applications of xafs to materials and nano– science · applications of xafs to materials and...

Applications of XAFS to materials and nano sciencematerials and nano– science

Federico BoscheriniD f Ph i Department of Physics

University of Bologna, Italyfederico boscherini@unibo [email protected]

www.df.unibo.it/fismat/rad-sinc

1/67SILS School on SR, Duino (Trieste)Italy, September 2009

Plan

• Introduction: why use XAFS in materials Introduction: why use XAFS in materials science & nanostructure research?

• Examples of applications, using bothresults which have “stood the test of time”– results which have stood the test of time

– recent results

2SILS School on SR, Duino (Trieste)Italy, September 2009

X-Ray Absorption Fine StructureX y p FEn

ergy

“ E ” b b d l / d 1s2s2p ωh

ωh“XANES”: Absorber symmetry and valence/oxidation state

Electronic structure of unoccupied statesMedium range structurem g

“EXAFS”: Coordination numbersInteratomic distancesDisorder of distancesDisorder of distances


EXAFSEXAFS• Extended X-ray Absorption Fine Structure• Fit fine structure oscillations with• Fit fine structure oscillations with

From ab initio calculations or from reference compounds

2222 ]22[)()( jkkSikANSk σδ −∑

From ab-initio calculations or from reference compounds

120 ]22[)()( j

jjjshellsj

j ekrSinkANSk δϕχ=

++= ∑

M s :

( )h

h BEmk

−=

ω2

Debye Waller factor-thermal vibration

d d

Coordinationnumber

Interatomic distance

Measure:

-static disorder4SILS School on SR, Duino (Trieste)

Italy, September 2009

XANESXANES• X-ray Absorption Near Edge Structure

(also NEXAFS)

)(ˆ422

fEfri ρεωαπσ rh •=

)(

0,1

d

m =±=light) pol. (lin.

ll ∆∆

• “Molecular orbital” approach: 1 electron approximation, i l b i d

)(, sdpps →→

constant matrix element: probe site and symmetryprojected density of states of final states

• “Multiple scattering” approach: structural interpretation Multiple scattering approach: structural interpretation through simulation


Characteristics of XAFS• Atomic selectivity• High resolution for first few

di ti h ll

• Today’s topics– Dopants, defects

coordination shells• Sensitivity to high dilutions &

surfaces/interfaces

– Alloys• local distortions • orderingsurfaces/ nterfaces

• Equally applicable to ordered or disordered matter

ordering– Phase transitions– Thin films, interfaces

• XANES:– valence/oxidation state– 3D structural sensitivity

– Nanostructures• Semiconductor dots• Metallic clusters3D structural sensitivity

• µm spot size now available• Metallic clusters


Role of XAFS in Materials ScienceGrowth Physical Properties

Structure

Nakamura et al., Jpn. J. Appl. Phys. 35, L217, 1996MBE@TASC

XAS


XAFS and dopants/dilute elementsXAFS and dopants/d lute elements• Only the structure around the photo-

excited atom is probed• Fluorescence detection greatly g y

enhances sensitivity• Present sensitivity limitPresent sensitivity limit

– dopants in the bulk ∼ 1018 at/cm3, – thin films (single layer) ∼ 1014 at/cm2thin films (single layer) ∼ 10 at/cm


Arsenic in a-Si:H• As in a-Si expected to be

3-fold coordinated

ber

(CN

)

3.0 • But: doping is observed• Doping in c-Si due to 4-fold As

Knights et al observe increase ion

Num

b

• Knights et. al. observe increasein CN at 1% As concentration– evidence of active As oo

rdin

ati

• Knights, Hayes, and MikkelsenJr.Phys Rev Lett 39 712 (1977)

As

Co

1 % 10 %2.5

Phys. Rev. Lett. 39, 712 (1977)


Si/GaAs δ-doped layers and superlatticesB h i i F tti B i O i

• δ doping: physical separation

Boscherini, Ferretti, Bonanni, Orani, Rubini, Piccin, and Franciosi APL 81, 1639 (2002)

• δ-doping: physical separationof dopants and chargecarriers– Si-GaAs a technologically

relevant caseOp n issu : Numb f ch• Open issue: Number of chargecarriers not proportional tonumber of dopants above a number of dopants above a critical concentration. Structural origin?


Si-GaAs: samplesp• Growth conditions optimized for Si/GaAs

superlattice formation: p– MBE, 540 °C, 4 nm/h, @TASC

• Si thickness: 0.02 ML – 4 ML; repetition optimized for XAS

• Fraction of electrically active Si atoms:4 22×10–4 – 6× 10-2


Si-GaAs: dataSi GaAs data• Si K edge XAS @

LURELURE• Fluorescence

mode bulk

Si - As theory

-1)mode, bulk

sensitivity• Si – Si clustering

0.2 ML Si

2 ML Sik χ(

k) (Å

-

• Si – Si clustering apparent in raw data 1

2 ML Si

Bulk Siliconk

data2 4 6 8 10 12

k (Å-1)

0.5 Å-1 u S co


Si-GaAs: results

• Si – Si CN > 2 4 10-1

always• Presence of Si

l t3.5

10-2

N (Si) R*

Num

ber Fraction

clusters• This is the origin

of low 3

10

ordi

natio

n

n of activeof low concentration of charge carriers

2.510-3

Si -

Si C

oo

e Si atoms

2 10-4

10-2 10-1 100 101

S

Thi k (ML)Thickness (ML)


Fe:GaNM. Rovezzi, F. D’Acapito, A. Navarro-Quezada, B. Faina, T. Li, A. Bonanni, F. Filippone, A. Amore Bonapasta, and T. Dietl, Phys. Rev. B 79, 195209 (2009)

A Bonanni A Navarro-Quezada Tian Li M Wegscheider Z Matěj V Holý A. Bonanni, A. Navarro Quezada, Tian Li, M. Wegscheider, Z. Matěj, V. Holý, R. T. Lechner, G. Bauer, M. Rovezzi, F. D’Acapito, M. Kiecana, M. Sawicki, and T. Dietl Phys. Rev. Lett. 101, 135502 (2008)

– Aims:– Define the site of Fe in GaN for – Define the site of Fe in GaN for

different metal concentrations and growth rategrowth rate

– Determine the effect of Si co-dopant


Fe:GaN samplesFe:GaN samples

• Grown by MOVPE

SQUID

• Grown by MOVPE• Fe concentrations 4 × 1019 cm-3 - 3 × 1020 cm-3

SQUID

Ferromagnetic (FM) signal detected for all samples


Fe:GaN dataFe:GaN dataLow Fe content:• only two Fe N (R ) • only two Fe-N (R1)

and Fe-Ga (R3) bonds. • Fe substitutional;

b d l h i e bond length in agreement with DFT for Fe3+ea

sing

Fe

High Fe content• Appearance of Fe-

Incr

e

FTs of the EXAFS spectra at

• Appearance of Fe-Fe (R2) coming from a precipitated Fe3N hFTs of the EXAFS spectra at

various Fe content phase16SILS School on SR, Duino (Trieste)


Si Fe:GaN dataSi,Fe:GaN data

•For the same Fe ng F

e

content Si co-doping prevents the formation of Fe3Nnc

reas

i

formation of Fe3N•No evidence of the Fe-Fe bond at R2

InFTs of the EXAFS spectra at various Fe content and

codoped with Sicodoped with Si17SILS School on SR, Duino (Trieste)


ff h h f FSi affects the charge state of Fe

P l d f F 3 F 2 ddPartial reduction of Fe3+ ions to Fe2+ upon Si addition


Si F G N l iSi,Fe:GaN conclusions

• For high [Fe] FM due to Fe3N precipitates• For low [Fe] FM due to substitutional FeFor low [Fe] FM due to substitutional Fe• Si codoping prevents the formation of

Fe N aggregates and permits a higher Fe3N aggregates and permits a higher incorporation of Fe.Si l d t ti l d ti t F 2+• Si leads to a partial reduction to Fe2+


Metal precipitates in Si solar cellsMetal precipitates in Si solar cellsBuonassisi et al, Nature Mat. 4, 676 (2005)

• Supply of high purity Si << demand• Use of lower purity materialUse of lower purity material• Problem: impurities decrease efficiency

µ XRF and µ XAFS to characterize • µ-XRF and µ-XAFS to characterize metal precipitates and suggest processing to improve efficiencyprocessing to improve efficiency


µ-XRF & µ-XAFS: two defects~ 20 nm silicide

~ 10 µm oxideµm

2121SILS School on SR, Duino (Trieste)


Processing & characterization

Best process: slow cool (5 °C s-1) from 1200 °Cµm sized, low density precipitatesgreater carrier diffusion lengths

22

greater carrier diffusion lengths22SILS School on SR, Duino (Trieste)


Low Z dopants and XAS• C, N & O often used as dopants• Experimentally difficult: low fluorescence

i ld ft X UHVyield, soft X-rays, UHV100

10-2

10-1

nceY

ield

K edge yield

10-3

10

Fluo

resc

en

K d L d

LIII

edge yield

ALOISA beamline @ ELETTRA10-4

5 10 15 20 25 30Z

K edges LIII

edges

Z


Dilute nitrides: GaAs1-yNy, InxGa1-xAs1-yNy

Anomalous non-linear opticaland electronic proprieties of

d

3.25 eV

Eg (eV) GaN

III-V nitrides• Red shift of the band gap by adding

few % of nitrogen (≈ 0 05-0 1 eV

GaAs

1.42 eV

few % of nitrogen (≈ 0.05 0.1 eVper N atomic percent in InGaAsN )

• Huge and composition dependentoptical bowing

0.0 0.5 1.0y

optical bowing


Hydrogen – nitrogen mpl x s in dil t nit id scomplexes in dilute nitrides

• Hydrogenation leads to reversible changesy g g– compressive strain– opening of Eg

X-ray diffraction photoluminescence(004)opening of Eg (004)

GaAsN

EK~100 eV

GaAs

GaAsN


Hydrogen – nitrogen complexes in dilute nitrides

• Which is the hydrogen –nitrogen y g gcomplex responsible for these changes?g

Some candidate Some candidate low energy structures

2626SILS School on SR, Duino (Trieste)


Hydrogen – nitrogen complexes i dil t it idin dilute nitrides

ion

• DFT calculations to determine lowest energy geometries os

s-se

ct As grown

lowest energy geometries• Full multiple scattering XANES

simulationsA C lik l tio

n cr

o

N-HBC

N-H3

C• Answer: C2v – like complexes are mostly present

• 3-D sensitivity of XANES!! Abs

orp C

2vHydrogenated

y390 400 410 420 430

AEnergy (eV)


Annealing studiesAnnealing studies


XAFS and alloysy• High resolution in probing the local

di ti i fi t f di ti coordination in first few coordination shells

• Study, as a function of composition– Deviation of local structure from

average structure – Atomic ordering


Bond lengths in InxGa1-xAs• Does the local structure follow

the average structure suggested by Vegard’s law?y g

• NO! Bond legths stay close to sum of covalent radii

• Mikkelsen Jr and Boyce • Mikkelsen Jr. and Boyce, Phys. Rev. Lett. 49, 1412 (1982)


Disorder in Sr2FeMoO6 double perovskitesC. Meneghini, S. Ray, F. Liscio, F. Bardelli, S. Mobilio

λ

g , y, , ,& D.D. Sarma, Phys. Rev. Lett. 103, 046403 (2009)

SL

Fe

0.90.80.5

Mo

OSrS

0.40.2 A = Fe sublattice

B = Mo sublattice

long range order parameter Sdetermined from powder XRD

P(FeA) = N(FeA)/N(Fe) = xdetermined from powder XRD

S = 2x - 131SILS School on SR, Duino (Trieste)


Double perovskites: disorder from a local perspective

R d A i i d fRandom Antisite defects Defect clusters

S = 0.51S 0.51

Antiphase regions

S takes into account only for the number of defects (FeB or MoA), not their arrangement

32

not their arrangement

SILS School on SR, Duino (Trieste)Italy, September 2009

Double perovskites: XAFS very similar spectra for all Sspectra for all S

ξ = P(Fe-Mo)

ξ EXP

0.90.80.50.40.2

0.90.80.50.40.2

AS


ξ=0.63=S2 S=0.51 Double perovskites:a model for local disorder

Randomly distributed defect clusters

a model for local disorder

having average diameter DD is refined in order to obtain S and x close to their experimental values

ξ=0.85 S=0.51

1 5 nm close to the r exper mental values1.5 nm

ξ=0.99 S=0.51 Conclusion:ξ Conclusion:• Fe-Mo antisite defects appear as small (~1.5 nm)domains with antiphase Fe/Mo order.


XAFS and phase transitionsXAFS and phase trans t ons

• Measure local structure through the gphase transition

• XAFS has highlighted the difference XAFS has highlighted the difference between the real local structure and the average structurethe average structure


Ferroelectric Phase transitions in PbTiO3

low temp

Sicron, Ravel, Yacoby, Stern, Dogan and Stern, Phys. Rev. B 50, 13168 (1994)

low temp

• At Tc = 763 K PbTiO3 undergoes tetragonal to cubic At c 763 K Pb O3 undergoes tetragonal to cub c phase transition

• T < Tc it is ferroelectric (permanent dipole moment)• Phase transition believed to be purely displacive (no

local distortion for T > Tc)


Ferroelectric Phase transitions in PbTiO3

• Ti and Pb XAFS data • Local lattice parameters cp

and local distortionsdo not change at TcThe ferroelectric

a• The ferroelectric

transition has a large order – disorder character: the orientation of local distortions changes at Tchanges at Tc


La1-xCaxMnO3 Manganites

• For 0.2 < x < 0.5For 0.2 x 0.5– Metallic ferromagnet T < Tc

– Insulating paramagnet T > Tcg p g c

• Colossal Magneto Resistance• Double exchange model: g

hopping conductivity between adjacent Mn ions is enhanced f l dif Mn ions are spin aligned Mn3+

Mn4+


La1-xCaxMnO3 Manganites• Mn(3+)O6 octahedron in

LaMnO3 is Jahn-Teller distorteddistorted– Mn4+ is not

• Conductivity in the ypresence of polarons: an electron + a lattice distortiondistortion

• Range of polaronic distortion?


La1-xCaxMnO3 Manganites• Presence of

localized localized structural distortions at distortions at MI transition

Booth, Bridges, Kwei, Lawrence, Cornelius and NeumeierPhys. Rev. Lett. 80, 853 (1998)


XAFS and thin films / interfacesXAFS and thin films / interfaces• With specific detection schemes p

sensitivity to very thin films achievable– Grazing incidence– Electron / fluo detection

• Exploit linear polarization of SR to obtain directional informationobtain directional information


Using SR linear polarization to study thin films

Electron orbit

ε̂

θθσ 2)( Cos∝θ

ε


Cubic and hexagonal GaN• N K-edge XAS to study N K edge XAS to study

relative amounts of cubic and hexagonal GaN

• Exploit– linear polarization of SR

polarization dependence of – polarization dependence of cross-section

• XAS signal must exhibit (at g (least) point group symmetry of the crystal

T : isotropic si nal– Td: isotropic signal– C6V: σ tot(E,θ)=σ iso(E)+(3Cos2θ −1)σ1(E)


GaNCUBIC HEXAGONAL

• Katsikini et. al., APL 69, 4206 (1996); JAP 83, 1440 (1998)


The Fe/NiO(001) interfaceThe Fe/NiO(001) interfaceP. Luches, V. Bellini, S. Colonna, L. Di Giustino, F. Manghi, S. Valeri, and F. Boscherini, Phys. Rev. Lett. 96, 106106 (2006).

heating up to TN

FM +cooling in H

AFM

h biexchange bias


The Fe/NiO(100) interface: structurebcc

FeFM TC=1040 K

rock-salt

aFe= 2.866 ÅdFe = 4.053 Å

m=-2.8%

NiOAF TN=520 K

aNiO= 4.176 Å46SILS School on SR, Duino (Trieste)


Fe/NiO XANES

• deviation from deviation from metallic character

• shift of the hit lin white line

towards FeO


Fe/NiO XANES: polarization dependence

kE

θ

• planar FeO phase


Fe/NiO EXAFS-quantitative analysis

N BCC Fe, R (Å)

BCT Fe, R (Å)

fit results, R (Å)

1st shell 8 2.48 2.50 2.50

2nd shell 2 2.77 2.81the epitaxial strain is partially 2.87

3rd shell 4 2.95 2.87

4th shell 84.05

4.04 3.92

5th shell 4 4.17 4.07

6th h ll 8 4 64 4 74

released at 10 ML

SILS School on SR, Duino (Trieste)Italy, September 2009

49

6th shell 84.75

4.64 4.74

7th shell 16 4.87 4.84

8th shell 8 4.96 5.01 5.09

Fe/NiO 2 ML thickness


Fe/NiO: atomic structure of interface

BCT Fe

buckled pseudomorphicp pFeO layer

• expanded FeO-NiO and FeO-Fe distance


XAFS and nanostructuresF l l h ff• XAFS is a local, short range, effect

– Origin: core hole lifetime (τhole = 10-16 – 10-15 s) and electron mean free path (5 – 10 Å)electron mean free path (5 10 Å).

• Same formalism applies to molecule, cluster or crystalline solid– insenstive to variations of morphology– sensitive to low thicknesses, high dilutions

Ex ll nt p b f i ti ns in l l n i nm nt • Excellent probe of variations in local environment due to– Size effectsSize effects– Change 3D / 2D / 1D


S i d dSemiconductor quantum dots

• Self organized hetero-epitaxial growth can lead to quantum dots with g qnarrow size distribution

• Due to small size the quantum dots Due to small size the quantum dots exhibit new physical properties


Ge Quantum DotsBoscherini, Capellini, DiGaspare, Rosei, Motta, and Mobilio, Appl. Phys. Lett. 76, 682 (2000)

• Need for understanding of

p p pp y

Need for understanding of local bonding

• Preparation:p– Ge/Si(001) by CVD @ 600 °C,

Univ. Roma TreE it AFM d t h t i • Ex-situ AFM used to characterize degree of relaxation

– Ge/Si(111) by MBE @450 - 550 y°C, Univ. Roma II

• In-situ STM/AFM


Energetics of island formationEnergetics of island formation• Competing energies:

– strain• Contributions from:

- wetting layer– strain– surface– dislocations

wetting layer- islands

d s ocat ons

Wetting layer WL+Strained island WL+Relaxed islandWL+2D plateletWetting layer WL+Strained island WL+Relaxed island

"Coverage"

WL+2D platelet


Q. Dots: AFMQ• Analysis of aspect ratio

provides measurement 80

100

%)

Ge/Si(001)provides measurement of relative amount of relaxed islands

60

80

volu

me

( Ge/Si(001)

• Ge/Si(001): Full range of relaxation examined 20

40

Rel

axed

v

00 5 10 15 20 25 30 35 40

R

equivalent thickness (nm) (1 ML = 0 135 nm WL = 3 ML)(1 ML 0.135 nm, WL 3 ML)


Q. Dots: Ge edge dataGe in Si

ansf

4

Ge:Si

dots

Fou

Tra

2

3

(Å-1

) (111)1 nm

(001)Ge bulk

Ge

F

1

2

k χ(

k) ( (001)

7.8 nm

(001)38 nm 0 2 4 6

interatomic distance (Å)0

38 nm

Ge

2 4 6 8 10 12 14 16k (Å-1)

•Assuming random alloy average composition is Ge0 70Si0 30is Ge0.70Si0.30


Conventional SK growthConventional SK growth


SK growth with interdiffusion SK growth with interdiffusion


Metallic nanostructuresMetallic nanostructures

• One of the first applications of XAFS• Exploits high resolution in first Exploits high resolution in first

coordination shells


Clusters: bond length contraction

Apai et. al.Phys. Rev. Lett. 43, 165 (1979) Montano et. al.

Phys. Rev. Lett. 56, 2076 (1986)• A bond length contraction has been

found for weakly supported metallic l st s (Ni C A ) f d 100 Å

y , ( )

clusters (Ni, Cu, Au……) for d < 100 Å61SILS School on SR, Duino (Trieste)


Bond length contractiong• A macroscopic surface tension interpretation

(“liquid drop”) can explain the bond length ( liquid drop ) can explain the bond length contraction– f surface tension fR κ2∆f surface tension, κ bulk modulusr radius of spherical particleM l

rf

RR κ

32

−=∆

– Montano et. al.Phys. Rev. B 30, 672 (1984)Balerna et. al.,h ( )Phys. Rev. B 31 5058 (1985)


Dynamic properties of Au clustersy p p• In the harmonic approximation,

the Mean-Square-Relative-qDisplacement, σ2, damps the EXAFS signal with a term

222 σke−

2( ) 2

0020

ˆjjj Ruu •−=

rrσ

Bulk Au63SILS School on SR, Duino (Trieste)


Dynamic properties of Au clusters h l d • As the cluster dimensions

decrease an enhacement of σ2 is evidentσ is evident

• Surface atoms have less motion constraints

• High surface-to-volume ratio for nanoclusters

• Values reproduced by numerical model for free sphere phonon DOS which sphere phonon DOS which includes surface modes

• Balerna and Mobilio, Phys Rev B 34 2293 (1986)Phys. Rev. B 34, 2293 (1986)


Cross-over from thermal expansion to contraction

Comaschi Balerna and Mobilio Phys Rev B 77 075432 (2008)

Extremely good

Comaschi, Balerna and Mobilio, Phys. Rev. B 77, 075432 (2008)

spectra of Au nanoparticles (error bar on bond length = 0.2 pm)

6518 Å-1

Cross-over in expasion / pcontraction behaviour

R d d di i • Reduced dimensions increase splitting of l t i t telectronic states

• Change of total energy dependence on bond length (R)

40 Å diameter NP • At high T states with low R are accessed


ConclusionsXAFS h b d dd i• XAFS has been used to address importantstructural issues in materials/nano science

• It has specific advantages especially• It has specific advantages, especially

• Atomic selectivity• High resolution for first few coordination shells• Sensitivity to high dilutions & surfaces/interfaces• Equally applicable to ordered or disordered matterEqually applicable to ordered or disordered matter• XANES:

– valence/oxidation state3D structural sensitivity– 3D structural sensitivity

• µm spot size now available

67/67SILS School on SR, Duino (Trieste)Italy, September 2009

applications of xafs to materials and nano– science · applications of xafs to materials and...

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