Spectroscopy - Aalborg Universitethomes.nano.aau.dk/kp/PhDcourse08/Spectroscopy.pdf · ~200 nm ~200...
Transcript of Spectroscopy - Aalborg Universitethomes.nano.aau.dk/kp/PhDcourse08/Spectroscopy.pdf · ~200 nm ~200...
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SHG Spectroscopy
•Clean surfaces•Clean surfaces•Oxidation•SOI wafer
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SHG set-uppScan regions
Idler: 730-1050 nm
1000- 1400 nm
Signal: 680-550 nm
600 500600-500 nm
Ti:Sapphire:
700-1000 nm
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SHG from GaAs
Bergfeld, Daum, PRL 90, 2915
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Spectroscopy on Si/SiO interfacesSpectroscopy on Si/SiO₂ interfaces
3.4 eV: E₀/E₁ transition at Γ-point4 2 eV: E transition at Γ point4.2 eV: E₂ transition at Γ-point3.6 eV: Interface transition
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Si(100) Si/Ge SiOSi(100)‐Si/Ge‐SiO₂
Growth of strained layer
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SHG Spectroscopy
8
10
Si(111)7x7
Interband transitionsSurface states below 3.4 eV
4
4
6
8
Clean
arb.
uni
ts) p to p
2
3
4
0
2
10 L O2
SH
G (a
0
1
2
U /S
U2
ergy
(eV
)
4
2,0 2,5 3,0 3,5 4,0 4,5
units
)
Cleanp to s
2
-1
0
S3
S2
U1/S1En
2
SH
G (a
rb. Clean
10 L O
-3
-2
MK Γ
2,0 2,5 3,0 3,5 4,0 4,5 5,00
S
SH Photon Energy (eV)
10 L O2
2.8 eV (1.4 eV): 1ω S2→U12.0/2.4 eV: 2ω S3→U13 4 eV: 2ω E
Surface states3.4 eV: 2ω E14.4 eV: 2ω E2
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Pump-probe:Dynamics of surface statesDynamics of surface states
3
4
1
2U2
gy (
eV)
-1
0
S2
U1/S1Ene
rg
-3
-2
S3
MK Γ
Saturation and recovery of unoccupied ad-atom stateby pump pulse.Intra band ∼100 fs, interband (surface)∼1-2 psSurface states to bulk ∼100 psHöfer et al. PRB 73 245305 (2006)
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SH resonances
∑⎭⎬⎫
⎩⎨⎧
+∝ mmnns
ifif)2(
)½2()exp(
)½()exp()2( φφωχ
10
∑⎭⎬
⎩⎨ +−+−mn
mmnn
s ii, )½2()½()(
γωωγωωχ
7
8
9
Clean p to p polarization
Resonances:
2.05 eV SS 1ω resonance3 06 V SS 2
5
6
7
arb.
uni
ts] 3.06 eV SS 2ω resonance
3.34 eV E1 2ω resonance4.34 eV E2 2ω resonance
3
4
SHG
[a
1ω or 2ω resonances?
0
1
2
10 L oxygen Constructive / destructiveinterference
1.5 2 2.5 3 3.5 4 4.5 50
SH Photon Energy [eV]
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SH resonances
8
9
p to p polarization
Resonances:
2.15 eV SS 1ω resonance
6
7
8
its]
p to p polarization 2.15 eV SS 1ω resonance3.06 eV SS 2ω resonance3.34 eV E1 2ω resonance3.55 eV Si-O 2ω resonance
4
5
G [a
rb. u
n
10 L oxygen
Clean
4.34 eV E2 2ω resonance
2
3SH New resonance from Si-O bondsat the interface
1.5 2 2.5 3 3.5 4 4.5 50
1
SH Photon Energy [eV]
Daum et al. PRL 93 097402
gy [ ]
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Ge(111) – linear response
Lowest direct transition very weakSpin-orbit splitting of 2 1-eV transition ~0 2 eVSpin orbit splitting of 2.1 eV transition 0.2 eV
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Ge(111) spectra ‐ comparison
2.1-2.4 eV transitions: spin-orbit or interface states?2 8-3 0 eV transitions: sensitive to surface treatment2.8 3.0 eV transitions: sensitive to surface treatment.
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Ge(111) spectra ‐ comparison
2.1-2.4 eV transitions: spin-orbit or interface states?2.8-3.0 eV transitions: sensitive to surface treatment.Additional peak at 3.1 eV?What happens below 2.0 eV?
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Second harmonic generationSecond harmonic generation spectroscopy on SOI wafersp py
Kjeld PedersenThomas Garm PedersenAalborg University
Denmark
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Sn nanocrystals in Si
1,0
1,2Sn in SiB lk l
0,6
0,8
nits
)
Si d i lSi epilayer
0,8
1,0
Si epilayer Snunits
)
Bulk sample
0,4
,
HG
(arb
. u Si device layer
Si waferOxide
0,4
0,6Si wafer
Si epilayer Sn
HG
(arb
. u
700 800 900 1000 1100 1200 1300 14000,0
0,2SH
700 800 900 1000 1100 1200 13000,0
0,2SH
Pump Wavelength (nm)
Samples from Arne Nylandsted
700 800 900 1000 1100 1200 1300
Pump Wavelength (nm)
Conclusion: Don’t use SOI wafers for structures for optical characterization(Unless you want to probe the SOI interfaces)
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SOI – SHG from interfaces1
380 nm Si400 nm SiO2
0,1
Log scale!
b. u
nits
) Si device layerOxide
~380 nm~400 nm
SH
G (a
rb
Si handling wafer
700 800 900 1000 1100 1200 1300 1400
0,01
Pump Wavelength (nm)Pump Wavelength (nm)
SHG oscillationsLinear properties
SHGSHG sources
Can the oscillations be used to isolate SHG from buried interfaces?Can the oscillations be used to isolate SHG from buried interfaces?
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SOI wafers
• Fast IC’s – transistor insulation
• CMOS
• MEMS
• Si Photonics
• ….
Si device layer
Oxide
~200 nm
~200 nm
Si handling wafer
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SHG measurements on SOI interfaces
B. Jun, IEEE Trans Nucl Sci, 51, 3231B. Jun, Appl. Phys. Lett. 85, 3095N. Tolk, Microelectronic Engineering 84, 2089
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Linear propertiesEllipsometryEllipsometry
0,8 SOI waferFit to eliipsometric data:209 nm Si410 nm SiOan
d s 2)
0,0
0,4 410 nm SiO2
eter
s (s
1 a
-0,4
c pa
ram
e
-0,8
psom
etri
400 500 600 700 800-1,2E
lli
Wavelength (nm)
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Linear propertiesReflection multilayer structureReflection – multilayer structure
1 2 3 4
Si SiSiO₂
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1,0Linear reflection ‐data from Palik
0 4
0,6
0,8 θ=30oec
tion
θ=40o
600 800 1000 1200 1400 16000,0
0,2
0,4
600 800 1000 1200 1400 1600
Ref
le
600 800 1000 1200 1400 1600 600 800 1000 1200 1400 1600
0 4
0,6
0,8
θ=50o
ectio
n θ=60o
600 800 1000 1200 1400 16000,0
0,2
0,4
400 600 800 1000 1200 1400 1600
Ref
le
600 800 1000 1200 1400 1600 400 600 800 1000 1200 1400 1600
0,6
0,8
θ=70o
ectio
n
θ=30o
Fit to data: dSi=203 nm doxide=402 nm
0,0
0,2
0,4
Ref
le
θ=70o
600 800 1000 1200 1400 1600,
Wavelength (nm)600 800 1000 1200 1400 1600
Wavelength (nm)
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SHG set-upScan regions
Idler: 720-1400 nm
Signal: 680-500 nm
Excitations: 1-5 eV
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Refractive index ‐ absorption
7
2ωω Si
5
6
7 ω Si
ex
3
4
5
nr
ve In
de
1
2
3
niRef
ract
i
200 400 600 800 1000 1200 1400 1600 1800
0
1 iR
Wavelength (nm)
Transparent to pump light in whole regionTransparent to pump light in whole regionStrong absorption of SHG for pump shorter than 800 nm
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12
SHG spectra
10
11
12
E2
SOI wafer, 200 nm Si, 400 nm SiO2
8
9
Interface
2
Eunits
)
70o
60o
5
6
7 E1
G (a
rb. u
60 50o
40o
30o
3
4
SH
G 30o
0
1
2
500 600 700 800 900 1000 1100 1200 1300 14000
Pump Wavelength(nm)
Resonances at critical points + oscillations
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SHG spectra10
SOI wafer, 200 nm Si, 400 nm SiO2
1
70o
60o
50oLog scale!
1
SH
G
40o
30o
0,1
Interface
E2 E1
500 600 700 800 900 1000 1100 1200 1300 1400
0,01
Pump Wavelength(nm)Pump Wavelength(nm)
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SHG sources
Sum of radiations from dipole sheets
ω ω 2ω
Interface contributions
Bulk contributions
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Rotational anisotropyBulk contributionBulk contribution
1,0
0,8ts
)
0,6
p to parb.
uni
t
0 2
0,4p pλp=750 nmθ=60o
SH
G (a
0 45 90 135 180 225 270 315 3600,0
0,2y=(0.9+0.1*cos(4*pi/180*col(A)))^2
S
0 45 90 135 180 225 270 315 360
Rotational Angle (deg.)
Bulk contribution ~10%
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Effect of linear reflections1st interface1st interface
1
R( )
R(2ω) + Min at 550 nm+ Min at 1000 nm
0,1
R(ω)
θ=50o
on
+ Peak at 750-800 nm
÷ Min at 620 nm
0,01Ref
lect
io Min at 620 nm
0,01SHGR
500 600 700 800 900 1000 1100 1200 13001E-3
Wavelength (nm)Wavelength (nm)
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SHG Si/SiO₂ interface response
Natural oxide
W. Daum, PRB 59, 2915
Si(100) interface resonance
oxide
( )
Depends on interface formation60 nm thermal oxide
Interface resonance part of responsefunction in near IR (eg. 800 nm)
105 nm dry/wet/dry+annealing+annealing
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Si(111)/oxide interface
8
10
Si(111)7x7
4
6
8
Clean
arb.
uni
ts) p to p
2 0 2 5 3 0 3 5 4 0 4 50
2
10 L O2
SH
G (a
4
2.0 2.5 3.0 3.5 4.0 4.5
b. u
nits
)
Cleanp to s
2
SH
G (a
rb10 L O2
2.0 2.5 3.0 3.5 4.0 4.5 5.00
SH Photon Energy (eV)
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Model – SHG from dipole sheets
1
zSipe’s model:J. Opt. Soc. Am. B4, 481 (1987)Phys. Rev. B35, 1129 (1987)
2
••
y ( )
Field in medium 1 from source in medium i
iP
zi
••
n
∑ += EE
Other models: Bethune, J. Opt. Soc. Am. B6, 910
∑=i
iEE 1,1
, p ,Yeganeh, Phys. Rev. B46, 1603Wierenga, Physica B204, 281
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SHG model (continued)
)()()2( )2( ωωχω kjijki EEP =
I fEEl∑= ll if φχ )2( )exp(InterfaceEEl ,,: 21∑ −−
=l ll iγωω
χ 22)(
2
xyzzzzzxxeff aaaaa ∂+∂+∂++= 54321 γξχ
BulkAnisotropic
B lk
SurfaceIsotropic
BulkIsotropic Depends on angle of incidence
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Interface contributions
10
8
9χ(2) constant θ=50o Only 1st interface
for λ<850 nm
6
7 Exp 1st interface3rd interfaceun
its)
Peak at 900 nm:2nd and 3rd interface
3
4
53rd interface 2nd interface
G (a
rb.
1
2
3
SH
G
500 600 700 800 900 1000 1100 1200 13000
1
Wavelength (nm)
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Fit to experiments10 SHG model: 3 interfaces
θ=50o 10θ=30o
1 2nd and 3rd interface
b. u
nits
)
1
b. u
nits
)
0,1
SH
G (a
rb
0,01
0,1
SH
G (a
r
500 600 700 800 900 1000 1100 1200 1300
0,01
Wavelength (nm)
500 600 700 800 900 1000 1100 1200 1300Wavelength (nm)Wavelength (nm)
S ² f b th l d f ll 3 i t fd l1st
Same χ² for both angles and for all 3 interfaces
No bulk contribution
Si device layerOxide
Si handling wafer
2nd
3rdg
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Looking for buried interfaces
0,8
1,0 1st interface 2nd 3rd
dSi=200 nmdSiO =400 nms)
0,8
1,0
dSi=100 nmdSiO =200 nm)
0,4
0,6
SiO2
χ(2)=1
G (a
rb. u
nits
0,4
0,6
SiO2
χ(2)=1
G (a
rb. u
nits
)
500 600 700 800 900 1000 1100 1200 1300 14000,0
0,2SH
G
Wa elength (nm) 500 600 700 800 900 1000 1100 1200 1300 14000,0
0,2SH
G
Wavelength (nm) 500 600 700 800 900 1000 1100 1200 1300 1400
Wavelength (nm)
0,8
1,0 1st interface 2nd 3rd
dSi=300 nmdSiO2
=300 nm)
We can find a wavelength to test 3rd interface
0,4
0,6
2
χ(2)=1
G (a
rb. u
nits
)
2nd interface is difficult to reach
1 t
500 600 700 800 900 1000 1100 1200 1300 14000,0
0,2SH
G
Si device layerOxide
1st
2nd
Wavelength (nm) Si handling wafer 3rd
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Conclusions
• Oscillations in SHG from SOI wafer– Multiple reflections in linear fieldp
– Multiple reflections in SH field
Variations in χ² of Si– Variations in χ² of Si
• Only first interface for λ<800 nm• Wavelength for2nd and 3rd interface can be foundfound– λp ,θ