X-ray Absorption studies ofatomic environments in
semiconductor nanostructures
Federico Boscherini
INFM and Department of PhysicsUniversity of Bologna, Italy
University of Bologna INFM
• Introduction– Why XAS for nanostructures– How XAS for nanostructures
• Interdiffusion in quantum dots and islands• Relation between local and long-range
elasticity in an “ideal” alloy: (InGa)As• Growth of nitride epilayers: a local view
University of Bologna INFM
Atomic structure in nanostructures
• As a result of reduceddimensions:– atomic intermixing
• in the “core”• at the interfaces
– variations in bond lengths
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Sn cluster in SiO2 MDM-INFM
XAS to study nanostructures• XAS is a local, short range, effect
– same formalism applies to molecule, cluster orcrystalline solid
– insenstive to variations of morphology– sensitive to low thicknesses, high dilutions
• Excellent probe of variations in localenvironment upon reduction of dimensionsand/or dimensionality
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Evolution of X-ray Absorption Spectroscopy• X-ray Absorption
Spectroscopy has greatlybenefited from– third generation SR sources
• high brilliance, extended energyrange, stability, reliability
– full theoretical understandingand reliable analysis programs
• XAS a reliable tool σ = σ 0
3 Sin2δ l =10 Im{T 1 −TG( )−1}1m ,1m
0,0
m∑
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GILDA beamline at ESRF• Dynamical sagittal focussing over wide energy range• 13-element HP-Ge detector with digital electronics
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GILDA experimental chamber• Transmission /
fluorescence• LNT - 150 °C
– reduce thermal damping• Rotatable holder
– polarization studies• Vibrating holder
– “smooth” Bragg peakeffects in single crystalepilayers
University of Bologna INFM
Quantum dots
– F. Boscherini, G. Capellini, L. DiGaspare, F. Rosei, N. Motta,and S. Mobilio, Appl. Phys. Lett. 76, 682 (2000)
• Stranski-Krastanov growth leads to narrow sizedistribution of dots
• Need for understanding of local bonding
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Energetics of island formation• Competing energies:
– strain– surface– dislocations
Wetting layer WL+Strained island WL+Relaxed island
"Coverage"
WL+2D platelet
• Contributions from:- wetting layer- islands
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Preparation of Ge dots• Ge/Si(001) by CVD at University of Roma Tre, 600 °C
– Ex-situ AFM used to characterize degree of relaxation
• Ge/Si(111) by MBE at Tor Verg., 450 - 550 °C– In-situ STM/AFM
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Ge/Si(001): AFM probes relaxation• Analysis of aspect ratio
provides measurementof relative amount ofrelaxed islands
• Ge/Si(001): Full rangeof relaxation examined
0
20
40
60
80
100
0 5 10 15 20 25 30 35 40
Rel
axed
vol
ume
(%)
equivalent thickness (nm)
Ge/Si(001)
(1 ML = 0.135 nm, WL = 3 ML)
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Ge/Si(001): Ge K-edge XAFS
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0
1
2
3
4
2 4 6 8 10 12 14 16k (Å-1)
k χ (
k) (Å
-1)
Ge:Si
(111)1 nm(001)
7.8 nm
(001)38 nm
Ge
Ge/Si(001): XAFS results
• N(Ge-Si)=(1-1.5)±0.3• Assuming random alloy
average composition isGe0.70Si0.30
0
1
2
3
4
0 5 10 15 20 25 30 35 40
para.perp.
equivalent thickness (nm)C
oord
inat
ion
num
ber
N(Ge-Ge)
N(Ge-Si)
Ge/Si(001)
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Ge/Si(111): XAFS
0
2
4
6
8
10
12
0 1 2 3 4 5 6
Mag
. Fou
. Tra
nsf.
[ k2 χ
(k)]
(Å
-3)
R (Å)
Ge in Si
Ge bulk
fed 5: 12.5 Å, 530 C
s1: 220 Å, 530 C
fed 3: 15 Å, 530 C
fed 11: 60 Å, 450 C
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0,8
0,9
1
2
1 10 100
450 deg C530 deg C
N (G
e - S
i)thickness (nm)
Ge/Si(111): STM• A trench develops around the islands
– Si from the substrate diffusing into the island
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InAs/GaAs(001): In K-edge XAFS• ALMBE, 10 ×
(3 ML InAs + 15 nmGaAs)
• Interdiffusionevident in 2nd shellsignal– Contraction of
bond length• In concentration in
dots: 0.25 - 0.450 1 2 3 4 5 6
R (Å)
InAssample
Mag
. Fou
. Tra
nsf.
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2 4 6 8 10 12 14 16
k χ (
k) (Å
-1)
k (Å-1)
InAs
sample
0.0-0.1
0.10.20.3
Intermixing and strain• Es ∝ ε2
• Alloying will decrease ε linearly withconcentration xaalloy = x a1 + (1- x) a0
• Intermixing must be considered in realisticmodels of Stranski-Krastanov growth
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Local strain in 2Dsemiconductor epilayers
– Romanato et. al., Phys. Rev. B 57, 14619 (1998)– Tormen et. al. , J. Appl. Physics 86, 2533 (1999).– Tormen et. al. , Phys. Rev. B 63, 115326 (2001)
• What is the microscopic mechanism ofaccommodation of strain?
• What is the relation between long-range andshort range description of the elasticity ofsolids?
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Long vs. short -range elasticity
• Long range distortion, probed by diffraction:–
• Short range distortion, probed by XAFS:
as
af
ε⊥= - [ 2 c12/c11 ] ε||
asas
af afa⊥
as
as
ε⊥ = – 2
C12C11
ε ||
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V({Rij}, {θ ijk}) = α2 (Rij – Rij
0)2Σij
+β8 Re
2 (Cos θijk + 13)
2Σijk
Local distortions in unstrained alloys
V({Rij}, {θ ijk}) = α2 (Rij – Rij
0)2Σij
+β8 Re
2 (Cos θijk + 13)
2Σijk
• Local distortions in unstrained alloys are well understood • In semiconductors β/α = 0.1 - 0.2
• Bond lengths are “rigid”• Fundamental behavior can be obtained with
• no disorder in force constants• disorder in atomic radii
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Samples used• High quality InxGa1-xAs/InP(001) layers were
used to study this issue• Samples deposited by MOCVD at ICTIMA-CNR• Characterized by AFM, HRXRD, RBS, TEM• Two series:
– pseudomorphic as a function of strain, with- 1.42 < ε||< 1.98
– fixed x=0.25, as a function of thickness, with- 1.42 < ε||< 0
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Variation of bond lengths• A linear variation of bond lengths with strain is clearly
detected
2.44
2.46
2.48
2.50
2.52
2.54
0 0.2 0.4 0.6 0.8 1
R(G
a-A
s) (Å
)
xUniversity of Bologna INFM
-0,02
-0,01
0
0,01
0,02
-1,5 -1 -0,5 0 0,5 1 1,5 2δ
R (
Å)
strain (%)
Polarization dependence• Interatomic distances parallel and
perpendicular to growth plane canbe obtained
• In the plane wave approximationeach correlation contributes with aweight equal to
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3 Cos2 ( ˆ E • ˆ r ij)
E
B krij
PARALLEL
PERPENDICULAR
Variation of 2nd shell distances
• A strain - inducedsplitting [δrPAR(x) -δrPER(x)] is detected
• The splitting is equal forall atomic distances
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4,05
4,1
4,15
4,2
4,25
0,2 0,3 0,4 0,5 0,6 0,7 0,8
InxGa
1-xAs
Indium concentration, x
Ga
- cat
ion
dist
ance
(Å)
Ga(As)In
Ga(As)Ga
PERPENDICULAR
PARALLEL
Local Effect of Strain• The strain tensor is
• Application of this tensor to 1st and 2nd shellinteratomic distances yields
ε|| 0 00 ε || 00 0 ε⊥
ε ⊥ = −γε || γ = 2C12C11
δr(1)(x) = a(x)2 −γ (x)[ ]
4 3ε||
δrOUT(2) (x) = a(x) γ (x) −1[ ]
2 2 ε ||
δrIN(2)(x) = a(x) ε ||
2
Local effect of strain
• The XAFS results can be reproduced bytransfering the macroscopic strain tensor to theatomic scale, neglecting the type of bond
• The effect of strain is linearly summed to that ofalloying
• CONCLUSION: Macroscopic elasticity theory isapplicable to the local scale
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Growth of GaN on SiC and AlN– F. Boscherini, R. Lantier, A. Rizzi, F.
D’Acapito, and S. Mobilio, Appl. Phys. Lett.74, 3309 (1999) and submitted to PRB (2001)
• Nitrides are of great interest forrealization of blue-violetoptoelectronic devices
• Understanding of basic physics andgrowth mechanisms still matter ofactive research– No substrate with similar lattice
constant readily available
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Issues addressed• Nitrides exhibit
– spontaneous polarization– strain-dependent piezoelectric polarization
• GaN/SiC abrupt is expected to be charged and unstable– intermixing is expected (Ga/Si or C/N mixed planes)
• Relation between morphology and local strain• Probe of elastic constants
• Samples grown at ISI, Forschungszentrum Jülich byMBE with rf plasma source N2
• XPS determination of band bending and band offsets
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GaN/SiC: Ga XAFS
0.0
1.0
2.0
3.0
4.0
5.0
Mag
. FT
( χ (
k) k
2) (
Å-3
)R (Å)
0 1 2 3 4 1 2 3 4
PERPENDICULAR PARALLEL
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Rbasal(2) = a
Roff − basal(2) = 1
3 a2 + 14 c2
Thickness: 0.7 - 150 nmR(2)(basal)
R(2)(off-basal)
PERP
END
ICU
LAR
PARALLEL
a
c
GaN/SiC• No change in 2nd shell distances, no
asymmetry between parallel andperpendicular directions– Relaxed growth (no piezoelectric polarization)
• No extensive intediffusion– High quality growth
• No Ga-Si mixed interface plane (C/N mixedplane cannot be excluded)
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GaN/SiC: Impact• These results provide basis for:
– interpretation of XPS data– discussion and input to ab-initio electronic structure
calculations• Band-bending observed in XPS compatible with
relaxed growth• Valence band discontinuity of 0.8 ± 0.1 eV must
be attributed to relaxed interface• Note that all ab-initio calculations assume
pseudomorphic growth
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GaN/AlN: dependence of morphologyon growth temperature
• “Smoother” epilayers for lower growth temperature
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7 nm620 º C
7 nm790 °C
500 nm
GaN/AlN: Polarizationdependent Ga XANES
• Fingerprint ofwurtzite structure– 6 nm layers are
hexagonal• Reproduced by full
multiple scatteringcalculations
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10360 10380 10400 10420 10440E (eV)
Abs
orpt
ion
coef
ficie
nt
PAR
PER
Simulation
Experiment
Simulation
Experiment
GaN/AlN: XAFS probes localelastic properties
• By measuring R(2) in twopolarizations we obtain ε||and ε⊥
• Strain increases for“smoother” epilayers butnever reaches -0.024
• Values ofin good agreement withspread of values inliterature
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0,000
0,002
0,004
0,006
0,008
0,010
-0,015-0,01-0,0050ε z
εx
620
790
740660
700
ε⊥ε ||
= −2C13C33
Thanks• Ge islands
– G. Capellini, S. Mobilio, INFM +Univ. Rome 3– N. Motta, F. Rosei, INFM +Univ. Roma 2
• InAs dots– M. Capizzi, INFM + Univ. Rome 1– P. Frigeri and S. Franchi, MASPEC-CNR
• Local elasticity– D. De Salvador, F. Romanato, M. Tormen, A. Drigo,
INFM + Univ. Padova• GaN epilayers
– A. Rizzi, Jülich and INFM + Univ. Modena– F. D’Acapito, INFM Grenoble– S. Mobilio, INFN and Univ. Rome 3
University of Bologna INFM
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