IMPRS-Course, March 2004w0.rz-berlin.mpg.de/imprs-cs/download/bc04_winter.pdf · 2012-04-17 · •...
Transcript of IMPRS-Course, March 2004w0.rz-berlin.mpg.de/imprs-cs/download/bc04_winter.pdf · 2012-04-17 · •...
IMPRS-Course, March 2004
Part1
Part2
Coffeebreak11:00 – 11:30
„Studies of properties of surfaces with ion-beams“
H. Winter
Humboldt-Universität zu Berlin, Germany
• some general features: classical trajectories and „shadow cone“
• structure of surfaces: Ion Scattering Spectroscopy (ISS)
• grazing ion/atom surface scattering: „surface channeling“
• studies on growthchemical composition
structure, and magnetismof ultrathin films by using fast ions
Rutherford scattering
predominant forward scattering
~ differential cross section forpure Coulomb scattering4He, 5.5 MeV → Au
concept of „shadow cone“
impact parameter angle of scattering
500 eV He – Mo atoms
d = 3.147 Å
studies on surface structure
Niehus und Spitzl, 1991
He+ - Pt(111)
Ion Scattering Spectroscopy (ISS)
element specific analysis
• momentum and energy transfer in classicalbinary elastic collisions
• He: M1 = 4 amuAl: M2 = 27 amu E1 = 0.550 EoNi: M2 = 58 amu E1 = 0.759 Eo
• time-of-flight: tAl / tNi = (E1,Ni / E1,Al )2 = 1.175
2 2
2 21 2 1
1 1
180 : 1 1 ;oo
M ME E M MM M
ϑ⎛ ⎞ ⎛ ⎞
= = − + >⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠
surface structure of NiAl(111)
Cu3Au (001)
H. Niehus et al.
relaxation of surface layers of Ag(110)
MEIS-study: Busch and Gustafsson, Surf. Sci. 407 (1998) 7
review paper: H. Niehus, W. Heiland, and E. Taglauer Surf. Sci. Reports 17 (1993) 216
H. Winter
Grazing collisions of atoms
and ions with surfaces
Physics Reports367 (2002) 387
superposition of „shadow cones“ → „critical angle“ for channeling
grazing ion surface scattering
„planar channeling“
(conservation of energy normal to
surface plane)
zmin
• well defined trajectories- distance of closest approach zmin
• interaction with electrons of selvage only
• non-destructive scattering
chemicalcomposition
magnetism
magnetism
growth
ion beam: H+, He+ ; 25 keV ; 1o – 2o
experimental setup
Auger-electron-spectroscopy
spin-polarized electron emission
ion yield
spin polarized electron capture
structure
ion-beam triangulation
growth
ion yield
θ = „random“
θ=60°
„rainbow scattering“
A. Schüller (2003)
effects of surface defects on trajectories
25 keV He+ 1.7° → Fe (100)
description by computer simulations(based on classical mechanics)
determined by „defects“ on topmost layer(thermal vibrations, steps, etc.)
angular distributions of scattered ions
angular distributions of scattered ions
25 keV He+ - Fe (001)
R. Pfandzelter, Phys. Rev. B57 (1998)
angular distributions of scattered ions: reflection from terraces
25 keV He+ → Fe / Fe (100)( homoepitaxy )
computer-simulations:- perfect 2D-growth - constant density of islands
angular distributions of scattered ions
T. Igel and R. Pfandzelter
growth of ultrathin films
homoepitaxy Fe / Fe(100)
growth at step edges
layer-by-layer growth
quasi-layer-by-layergrowth
intensity of specularly reflected projectiles
→ periodic change of surface morphology
→ oscillations of intensity(compare: RHEED)
N = η · ( D / F ) –i / (i+2) · exp { Ei / kT(i+2)}
nucleation theory: N saturation island densityD diffusion coefficientF evaporation rateEd diffusion barrieri critical cluster sizeEi binding energy of cluster
Ed = 0.49 eV
i = 1 (420 K)
i = 3 (580 K)STM – dataStroscio et al.PRL (1993)
real time studies on nucleation and growth of Fe / Fe (100)
D = ¼ ν0 exp (-Ed / kT)
step-flow growth
layer-by-layergrowth
initialdouble-layer
growth
statistical growth
growth of Co / Cu (100)
25 keV He0 → Co / Cu(100)( heteroepitaxy )
T. Bernhard and R. Pfandzelter
no monotonicArrhenius-behaviour !(„Z-shape“)
DFT-theory: (Pentcheva and Scheffler, FHI)
Experiment
growth of Co / Cu (100)
low T: diffusion of Co-adatomshigh T: diffusion of Cu- adatomsother T: competition of substitutionaladding of Co-Atoms with pinningof Cu-adatoms
• Below 320 K: Arrhenius-behavior. Hopping of Coatoms. Hopping barrier 0.59 eV(experiment: 0.54 eV). Small, rectangular Co islands.
• 320 K:Activation of atomic exchangebetween Co and Cu (Fig.2.2). Exchange barrier 1.00 eV. Enhanced island density due to pinning ofmigrating atoms at substitutional Coatoms
• Above 320 K :About 50% of Co atoms exchanged
into substrate. Combined growth oflarge, Co-decorated Cu islands andsmall Co islands
chemicalcomposition
Auger-electron-spectroscopy
surface sensitivity
excitation: first layer many layers
electron escape: some layers (dependent on Ee)
overall sensitivity: first layer some layers
surface sensitivity of Auger electron spectroscopy
Mean free path of electrons in metals
Auger electron spectroscopy
combination of excitation: - grazing ion-surface scattering- electron bombardment
25 keV protons → Ir / Fe(100)
Fe
Ir
Fe
Ir
I (n) = Io exp (−n /λ)
H+, Fe M 23VV λ → 0e-, Fe M 23VV λ ≈ 2.2 ΜLe-, Fe L 23VV λ ≈ 5.9 ML
intermixing in surface layers→ combination of proton- and electron-induced AES
1 ML Cr / Fe
M 23VV – Auger transitions
45 at% Cr55 at% Cr
Cr / Fe (100)
2 ML Cr / Fe
T=140K
prot
on-in
duce
dM
VV A
uger
line
elec
tron-
indu
ced
MVV
Aug
erlin
eel
ectro
n-in
duce
dLM
M A
uger
line
T=410K
1ML Co
2ML Co
3ML Co
4ML Co
5ML Co
Co / Cu (001) Ab initio DFT calculations of Co/Cu(001): propose capping of Co films with Cu
Pentcheva, Scheffler, Phys. Rev. B 61(2000)2211).
T. Bernhard and R. Pfandzelter
structure
„ion-beamtriangulation“
target current(total electron yield)
grazing ion surface scattering
critical angle for channeling ~ d-1/2
d
fcc(111)
fcc(111)
fcc(111)
fcc(111)
fcc(111)
fcc(111)
θ=60°
θ=30°
transition from „planar“ to „axial“ surface channeling
[10]
[21]
[11][12][01]
target current vs. Θ : H+, 25 keV – Cu(001)
T. Bernhard (2002)
inspection of growth: 25 keV He+ ions
variety of superstructures(Wuttig et al., 1992)
T > 270K : c(2x2) up to about 2 ML(1x1) over 2.5 ML
T < 270K : c(8x2) up to 1.5 MLc(12x8) over 1.5 ML
Mn / Cu (001)
[01]
[10]
target current vs. azimuthal angle
25 keV H+ - clean Cu(001) (1x1)
- Cu(001) c(2x2) Mn(Wuttig et al., 1992: ordered surface alloy)
[01]
[10]
[01]
structure: LEED (Wuttig et al.)
formation of Cu(001) c(8x2) Mn
(1x1)
c(2x2)-Mn
c(8x2)-Mn
c(12x8)-Mn
(1x1)-CoMn
transition
Cu(001)
different phases of
Mn / Cu(001)
studied by
„ion beamtriangulation“
T. Bernhard and R. Pfandzelter
hexagonal
quadratic
T. Bernhard, to be published
ion induced phase transition ( ~ 1013 He+ ions / mm2)
Mn / Cu(001)
0 2 4 6 8 10 12 1410
100
1000
10000
100000
1000000
1E7
He0 => Al(111) 16keV
coun
ts
electron number0 2 4 6 8 10 12 14
10
100
1000
10000
100000
1000000
1E7
He0 => Al(111) 16keV
coun
ts
electron number0 2 4 6 8 10 12 14
10
100
1000
10000
100000
1000000
1E7
He0 => Al(111) 16keV
coun
ts
electron number
electron number distribution
surface barrier detector
meshT ~ 98
%
target
biasvoltage
25 – 30 kV
Aumayr et al.Appl. Surf. Sci. 47 (1991) 139
without target:background signalcorrected signal
0 2 4 6 8 10 120
400
800
1200
1600
2000
2400
Kanal random
He0 => Al(111) 16keV Φin=1,88°
Zähl
rate
Elektronenzahl
0 10 20 30 40 50 601
10
100
1000
10000
100000
1000000 Kanal random
He0 => Al(111) 16keV Φin=1,88°
Zähl
rate
Elektronenzahl
electron number spectra
He, 16 keV - Al(111) , Φin = 1.88o
coincident noncoincident
pulse height spectrum of surface barrier detector biased to 25 keV- in collaboration with Aumayr and Winter (TU Vienna) -
counts of electron detector vs.azimuthal angle
(for different settings of discriminator level)
• at-sight information- simple analysis of data
• in-situ and real time analysis- studies under conditions of growth
• „simple“ experimental technique- measurement of target current (or electrons)
• method is sensitive to topmost surface layer(s)- scattering under „surface channeling“
„Ion beam triangulation“ of surfaces and ultrathin films
magnetism
magnetism
spin-polarized electron emission
spin polarized electron capture
Grünberg et al., PRL 57(86)2442
antiferromagnetic coupling between Fe-filmsexchange coupling oscillates with Cr film thicknessgigant magneto resistance (GMR)
Unguris, Celotta, Pierce, PRL 69(92)1125
layer-by-layer antiferromagnetic coupling
Fe / Cr / Fe(100)
Cr / Fe(100)
Secondary-Electron-Microscopywith Polarisation-Analysis
problem: background signal from substrate⇒ smaller probing depth needed
Cr / Fe (100)
Siegmann, Surf.Sci.307(94)1076
P ∝ Σi ⟨mi⟩ exp(−i/λ)
Cr / Fe (100)
λ = 4.2 ΜL
λ ≈ 0.5 ΜL
electronic orientation in free atoms: → emission of circularly polarized light
circularly polarized lightStokes‘ parameter: S/I = (I--I+)/(I-+I+)
spin polarized electron capture
Na I 3s2s - 3p2P
rough growth at 300 K
Cr / Fe (100)
electron-inducedelectron-emission
proton-inducedelectron-emission electron capture
Mn / Fe (100)
300 K: (metastable) layer-by-layer growth570 K: Stranski-Krastanov growth
(„Bauer‘s criteria“)
25 keV He+ → Mn / Fe(100)
AES 4 keV e-
25 keV He+ ions → He I 1s2s3S – 1s3p3P
in-planeantiferromagnetic
order
ferromagneticorder
Mn / Fe (100) and V / Fe (100)
MO Kerr-Effekt
sp-electron emission
sp-electron capture
P ( T ) ∝ Σi < m i > · exp (– i / λ )
< m i > magnetization of i-th layer of Fe(100) from molecular fieldapproximation
λ probing depth in ML
kinetic electron emissioninduced by 4 keV electrons orgrazingly scattered 25 keV protons
grazing scattering of 25 keV He+ ionscapture of electrons to He I 1s3p3P
Surface sensitivity: magnetization of Fe (100)
M. Potthoff and R. Pfandzelter
Fe / Cr / Fe (100)
grazing scattering of 25 keV He+ ionselectron capture in excited He I 1s3p3P term
emission of polarized light
ferromagnetic
antiferromagnetic
oscillatory interlayer-exchange splittingbetween Fe film and Fe substrate