Structural properties of semiconductor nanostructures ...M.G. Proietti Universidad de Zaragoza IUCR...

M.G. ProiettiUniversidad de Zaragoza

IUCR 2005

Structural properties of semiconductor

nanostructures determined by X-ray

Anomalous Diffraction (DAFS) and Absorption

(EXAFS)

M.G. Proietti, J. Coraux, V. Favre-Nicolin, H. Renevier, B. Daudin, A. Letoublon, M. Gendry, L. González, J.M. García, J.F. Bérar, S. Arnaud, B. Caillot

BM2 CRG-ESRF & LdC, CNRS, Grenoble , France .

Ecole Centrale de Lyon, LEOM UMR 5512, Ecully, France

Instituto de Microelectrónica de Madrid, CSIC,Tres Cantos, Madrid, Spain

ICMA, CSIC-Universidad de Zaragoza, Spain

Commissariat à l'Energie Atomique, DRFMC, SP2M, Grenoble,France


IUCR 2005

Talk outline:

MAD and DAFS spectroscopy

Grazing-incidence DAFS (GIDAFS) and EXAFS of

InAs/InP self-assembled Quantum Wires (QWrs)

Grazing-incidence DAFS (GIDAFS) of

GaN/AlN self-assembled Quantum Dots (QDs)

Conclusions


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Resonant scattering

Multi-wavelength AnomalousDiffraction (MAD)

Diffraction Anomalous Fine Structure spectroscopy (DAFS)

Diffracted intensity in the RS at several energies close to an

absorption edge

Diffracted intensity at fixed scatteringvector, as a function of energy

across an absorption edge

|FT |, |FA |, ∆φ=(φT - φA)Electronic structure (DANES)Local environment (EDAFS)

Non destructive methodsChemical (electr. state resonance) and site (interference) selective probeStatistical structural properties (long-range order+short-range order)Buried nanostructures

(See also talk MS82.29.4 by V. Holý)


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FA

fA(Q,E) = f0A(Q) + f’A(E) + if’’A(E)

FT : Structure factor includingall atoms

FA : Structure factor of resonantatoms

|FT |, |FA | and ∆φ=(φT - φA)

Multi-wavelength Anomalous Diffraction (MAD) FT & FA


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Diffraction Anomalous Fine Structure spectroscopy

« fine structure »χ = χ’A +iχ’’A

Local environment

II II

Abs.II

• Probes the local environment and electronicstructure (empty states) of resonant atomsselected by diffraction (scattering)

H. Stragier et al., Phys. Rev. Lett. 21, 3064, (1992) I.J. Pickering et al., J. Am. Chem. Soc. 115, 6302, (1993)

where

11600 11800 12000 12200 12400Energie (eV)

0

1

2

3

4

5

f'' (

unité

éle

ctro

niqu

e)

χ’’A

E(eV)11600 11800 12000 12200 12400Energie (eV)

-10

-8

-6

-4

-2

0

f' (u

nité

éle

ctro

niqu

e)

χ’A

E(eV)

(Ex. As atoms in bulk InAs)

As K-edge


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«smooth contribution » « oscillatory contribution »First order I(E) :

Formally equivalent to EXAFSEXAFS multishell fit codes (Ifeffit)

EDAFS analysis

M.G. Proietti, H. Renevier et al., Phys. Rev. B 59, (1999), 5479


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detector

site/spatial selectivity(Q)

chemical selectivity(f(E))

+surface sensitivity

+

Grazing incidence DAFS (GIDAFS)

7 cercles diffractometerat BM2-D2AM, ESRF

H. Renevier et al. J.Synchrotron Rad.(2003)

EXPERIMENT

Diffuse scattering/diffraction intensity vs Energy across absorption edges

- at fixed scattering vectors

- over a 1000eV range with a high S/N ratio (atleast 103)

- incidence angles close to αc

αi~ αc


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Application of GIDAFS to free standing nanostructures

• Δa/aInP =-3,3%• λ ≅ 20nm, height = 0.6-2.2nm• [110] : relaxed• [1-10] : pseudomorphic to InP

• 3D self organised growth (Stranski-Krastanow regime)• Grazing incidence Diffraction Anomalous Fine Structure (GI-DAFS)

[110]

[110]-

InAs/InP Quantum Wires

As K-edge

E(eV)

(440)

(420)InP(420)

Qr


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II II

Abs.II

R(Å)

GI-XAFSInAS QWrs

Surface Oxides

Diffraction Anomalous Fine Structure :

- Insensitive to the amorphous oxyde layer

- QWrs lattice accomodation to strain:Elastic tetragonal deformation

- local composition:Pure InAsLow As/P intermixing

S. Grenier et al., EurophysicsLetters, vol. 57, 499, 2002

EXAFS InAS bulk

k(Å-1)

DAFS InAS calc.

GI-DAFS QWrs (420)


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Sample 1:High Tg (520º), early 2D/3D trans.

Dep. thick. > H2D/3D, no annealing, immediate cap growth at As/P switchSample 2: Lower Tg (400º), InP buffer roughness Annealing for QWrs formation

a>aInP

l

InAs QWrs

h=k

l

h=k

Application to buried InAs/InP QWrsStrain, size, composition ?

2 samples grown by MBE in differentconditions: LEOM Lyon (1), IMM Madrid (2)

l-scans

InAs QWrs

InP (442)

surface

InAs QWrs

a>aInPa<aInP

αi

TEM(CP1276)

10nm InP capping

X-ray

cp1276

I3701

Reciprocal space maps

l scans at E values close to As K-edge

a<aInP

A. Letoublon et al., Phys. Rev. Lett., 92, (2004)

(442) InP

(442) InP

l-scans


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GIDAFS at FA max

MAD extraction of FT & FA

Diffraction selected local structure: • lattice strain accomodation• composition

• strain : 6% ([001])• height : 2.4 nm

As K-edge (I3701)l=1.9h=k=3.98

(CP1276)

(NanoMAD code, V. Favre-Nicolin)


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11,7 11,8 11,9 12Energy (keV)

30

40

50

60

70

80

Dif

frac

tion

Inte

nsity

(a.

u.)

E(keV)

GIDAFS oscillations analysis

Δφ/β fit (DPU code)

Phase and amplitude factorsSD , (φ0-φA)

EDAFS fit (Ifeffit code)

As K-edge CP1276l=1.9h=k=3.98

I3701

CP1276

sim DAFS from XAFS


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EDAFS vs EXAFS EXAFS measurements at theAs K-edge (ESRF- BM30)ε// & ε ⊥Multifit by Ifeffit code

ε ⊥

ε ⊥

Differences between the two samples show up in EDAFS

but not in EXAFS

Probing different As environments

I3701

CP1276

EDAFS measurements at theAs K-edge (ESRF- BM2)

CP1276

I3701


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II II

Abs.II

0.4±0.30.6±0.10.4±0.20.5±0.1--%P⊥

-0.4±0.1-0.3±0.1--%P//

-4.94±0.06-4.93±0.065.135.023(Asabs-InIII) ⊥

-4.87±0.03-4.88±0.034.875.023(Asabs-InIII)//

4.19±0.064.18±0.034.20±0.064.17±0.03--(Asabs-PII) ⊥

-4.15±0.07-4.19±0.07--(Asabs-PII)//

4.22±0.044.25±0.044.30±0.044.23±0.044.294.284(Asabs-AsII) ⊥

-4.15±0.06-4.16±0.064.154.284(Asabs-AsII)//

I3701(DAFS ε⊥ )

I3701(EXAFS ε⊥ & ε //)

CP1276(DAFS ε⊥ )

CP1276(EXAFS ε⊥ & ε //)

InAs/InP(pseud.)

InAsBulk

Sample→paths (Å)↓

EXAFS and EDAFS best fit results

InAs,P II shell

distancesP content

Strain Pure strained InAsor

InAsP alloy ?Abs


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EXAFS (ε// & ε⊥) EDAFS (ε⊥)

Similar results for the 2 samplesMixing of two different As environmentsStrained InAs & InAsP alloy

Intermixing As/P at the QWrs/capping

interface

P diffused QWrs

?

Different results for the 2 samplesDifferent As-As and As-P II shell dist.

CP1276: InAs QWrs + As/Pintermixing at interface

& capping

I3701: InAsP QWrs + As/Pintermixing at interface

& capping

Nice example of DAFS spatial selectivity and of strongEXAFS/DAFS complementarity

GIDAFS probes the QWrs As atoms (thecontribution of each atom depends on Q)EXAFS probes all As atoms


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~ 30o

AlNGaN dot

h= 4 nm

5 nm

}{ 0311

Db= 30 nm

Confinement: l = 1/ kF ~ 10 nm

60nm

Application of GIDAFS to GaN/AlN quantum dots

Ex : LEDs based on QDs doping withrare earth (RGB lights)

- Dots are dislocations free- Exciton confinement

- better luminescence efficiency- T independent luminescence

- Wurtzite (hexagonal system) : non centrosymmetric structure

- Strong piezo-electricity (red shift of luminescence energy)

- Strain is an important parameter

- Diffraction to study strain field

P[001

]

TEM


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GaN/AlN quantum dots : effect ofcapping

wetting layer (2ML)

buffer layer (40 ML)

AlN substrate (NGK)

Samples : 1 layer of QDs- free standing (0ML AlN)- cap layer of 2ML AlN- cap layer of 10ML AlN

GaN QDs (4ML)

Ga + N

GaN QDs

AlN

- Modified Stranski –Krastanow growth mode- 2.4 % mismatchbetween AlN and GaN

GaN thickness >critical thickness

RHEED

See also poster P.25.07.5, V. Favre-Nicolin et al.IUCR conf. MS.82


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GaN/AlN quantum dots : Strain vs the AlN cap thickness

AlN (30-30)

0 2 10

(αi=0.15°<αc)GaN

3.16

3.14

aGaN from themax. of FA

aGaN_bulk=3.188ÅaAlN_bulk=3.11Å

a GaN

√I

|| S

truc

ture

Fac

tor

||

h

10 ML

√I√I

3 32.92.9

FA

FT

h

0 ML

2 ML


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EDAFS of GaN/AlN quantum dots

Energy scan at fixed Q :Max. of FA (h=k=2.945, l=0)

Ga K-edge (10367eV), BM2, ESRF

k(Å-1)

Energy (eV)

2 ML

Ga

N

Wurtzite structure

ε[0

001]

All paths are expressed as a function of a and c (assuming bi-axial deformation)


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EDAFS fit results

εzz

1,671,661,661,691,626c/a0,05±0,10,0±0,10,1±0,1--x_Al

5,255,235,255,265,186C (Å)3,14 (diff.)3,147 (diff.)3,156 (diff.)3,113,188a (Å)10 ML AlN2 ML AlN0 ML AlNGaN biaxialBulksample

a lowers with increasing AlNcap thickness

c is close to cGaN biaxial

Biaxial strain accomodation seemsnot to be valid

Finite element simulations andnew diffraction experiments

are needed

GaN biaxial10ML

2ML

0M L

(5.186 Å) (3.188 Å)

Biaxial deformation

εxx = (a-aGaN_bulk)/aGaN_bulk

(EXAFS exp. with ε// & ε⊥foreseen at ESRF)


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ConclusionsDiffraction Anomalous Fine

Structure (DAFS)

FA Structure factor ofresonant atoms, FT, Δφ

-average strain, size, (3D shape)- composition (model)

Diffraction selected local structure:- local lattice accomodation of strain- local composition

Multi-wavelength AnomalousDiffraction (MAD) & reciprocalspace mapping

EDAFS combined to EXAFS

Strong complementarityDifferent views

EXAFS supports the EDAFS results

Chemical & spatial selectivity + quasi-surface sensitivityNon destructive probe

Grazing incidence


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H. RenevierJ. CorauxV. Favre-NicolinB. DaudinA. LetoublonM. Gendry

L.GonzálezJ.M. GarcíaS. ArnaudJ.F. BérarB. Caillot

Collaborators

Commissariat à l'Energie Atomique, Département de Recherche Fondamentalesur la Matière Condensée, SP2M, Grenoble,France.

Laboratoire de Cristallographie, Centre National de la Recherche Scientifique, Grenoble Cedex, France .

Ecole Centrale de Lyon, LEOM UMR 5512, Ecully, France

Instituto de Microelectrónica de Madrid, CSICTres Cantos, Madrid, Spain


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Electron and hole ground states in QDs

(A.D.Andreev et E.P.O’ Reilly, APL 79,521(2001)

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

PL In

tens

ity (a

. u.)

Photon Energy (eV)

Small QD's Large QD's

Gap GaN


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Influence of surface energy: the case of GaN QDs, modified SK

Ga + N

Ga

(a)

Ga

GaN

AlN

(b)

(c)

(d)

Ga

C. Adelmann et al APL 81, 3064 (2002)N. Gogneau et al JAP 94, 2254 (2003)


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GaN/AlN QDs (in progress)

Grazing incidence(in-plane Q vector) ID1, ESRF

Q//

(300) SiC

AlNGaN

Inte

nsity

(a.u

.)

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsité

PL

Nor

mal

isée

Energie (eV)

3 couches 7 couches 10 couches

1 10 100200

400

600

800

FWH

MPL

(meV

)

Nombre de couches

Photoluminescence (PL)

Energy (eV)

What is the strain, shape, size and atomic mixing after encapsulation ?As suggested by PL,

are the QDs size distribution decreasing andthe QDs size increasing, as a function of number of layers ?

Is there a vertical QDs correlation ?

?

Superlattices of GaN/AlN Quantum Dots

- 1 layer capped- 1 layer uncapped- 10 layers capped

- 3 layers- 7 layers- 10 layers


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Buried InAs/InP QWrs

Finite Element Method, A. Letoublon (SP2M/NRS), coll. C. Priester (Lille)

surface

InAs QWrs

a>aInP

InP Substrat

a<aInP

InP Substrat

Finite Element Method Simulation(hypothesis : no atomic mixing at interfaces)

εxx[110]=Δa/aInP

εzz[001]=Δa/aInP

surface

InAs/InP QWrs + 10nm InP capping

Strain fields


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1st order EDAFS :


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GaN/AlN QDs EDAFS FIT RESULTS

1,671,661,661,641,661,641,691,626c/a

0,05±0,10,0±0,10,1±0,10,15±0,10,1±0,20,09±0,2--x_Al

5,25±0,045,23±0,035,25±0,025,19±0,0055,25±0,075,24±0,065,265,186c

3,1863,183,1903,173,193,196-3,180R2⊥(Ga-Ga) ( Å)

7x10-38x10-36x10-37x10-38x10-35x10-3--s2 ( Å)2

3,14 (diff.)3,147 (diff.)

3,156(diff.)

3,164 (diff.)

3,157 (diff.)

3,19(diff.)3,113,188R2// (Ga-Ga) (a) (Å)

1x10-34x10-32x10-315x10-39x10-35x10-3--s1 (Å)21,941,941,931,92±0,011,95±0,011,94±0,01-1,95R1(Ga-N) (Å)

fit par.

S1959(1pl.+10MLcap)

(AlN)

S1956(1pl.+2MLca

p)(AlN)

S1967(1pl. no

cap)(AlN)

S1583(7pl )(SiC)

S1582(1pl cap)

(SiC)

S1568(1pl no cap)(SiC)

GaN/AlNBulk

sample


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Anomalous diffractionl-scans, As K-edge (11867eV)|FT |, |FA | and (φT - φA)

Strain, size, composition ?

• strain : 6% ([001])• height : 2.4 nm

a>aInP a<aInP

l InP (442)

BM32, ESRF

InAs QWrs

h=k

surface

InAs QWrs

a>aInPa<aInP

αiRX

TEM

|FA|= |FAs| QWrsooo : exp--- : cal..

l-scan √I- exp.-- cal.

E=10367eV

Application to nanostructures : buried InAs/InP QWrs

10nm InP capping

l

|FT| : QWrs+ InP matrix

- : exp.--- : cal.


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Sample→

paths (Å)↓

InAs

Bulk

InAs/InP

(pseud.)

CP1276

(EXAFS ε⊥ & ε//)

CP1276

(DAFS ε⊥ )

I3701

(EXAFS ε⊥ & ε//)

I3701

(DAFS ε⊥ ) Asabs-InI 2,632 2,60 2,593±0,003 2,57±0,02 2,593±0,003 2,62±0,02

(Asabs-AsII)// 4,284 4,29 4,16±0.06 - 4,15±0.06 - (Asabs-AsII) ⊥ 4,284 4,15 4,23±0,04 4,30±0,04 4,25±0,04 4,20±0,04 (Asabs-PII)// - - 4,19±0,07 - 4,15±0,07 - (Asabs-PII) ⊥ - - 4,17±0,03 4,20±0,06 4,18±0,03 4,15±0,06

Asabs-InI-AsII 4,765 5,26 4,71 4,72 4,72 4,73 Asabs-InI -P II - - 4,66 4,66 4,66 4,68 (Asabs-InIII)// 5,023 4,87 4,88±0.03 - 4,87±0.03 - (Asabs-InIII) ⊥ 5,023 5,13 4,93±0.06 - 4,94±0.06 -

%P// 0,3±0,1 - 0,4±0,1 - %P⊥ 0,5±0,1 0,4±0,2 0,6±0,1 0,4±0,3 SD - - - 0,6 - 0,8

EXAFS and EDAFS best fit results

Structural properties of semiconductor nanostructures ...M.G. Proietti Universidad de Zaragoza IUCR...

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