Ion Beam Analysis and Modification for Current Issues in...

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Surface Canada 2013 Tutorials, May 11, 2013 1

Ion Beam Analysis and Modification for

Current Issues in Surface Science

Lyudmila V. Goncharova

Department of Physics and Astronomy,

Western University, London, Ontario

[email protected]


Outline

• Production of Ion Beams

• Basics of Ion-Solid Interactions

I: Ion Beam Analyses - Rutherford Backscattering Spectrometry

- Elastic Recoil Detection

- Medium Energy Ion Scattering

- Research Examples: interfacial analysis of complex oxide thin film stacks; diffusion and oxidation processes with sub-nm resolution

• Conclusions

• References

II: Ion Beam Modification - Implantation

- Research Examples: formation of Si and Ge quantum dots

Surface Canada 2013 Tutorials, May 11, 2013

Tandetron Ion Scattering facility at UWO

3

Rutherford Backscattering (RBS) and Medium Energy Ion Scattering (MEIS)

Elastic Recoil Detection (ERD)

Nuclear Reaction Analysis (NRA)

Particle-Induced X-ray Emission (PIXE)

Various implantation capabilities…

2


4


Tandetron operating principle

(1) Begin with negative ions via sputtering for most species

(2) Accelerate to kinetic energy = qVt where Vt = terminal voltage (MV)

and qi = -1 so that Et ≡ Vt [MeV]

(3) Ions traverse a stripper gas at the high voltage terminal to produce

a charge state distribution of positive ions

(4) Accel/decel mode is available when the stripper gas is OFF: used

for Eion≤100 keV and the incident ions then have qi = -1


Inside Tandetron…

6

3


Ion Beam Analysis

7

(1) elastic scattering

Rutherford Backscattering

(2) fast recoils arising from elastic

scattering

Elastic Recoil Detection

(3) steering effects due to the crystalline

structure of target atoms (channeling)

(4) inelastic processes: energy loss as a

function of depth

(5) X-ray emission (PIXE) and nuclear

reactions (NRA)

He+, H+


Rutherford Backscattering Spectrometry

Elastic Collisions!

8

M2

Z4,M4

M1

E0

θ

E0=0

E1

M2

M1

E2

z1

Z2

(Eq.3) sinsin0

(Eq.2) coscos

(Eq.1) 2

1

2

1

2

1

2211

22111

2

22

2

11

2

1

vMvM

vMvMvM

vMvMvM

2

12

1

2/122

1

2

2

01

cossin

MM

MMMEE


Charged Particle Detectors

Schematic diagram of the operation

of a surface barrier detector (SBD)

• Silicon disc with gold film

mounted in the detector housing

•

• He++ particle is forming holes

and electrons over its penetration

path.

• The energy band diagram of a

reverse biased detector (positive

polarity on n-type silicon) shows

the electrons and holes swept

apart by the high electric field

within the depletion region.

9

4


Scattering kinematics: example 1

2MeV 4He+, =165o

Backscattered from

C, O, Fe, Mo, Au

31016 atoms/cm2

each

on Si substrate


Key features of RBS

11

2

12

1

2/122

1

2

21cossin

MM

MMM

E

Ek

o

Ability to quantify depth profile of buried species with a

precision of ~ 3%

Qualitative information: kinematic factor, k

Quantitative: scattering cross section, s

2

2

2

21

2sin4

)(

s

s

E

eZZ

d

d


Scattering kinematics: example 2

12 ⇒ Decreased mass resolution for heavier elements

5


Rutherford Cross Section

• Neglecting shielding by electron clouds

• Distance of closest approach large

enough that nuclear force is negligible

Rutherford scattering cross section

Note that sensitivity increases with:

• Increasing Z1

• Increasing Z2

• Decreasing E

2

2

2

21

2sin4

)(

s

s

E

eZZ

d

d


RBS spectra from thin and thick films

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Experimental

Simulated

O

Si

Ti

Au

Channel1,8001,6001,4001,2001,000800600400

Co

un

ts

5,400

5,200

5,000

4,800

4,600

4,400

4,200

4,000

3,800

3,600

3,400

3,200

3,000

2,800

2,600

2,400

2,200

2,000

1,800

1,600

1,400

1,200

1,000

800

600

400

200

0

400 600 800 1000 1200 1400 1600 1800

Energy [keV]


Ion dose (fluency), solid angle, cross section

Ion dose (fluency), the number of incident particles (collected charge)

- measured by Faradey cup

- Q = I t

Solid angle, in steradians, sr

- stays constant for a particular detector/detector slit

- need to be verified by the calibration standard measurements

Cross section (or differential cross section), in cm2/sr of the element

- well known (tabulated) in Rutherford cross section regime

6


Areal density: note about units

Areal density = t [g/cm2],

where = g/cm3, t = cm

N0 t

M [at./cm2]

where M = atomic mass [amu], N0 = Avogadro’s number

In absolute numbers – close to thickness in Å


Thickness measurement


RBS Spectrum of a thick film

• Target is divided into thin sublayers (“slabs”)

• Calculate backscattering from front and back side of each sublayer taking

energy loss into account

• For each isotope of each element in sublayer

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Stoichiometry

2MeV 4He+, backscattered from ceramic films on Si substrate

19

Experimental

Simulated

O

Na

Si

K

Cd

Ba

Channel900800700600500400300

Co

un

ts

8,500

8,000

7,500

7,000

6,500

6,000

5,500

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Energy [keV]

Ba0.05Cd2KSi2NaO4/Si

Lecture 15 20

Ion channeling and blocking

• Si(111) Si (diamond structure)

• Si(111) – side view

)110( )112(

)110(

)112(

Use crystal structure of the substrate

• Substrate can be aligned to a major crystallographic direction to minimize

background signal in some cases

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Experimental

Simulated

H

O

Na

Si

K

Cd

Ba

Channel900800700600500400300

Co

un

ts

7,500

7,000

6,500

6,000

5,500

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Energy [keV]

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Elastic Recoil Detection (ERD)

Heavy Elements by MEIS or RBS

Light Elements by Elastic Recoil Detection

Detector

Light elements (He+ or H+)

Detector

He+

H+, He+ “Classical” ERD

Incident energy = 1.6MeV He+

Incident angle = 75o

Recoil Angle = 30o

Al-mylar (range foil)

200 250 300 350 400 450 500 550 6000

50

100

150

200

Yie

ld

Energy [keV]

Kapton

1034

1051

1085

1091

1097

~150nm SiONH/Si(001)

a


ERD Principles and Limitations

2 0

21 2

2

1 2

4c o s

( )

E k E

M Mk

M M

Some advantages of ERD:

good dynamic range;

excellent hydrogen sensitivity;

very well suited for analysis of light elements

Some disadvantages:

Resolution (limited by detector, ~10-15keV);

sensitivity to surface contamination


RBS plus ERD Full Stoichiometry!!!

RBS and ERD results for VSxOyCz:H

Assumption: ~ 900Å V0.03S0.03O0.25С0,44H0.25/(bulk) V0.03S0.03O0.13С0,44H0.37

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V_RBS

Simulated

H

C

O

S

V

Channel800700600500400300200100

Co

un

ts

540

520

500

480

460

440

420

400

380

360

340

320

300

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Energy [keV]

V_1s.dat

Simulated

Channel3403203002802602402202001801601401201008060

Co

un

ts

700

650

600

550

500

450

400

350

300

250

200

150

100

50

0

150 200 250 300 350 400 450 500 550 600

Energy [keV]

9


A comparison between RBS and MEIS

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

Close to maximum of ~ 14 eV/Å at ~ 100 keV!

This helps, but the greater advantage is the

use of better ion detection equipment!


Medium Energy Ion Scattering (MEIS)

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• mass (isotope) specific

• quantitative (2% accuracy for high-Z)

• depth sensitive (at the sub-nm scale)

Energy distributions:

77 84 910

500

1000

1500

O(buried)

Zr(buried)

O(surf)

Ge(buried)Si

(surf)

Yie

ld

Energy [keV]

SiO2/ Si /ZrO

2/GeO

x/Ge(001)

Experiment Total Spc

100keV H+, SiO2/poly-Si/ZrO2/Ge(100)

H+ E

ne

rgy [ke

V]

Angle 115 120 125 130 135 140

H+ Y

ield

Angle [degree]

Energy distribution

for one angle

Angular distribution for

one element


MEIS analysis of as-deposited films

104 108 112 116 128

16O

In

ten

sity (

a.u

.)

Energy (keV)

as is

Hf

Si

98keV H+

Sample Alignment:

Si(001) incident; Si(110) outgoing

HfO2

SiO2

Si

29Å

7Å

TEM:

2.8nm HfO2/1nm SiO2/Si(001)

10


Basic concept: Depth profile is based on the energy loss of the ions traveling

through the film (stopping power dE/dx).

Example: Depth resolution for 95 keV protons

With MEIS spectrometer 180 eV vs RBS detector 15keV

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

Depth resolution and concentration profiling

7 5 8 0 8 5 9 0 9 5

× 5

d e p th

Z r

S iO

Sc

att

ere

d Y

ield

(a

.u.)

P ro to n E n e rg y (k e V )

ZrO2

Si(100)

98 keV p+ Backscattered proton energy spectrum

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

Si

ZrO2.04

Zr

O

Co

nce

ntr

atio

n

Depth (Å)


Oxidation temperature dependence: 16O and 18O

0 15 30 45 60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 SiOxN

y

950oC

700oC

16O

pro

file

s

Depth [Å]

as grown

500oC

HfSiOx

0 15 30 45 60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 SiOxN

y

950oC

700oC

18O

pro

file

s

Depth [Å]

500oC

HfSiOx

SiO1-xN

Si

HfSiOx Surf

Int 1

Int 2

104 106 108 110 112 114 116 118

as grown

Sisurf

SiInt 1SiInt 2

18Osurf

18OInt 1

18OInt 2

16Osurf

16OInt 116OInt 2

500oC

700oC

Energy (keV)

950oC

O reaction with Si deeper than N distribution

16O

18O

N

N


Oxygen diffusion in oxides

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SiO2 films:

• amorphous after annealing

• molecular O2 transport in SiO2

• decomposition by SiO desorption

Si-substrate

Atomic oxygen (O) transport in metal oxide films

SiO2 growth,

O-exchange

at interface

O-diffusion and

exchange in bulk

of oxide

MOx

O2 decomp.

at surface

O2 O Si-substrate

O-exchange

in surface

layer

SiO2

growth

at interface

Oxygen (O2) transport in SiO2

O2

(Many) metal oxide films:

• tend to crystallize at low T

• atomic O transport in the film

• high oxygen mobility

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Diffusion and interface growth in HfO2 and

HfSiOx ultrathin films on Si(001)

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L.V. Goncharova, M. Dalponte, T. Feng, et al, PRB 83 (2011) 115329

• Faster interfacial SiO2 growth in case of high-k

oxides in comparison to the SiO2 thickness

growth for bare Si

T (oC) Time (min) Oxide growth

(Å)

High-k

700 30 11

800 30 18

950 30 25

SiO2*

750

165 5

2640 10

900

60 10

1860 27

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


Part II: Ion Implantation

• Implantation chamber and implantation stage

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Periodic Table

• We can produce beams of all those elements shown in yellow !

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12


Ion Implantation

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Si- ions

aperture

i ≤ 4 μA

SiO2

(e.g.

area = A = 1.13 cm2)

Dose = i Δt / A (ions/cm2)

(from

Tandetron)

(raster beam in X,Y)

Sub

stra

te

(@ 25-2,000keV)

X

Y

sweeping


Stopping and Range of Ions in Matter (SRIM)

http://www.srim.org/ Download SRIM-2008


SRIM Setup Window

http://www.srim.org/

http://www.srim.org/SRIM/SRIMLEGL.htm



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Calculated Ion Trajectories

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2MeV He+ in Si 50keV He+ in Si 50keV Au+ in Si


Ion-implanted Si and Ge quantum dots in dielectrics

• Second generation Si and Ge photonics

• Strong light emission from nanocrystals or quantum dots (QD) by

reducing the size of Si to < aBohr (Si ~3-5nm; Ge ~ 24nm)

• Porous Si and crystalline QW

• Bonafos et al. used TEM to relate Si QD to excess Si (10, 20, 30%)

Cho et al, JAP 2007 Bonafos et al, NIMB (2001) Barbagiovanni et al, MRS (2009)


Si(001)

SiO2

Si ions

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Growth of Si-QD

• RT Implantation Si- (Ge+) 90keV 5x1016 -1x1017ions/cm2

• 120min @11000C (Si) or 9000C (Ge) in furnace,

• 60 min @5000C in N2/H2 gas

• Early stage of formation governed

by diffusion

• Eventually Ostwald ripening

)(4solSi

SiCCrND

t

C

Link between defects in the

SiO2 and formation of Si-QDs

Mokry C.R., Simpson P.J., Knights A.P. J. Appl. Phys. 105 (2009) 114301

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Ge in Al2O3(0001): crystallization and ordering

E.G. Barbagiovanni, S.N. Dedyulin, P.J. Simpson, L.V. Goncharova, NIMB 272 (2012) 74–77


XPS

• Shift of Ge peak towards the surface (RBS)

• GeOx peaks in XPS Ge loss via GeO desorption

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Ar sputtering prior to XPS analysis:

Ge layer is 3-5nm deep

Al2O3(0001)

GexO

disordered Al2O3

Tx>1100oC

N2 Al2O3(0001)

Ge-QD


Cross-sectional TEM micrographs

• Contrast arising from stress

fields and end of range

implantation damage

• Moiré fringes become visible

from the overlap of the

crystal planes of Ge QD and

the sapphire matrix

15


Ge in Al2O3(0001): crystallization and ordering

I.D. Sharp, Q. Xu, D.O.Yu, et al. JAP 100 (2006) 114317

• Slow diffusion rate of the alumina matrix atoms at < Tmelt

• Ge blocking minimum can be related to the stereographic projection of the sapphire crystal and corresponds to the [111] scattering plane:

(1104) Al2O3 // (111)Ge and [211] Al2O3 // [112] Ge


Conclusions and future directions:

• Ion Beam Analysis is an enabling technology for thin film

scientists and engineers

• Our goals are to initiate collaborative research projects

and stimulate multidisciplinary interactions, To enable

the use of ion beams, including the introduction of ion

beam methods to new discipline areas

• Development of novel ion beam analyses techniques

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References:

1) L.C. Feldman, J.W. Mayer (1986) Fundamentals of

Surface and Thin Film Analysis.

2) Y. Wang, M. Nastasi (2010, or previous edition)

Handbook of Modern Ion Beam Materials Analysis.

3) The Stopping and Range of Ions in Matter (SRIM),

http://www.srim.org/

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Elastic recoil detection for negative ions

Crucial points for detecting H ion

recoils directly are:

• To increase the recoil cross-section

• To reduce (to suppress) the

background originating mainly from

elastically scattered incident ions

Toroidal Ion Energy Analyzer (HVEng, Amersfoort,

The Netherlands)

V-

V+

MEIS

V- V+

ME-ERD

Only charged particles are detected

by TEA

use incident beam ions without

negative ion fractions and detect

negative H- recoils

X+ H+,H, H-


Control QD Distribution with Mask

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Si QD nucleation and growth by Si ion implantation and anneal

Lateral separation between implanted regions


Thank you! Lyudmila V. Goncharova

Department of Physics and Astronomy,

Western University, London, Ontario

[email protected]

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