Cfm Surface Lectures 2012 Notes 1-9 Final

7/25/2019 Cfm Surface Lectures 2012 Notes 1-9 Final

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Surface Physics :

Structure and Composi t ion & How to study them !

Course structu re

Lecturing schedule

Comments :

PowerPoint: - hand-outs; research & past paper; problem workshop; lab tour

Texts: Woodruff & Delchar, Modern Techniques of Surface Science, CUP:

Zangwill, Surface Physics, CUP: McCash, Surface Chemistry, Oxford

Briggs & Seah Practical Surface Analysis ,Vols: 1 & 2 , Wiley

Prof Chris McCon vil le P431 c.f.mcconvil [email protected]

week Tues 10-11PS128

Wed 11-12B2.02

Thurs 9-10F1.11

Fri 12-1B2.02

8 CFM1 - CFM2 CFM3

9 CFM4 CFM5 - CFM6

10 CFM7 - CFM8 CFM9

1. Why surfaces are important & how can we study them?

2. Surface science techniques - phenomenology & determination

3. Surface crystallography & structural / chemical determination


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SURFACE PHYSICS

Why are surfaces interesting?

Fundamental:a surface is a special kind of defectthe mostextreme kind of defect - in a perfect 3-D periodic solid withdifferent geometrical (atomic) and electronic structure

Practical:

1. all gas-solid and liquid-solid interactions occur at thesurface. e.g. corrosion, adhesion, wear, heterogeneouscatalysis (surface reactions, chemistry), electronic junctions

2. the surface chemistry (compound formation) and electronic

structure of solid-solid interfaces can dominate the performanceof a reaction or the operation of an electronic device

3. surfaces and interfaces can also be modified by adsorption(segregation) from the bulk - e.g. grain boundary segregationand intergranular brittle fracture or by alloying at the surface


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grain boundary segregation and intergranular brittle fracture!!

a 2nd world war Liberty ship


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What is difficult about studying surfaces?

Experiment:

2. Surface Specificity

Need to detect these small amounts of material (very few atoms)in the presence of the under ly ing bulk sol id.

e.g. 1 mm thick sample has 5 x 106atomic layers

so 1% of a monolayer is 1 part in 5 x 108of the total no. of atoms

Need to use a very, very local clean environmentto ensure the surface stays clean

ULTRA-HIGH VACUUM (UHV)


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3. The need for ultra-high vacuum (UHV)consider the rate of arrival of molecules at a surface from the surrounding gas

kinetic theory of gases: rate of arrival of molecules ofno. density n /unit. vol., average velocity ca, is r = n ca

now c2rms= 3 kBT/m & ca= (8/3)1/2crms(kB- Boltzmanns constant, T - absolute temp., m - molecular mass)

and P = nkBT (cf P = RT/V) so r = P/(2kBTm)1/2

substitute kB

= 1.38 x 10-23J.K-1and m = 1.66 x 10-27M kg

gives r = 2.63x1024P/(MT)1/2molecules m-2

take M=28 (N2, CO), T = 300 K and convert P (in Pa) to p in mbar (1 mbar = 100 Pa)

so r = 2.87 x 1024p molecules m-2

1 ML1019molecules m-2so with unity sticking factor the monolayer time is= 3.48 x 10-6/p s or 3.48/p s

e.g. p = 1 mbar, = 3.5 sp = 3.5 x 10-6mbar, = 1 s

p = 3.5 x 10-10mbar, = 104s or 3 hrs

MORAL : need UHV for

realist ic experimental

t imescales on c lean and

created surfaces


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Diffusion

Pump

(low vapourpressure oil)

Turbo

Molecular

Pump

(high speed fan,oil free)


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What kind of surface?Simple- low Miller index single

crystal surfaces

e.g. face centred cubic (fcc)

General- especially vicinalsurfaces (low index + steps &kinks)

Can do the same for body-centredcubic (bcc), hexagonal close-packed (hcp), wurtzite (wz), and

zinc-blende (zb) structures


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wz

z


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Examples of high-index (stepped) surfaces

step atoms shaded for clarityfcc(410)

Terrace: (100)

Step

Primitive unit mesh

Centred-primitive unit mesh


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Surface structurewhy the interest?

Surface relaxation & atomic reconstructionatomic rearrangement for energy minimisation

Adsorptionwhere do adsorbed atoms and

molecules sit?active sites in heterogeneous catalysis

Adsorption bond lengths

implications for bonding and reactivity


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Surface structural phenomena - a brief overview

Clean sur face (e.g. metal): su rface relaxation

(i.e. the outermost layer spacing changes)

WHY?Electron charge depletion in surface layerdue to spill over into vacuum & smoothing -typically causes outer layer contract ion.

Damped oscillatory layer spacing changeswith depth due to propagation of chargefluctuations.

Sem iconduc tor sur face reconst ruct ionWHY? Covalently-bonded networks (e.g. Si, Ge, GaAs, InSb,.) -

surface leads to dangling bonds several surfacereconstructions possible to reduce no. of dangling bonds (onthe polar surfaces) and depending on surface composition.

Leads to depletion / accumulation of charge at the surface


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Surface Physics :

Structure and Composi t ion & How to study them !

Course structu re

Lecturing schedule

Comments :

PowerPoint: - hand-outs; research & past paper; problem workshop; lab tourTexts: Woodruff & Delchar, Modern Techniques of Surface Science, CUP:

Zangwill, Surface Physics, CUP: McCash, Surface Chemistry, Oxford

Briggs & Seah Practical Surface Analysis ,Vols: 1 & 2 , Wiley

Prof Chris McCon vil le P431 c.f.mcconvil [email protected]

week Tues 10-11PS128 Wed 11-12B2.02 Thurs 9-10F1.11 Fri 12-1B2.02

8 CFM1 - CFM2 CFM3

9 CFM4 CFM5 - CFM6

10 CFM7 - CFM8 CFM9

1. Why surfaces are important & how can we study them?

2. Surface science techniques - phenomenology & determination

3. Surface crystallography & structural / chemical determination


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Si(100)-(2x1)

Ideal termination has 2 dangling bonds persurface Si atom - surface atoms pair (formdimers) to reduce this to 1 dangling bond

per surface Si atomNB: dimers are asymmetric (buckled)

Si(111)-(7x7)

Ideal termination has 1 danglingbonds per surface Si atom -

reduced by:1. surface dimer formation -removes these dangling bonds

2. adatoms bond to groups of 3 Sisurface atoms (reduce 3 danglingbonds to 1

3. stacking fault appears in 1/2 ofsurface unit mesh


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Si(111)(7x7) surface Si(100)(2x1) surface

LEED


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Surface struc tural phenomena - a brief overview

Ionic solids (e.g. oxides or III-V semiconductors)

- instability of polar surfaces

WHY? dipole interaction energy becomes infinite soreconstruct to remove dipole interactions(e.g. nano-facets to non-polar orientation)

e.g. NiO(111) (100) nano-facet

MgO(111) - microfacets

ideal bulk-terminatedreconstructed


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Some other metal surface reconstructions

(111)nanofacets zig-zag rows

Examples of clean metal surfacemissing row and close packed


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Atom ic adsorp t ion on metals - usually highest-coordination site

The so-called clock reconstruction of a Ni(100) surface followingthe adsorption of atomic C or N ( but not O).


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Molecular adsorbates commonly form local directional bonds

Si(100)(2x1)-OH+H

H2O OHa+ Ha

Cu(110) + glycine

2NH2CH2COOH 2NH2CH2COOa+ H2

Need techniques that will allow thisdetailed information to be determined

(with sufficient accuracy!)


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Surface Structure & Surface Crystallography

NB: a surface is a 3-D ob ject but has only 2-D period ici ty

Terminology

substrate

Structure identicalto that of infinitebulk solid

selvedgeOutermost atomic

layers which differdue to presence oftermination of solid

adsorbate

Layer(s) of differentspecies from solid orgas phase or bulk

surface


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Surface Structure & Surface Crystallography

NB:a surface is a 3-D ob ject hence;

Surface = Adsorbate + Selvedge (+ Substrate)

- but has only 2-D period ici ty

and remember - ALL symmetry pro pert ies considered

apply to the 3D ob ject

Classi f icat ion o f stru ctures

Identify both t ranslat ionaland point symmetry operations

Devise a convenient notat ionrelating surfaceperiodicity (unknown) to substrate periodicity (known)

Classify structural types by relationship of surface andsubstrate periodicities


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Classify the 2-D periodic surface according to:(1) their periodicity; 2-D Bravais nets


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Stereograms of the ten two-dimensional point groups. On the left areshown the equivalent positions, on the right the symmetry operations. The

names follow the full and abbreviated 'International' notation.

Classify the 2-D periodic surface according to:(2) their point group symmetry; Ten 2-D point groups

Elements: 1,2,3,4,6 - fold rotation &mirror - al l operat ions act with in aplane parallel to the surface

Classify the 2-D periodic surface according to


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17- 2D Space GroupsClassify the 2-D periodic surface according to(3) their space group symmetry; 2D space groups

combine: Bravais nets &p.g. operations + glide


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alsoweclassify structures according to the relationshipbetween the surface and substrate periodicity

substrate primitive translation vectors (2-D) aand b

surface primitive translation vectors (2-D) aand b

hence can write a=G11a+ G12b and b = G21a+ G22b

where Gijare four coefficients which form a matrix G

so and

Note: thearea of substrate mesh is |ax b| so detG is the ratioof the areas of the surface and substrate primitive unit meshes


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Classification:- several possibilities

a) det.Gis integral and all matrix components are integral

the two meshes are simply related- mesh of surface+substrate is the

same as the mesh of the surface alone i.e. same translational symmetry

b) det.G is a rational fraction (or det.G is integral and some matrix

components are rational fractions)the two meshes are rationally related- structures are commensura te

e.g. in 1-D

so now the mesh of surface + substrate is a, b

e.g. in 1-D

det det

detG

P

QdetP and detQ have the

smallest integral values


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Classification:

c) det.G is irrational

incommensurate structure- implies surface atoms do not see the

corrugated potential of the substrate (may occur for adsorbed layerswith very strong adsorbate-adsorbate interactions)

Nomenclature

Most general: - use matr ixnotat ion- G

More convenient: - use Wood notation

e.g. adsorbate A on{hkl} surface of materialX

primitive translation vector lengths related by |a|=p|a| ,|b|=q|b |

surface mesh rotated by

structure is X{hkl}(pxq)R -A

NB:- cannot use Wood notation when the included angles ofthe surface and substrate mesh differ

e.g Ni{100}(2x2)-O andNi{111}(3x3)R30-O


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Dif f ract ion from surfaces how to make the signal

su rface speci f ic

Two contributing processes

inelastic scattering removes signal from any technique

which only detects electrons which have no tlost energysuch as elastic scattering (diffraction)!

1. inelastic electron scatter ing

Mechanisms

a. electron-hole pair excitations - low energy excitations -

dominant at low (


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Inelastic

scattering

mean-free-path

(Angstrom)

Electron energy above EF(eV)

Theoretical calculation of inelastic scattering in jellium with a charge

density appropriate for Al (devised by Quinn in 1962)

plasmone-h

plasmon scattering dominates for electron energies above about 30 eV

E i t l d t i ti f l t t f l t


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Experimental determination from electron spectroscopy of electron

attenuation lengths in many different materials (Seah & Dench, 1979)

Electron energy above EF(eV)

AttenuationLength(Angstroms)

d AB

e-

detect attenuationof electronsemitted from B as

function ofthickness ofoverlayer film of A

I I d 0exp( / )


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2.elast ic electron scatter ing

Part of the incident flux is elastically (back-)scattered out of the crystal by each atomic

layerso incident flux is attenuated as itpenetrates the solid due to elasticscattering as well as inelastic scattering

Elastic scattering can alsocontribute to attenuation lengthmeasurements by increasing theaverage escape distance

Quanti tat ive su rface stru ctu re determin at ion


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conservation of energy so

where is a reciprocal lattice vector

Quanti tat ive su rface stru ctu re determin at ion

(Electron) Diffraction at Surfaces & the Reciprocal Net

First, recall the situation in 3-D

periodic solids

conservation of reducedelectron wavevector (momentum)

k k

k k g' hkl

g a b chkl h k l * * *

a b c

*

2V

b c a

*

2V

c a b

*

2V

V a b c.

conservation of energy k k'2 2

In a 2-D periodic system

where is a reciprocal net vector

conservation of reducedelectron wavevector parallelto surface

k k g'/ / / / hk g a bhk h k * *

a

b n

*

2 A b n a

*

2 A A a b n.

k k'

2 2

k k k k ' '/ / / /2 2 2 2

perp perp

nis a unit surfacenormal


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Ewald Sphere Construc t iona convenient representation of these conservation equations

3-D case

1. Draw the vector kto the origin of thereciprocal lattice

2. Draw a sphere, radius |k| centred on the

start of the vector k3. Diffracted beams kcorrespond to thevectors from the centre of the sphere to theintersections of the sphere with reciprocallattice points

NB - the sphere ensures energy conservation, the reciprocal latticepoints ensure reduced momentum conservation

E ld S h C t t i


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Ewald Sphere Construc t iona convenient representation of these conservation equations

2-D case

1. Draw the vector kto the origin of thereciprocal net

2. Draw a sphere, radius |k| centred on the

start of the vector k3. Diffracted beams kcorrespond to thevectors from the centre of the sphere to theintersections of the sphere with reciprocal netrods drawn through reciprocal net points and

perpendicular to the surface

NB - the rods show that electron momentum transferperpendicular to the surface is a continuous variable - but theparallel component is discrete


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de Broglie and so

Low Energy Electron Dif f ract ion - LEED

typical energies 30-300 eV - WHY?

Wavelengthinteratomic distances

h

p

p

mE

2

2

h

mE2

with E in eV, in ngstroms, we have

e.g. if E=150.4 eV, = 1.0

Surface specificity- this energy range gives highest elasticand inelastic electron scattering cross-sections

1504.

E

Practical implementat ion : -


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Practical implementat ion : -

LEED optics

Field-free space Retarding-field region

Acceleration region - electronshit fluorescent screen

al l in ul tra-high

vacuum

~+5 keV

high-pass filter

NB:- only elastically scatteredelectrons are coherent and soform diffraction pattern


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LEED pattern is a projectionof the reciprocal net with a

magnification determined bythe electron energy (and thusEwald sphere radius)


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Low energy electron di f f ract ion (LEED)

Retardin g field analys er (RFA)


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Cu(110)

Cu(100) Cu(111)

E0~ 60 eV forall 3 surfaces


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LEED pattern is simply aprojection of the reciprocalnet with a magnification

factor determined by theelectron energy (and thusEwald sphere radius)


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Clean GaAs(001)

Surfaces


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Real space Diffraction pattern (k-space)

square(1x1)

centredrectangle

(1x1)

squarec(2x2)

square(2x1)

from thediffractionpatterndetermine the

reciprocal netand henceinvert toobtain realsurface net.

NB:

Nomenclature fordiffracted beams- indexed relativeto substratereciprocal net


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Overlayer Structures

with LEED

(4x2)

c(4x2)

Two domains ofthe (4x2) rotated90o(a) and 45o

(b) relative to eachother


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Multiple domains can also have an effect when point group


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Multiple domains canalso have an effect when point groupsymmetry of the surface structureis lower than that of the substrate

both domains havesame diffracted beamlocations, but different(2-fold symmetric)

relative intensities.

Sum of two is 4-foldsymmetric

General con clusio n the effect of domains ensures that the point groupsymmetry of the surface diffraction pattern is always the same as that ofthe substrate.


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Reflect ion High Energy Electron Diffract ion (RHEED)

- High energy electrons (>15 keV) used with grazing incidence geometry (< 2o)

- Surface sensitivity due to low component of momentum normal to surface (few )

In LEED, low energy electrons used- provide large elastic scattering cross-section for back-scattered electrons- keep the penetration depth of the electrons short

In RHEED, another solution is used- provide large elastic scattering cross-section for forward-scattered electrons- keep penetration depth small by using grazing incidence

(LEEDs high energy cousin)


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Kinematic Basis o f RHEED

Conservation of energy (E = 2k2/2m):|ki| = |kf|

Conservation ofparallelmomentum (Laue condition):

k||= kf||ki||= G(h,k)

G(h,k)= ha1*+ ka2*is the 2D reciprocal lattice vector

symmetry of reciprocal 2D lattice and real space 2D lattice are the same

kf||ki||

ki kf

No translational symmetry in surface normal- surface is 2D periodic

- 5 Bravais lattices; (square, rectangular, centred rectangular, hexagonal, oblique)

2D unit mesh (cell) defined by lattice vectors a1and a2

2m

pmv

2

1E

mv

h

p

h

22

2mE

h

E150

convert to and E to eV


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Kinematic Basis o f RHEED

No translational symmetry in the surface normal (2D)- reciprocal lattice rods which are perpendicular to the surface

Ewald sphere is constant energy sphere of radius ki- diffraction occurs when sphere intersects a reciprocal lattice rod (at kf)

streaked RHEED patterns generally observed from flat surfaces

- due to thermal broadening of the lattice rods and surface imperfections

kf

ki

kf

kf

kf

kf

(00)

G (h,k)

Ewald sphere constructionfor 2D diffraction

|ki| = |kf| k||= kf||ki||= G(h,k)reciprocal lattice

rods

Ewald sphere

diffraction

pattern


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Theory s im i lar to LEED:-

bu t the incident wavevector IkIis now very large


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Surface Recons truc t ion

Semiconductor surfaces are characterised by dangling bonds (unsatisfied bonds) dueto lower coordination of surface atoms compared to bulk

Many semiconductor surfaces (polar surfaces) undergo a reconstruction to reducenumber of dangling bonds and minimise the surface energy

new larger 2D unit cell

Formation of surface dimers is common - change of periodicity:

e.g. GaAs(001)-(2x4)

GaAs

Other examples include:

Si(001) - (2x1) and Si(111) - (7x7)

GaAs(001) - c(4x4), c(8x2), (4x2), (2x3),

InSb(001) - c(4x4), (1x3), c(8x2), (1x1).

x2x1

Origin o f RHEED Patterns


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Origin o f RHEED Patterns

e.g. GaAs (001)

RHEED patterns along different azimuths

GaAs(001)-(2x4) Sur face Struc tu re


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GaAs(001) (2x4) Sur face Struc tu re

[110]

[110]

orderstreaks

orderstreaks 3rdlayer As

2ndlayer Ga

1stlayer As

4thlayer Ga

[110]

[110](2x4) unit cell

2x periodicity due to As dimer formation 4x periodicity due to existence of 2 dimers

and by 2 missing dimers reconstructed surface characterised by (2x4)

unit cell

streaked diffraction pattern

unit cell for

ideal terminated

surface

planview

sideview


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Clean GaAs(001)

Surfaces

R fl t i Hi h E El t Dif f t i (RHEED)


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Reflect ion High Energy Electron Diffract ion (RHEED)

High energy electrons (>15 keVup to 35 keV) used with grazing incidencegeometry

Ideal geometry for in-situ monitoring of growth Diffraction pattern obtained during growth provides information regarding

surface structure (2D unit cell) and surface quality during deposition

Intensity of diffraction features can be monitored and provide calibration ofgrowth ratesRHEED intensity oscillation technique

Ga As2/As4

electron gunRHEED pattern

GaAs

In-s i tu mon i tor ofMBE growth

Surface reconstruc t ions obs erved du r ing GaAs (001)


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Surface reconstruc t ions observed du r ing GaAs (001)grow th by m olecular beam epi taxy (MBE)

Growth diagram obtained in-situ byRHEED

Strong dependence on substratetemperature and incident flux ratio

(BEP = beam equivalent pressure)

14 different surface reconstructions - bothAs and Ga terminated

Most common growth surface is (2x4) Asterminated surface

~ 500-600 C and As/Ga flux ratios < 1:1

Classic Thin Film Grow th Modes


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Frank van der Merwe (FvdM) Volmer-Weber (VW)

2D layer-by-layer 3D is land g row th 2D3D growth

Stranski-Krastanov (SK)

Dg= gf+ gi- gs

gs,f = surface energies of substrate and epilayergi = interfacial energy

g

< 0; 2D grow thg

> 0; 3D growth

2D growth occurs when the atoms of the deposit material are more stronglyattracted to the substrate than to themselves

3D growth occurs when the deposit atoms are more strongly bound to eachother than to the substrate

2D-3D growth occurs in lattice mismatched heteroepitaxial systems: increasedstrain leads to an increase in gi as the film thickness is increased

e.g. Ge/Si (~ 4%), InAs/GaAs (~7%)


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Sem iconduc tor Quantum Dots (QDs) formed by


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Q (Q ) y

Sel f -Assembly

Deposition

Substrate

2D wetting layer

Self-assembled

3D islands

Formed during lattice mis-matched heteroepitaxial growth

Examples:

InAs / GaAs 7.2%Ge / Si 4.1%InAs / InP 4.0%InSb / GaAs 14.6%InSb / Si 20.2%

Stranski-Krastanov growth:strain plays an important role

Initial 2D layer formedso called wett ing layer

Coherent (dislocation free)3D islands occur at somecritical coverage crit

Growth mode transition easilymonitored in MBE using RHEED

Important factors; size, shape,density, composition (alloyingcan be important)


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Summary o f e lectron di f f ract ion techn iques:


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LEED Typically 30 -150 eV used to maximize the surface sensitivity.

Electrons elastically scattered-information on surface periodicity /reconstruction. Typically used to monitor static structures and determine the structure of

clean and adsorbate covered surfaces. Can be used in I-V (I-E) mode in conjunction with multiple scattering

calculations for a full structural analysis approach

Summary o f electron di f f ract ion techn iques:

RHEED Typically uses 15 keV electrons at very grazing incidence to maximize

the electron path length in the near surface. Used to monitor growing surfaces (geometry in separate azimuths)

Used for calibrating deposition rates (RHEED oscillations) Can detect 2D to 3D transitions in growth e.g. layer-by-layer to quantumdot growth

Can also be used in higher pressure environments (


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Su ace s uc u e de e a o us g

From diffraction patterncan determine the reciprocal net andhence invert to obtain the real surface net.

Try to extract any additional symmetry information from thediffraction pattern (point group, space group) - i f possib le!

Can (potentially) determine atomic positions within the unit net byanalysing the diffraction beam intensi t ies

In bulk structure determination by X-ray diffraction - measure intensitiesof many diffracted beamscan use Fourier transform of the intensitiesto get some directinformation on the structure

Surface structure determinat ion u sing LEED I-E

Dkperp is a continuous variable - measure intensity-energyspectra of diffracted beams, i.e. I-E plots

effect of com plex scatter ing factorsand mult ip le scatter ingprecludes use of Fourier transforms - use tr ial-and-errormethod

with multiple scattering simulations


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General c lassi f icat ion o f electron spectroscopy method s


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General classi f icat ion o f electron spectroscopy method s

B i l i t t i f th h t l t i f f t


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K L M

h

Basics: exp loi tat ion of the pho toelectr ic effect

What is the response of an atom to the loss of an electron?

1. Atom becomes a positive ion.2. Coulombic screening of the nuclear charge is reduced3. Energy levels adjust to new screening and chemical environment

KE = h- BE


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Avai lable pho ton energies for XPS


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XPS - hvalues used - 1487 eV (Al K) & 1254 eV (Mg K

)

Accessible corelevelsessentially for all elementsand (low) photoelectronkinetic energies

i.e. those whichcorrespond to shortattenuation lengths

Avai lable pho ton energies for XPS

valence levels

deep core levels

Photoionisation cros s-sect ions fo r XPS


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Essentially all elements

have one (or more) corelevels with similarly large

photoionisation cross-section (hence similarsensitivity).

XPS cross-sections:

photoionisation cross-sections athu=1500 eV (cf. Al K)

uppermostfactor of 10 incross-section

Photoionisation cross sect ionsfo r XPS


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What is the photoelectron binding energy measured in XPS?


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The simple view: KE = h- Eb Ebis the one-electron BE

Koopmans theorem but this is NEVER observed!

The apparent (photoelectron) binding energy is the difference betweenthe energy in the init ial state and the energy in the f inal state

initial state - neutral ground state atom + photon

final state - core-ionised atom + energetic (KE) electron

What is the time-scale of the emission process? - how doesthe core-ionised atom respond?

Free atom - adiabat ic appro ximation(fully relaxed)

KE = hu- Eb + Ea Ea= intra-atomic relaxation energy

This would be fine if photoemission was a slow processin reality it israpid so valid to use the Sudden approximation

i.e. Final state is one where electron is in an excited bound state of atom

or is ejected into the continuum of unbound states above Evacuum

What is the photoelectron binding energy measured in XPS?

F t di b t i i t i (f ll l d)


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Free atom - adiabat ic approxim ation(fully relaxed)

KE = hu- Eb + Ea

Ea= intra-atomic relaxation energyFree atom - sudd en approx imat ion

shake-up- other electrons in excited bound states

shake-off- other electrons in continuum (for atoms

= multiple ionisation)

Solid - adiabat ic approxim ation(fully relaxed)

KE = hu- Eb + Ea + Er

Er= inter-atomic relaxation energy

Solid - sudden approx imation

shake-upand shake-off- but metals have acontinuum of states above Ef- so even excitation

to states below vacuum level are shake-off-like


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Mg K=1253 eV XPS spectra show


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g

Zn-Cu-Co alloy

A l K

=1486 eV

Cu(100) sample

BOTH photoemiss ion &

Auger electron peaks

Enhances the spectralfinger-print of the atom(element) - several peakswith characteristic relativeintensities on staircase

background.Distinguish photoemissionand Auger peaks bychanging photon energy

KEphotoemission= h- EAKEAuger= EA- EBEC

Photoemissionintensity

inelastically-scatteredbackground


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Chemical state information in core level spectroscopy -especial ly XPS


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especial ly XPS

kinetic energy of thepeaks in the photoelectronenergy spectrum identifiesthe emitting atom

Recall - basis of XPS is that core level

binding energies are characteristic ofthe atomic sp eciesand so can beused for elementidentification/quantification

However this is not thewhole story ..

Chemical effects in XPS


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How can the photoelectron binding energy of an atomic core level be influencedby the surrounding atoms (chemical bonding = electronic structure)?

init ial s tate effect- change in one-electron binding energy Ebdue to change in valence electron environment

f inal s tate effect- change in inter-atomic relaxation energy Er

combined effect leads to an experimentally-observedchemical shift

NB: - the two con tr ibuto ry effects are NOT separable exper imental ly

Use the chemical shift as a spectral fingerprint of differentbonding environments

Auger Electron Spectroscopy (AES) is a dif ferent techn ique


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Use incident energetic electron sto produce core ionisation

allpeaks are due to Auger electron emissionNo equivalent of photoemission peak because in electronionisation incident electron does notgive up allits energy

Surface specif ic i tydue to inelastic & elastic scattering of outgoing

electrons - essentially same as XPSPract ical p roblem:use of incident electrons creates a largebackground of inelastically-scattered and secondary electrons - sosignal-to-background ratio is poor

Solut ion:use electronic differentiation of spectrum to suppressbackground and allow amplification

Consequence:peak in N(E)becomes double feature indN(E)/dE (or N(E))

E

typical Auger electron spectra of Si(001)


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incidentelectrons

3-10 keV

e-electron

energyanalyser

NB: unlike with photons, theelectron ionisation cross-section

peaks around Ep = 3 x B.E.

AES - an app l icat ion - character is ing layer-by layer grow th


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Ag

V

knees in the AES

amplitude vs. exposureshow monolayercompletions andprovide an absolutecoverage calibration

AES - an appl icat ion - ident i fy ing grain

b d t i i i t l f t


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boundary segregat ion in intergranular fracture

200 m

0.45 ML of P foundhad segregated tograin boundaries

Turbine rotor failure - Hinkley Point power station, UK.

Quant i ficat ion of sur face composi t ion


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XPS:-Measured intensity of emission depends on:

Surface composition (how much of an element) Photoionisation cross-section

Instrumental factors (analyser efficiency, sensitivity etc.)

Electron attenuation length

Depth distribution of elements

many of these factors can be accounted for by using standardreference spectra for relative peak intensities

AES:-Also need to think about: Auger branching ratio

Contribution to ionisation from backscattered electrons

depends of nature of substrate


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Wide Scan XPS of GaN AsXP

S of a d ilu te III-N-V alloy GaN0.104As0.896


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0 100 200 300 400 5000

500

1000

1500

2000

2500

3000

3500

Ga 3s

As 3p

Ga 3p

As 3s

Wide Scan XPS of GaN0.104

As0.896

Ga (LMM)

N 1sAs (LMM)

As 3d

Ga 3d

Intensit

y

(arb.units)

Binding Energy (eV)


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96/178


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Analysis of carbon f ibre based po lymer com pos i te


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6CF3

(5CF2)2

3

CH2

O

4C=O

[1CH2

2C ]n

1CH3

h e-

Fluorinated Side Chain Polymer - Angle

Resolved C 1s.

49%

33%

3% 18%

y p y p

material by XPS

Woven carbonfiber composite

XPS analysis identified the functionalgroups present on composite surface.

Chemical nature of fiber-polymer interfacewill influence its properties.

Analysis of m ater ials for s olar energy col lect ion


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y gy

XPS Depth Prof i l ing- The amo rpho us-SiC/SnO2interface

Profile indicates a reduction of the SnO2at theinterface during deposition. Such a reductionwould effect the collectors efficiency.

Photo-voltaic Collector

Conductive Oxide- SnO2

p-type a-SiC

a-Si

Solar Energy

SnO2

Sn

Depth500 496 492 488 484 480

Bind ing Energy, eV

XPS analys is of p igm ent f rom Egypt ian Mummy


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150 145 140 135 130Binding Energy (eV)

PbO2

Pb3O4

500 400 300 200 100 0Binding Energy (eV)

O

Pb Pb

Pb

N

Ca

C

Na

Cl

XPS analysis showed

that the pigment usedon the mummywrapping was Pb3O4rather than Fe2O3

Egyptian Mummy

2nd Century AD(World Heritage MuseumUniversity of Illinois)

The impact o f XPS?


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Phys. Rev. 105, 1676, (1957)

Kai Siegbahn (1918 - 2007 )

1981 Nobel Prize in Physics

X-ray Photoelectron Spectroscopy (XPS),also known as Electron Spectroscopy for

Chemical Analysis (ESCA) is a widely usedtechnique to investigate the chemicalcomposition of surfaces.

p

for his contribution to the developmentof high-resolution electron spectroscopy "


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Instinct says beer. Reason says Carlsberg

Instinct says Surface Analysis. Reason says XPS

Electron s cat ter ing m ethods for su rface structure determ inat ion


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103

LEED - remote electron source - incidentplane waves - interference - diffraction.

NB - much of interference comes fromsubstrate layers

Alternative approach - use photoemission from a core level of a surface

(adsorbate) atom as the sourceof the electrons which interfere

Photoelectron Diffraction -detect photoelectrons

(Surface) Extended X-ray AbsorptionFine structure - SEXAFS - detect

photoabsorption

Sur face st ructure & composi t ion us ing s cattered ions


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Factors governing ion -surface interact ion

e.g. Energy, mass, charge exchange, etc. Surface structure & composi t ion

Low energy ion scattering (LEIS)

Coaxial impact collision ion scattering spectroscopy (CAICISS)

Sub-sur face structure & c omposi t ion

Medium energy ion scattering (MEIS)

Secondary ion mass spectrometry (SIMS)

Combinat ions of techniques

First UK MEIS data

Factors governing ion-sur face interact ion


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Energy: LEIS: 0.5 - 10 keV

MEIS: 50 - 400 keVRBS: >1 MeV

SIMS: 250 eV - 2 keV

Mass: Hydrogen (H+)

Helium (He+) or Neon (Ne+)

Alkali metals (e.g. Li+)

Oxygen (O+) and Caesium (Cs+)

Momentum transfer: (e.g. sputtering)

Charge transfer: (e.g. neutralisation)

Binary collision model in ionTwo body billiard ball collision:

Ion scat ter ing spectros cop y: basic pr incip les


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106

yscattering

Two-body billiard ball collision:

E

E AA1

0 2 1

2 21

1 2 21

1

( )[cos ( sin ) ]/

whereAis the ratio of themasses A = M2/M1

EE

AA

2

02

224

1

( )cos

Conversation of energy

E0= E1+ E2

E0= M1v02/2 etc.

Conservation of momentum

M1v0 = M1v1cos1+ M2v2cos2

M1v1sin1+ M2v2sin2

Why is the binary collision modeladequate (ignore solid)?

Duration of collision is short(cf. vibrational timescale)

Energy transfer large (cfbinding energy of the atomto the solid)

Low energy ion s catter ing (LEIS)


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1-10 keV He+, Ne+ or Li+

Fixed scattering angle(usually as small aspossible or 90)

Compositional informationfrom ion energy losses

Surface structure determinationvia shadowing & blocking cones

Low energy ion s catter ing (LEIS)


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1-10 keV He+, Ne+ or Li+

Fixed scattering angle(usually as small aspossible or 90)

Compositional informationfrom ion energy losses

Surface structure determinationvia shadowing & blocking cones

Low energy ion scattering spectroscopy - elementidentification through scattered ion energy spectra


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109

Typical 1 keV He+LEIS spectrum

scattering angle 90o so

Note: peaks get closer as M2

increases- mass resolution best for largescattering angles and smallAvalues

E

E

A

A1

0

1

1

(O atom)

1 keV He+ions

Surface specificity & shadow cones

NB: shadow cone widths are close tointer-atomic spacing at low energies

Focussing effect at edge of cones

Several different energ y regim es of ISS

in addition to LEIS - Low energy (~500-5000 eV, He+, Ne+..) also have


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110

MEIS - Medium energy

HEIS - High energy

50-400 keV, H+, He+...

1-2 MeV, H+, He+...

At these high energies:

shadow cones much narrower

ions penetrate below surface layerShadow cone width (as characterised here by 5oimpact parameter) decreases with :-

increasing energy

decreasing ion mass (nuclear charge)decreasing scatterer atom mass (nuclear charge)

MEIS & HEIS are notintr ins ically

su r face speci f ic


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Mult ip le scat ter ing in LEIS - a sou rce of struc tura l information


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112

scattering from atomic chain

treat as succession of binary collisions

At grazing incidence skimming

trajectories lead to minimum andmaximum possible scatteringangles which are related to theatom spacing in the plane of

incidence

pseudo-singlescattering

pseudo-doublescattering

BUTneutralisation maysuppress scattered ion signalfrom these trajectories

Inf lu ence of charge exchange on m ult ip le scatter ing in LEIS


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113

Augerneutralisation

resonantchargeexchange(ionisation &neutralisation)

mechanisms

especially He+scattering- the longer the trajectoryclose to the surface, themore neutralisation

trajectory-dependentneutralisation probability

suppresses multiplescattering signal

especially Li+scattering -achieve chargeequilibrium whenclose to thesurface

final chargestate determinedby outgoingtrajectory only(point of

equilibrium loss)

no suppressionof multiplescattering signal

Newtonian scattering of a classical particle?


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0 30 60 90 120

0 30 60 90 120

and at the atomic scale .


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i .e. Life gets s imp ler if the total

scattering ang le is 180!!

E1

E0

=(A 1)2

(A + 1)2

NB:- Remember A is the ratio of the masses of the scattered & target atoms

Shadow ing & Block ing


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Block ing co ne formationshown by ion trajectoriesstarting from a point source(in the third layer). Note theblocking cones are broader.

Shadow ing coneformation for calculatedion trajectories at grazingincidence (here shown at

cthe critical angle)

i.e. top layer specific!

CAICISS(Coaxial Impact Col l is ion Ion Scatter ing Spectro sco py )


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(Coaxial Impact Col l is ion Ion Scatter ing Spectro sco py )

Conceived b y M. Aono et al. RIKEN, Japan

Pulsed ion beam

Beam chopping

and steering

Ion beam

Source (Neilson)

He or Ne

Sample

angle (polar/azi) Time of flight

MCP detector

Time of flight

(a)

(c)

(b)

(d)

Pulsed ion beam

Beam chopping

and steering

Ion beam

Source (Neilson)

He or Ne

Sample


MCP detector

Time of flight

(a)

(c)

(b)

(d)

Pulsed ion beam

Beam chopping

and steering

Ion beam

Source (Neilson)

He or Ne

Sample


MCP detector

Time of flight

(a)

(c)

(b)

(d)

CAICISS @ Warw ick


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119/178


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Interaction o f H w ith Si (111) - (

3 x

3) - Ag


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Series of time-of-flight CAICISS spectra each showing twopeaks (Ag and Si) for (a) exposure to H*at R.T. and

(b) following the H*desorption sequence

CA ICISS o f Si (111) - ( 3 x 3) - Ag atom ic hydro gen (H*)


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Separation of ions and neutralsusing a synchronised pulsedvoltage.

At 2 keV the probability of ionneutralisation for sub-surface

scattering events is ~100%

Crystalline Ag and an Ag atom of the(3 x 3) structure have the same

neutralisation efficiency for He+ions.

Hence the intensity ratio in the Ag(ion) peak is ~0.25 implying that theAg clusters are 4 atomic layers(~7) high.

Medium energy ion scatter ing (MEIS)


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Surface relaxation by shadowing andblock ing

MEIS/HEIS - much narrower shadow cones - must use

specific incidence directions for surface specificity


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124

sub-surfaceshadowing(surface-specificsignal) incrystallographic

(channelling)directions

2 MeV He+scattering fromW(100)

surface peak

incidencedirection

Subsurface scattering

elastic +inelastic

Channell ing in MEIS


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MEIS - more precise structural informationfrom double-alignment experiments


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126

Measure blocking

curves of

elasticallyscattering ion flux(surface peak) as

function ofoutgoing direction -bulkand surfaceblocking dips aredisplaced for arelaxed surface

101 keV H+

ions from Ni(110)

incident ionshadowing

scatteredionblock ing

Analys is of MEIS spectra


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MBE grown & ion implanted samples


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Anneal ing a s ingle Sb dlayer : moni tor ing di f fus ion


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Difference spectra indicate Sb in lattice

sites.

Lattice site occupation high (~75-80%) evenafter 450C anneal.

At 640C non-lattice site Sb appears alongwith surface segregated material, althoughsubstitutional Sb remains at the same depthwith no apparent broadening.

By 800C very little sub-surface Sbremains and only a peak at the surfacecan be seen.

MEIS from an A l-Pd-Mn quasi-cry stal


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LEED pattern at 75 eV showingthe 5-fold icosahedral symmetryof a Al70Pd21Mn9quasi-crystal.

Azimuthal Angle (deg)

Inten

sity

(counts)

4200

3800

3400

3000

0 60 120 180

MEIS data showing local structureincluding 5-fold symmetry at 72intervals

Absolute scattering yields in MEIS - provide quantitativeinformation on the number of displaced atoms induced byadsorption


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131

adsorption

e.g.Cu(111)+SCH3pseudo-(100) reconstruction - howmany reconstructed Cu

layers?

blocking

curves

theory:

2-layerreconstruction

1-layerreconstruction

clean surface

Daresbury MEIS Facil i ty


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Scattering

chamber

Preparation

chamber

Electrostatic

lensCollimator

500kV power supply Accelerator tube Ion sourceplatform

Dipole magnetExperimental station

Secondary Ion Mass Spectrom etry (SIMS)


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Near Sur face Processes in SIMS


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Several pro cesses :- removal of the matrix & impurity incorporation:- implantation of the incident ion beam:- charge exchange & neutralisation:- Altered layer forms prior to steady-state erosion

Also, ionisation is influenced by the electronic state of the surface, sosecondary ion yields can vary by 102 - 103for different elements.

As dopant profiles in semiconductors get shallower, the need for thinneraltered layers increases, hence the need for SIMS at ultra-low energies

(i.e. < 250 eV)

Summary - surface struc ture determ ination by ion scat ter ing

Key underlying physics is the use of the shadow cone


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135

LEIS- shadow cone is wide (atomic separation), low sub-

surface penetrationmultiple scattering useful for interatomic determination insurface plane - but these trajectories may have enhancedneutralisation probability (especially for He+)

MEIS/HEIS- narrow shadow cones, strong sub-surfacepenetration possible in non-shadowing incident directions

absolute scattering yields equate to number of illuminatedlayers - so can give structural information from sub-surfaceshadowing

use of ingoing shadowing and outgoing blocking(double alignment) enhances specificity of method

SIMSerodes the surface and analyses the emitted ions todetermine the composition

Scann ing probe m icros copy (SPM) techniques:


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Scann ing probe m icros copy (SPM) techniques:

Scanning tun nel l ing microsc opy (STM)

Metal surfaces & atomic manipulation

Semiconductor surfaces & epitaxial growth

Atom ic force microsco py (AFM)

Conducting & non-conducting surfaces

Biological surfaces

Other app l icat ion s

Scanning ... force, magnetic, capacitance, electrochemical ... microscopy

Scanning p robe micro scopy (SPM)

Scanning Probe Microscopy


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Scanning p robe micro scopy (SPM)

Covers a range of imaging from several 100 m to 100 pm

Surfaces can be resolved with atomic resolution (STM & AFM)

Resolves structural features, steps, defects, adsorbatesin vacuum,air or liquid

Imp act of SPM

SPM has become an essential tool in nanoscience and nanotechnology

Local experiments on single atoms or molecules can be performed

Force measurements of single chemical bonds, biological systems oroptical spectra of single molecules can be performed

Local probe can manipulate materials on the atomic and molecular

scale - build artificial structures on the atomic scale

Introduction

Scanning Tunneling Microscope (STM) was invented by Gerd Binnig and


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Scanning Tunneling Microscope (STM) was invented by Gerd Binnig andHeinrich Rohrer at IBM Zurich in 1981 (Nobel Prize in Physics in 1986).

Binnig also invented the Atomic Force Microscope (AFM) at Stanford University,with Quate and Gerber, in 1986.

All other applications and techniques flow from these two designconcepts.

Th STM i l t i th t h ( i l t )

The Scanning Tunneling Microscope (STM)


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The STM is an electron microscope that uses a sharp (single atom)tip to attain atomic resolution images of the surface electron density

since you cant actually see atoms.

The STM is an electron microscope that uses a sharp (single atom)

The Scanning Tunneling Microscope (STM)


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The STM is an electron microscope that uses a sharp (single atom)tip to attain atomic resolution images of the surface electron density

since you cant actually see atoms.


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Si(111)(7x7) su rface recon stru ct io nSince you are measuring the electronic states, images of the same surface can vary!


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Filled states image 49 atom unit cell model for (7x7)

Si(111)(7x7) su rface recon stru ct io nSince you are measuring the electronic states, images of the same surface can vary!


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Filled states image High resolution image

BUT why not see the atomic positions with STM?

Pr incip le of scanning tunnel ling m icroscop y (STM)


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144

Bring conducting (usually W) tip to

within atomic dimensions of surface.

Measure current tunnelling throughvacuum gap between tip and surface - thisdepends exponentia lly on separat ion

Scan tip parallel to surface - currentwill vary (exponentially) with separation- either map the currentas a functionof position or - more usually - map theheight variations applied to the tip to

maintain constant tunnelling current

Atomic-scale images of the (filled orunfilled) electronic states at thesurface

unfilled

states

filled states

tip

surface

E

Scanning Tunnel l ing Microscopy (STM)


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Requirements:

Conducting substrate (metal, oxideor semiconductor)

Operates in vacuum or air (orsolution)

Ultra-high vacuum allows atomic

resolution Provides real-spaceinformation

Need good vibrational isolation (!)

Practical:

Sharp metal tip (W or Pt/Ir) is brought close to conducting substrateMotion of tip is controlled by 3 piezoelectric drives (x,y,z)

Bias voltage is applied between the substrate and tip

Quantum tunneling occurs and current flows between the two

V

x

y

z

substrate

computer

tip

displayset tunnellingvoltage

tunnellingCurrentamplifier

w

Scanning Tunnel l ing Microscopy (STM)


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Requirements:

Conducting substrate (metal, oxideor semiconductor)

Operates in vacuum or air (orsolution)

Ultra-high vacuum allows atomic

resolution Provides real-spaceinformation

Need good vibrational isolation (!)

Practical:

Sharp metal tip (W or Pt/Ir) is brought close to conducting substrateMotion of tip is controlled by 3 piezoelectric drives (x,y,z)

Bias voltage is applied between the substrate and tip

Quantum tunneling occurs and current flows between the two

V

x

y

z

substrate

computer

tip

displayset tunnellingvoltage

tunnellingCurrentamplifier

w

Quantum mechanical tunneling


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W

Tunnelling current, I ~ exp (-2kw)

where k = (2m)1/2/h , = work function

change in barr ier width o f 1 leads to x10 chang e in tunn el ing cu rrent

STM: basic pr inc iples

Treat sample and tip as metalsSimple 1D potential model; barrier width = w

Electron in state yn with energyEn lying between EF-eV and EF


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p p ; Enlying between EF eV and EFhas a chance to tunnel into thetip. The contribution to the tunnelcurrent is proportional to;

wkeon '22|)(| y

Total current I is proportional to

the number of states within theenergy interval, eV;

f

eVfn

E

EE

wkeonI '22|)(|~ y

For small V the summation can be written in terms of the local density of statesat the Fermi level

E

EnEn zEz

y

2|)(|

1),(

Hence current is proportional to wfwk

f eEVeEV 025.1'2 ),0(~),0(

mk

2'

Sample

tip

Z=0 Z=w

w

EF- eV

EF

i .e. tunnel cu rrent is d etermin ed by the LDOS of the s ample at the Fermi energy

STM modes of Operat ion


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lateraldistance

currentsmall w, largecurrent

lateraldistance

z-piezovoltage

Constant current-tunneling current is keptconstant by changing theheight of the tip and the z-piezo voltage is plottedversus lateral position. Mostcommon method.

Constant height- tip isscanned in x,y plane andremains at constantheight in the z-direction.Variation in tunneling

current is measured.Good for very flatsurfaces.

Calibrat ion o f t ip shapecare needed!(a good moto whenever

interpret in g SPM images!)


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How does the image

formed relate to whatyou are actuallylooking at?

STM imaging d if ferent mater ials


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Metals- valence electrons generally highly delocalised - strong effect

due to smearing out (Smoluchowski smoothing) - STM images of cleanmetals show very weak corrugations (< 0.1 ) - mainly above atoms.

Usually requires low temperatures for high resolution.

Semiconductors- valence electrons more strongly localised and directionalbetween atoms (covalent bonds) - STM images of clean semiconductorsshow strong corrugations (up to 1 or more) - protrusions especially abovedangling bonds

Insulators- cannot image with STM

Compounds & adsorbate-covered sur faces- atoms of different elementsmay appear quite different due to electronic effects. Some atoms may even

image as dips, not protrusions

Look again @ STM from Si(111)-(7x7) surface

typical STM image


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152Si(111)(7x7) structure

STM is anelectronicprobe, so for

surface structural applications thereare key problems:

Do the protrusions correspond toatomic positions?

Do you see all atoms in the same

way? (chemical effects?)Are the relative heights and lateralpositions reliable (quantitative)?

NB: valence states at the surfaceof a covalently-bonded solid arelocalised, so STM shows largecorrugations

STM from elemental metal su rfaces


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153

Generally corrugations are veryweak but protrusions are abovesurface atoms - image appearanceessentially independent of biasvoltage - but best contrast at lowvoltage, very close tip-sample

separation

Strong bias voltage effects (and

hence misleading images) do occurin special cases - e.g. bcc (110) -see simulations on W(110)

STM from an adsorbate on a metal - C on Ni(100)

Isolated C


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154

Ni(100)(2x2)-C p4gclock reconstruction

low coverage

clock

atoms imagedas deep dips

STM from adsorbates on Pt(111) - sim ulations


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155P. Sautet, Surf. Sci. 374 (1997) 406

Added row structure formed by O reaction with Cu(110)

O atoms


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156

153 x 128 STM image

Cu(110)-(2x1)-O

outermost layer Cuatoms

lower layer(s) Cuatoms

Cu atoms are removedfrom the atomic steps to

h dd d

Added row structure formed by O reaction with Cu(110)


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157

153 x 128 STM image

Cu-O-Cu-O chains

are added to thesurface and movetogether

235 x 256 STM images -increasing O2exposure

create the added rows -so the steps recede as

the added rows grow

F. Besenb acher, Rep. Prog . Phys .

59 (1996) 1737

Clean metal su rfaces - Cu(111)

Low temperature STM (>10K)


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Low temperature STM (>10K)

Observe Friedel oscillations

Molecu lar im aging w ith STM


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Cyclopentene molecules on aAg(110) surface at 80K

Most famous STM imageSeptember 28th1989


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Xe atoms on a Ni(110) surface at 4K

Positioning Atoms with an STM


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D.M. Eigler & E.K. Schweizer, Nature 344, 534 (1990)

Fe atoms on the surface o f Cu (111) at 10K


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M.F. Crommie, C.P. Lutz, D.M. Eigler, Science 262(1993) 218

Molecu lar manipu lat ion

w ith STM


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P. Zeppenfeld, C. P. Lutz and D. M. Eigler, Ultramicroscopy 4244, 128 (1992)

In-si tu STM-MBE system

RHEED

STM

MBE


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Ga/In As2/As4

electron gun

pattern

GaAs

in-situ monitoring of growth by reflection highenergy electron diffraction (RHEED)

atomic scale snapshots of growth by scanningtunneling microscopy (STM)- 1 cm2substrates

- rapid quenching

MBE

Ga/In

- Ga, In, As, Sb, Si solid sources- N plasma source

- atomic H source

Quench grownsample in vacuum

As2/As4

GaAs (001)

STM imaging o f MBE grown GaAs(110) su rface

(110) surface is non-polar and there is no surface reconstruction


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0.2 nm

0.57 nm

0.4 nm

Top view

Side View

unit cell

Filled states STM image of GaAs(110)- lone pair on surface As atoms

200 nm x 200 nm

Example of vol tage dependent imaging

Non-polar GaAs(110) surface


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+1.9 V -1.9 Vsample bias

Plan view of surface

(1x1) unit cell

Atom select ive imaging:

Charge transfer from Ga to As atoms in surface Ga dangling bonds empty, As dangling bonds full

Unoccupied state density - Ga (empty states imaging)

Occupied state density - As (filled states imaging)

InSb(001)-c (4x4) surface

Same symmetry and periodicity as for


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y y yGaAs(001)c(4x4) surface

High resolution filled states image(bias = -2.5 V, tunnel current = 1 nA)

Improved image resolution relative to GaAs.InSb lat t ice constant > GaAs lat t ice cons tant

10

GaAs(001)c(4x4)

Sem iconductor Heteroepi taxy

Optoelectronic devices requirecombinations of different materialswith different band gaps


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7

low lattice mismatch (typically < 1%)

AlAs/GaAs, CdTe/InSb, InAs/GaSb

high lattice mismatch (typically > 3%)

Ge/Si, InAs/GaAs, InSb/GaAs

stra in

dis locat ions

growth mod e changes

% misfit between two different materials:

= (aeas)/asx 100

ae= lattice parameter of epilayer

as= lattice parameter of substrate

with different band gaps

Semiconductor heteroepitaxialgrowth allows band gap engineering

Relative lattice constants of differentmaterials play important role ingrowth behaviour and quality

STM evidence for 2D Layer-By-Layer growth mode

GaAs(111)A homoepitaxy - 2ML GaAs growth


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0 0.5 1 1.5 2 2.5

Surfacestep

density

Coverage (ML)

Oscillation in step density correlates withmeasured RHEED intensity oscillations

0.5 ML

0.25 ML

0.75 ML

1.0 ML

1.25 ML

1.5 ML

1.75 ML

2.0 ML 1000

Direct evidence for 2D island nucleation, coalescenceand completion of each layer

Surface step density measured from STM images

Effects of Lat t ice Mismatch on grow th mode

E.g.InAs growth on different low index GaAs substrates (= 0.0756)

(001) (110) (111)A


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7

(001) (110) (111)A

GaAs(001) - 2.5 ML InAs

Stranski-Krastanovgrowthstrain relief - 3D islands(Quantum dots)

GaAs(110) and (111)A - 5 ML InAs

Strain relief - dislocations

2D layer by layer growth

Plan view STM images

Stranski -Krastanov g row th & QD formation w ith STM

InAs growth on GaAs(001):- 7.2% lattice mismatch

( )


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(a) (b)

(c) (d)

(a) 1.4 ML: InAs 2D wetting layer

Different grey levels correspondto step edges (0.3nm) of 2Dwetting layer

(b) 1.7 ML: Initial stages of 3Disland formation. Coexistence of

small and large 3D islandsrandomly nucleated on surface

(c) 2.0 ML: Evolution of 3D islandsand rapid increase in numberdensity

(d) 2.7 ML: Coalescence of 3Dislands into larger 3D islands

Shape of quantum dots f rom high reso lu t ion STM imaging

InAs/GaAs(001)


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Atomic scale STM imaging reveals crystallographic facets and the shapes of the 3Dislands

Number of facets formed, e.g. {137}, {111}, {110} Shape and island aspect ratio depend on growth conditions


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Tip-Samp le Interact ions in AFM

Repulsive:

Several different forces contribute to cantilever deflection


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- short distances (few Angstroms)

-due to overlap of electron clouds associatedwith atoms in tip and sample

contact regime

Contact AFM mode

- tip makes soft physical contact with sample (forces ~ 10-8- 10-6N)- repulsive forces cause cantilever to bend to accommodate topographic changes

- stiff cantilevers allows nano-patterning via surface deformations

Attractive:

- long distances (10-100 Angstroms)- long range van der Waals interaction

non-contact regime

Non-contact AFM mode- cantilever vibrates near sample surface close to its resonant frequency- detect change in frequency or amplitude of tip oscillation- minimal contact and low forces (~10-12N) allows study of soft samples

Examples of AFM images from Sem iconduc tors

InAs/GaAsdensity & distributionof quantum dots grown by MBE

Si/Ge heterostructure (30% Ge)


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of quantum dots grown by MBE

0

10

20

30

40

10 20 30 40 50 60

Provides information on large area surface morphology on near atomic scale

UHV-AFM imageSi(111)(7x7) surface

(Scan size:13nm x 13nm, Cantilever: PiezoResistive type, Frequency shift:-33Hz)

CVD (550C)

Roughness (rms)4.200 nm

Cross-hatch pattern dueto misfit dislocations

MBE (550C)

Roughness (rms)15.856 nm

High reso lut ion bio log ical imaging with AFM


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"Nano-Alps" AFM image of a monolayer ofa bacterial surface-layer protein (Bacillus

sphaericus CCM2177).

The layer exhibits square lattice symmetrywith a lattice constant of 13.1nm. The 50nm image was obtained in contact mode

under water.

AFM reconstruction based on averaging over100 plaque particles each for the luminal and

cytoplasmic face of an asymmetric unitmembrane (AUM) of urine bladder epithelium.

The luminal side (left) of particles protrudes

about 6.5nm relative to the lipid bilayer and thecytoplasmic face (right) at only 0.5nm; hence thename ''asymmetric unit membrane.

Averages are based on contact mode AFMimaging in buffer solution. These particles form2-D crystalline plaques in situ. Centre-to-centre

distance is 16nm

Why, how, what surfaces? Surface sensitivity &ifi it

Course Summary: the topics


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177

specificity.

Surface structures - 2D symmetry & phenomenology Diffraction from surfaces, reciprocal net, electrondiffraction (LEED) and RHEED

Chemical structure of surface determination usingelectron and photoelectron spectroscopy - XPS & AES -including chemical state specificity used for structuraldetermination e.g. photoelectron diffraction

Ion scattering methods of surface structuredetermination (LEIS, MEIS, SIMS) Scanning probe microscopySTM and AFM

Richard Feynman - 1959


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Documents

Cfm Surface Lectures 2012 Notes 1-9 Final