S f Ch i iSurface Characterization

LEED

PhotoemissionPhotoemission

Linear optics

Surface characterization with electrons

M P S h WA D h S f I t f A l 1 (1979) 2M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2

LEED low energy electron diffractionLEED – low energy electron diffractionDe Broglie wavelength: λ= h/(mv)g g /( )For electrons: λ = 150 / E0 E0 in eV, λ in Å.For 100 eV‐electrons: λ(100) = 1.22 Å (low energy)Corresponds to atomic dimensions, similar to XRD

Analogy to optical grating.

Constructive interference:Enhancement of intensity only

a ogy to opt ca g at g

y yin certain directions:

n λ = d sinθn λ d sinθ

For 2D arrangement (plane lattice):scattering conditions have to bescattering conditions have to befulfilled in both directions.

h,k(order)

20 ML Ge

Ge onSi(111)7 7Si(111)7x7

Transition from 7x7 to 5x5

Si(111)7x7

Synchrotron radiation photoemission spectroscopy

• ISA Aarhus University• ISA Aarhus University• Maximum Energy 580 MeV• Max Current 250 mA

Lif ti 15 h• Lifetime 15 hours

SGM130‐400 eV1010 photons/sec @130 eV

Photoelectron spectroscopy• Core levels

– Chemical reactions/mixing

– Growth modesGrowth modes

• Valence bands– Electronic levels relevant for optics

hν=130 eV

hν

arb.

uni

ts) Al2p

sp bandSecondary

Inte

nsity

(a

Binding Energy(eV)

Secondaryelectrons

EF Evac Kinetic Energy (eV)g gy( ) gy ( )

Wedge‐shaped metal filmAFM

18 18 218 18 2. .× μmSample moved into shadow of shield. Evaporation rate ~1 ML per minute Triangular domains ~200 nm

Ag(111) LEED-pattern -Ag(111) LEED pattern only one type of domains

25Convolution fit ofts

)

15

20

Convolution fit ofGauss beamand step functiond=150 μm

ght (

arb.

uni

S i fil hi k b i 38,80 38,85 38,90 38,95 39,000

5

10

i2p

peak

hei

Scanning film thickness by movingwedge through laser or synchrotron beam

38,80 38,85 38,90 38,95 39,00

Si

Sample position (mm)

Width of synchrotron beam

The Si(111) 7×7 structure

Structure known from STMStructure known from STM

and various spectroscopies

• 12 adatoms

• 6 rest atoms

Surface states:

Occupied • S 0 3 eVOccupied  • S1 ‐0.3 eV

• S2 ‐0.8 eV

• S 1 7 eV• S3 ‐1.7 eV

Empty      • U1 0.5 eV

• U 1 7 eV• U2 1.7 eV

Si2p core levels

24 Si2p 130 eV

20

24

Si(I)Si(III)Si(IV)

7x7Si2p 130 eV

nits

)

12

16Si(II)

( )

arb.

un

100 L O2 Decomposition ofspectra

8

12 Sum Bulk Ad-atoms Rest atomsns

ity (a

7x7:•Bulk

0

4 Background ExperimentsInte •Ad‐atom

•Rest atom

106 104 102 100 98 96

Binding energy (eV)

Si‐surface oxide:•Bulk•Oxidation stages

The Si(111) 7×7 structureThe Si(111) 7×7 structureStructure known from STM and various spectroscopies:Structure known from STM and various spectroscopies:

12 adatoms  +  6 rest atoms

Surface states:

Occupied: S ‐0 3 eV S ‐0 8 eV S ‐1 7 eVOccupied: S1 ‐0.3 eV,  S2 ‐0.8 eV,  S3 ‐1.7 eV

Empty: U1 0.5 eV,   U2 1.7 eV

20Si(111)7x7 38 eV

15

b. u

nits

)

300K

Si(111)7x7 38 eV

10

ensi

ty (a

rbS

S3

300K450Kθ=10o

6 4 2 00

5In

te

S1

S2

6 4 2 0Binding Energy (eV)

Film growth ‐ Si2p spectrag p p

8

10Growth at 170 KNo annealing

130 eV

s)

7x7

Growth at 170 KAnnealing atroom temperature

10 Bulk 300 K Surface 300 K Bulk 170 KSurface 170 K)

6

8

(arb

. uni

ts

1

Surface 170 K

(arb

. uni

ts

2

4

nten

sity

(

0,1

Inte

nsity

102 101 100 99 98

0

101 100 99 98 97

In

Binding Energy (eV)

0 2 4 6 8 10 120,01

Coverage (ML)Binding Energy (eV)

Growth at 170 K leads to exponential decay of Si2p levels with ~5Å decay rate.Room temperature annealing of the film leads to growth of large atomically flatdomains Areas with low Ag coverage are formeddomains. Areas with low Ag coverage  are formed.

⇒A liAnnealing

Au on 6 ML Cu/Si(111)  Au4f spectra

10

1216 ML Au on Si(111)

Au4f Bulk

Au on Si(111):Reacted component

hi h

4

6

8 Au4f

its)

BulkReacted

at higher BE

AuReacted layer

0

2

4

arb.

uun Si

16

82 81 80 79 78 77 76 75

20 ML Au on 6ML Cu/Si(111)

Surfaceensi

ty (a

Au on Cu/Si(111):Surface component

8

12 Surface

Inte

AuAu

at lower BE

6 ML C

0

4SiSi

6 ML Cu

82 81 80 79 78 77 76 75

Binding Energy (eV)

Valence band spectraValence band spectra

• Excitation energy typically 30‐100 eV

Bi di 0 10 V

2520 ML Au 15

10

d bands47 eV

• Binding energy: 0‐10 eV

– sp electrons in Ag, Au Cu Al

2010

5

4b. u

nits

)

SP bandsAu, Cu, Al

15 3

2

sity

(arb Surface

state

10

1 MLInte

ns

0

5

6 ML Cu/Si(111)

8 6 4 2 00

Binding Energy (eV)

STMSTM

Difference STM images of occupied surfaceSi(111)7 7 fstates on a Si(111)7x7 surface:

a) topographic imageb) adatom states at ‐0.35eVc) dangling bond states at ‐0.8eV) g gd) back bond states at ‐1.7eV [15].

Topographic image of Si(111)7x7 surface asrecorded using Scanning TunnelingMicroscope (STM) with a bias voltage of + 2 V[8].

Reflection-Anisotropy Spectroscopy (RAS)

Wolfgang Richter, TU‐Berlin

RAS - Probe for solid-liquid interfaceAu(110) Th l d l

UHVAu(110) Three-layer model

McIntyre and Aspnes, Surf. Sci. 24, 417 (1971

Ambient

Surface

Bulk

Electrolyte[ ]'''8 ss BAd

RR εε

λπ Δ−Δ=⎟

⎠⎞

⎜⎝⎛ Δ

Electrolyte[ ]

expssR λ⎠⎝

Surface states at 2.5 and 3.7 eV

Sheridan et al. PRL 85, 4618 (2000)

Surface optics - Differential reflectionSurface optics - Differential reflection

f1 ~100 Hzf2 ~10 Hz

Selci et al. JVST A 5, 327 (1987)

Ellipsometry - Optical properties of SiEllipsometry Optical properties of Si

E0+E₁ E20

εIεR

Aspnes and Studna, PRB 27, 985 (1983)p , , ( )

Depth sensitivity of linear optics

The probe depth can bevaried through the wavelength

SDR – Si(111)2×1

Difference between clean and d loxygen exposed sample

Excitation of surface statesExcitation of surface states

Anisotropy‐ confirm chain model

SDR on Si(111)7×7SDR on Si(111)7×7

Experiments 1.7 eV: Ad‐atom dangling bonds

2‐3 eV: Ad‐atombackbonds to bulk states

Tight‐bindingmodelmodel

Noguez et al. PRL 76, 4923 (1996)