Optical Spectroscopies of Thin Films and Interfacesrote/Zahn/Vibrational_Spectroscopies.pdf ·...
Transcript of Optical Spectroscopies of Thin Films and Interfacesrote/Zahn/Vibrational_Spectroscopies.pdf ·...
Dietrich R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Germany
Optical Spectroscopies of Thin Films and Interfaces
1. Introduction
2. Vibrational Spectroscopy, i.e. Raman
3. Spectroscopic Ellipsometry
4. Reflectance Anisotropy Spectroscopy
Principle of Raman Scattering
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
Raman Spectroscopy
hωs=hωi+hΩ
200 250 300 350
ZnSe LO
Intensity / ctsmW-1s-1
GaAs LO
Raman Shift / cm-1
Raman Spectroscopy
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
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
Resonance Raman scattering
0ij0
I∝0 Light j j phononi i Light 0
hωL −hωphonon−Ej
hωL−Ei
ij
∑
2
LightLight Phonon
Sub-Monolayer Sensitivityvia Resonance Enhancement
Growth Chamberultra-high vacuum: base pressure<1⋅10-10mbar
up to 3 Knudsen cells
LEED/Auger
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
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
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
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
CdS Growth on InP(100)
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
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
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
CdTe Deposition at 300°C
no CdTe growth
strong interfacereaction
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.
CdTe Deposition at RT
no interfacereaction
Fabry-Perotmodulation
change in InSbLO/TO ratio
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
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
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
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
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
Raman Monitoring of ZnSSe Growth
ZnS- and ZnSe-like LO phononscatteringobservableup to up to third order
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)
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
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
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
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
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
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
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
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
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
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
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
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
• 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
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).
Raman Monitoring ofRaman Monitoring of PTCDAPTCDA Growth on Growth on SS--GaAs(100):2x1GaAs(100):2x1
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
∝
∝ =
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
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
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
200 400 600 1200 1350 1500 1650
Inte
nsity
/ ar
b. u
nits
Raman shift / cm-1
x0.1
Ag Raman Modes of In4PTCDA
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
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
• 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
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)
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
Raman Spectroscopy
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.