1 surface course 2015 - meis

Surface analysis with ion beams

Two parts: Surface structure determinations Thin film analysis L. C. Feldman, J. W. Mayer and S. T. Picraux, Materials Analysis by Ion Channeling, Academic Press (1982) L. C. Feldman and J. W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland (1986) 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)

Quantitative, somewhat elaborate equipment, surface specific

High energy (1 2 MeV)

Quantitative, simpler equipment

Advantages of ion beams*

Penetrating (can access buried interfaces!)

Mass specific

Known interaction law (cross sections are known)

Excellent depth resolution

*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

Now on to more detailed structural work using Medium Energy Ion Scattering (MEIS)!!

7

MEIS lab at Rutgers

ALD XPS

XPS

IR

NRP

MEIS

Electrostatic ion detector

8 Angle

Ener

gy

~20

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

Al

18O

16O

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

64

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 the S

i(110) 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 the S

i(110) pla

ne ()

[0111][0223][0112]

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 (co

unts

)

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

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:

d

12

d

23

50 60 70 80 90 1001101201302

4

6

8

10

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

II IIII

Yie

ld (M

L)

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)

The angular shifts from the first and second layers are displaced in opposite directions!

Surface relaxation and rumpling!

Melting of Pb(110) Frenken et al PRB 74, 7506 (1986)

Melting starts at the surface Partial disorder already at ~ Tm

bulk

At Tm 20 the top layer is disordered The thickness of the disordered layer diverges logarithmically

Gate oxide

Barrier?

Gate

Source Drain Silicon

CMOS Gate Structure

>10nm

The SiO2/Si system: interface of choice in microelectronics for

40+ years

Moores 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)

II. Nuclear Resonance Profiling - a light projectile (proton) penetrates the nucleus and induces a nuclear reaction; the reaction product ( or ) is detected.

106

0

105

104

103

102

101

100

10-1

10-2

10-3

10-4

10-5

100 200 400300 500 600 700 800 900

95

151

216

334

629 73084618 15

O(p, ) N

Seo d

e c

hoqu

e d

ifere

nci

al (

b/s

r)

Energia (keV)

Resonant nuclear reactions

18O 15N p

Reaction cross section (b/sr) vs. proton energy (keV)

106

0

105

104

103

102

101

100

10-1

10-2

10-3

10-4

10-5

100 200 400300 500 600 700 800 900

95

151

216

334

629 73084618 15

O(p, ) N

Seo d

e c

hoqu

e d

ifere

nci

al (

b/s

r)

Energia (keV)


p

E E r

mylar (13 m)detector

semicondut or

amost ra

detector (BGO)

p,

(a)

(b)raios


106

0

105

104

103

102

101

100

10-1

10-2

10-3

10-4

10-5

100 200 400300 500 600 700 800 900

95

151

216

334

629 73084618 15

O(p, ) N

Seo d

e c

hoqu

e d

ifere

nci

al (

b/s

r)

Energia (keV)

ER

ER

ER

E (> E )2 1

E1

Amostra



Some low energy nuclear resonances

Examples

0 2 4 6 8 10 120

1

429 431 433Energia dos Prtons (keV)

15N(p,)

12C

Re

nd

ime

nto

(u

.a.)

Profundidade (nm)

15N

(10

19.c

m-3)

0 2 4 6 8 10 120

1

429 431 433

15N(p,)

12C

Proton Energy (keV)

Re

nd

ime

nto

(u

.a.)

Profundidade (nm)

15N

(10

19.c

m-3)

15N profile

As prepared Vacuum annealed

15N 15N

Depth (nm) Depth (nm)

Nuclear resonance depth profiling of Al2O3 on Si(100)

(Baumvol et al., Porto Alegre)

Overlapping peaks in MEIS spectrum (M. Copel et al., IBM)

Some advantages of NRP:

Detects only one specie at a time

Very well suited for analysis of light elements

Some disadvantages of NRP: Detects only one specie at a time

If no suitable resonance exists

III. 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 ~ 3

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/ at ~ 100 keV!

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

Energy (MeV)

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

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 near surface 8 at depth of 40

?

Spectra and information content

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

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

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

Depth resolution and concentration profiling

Layer model:

raw spectrum

5

4

3 21

layer numbers

Energy, keV3

H+

SUBSTRATE

Layer 1

Layer 2

Layer 3

.

Layer n

Areas under each peak corresponds to the concentration

of the element in a 3 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


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


(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

400C anneal leads to minor broadening of the La, O and Si

distributions

800C 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




(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 Al2O

3 annealed in

3 Torr 18

O2 at 400, 500, 600, 700

oC for 5 min

yie

ld (

a.u

.)

proton energy (keV)

30 Al2O3 annealed in

3 Torr 18O2

0 200 400 600 800 10000

5

10

15

20

25

30 ZrO2

30 Al2O

3

SiO

2 G

row

th (

)

O2 Anneal Temp (

oC)

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.

49

Oxides

50

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

51

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

This model assumes an abrupt interface

52

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).

53

54

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

55

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

56

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

57

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

58

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

A novel instrument for surface structure studies using energetic ion beams: the Helium Ion Microscope - HIM

Ion source

Diffraction in He microscope and in SEEM

Advantages

Very high source brightness

Large depth of view

Very small beam spot

High secondary electron yield allowing currents as low as 1 fA (typically 0.2 pA).

Topographic, material, crystallographic, and electrical properties sampled.

Great for insulators

Does not rely on sample periodicity

Little sample damage due to relatively light mass of the helium ion (but ).

Overview of the Helium Ion Microscope

FOV: 200 nm

Sample: Asbestos fiber (crocidolite)

and carbon membrane FOV: 300 nm

Single Walled Carbon Nanotube

Imaging mechanism (so far): Secondary electrons

Why is Helium So Different?

Case Study: Imaging Structures in the Inner Ear

Three areas were studied, with the goal of obtaining direct observation and/or measurement of nano-scale structures that are related to the functioning of the ear.

Tectorial Membrane: structure and layout of collagen fibers.

Stereocilia, (sound transducers) including

The nature of their membranes

The tip links, which connect the tips of stereocilia in adjacent rows in their bundles

Otoconia: nanostructure of the calcite-containing minerals employed in the ear for balance.

Observe these in HIM without usual staining (OTOTO method) and conductive coating.

Helium Ion Microscope: What is there beyond imaging?

Lithography with a helium beam Beam chemistry with a helium beam Implantation of helium atoms Milling, drilling with a helium beam Analysis with a helium beam

Ion beam milling of graphene

SECONDARY ION MASS SPECTROSCOPY

SIMS

Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)

1 surface course 2015 - meis

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