

500 oC

400 oC

Si interfaceoxygen exchange Al

18O

16O

MEIS spectra of 30 Å Al2O

3 annealed in

3 Torr 18

O2 at 400, 500, 600, 700

oC for 5 min

yie

ld (

a.u

.)

proton energy (keV)

30Å Al2O3 annealed in

3 Torr 18O2

0 200 400 600 800 10000

5

10

15

20

25

30Å ZrO2

30Å Al2O

3

SiO

2 G

row

th (

Å)

O2 Anneal Temp (

oC)

Why GaAs (or Ge)?

• Potentially great advantages over Si-based devices for both high-speed and high-power applications

• The electron mobility of GaAs is 5x that in Si

• Much thinner interfacial oxide

HfO2 on GaAs: MEIS and TEM comparison

No etch

HF etch

*Grazul &

Muller,

Cornell

• TEM and MEIS results are consistent

GaAsa-C

HfO2

Ga-rich oxide

GaAsa-C

HfO2

Ga-rich oxide

TEM* MEIS

O

Hf

O

Hf

TEM of Al2O3 on GaAs

(J. Grazul and D. Muller, Cornell)

HF etch No etch

• no contrast between Al2O3 and GaxAsyO

n(Ga+As), Å-2

n(Al), Å-2

n(O), Å-2

n(O)/n(Al)

HF etch 0.33 1.48 2.23 1.51

No etch 0.55 1.30 2.22 1.70

MEIS data of Al2O3 on GaAs: one-parameter fitting

No etch

HF etch

• interfacial oxide is much thinner

for the HF-etched sample

1.51

1.70

Interfacial oxide: (Ga2O3)0.37(Ga2O)0.63(As2O3)0.17, porous oxide: = 0.5 bulk

80 85 90 95

H+ Energy (keV)

0 10 20 30 40 50 60 700.00

0.01

0.02

0.03

0.04

0.05

Ga

As

Al

O

N (

Å-3)

Depth (Å)

80 85 90 95

H+ Energy (keV)

0 10 20 30 40 50 60 700.00

0.01

0.02

0.03

0.04

0.05

Al

O

N (

Å-3)

Depth (Å)

0 5 10 15 20 25 30

0.85

0.90

0.95

1.00

No

etch

Etched

rms e

rror

Interface thickness (Å)

Epitaxial Oxides

Very versatile materials:

– Small changes in composition → big changes in properties (high-Tc!)

– Combining two different oxides may give multifunctionality and/or entirely new phenomena.

48

Oxides

49

Metallic LaAlO3/SrTiO3 (LAO/STO) interface

LaAlO3 (Eg = 5.8 eV) by PLD or MBE band insulator

SrTiO3 (Eg = 3.2 eV) band insulator

2D Electron Gas

• Metallic conductivity Ohtomo & Hwang, Nature (2004)

• Superconductivity Reyren et al, Science (2007)

• Magnetic order Brinkman et al, Nat. Mat. (2007)

Reyren et al., Science (2007) Brinkman et al., Nat. Mat. (2007) Ohtomo & Hwang, Nature (2004)

T(K)

Origin of metallic state at the interface

50

• Polar catastrophe: polar-nonpolar discontinuity Ohtomo &Hwang, Nature 427 (2004)

This model assumes an abrupt interface

51

Important points

Cationic interdiffusion Nakagawa et al, Nature 5 (2006), Kalabukhov et al, PRL 103 (2009); Willmott et al, PRL 99 (2007).

Oxygen vacancies – have been shown by many to influence carrier concentrations

Conductivity only observed on TiO2 terminated substrates

Samples

• We have investigated samples from three different labs.

• None of the samples were made at Rutgers.

• All made by PLD on TiO2 terminated substrates.

• Post-annealed in oxygen.

• Most data presented today from samples by Prof. H. Hwang, Tokyo (now Stanford).

52

53

He+ channeling spectrum of 4-unit-cell LAO/STO(001)

The measured Sr and Ti peaks fall

at significantly higher ion energies

than those calculated for a sharp

interface:

The energies for the Sr and Ti

peaks are those of both species

present within the outermost u.c.

of LAO outdiffusion of Sr and Ti

to the LAO film

4 u.c. LAO

STO (001)

198.6 KeV He+ [001] [111]

rastering beam

140 150 160 170 180

0

50

100

150

Co

un

ts

He+

Energy [keV]

Experiment

Simulation-no outdiffusion

of Sr and Ti

x4

Sr

Ti

LaTokyo-LO4

54

85 90 95

0

100

200

300

400

500

600

H+ Energy [keV]

Experiment

Simulation-no outdiffusion of Sr and Ti

Yie

ld

Tokyo-MID4 La

SrTi

Al

H+ channeling spectrum of a 4-unit-cell LAO/STO film

55

85 90 95

0

100

200

300

400

500

Experiment

Simulation-no outdiffusion of Sr and Ti

Yie

ld

H+ Energy [keV]

Al

Ti Sr

LaAugsburg

H+ channeling spectrum of a 4-unit-cell LAO/STO film

56

Angle dependent XPS shows poor agreement with sharp interface model (Chambers et al)

57

The LaAlO3/SrTiO3 interface shows evidence for substantial interdiffusion, a useful result for understanding the origin of the many interesting effects shown by this system. More details: Surf. Sci. Rep. 65, 317 (2010)

Conclusions