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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
Surface Canada 2013 Tutorials, May 11, 2013 2
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
Surface Canada 2013 Tutorials, May 11, 2013
4
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
Inside Tandetron…
6
3
Surface Canada 2013 Tutorials, May 11, 2013
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+
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
Scattering kinematics: example 1
2MeV 4He+, =165o
Backscattered from
C, O, Fe, Mo, Au
31016 atoms/cm2
each
on Si substrate
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
Scattering kinematics: example 2
12 ⇒ Decreased mass resolution for heavier elements
5
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
RBS spectra from thin and thick films
14
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]
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
Thickness measurement
Surface Canada 2013 Tutorials, May 11, 2013
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
7
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
21
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]
8
Surface Canada 2013 Tutorials, May 11, 2013 22
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
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013
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
24
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
Surface Canada 2013 Tutorials, May 11, 2013
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!
Surface Canada 2013 Tutorials, May 11, 2013
Medium Energy Ion Scattering (MEIS)
26
• 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
Surface Canada 2013 Tutorials, May 11, 2013 27
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
Surface Canada 2013 Tutorials, May 11, 2013 28
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)
Surface Canada 2013 Tutorials, May 11, 2013 29
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
Surface Canada 2013 Tutorials, May 11, 2013
Oxygen diffusion in oxides
30
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
11
Surface Canada 2013 Tutorials, May 11, 2013
Diffusion and interface growth in HfO2 and
HfSiOx ultrathin films on Si(001)
31
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.
Surface Canada 2013 Tutorials, May 11, 2013
Part II: Ion Implantation
• Implantation chamber and implantation stage
32
Surface Canada 2013 Tutorials, May 11, 2013
Periodic Table
• We can produce beams of all those elements shown in yellow !
33
12
Surface Canada 2013 Tutorials, May 11, 2013
Ion Implantation
34
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
Surface Canada 2013 Tutorials, May 11, 2013
Stopping and Range of Ions in Matter (SRIM)
http://www.srim.org/ Download SRIM-2008
Surface Canada 2013 Tutorials, May 11, 2013
SRIM Setup Window
13
Surface Canada 2013 Tutorials, May 11, 2013
Calculated Ion Trajectories
37
2MeV He+ in Si 50keV He+ in Si 50keV Au+ in Si
Surface Canada 2013 Tutorials, May 11, 2013 38
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)
Surface Canada 2013 Tutorials, May 11, 2013
Si(001)
SiO2
Si ions
39
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
14
Surface Canada 2013 Tutorials, May 11, 2013 40
Ge in Al2O3(0001): crystallization and ordering
E.G. Barbagiovanni, S.N. Dedyulin, P.J. Simpson, L.V. Goncharova, NIMB 272 (2012) 74–77
Surface Canada 2013 Tutorials, May 11, 2013
XPS
• Shift of Ge peak towards the surface (RBS)
• GeOx peaks in XPS Ge loss via GeO desorption
41
Ar sputtering prior to XPS analysis:
Ge layer is 3-5nm deep
Al2O3(0001)
GexO
disordered Al2O3
Tx>1100oC
N2 Al2O3(0001)
Ge-QD
Surface Canada 2013 Tutorials, May 11, 2013
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
Surface Canada 2013 Tutorials, May 11, 2013 43
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
Surface Canada 2013 Tutorials, May 11, 2013
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
44
Surface Canada 2013 Tutorials, May 11, 2013
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/
45
16
Surface Canada 2013 Tutorials, May 11, 2013 46
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-
Surface Canada 2013 Tutorials, May 11, 2013
Control QD Distribution with Mask
47
Si QD nucleation and growth by Si ion implantation and anneal
Lateral separation between implanted regions
Surface Canada 2013 Tutorials, May 11, 2013 48
Thank you! Lyudmila V. Goncharova
Department of Physics and Astronomy,
Western University, London, Ontario