Download - 1 Surface Course 2015 - MEIS
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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)
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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
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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
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Elementary considerations in ion backscattering (1)
k = the kinematic factor
Relation between incoming and backscattered ion energies
Cross section for backscattering
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Elementary considerations in ion backscattering (2)
Depth profiling
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Now on to more detailed structural work using Medium Energy Ion Scattering (MEIS)!!
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7
MEIS lab at Rutgers
ALD XPS
XPS
IR
NRP
MEIS
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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
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From the Ion beam analysis laboratory at Kyoto university; note the magnetic spectrometer.
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The Kyoto Kobelco very compact MEIS facility
Footprint:
~ 2.1 x 1.5 m
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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
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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
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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)
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An (oversimplified) picture of the origin of the oscillatory relaxation
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Reconstruction of the (110) surface of Au:
Possible structural models consistent with a (1x2) symmetry
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Conclusions:
Missing row structure
Large first layer inwards contraction
Buckling in the lower layers, results in charge density smoothing
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Another research example: Surface structure of TiC(001)
Y. Kido et al, PRB 61 1748 (2000)
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The angular shifts from the first and second layers are displaced in opposite directions!
Surface relaxation and rumpling!
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Melting of Pb(110) Frenken et al PRB 74, 7506 (1986)
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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
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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)
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II. Nuclear Resonance Profiling - a light projectile (proton) penetrates the nucleus and induces a nuclear reaction; the reaction product ( or ) is detected.
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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)
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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
p
E E r
mylar (13 m)detector
semicondut or
amost ra
detector (BGO)
p,
(a)
(b)raios
Reaction cross section (b/sr) vs. proton energy (keV)
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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
Resonant nuclear reactions
Reaction cross section (b/sr) vs. proton energy (keV)
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Some low energy nuclear resonances
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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)
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Nuclear resonance depth profiling of Al2O3 on Si(100)
(Baumvol et al., Porto Alegre)
Overlapping peaks in MEIS spectrum (M. Copel et al., IBM)
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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
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III. Medium Energy Ion Scattering (MEIS) - a low-energy, high resolution version of conventional Rutherford Backscattering (RBS)
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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
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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)
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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
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Determining interface strain using monolayer resolution ion scattering and blocking (Moon et al.)
Ozone oxide, no strain
Thermally grown oxide, significant strain
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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
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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.
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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)
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Schematic model
SiO2
Transition zone,
SiOx
Si (crystalline)
Surface exchange
Growth
Deal and Grove
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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
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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
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
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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!
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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)
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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
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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)
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Origin of metallic state at the interface
51
Polar catastrophe: polar-nonpolar discontinuity Ohtomo &Hwang, Nature 427 (2004)
This model assumes an abrupt interface
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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
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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
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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
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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
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57
Angle dependent XPS shows poor agreement with sharp interface model (Chambers et al)
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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
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A novel instrument for surface structure studies using energetic ion beams: the Helium Ion Microscope - HIM
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Ion source
Diffraction in He microscope and in SEEM
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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 ).
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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
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Why is Helium So Different?
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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.
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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
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Ion beam milling of graphene
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SECONDARY ION MASS SPECTROSCOPY
SIMS
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)
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Fundamentals of Surface and Thin Film Analysis, L.C. Feldman and J.W. Mayer, North Holland-Elsevier, N.Y. (1986)