Optical Spectroscopies of Thin Films and Interfacesrote/Zahn/Vibrational_Spectroscopies.pdf ·...
Transcript of Optical Spectroscopies of Thin Films and Interfacesrote/Zahn/Vibrational_Spectroscopies.pdf ·...
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Dietrich R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Germany
Optical Spectroscopies of Thin Films and Interfaces
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1. Introduction
2. Vibrational Spectroscopy, i.e. Raman
3. Spectroscopic Ellipsometry
4. Reflectance Anisotropy Spectroscopy
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Principle of Raman Scattering
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Raman SpectroscopyRaman SpectroscopyR - Rayleigh Scattering
S - Stokes Raman Scattering
ωi- ω(q)AS - Anti-Stokes
Raman Scatteringωi+ ω(q)
ωi
v=0v=1
ω(q)ω(q)
Virtual levels
qkk
qEP
i
i
S
S
rh
rh
rh
rhhh
rrrr
±=
±=
=
)(0
ωωωχε
ωi ωiωi+ ω(q)ωi- ω(q)
Inelastic scatteringInelastic scattering of the light mediated by the polarisabilitypolarisability of the medium.
ω
I
Reflected light
Incident light
Scattered light
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Raman Spectroscopy
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hωs=hωi+hΩ
200 250 300 350
ZnSe LO
Intensity / ctsmW-1s-1
GaAs LO
Raman Shift / cm-1
Raman Spectroscopy
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1,5 2,0 2,5 3,0 3,51
10
100
1000
laser lines
Info
rmat
ion
dept
h / n
m
Photon energy / eV
Information depth for GaAs= ½ of light penetration depth
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Resonance Raman excitation profiles
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
100 150 200 250 300Raman shift (cm-1)
1.65 1.70 1.75 1.80 1.85 1.90 1.95
Inte
nsity
(arb
. uni
ts)
Laser Photon Energy (eV)
hωL
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Resonance Raman scattering
0ij0
I∝0 Light j j phononi i Light 0
hωL −hωphonon−Ej
hωL−Ei
ij
∑
2
LightLight Phonon
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Sub-Monolayer Sensitivityvia Resonance Enhancement
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Growth Chamberultra-high vacuum: base pressure<1⋅10-10mbar
up to 3 Knudsen cells
LEED/Auger
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Inten
sity
/ ctsm
W -1
s-1
Raman Shift / cm-1
Inten
sity
/ ctsm
W -1
s-1
Raman Shift / cm-1
Frequency Position and Lineshape
frequency shift by
temperature ≈2cm-1/100°Cpressure ≈1cm-1/1kbar
lineshape:
asymmetric broadening and shiftoccurs as a result of latticedisturbance
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0 100 200 300 400284
286
288
290
292
+/- 10°C
+/- 0.2 cm-1
Pea
k Po
sitio
n in
/ cm
-1
Temperature / °C
Determination of Surface Temperature
Using temperature induced shift of substrate phonon peak:
cm-1/100°CInSb: 2.1InP: 2.0GaAs: 1.8Si: 2.2ZnSe: 2.4
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0.45 0.50 0.55 0.60 0.650
1
2
3
4
visi
ble
ligh
t
red
blue
(620 nm)
(414 nm)
InSb
CdTeInPSiGaAs
ZnSe CdS
ZnS
GaN
Ener
gy b
andg
ap /
eV
Lattice constant / nm
Eg vs Lattice Constant
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CdS Growth on InP(100)
substrate: ammonium sulfidepassivated InP
wafers annealed in UHV to 330°C for 10 min; TS=200°C compound source for CdS at 620°C
laser excitation:2.34 eV
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CdS Growth on InP(100)
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0 50 100 150 2000.0
0.1
0.2 calculation experiment
Inte
nsity
LO
CdS
/ co
unts
s-1
mW
-1
CdS Layer Thickness / nm
Determination of CdS Layer Thickness
Fabry-Perotinterferencescause intensitymodulation of Ramansignals
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200 300 400
∆d=4nm
Sca
tterin
g In
tens
ity
Raman Shift / cm-1
Initial Phase of CdS Depositionon InP(100) at 200°C
broad shoulderon low frequencyside of CdS LO phonon peakindicates an interfacialreaction leadingto an In-S richlayer
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CdTe Growth on InSb
substrate: cleaved n-type InSb(110) surface
CdTe deposition from single Knudsencell kept at 550°C
laser excitation: 2.41 eV
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CdTe Deposition at 300°C
no CdTe growth
strong interfacereaction
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100 150 200 250
In2Te
3
A1g
(Sb)
D
C
B
A
Experiment Fit
Sca
tterin
g In
tens
ity
100 150 200 250
77K
D
C
B
AIn
2Te
3
Scat
terin
g In
tens
ity
Raman Shift / cm-1
Interfacial Reaction Products
Reaction of Te with InSb leading to the formation of In2Te3 and liberatedSb confirmed.
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CdTe Deposition at RT
no interfacereaction
Fabry-Perotmodulation
change in InSbLO/TO ratio
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ZnSSe Growth on GaAs(100)
substrate:As capped MBE grownGaAs layer
compound sources for ZnSe and ZnS
atomic nitrogen provided by rf plasma sourcelaser excitation: 2.54 eV for doping at
TS=260°C2.66 eV for ZnSSe at
TS=250°C
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100 200 300 400 500 600 050
100150
200
0.02
0.04
0.06
0.08
0.10
Intensity / counts mW -1s -1
Thickness / nmRaman Shift / cm-1
Raman Monitoring of ZnSe Growth
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100 200 300 400 500 600 050
100150
200250
0.02
0.04
0.06
0.08
0.10
Intensity / counts mW -1s -1
Thickness / nmRaman Shift / cm -1
Raman Monitoring of ZnSe Growth: Nitrogen Doping
weak ZnSe2LO scatteringrevealschange in resonancecondition as a result of nitrogendoping
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0 50 100 150 200 250 300284.7
285.0
285.3
285.6
285.9
286.2
286.5
286.8
ZnSe:N ZnSe undoped
Ram
an S
hift
/ cm
-1
Thickness / nm
Dependence of GaAsLO Frequency on ZnSe Doping
Nitrogeninducescompressivestrain in GaAs
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125 150 175 200 225 250 275 300 325 350 375
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
5.7 cm-1
5.7 cm-1
20.2 cm-1
13.7 cm-1
ZnSe LO
GaAs LO
ZnSe:N
ZnSeundoped
TM =260°C
Eex
= 2.54 eV (488 nm)d = 200 nm
Ram
an In
tens
ity /
coun
ts m
W-1
s-1
Raman Shift / cm-1
ZnSe with and without Nitrogen
broadeningof ZnSe LO phonon mode indicateslatticedisturbancebynitrogenincorporation
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Raman Monitoring of ZnSSe Growth
ZnS- and ZnSe-like LO phononscatteringobservableup to up to third order
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0.0 0.2 0.4 0.6 0.8 1.040
60
80
100
120
140
Theory after Hayashi et al. measured peakdifference
at nominal x
LOZn
S-LO
ZnSe
/ cm
-1
sulphur content x
Determination of S Content in ZnSxSe1-x
dependence of the relative frequency shiftof ZnS- and ZnSe-like LO modes onsulphur contentK.Hayashi et al. ,Jpn.J.Appl.Phys. 30, 501(1991)
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200 220 240 260 280 300 320 340
LO1+LO
2
LO2
Sca
tterin
g In
tens
ity
Raman Shift /cm-1
460 480 500 520 540 560 580
xnom
= 0.05
LO1: ZnSe-like
LO2: ZnS-like
LO2-LO
1
2LO1
LO1
Composition of Ternary Compounds
increasing frequencysplitting of ZnS- and ZnSe-like LO modescan be seen in LO and 2LO features
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100 200 300 400 500 600 70050
100150
2002500.1
0.2
0.3
0.4 LOZnS LOZnSe+LOZnS
2 LOZnSeLOZnSe
Intensity / counts mW -1s -1
Temperature / °CRaman Shift / cm-1
with increasingtemperaturethe bandgapof ZnS0.05Se0.95approaches thephoton energyof 2.66 eV
typical gain oftwo orders ofmagnitude
Resonance enhancement
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GaN Growth on GaAs(100)substrate:As capped MBE grown
GaAs layer
atomic nitrogen provided by rf plasma source
Ga from Knudsen cell at 870°C
laser excitation: 3.05 eV
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Raman Monitoring of GaN Growth on GaAs(100) at 600°C
resonanceenhancement of scattering in thecubic modification:
Eex=3.05eV≈Eg,cub
at 600°C
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200 400 600 800 1000
T=600°C
E2
GaAs LO
GaN
E2
A1+LO
dGaN
=
230nm
30nm
clean GaAs
Sca
tterin
g In
tens
ity
Raman Shift / cm-1
GaN Growth on GaAs(100)
high sensitivityachieved for GaNdetection at elevatedtemperatures
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Substrate strain and GaN crystalquality
0 50 100 150 200 250
34
36
38
40
42 A1+LO GaN
FWH
M /
cm-1
GaN layer thickness / nm
281
282
283
284
LO GaAsPos
ition
/ cm
-1
shift of GaAs LO phonon again revealsthe evolution of compressive strain in the substrate
evolution of FWHM is related to thecompetitive growth of cubic and hexagonal GaN
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Raman Spectroscopy and OMBD
Dilor XY 800 SpectrometerMonochromatic light source: Ar+ Laser (2.54eV), Detector: CCD • resonance condition with the absorption band of the organic material.• resolution: ~ 3.5 cm-1.
1.5 2.0 2.5 3.0 3.5 4.0
0
2
4
6
Abso
rbtio
n co
effic
ient
*10
5
S0-S2 transition
S0-S1 transition
DiMe-PTCDI
PTCDA
Energy / eV
800 700 600 500 400
0
2
4
Wavelength / nm
Ar+ line
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PTCDA DiMe-PTCDI
Symmetry D2hRaman active: 19Ag+18B1g+10B2g+7B3g
IR active: +10B1u+18B2u+18B3u
Silent: + 8Au108 internal vibrations
Molecular Vibrational Properties
CC2424HH88OO66
• DiMe-PTCDI: Cambridge Structural Database.
• PTCDA: α- and β-phases: S. R. Forrest, Chem. Rev. 97 (1997), 1793.
Monoclinic crystallographic system in thin films:
CC2626HH1414OO44NN22
C2h44Ag+22Bg
+23Au+43Bu
+ 8Au132 internal vibrations
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2-fold
DavydovSplitting
internal molecular modes: external molecular modes (phonons):
200 300 400 500 600 700
1200 1300 1400 1500 1600 1700
Inte
nsity
/ a.
u.
x2
Raman shift / cm-1
CC--OOBBgg
CC--HH CC--CC
CC--CC
SymmetrySymmetry: : DD2h2h CC2h2h (monoclinic)(monoclinic)
25 50 75 100 125 Raman shift / cm-1
Inte
nsity
/ a.
u. 6 rotationalvibrations:3Ag+3Bg
19Ag+18B1g+10B2g+7B3g
BBgg
AAgg
AAgg
BBgg
AAgg
RamanRaman--active vibrations of active vibrations of PTCDA PTCDA ((CC2424HH88OO66))::Effect of crystal formation Effect of crystal formation
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200 400 600 1200 1350 1500 1650
Inte
nsity
/ ar
b. u
nits
Raman shift / cm-1
Raman Spectra of a PTCDA Crystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G).
x0.1
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external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg
SymmetrySymmetry: : CC2h2h (monoclinic)(monoclinic)
25 50 75 100 125 Raman shift / cm-1
Inte
nsity
/ a.
u.Phonons in PTCDA:
BBgg
AAgg
BBgg
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200 400 600 12
Inte
nsity
/ ar
b. u
nits
Raman sh
Raman Spectra of a Raman Spectra of a PTCDAPTCDA CrystalCrystal
• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).
Raman shift /cm-1
and a and a DiMeDiMe--PTCDIPTCDI
DiMe-PTCDI PTCDA
PTCDA DiMe-PTCDI
DiMe-PTCDI
PTCDA experimental
ω m= =0.97ω m
ω 221= =0.95ω 233
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• Molecules remaining at the surface:NPTCDAPTCDA(0.04nm) ~ 1013 cm-2
NddSiSi ~ 1012 cm-2
Strong interaction between PTCDAPTCDA molecules and defectsdefects mainlymainly due to SiSi at the GaAsGaAs surface.
Interaction of Interaction of PTCDAPTCDA with the with the SS--GaAs(100):2x1 GaAs(100):2x1 SurfaceSurface
Annealing of a 14 nm thick film at 623 K for 30 min:
1300 1400 1500 1600
Inte
nsity
/ ct
s m
W-1 s
-1
Raman shift / cm-1
0.00
2
40 nmx 0.01
0.45 nm(x 0.6)
0.18 nm
ann.x 4.4
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300 600 9000
10
20
30
1200 1400 16000
500
1000
1500
Inte
nsity
/ A
4 am
u-1
Raman shift / cm-1
Calculated Vibrational Properties:PTCDA
1340 1350
2.7 cm-1
• calculations with Gaussian `98 (B3LYP:3-21G).
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Raman Monitoring ofRaman Monitoring of PTCDAPTCDA Growth on Growth on SS--GaAs(100):2x1GaAs(100):2x1
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200 250 300 350 400
LO Ω−
Nd = 2.7 *1018 cm-3
Ram
an in
teni
sty
/ a. u
.
Raman shift / cm-1
0 2 4 60.00.20.40.60.81.01.21.4
Raman PES
S-GaA
s
Ban
d B
endi
ng /
eV
Film Thickness / nm
PTCDA/S-GaAs
Electronic Properties at Electronic Properties at PTCDAPTCDA//SS--GaAsGaAs
• Relative intensities of GaAs LO and PLP (Ω-) bands:
Band bending within the substrate: minor changes upon PTCDA adsorption.
Good agreement with photoemission (PES) studies: S. Park, D.R.T. Zahn, et al. Appl. Phys. Lett. 76 (2000) 3200.
J. Geurts, Surf. Sci.
Rep. 18 (1993), 1.
4882
( 0)
GaAsn
nmdLO
n s
I eI
V z
δ
δ
−Ω
∝
∝ =
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Determination of Molecular Orientation:Determination of Molecular Orientation:DiMeDiMe--PTCDIPTCDI
Azimuthal rotation of a 120 nm thick film; normal incidence.Periodic variation of signal in crossed and parallel polarization.
M. Friedrich, G. Salvan, D. Zahn et al., J. Phys. Cond. Mater. submitted.
γ=0°: x II [011]GaAs
γ=90°:x II [0-11]
γ
phononsphonons phononsphonons
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Determination of Molecular Orientation:Determination of Molecular Orientation:DiMeDiMe--PTCDIPTCDI
yx
xx
IDep =
I
56 4 ;,
θψ ϕ
= ° ± °
( ) ( )θ ψ ϕ θ ψγ ϕ γ⋅ ⋅g
-1g
m= R , ,A ,A, R , , Good agreement with IR and NEXAFS results
( )s igAI = e e⋅ ⋅r r
0 60 120 180 240 300 3600.0
0.5
1.0
1.5
2.0
2.5
Dep
olar
izat
ion
Rat
io/ a
.u.
Experimental angle (γ)/°
BreathingBreathing mode at 221 cmmode at 221 cm--11
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200 400 600 1200 1350 1500 1650
Inte
nsity
/ ar
b. u
nits
Raman shift / cm-1
x0.1
Ag Raman Modes of PTCDAwith In
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200 400 600 1200 1350 1500 1650
Inte
nsity
/ ar
b. u
nits
Raman shift / cm-1
x0.1
Ag Raman Modes of In4PTCDA
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In Situ Raman: Monitoring of IndiumDeposition onto PTCDA (15 nm)
1200 1400 1600
0.05
Raman shift / cm-1200 400 600
Inte
nsity
/ ct
s m
W-1s-1
0.005
43/5
In thickness / nm
00.4/0.71.1/1.52.8/135.0/288.0/3315.0/5826.0/10
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Influence of Indium on VibrationalSpectra of PTCDA
1200 1400 1600
0.0025
+ InB3g
B1u
Ag
Ag
B3g
B2u
Ag
B3g
(B3g)B1u
B3gAg
Raman shift / cm-1
PTCDA
200 400 600
B2uAg
B3g
Ag
Ag
In15 nm
Inte
nsity
/cts
mW
-1s-1
0.0025Ag
B2g
GaAs
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• Thin PTCDAPTCDA film: “first layer” SERS effect: molecules in contact with AgAg
• 15 nm PTCDAPTCDA film: mainly long range SERS:no AgAg diffusion into PTCDAPTCDA
S-GaAs(100)
AgAg//PTCDA:PTCDA: Evidence for Abrupt InterfaceEvidence for Abrupt InterfaceSimilar interface formation for AgAg//DiMeDiMe--PTCDIPTCDI
1350 1500 1650
Inte
nsity
/ ct
s m
W-1s-1
0.03
PTCDA(0.4 nm)
Raman shift / cm-11200 1350 1500
PTCDA(15 nm)
0.001
S-GaAs(100)
Ag:1.6 nm/minAg:5.5 nm/min
2.2 nm Ag
11 nm Ag
/ 30
/ 5
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Indium/PTCDA: Evidence for Strong Indiffusion
1200 1350 1500 1650
Inte
nsity
/ ct
s m
W-1s-1 0.03
PTCDA
PTCDA(15 nm)
Raman shift / cm-11350 1500 1650
PTCDA(0.4 nm)
0.001
x 0.017+ Inx0.045
In: 0 →100 nm
In: 1 nm/min
PTCDA~0.4 nm(~1 ML) S-GaAs(100)
~15 nm(~50ML)
PTCDA
S-GaAs(100)
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200 300 400 500 600 1300 1400 1500 1600
Inte
nsity
Raman shift / cm-1
5x10- 2
cts.mW- 1S - 1
5x10-3
cts mW-1s-1
+ Mg
DiMe-PTCDI
+ In
+ Ag
Comparison of In, Ag and Mg deposition on DiMePTCDI
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Raman Spectroscopy
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STM tip-enhanced Raman spectroscopyA new approach, tip-enhanced Raman spectroscopy (TERS), is explored that combines Raman spectroscopy at smooth surfaces with a local electromagneticfield enhancement provided by an optically active Ag STM or AFM tip. This optical activity is achieved by exciting local surface plasmon modes by focussing the laser light through a thin metal film onon a glass slide onto the tip apex. The local enhancement of the Raman scattering cross section in the vicinity of the tip opens promising avenues towards single molecule Raman spectroscopy.