Structural properties of semiconductor nanostructures ...M.G. Proietti Universidad de Zaragoza IUCR...
Transcript of 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
M.G. ProiettiUniversidad de Zaragoza
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
«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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IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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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IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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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IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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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IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
~ 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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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)
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
1st order EDAFS :
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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.
M.G. ProiettiUniversidad de Zaragoza
IUCR 2005
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