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