Download Spectroscopy?

Introduction to laser spectroscopy
techniques
• 黃建智
• Department of Physics
• Nation Taiwan University
P/NSTL@NTU
Outline
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What is spectroscopy ?
Absorption
Photoluminescense
Raman spectroscopy
Summary
Reference
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Spectroscopy?
• deals with the interactions of electromagnetic radiation with
matter as related to the transitions between specific energy states
of chemical species
• The data that is obtained from spectroscopy is called a spectrum
• energy-level structure of physical system
• The choice of spectroscopic method is primarily determined by
the energy range of the phenomenon to be studied
Wavelength
100cm 100m
1m
100nm
10nm
1nm
10-6eV 10-4eV 10-2eV
Photon energy
1eV
10eV
100eV
1keV
10m
RADIO
IR
MICROWAVE
Molecular
Vibrations
Finestruct
ure
VUV
Soft X-rays
UV
Extreme Ultraviolet (SXR)
(EUV)
0.1nm
10keV
Hard Xrays
Outer
Electron
Trans
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What Instruments Are Needed?
• Radiation source :
•Analyzer
•Detector
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Radiation Source and Detector
•Infrared Nernst filament
•Infrared :Semiconductors
(black body irradiation)
•Visible Tungsten-halogen lamp
(350 – 5000 nm)
•UV Deuterium discharge lamp
(190 – 350 nm)
•Xenon discharge lamp
(< 190 nm)
(HgCdTe, InSb, PbS,
InGaAs)
•Visible & UV :
Photomultiplier: PMT
Photodiode: Si
Diode array detector
Charge coupled device CCD
•Lasers UV-Vis-NIR
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Optics of Spectrometer
•Constructive interference:
•Resolution depends on :
1.grooves/mm in relation to 
•n = 1, 2, … is the diffraction order
2. Length of monochromator
•d is distance between the grooves
3. Slit width
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Types of Interaction
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Absorption
Po
Reflection\ Scattering
P
d
rayleigh
•
Luminescence
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Absorption
•transfer of energy from
photon to atom or molecule
which produces a transition
from a lower energy to a
higher energy level
•Recording the resultant
change in light intensity as a
function of wavelength (i.e.
energy) is what allows us to
characterize the structure of
the absorbing molecule.
when E = hv
hv
E

Ground
State
Excited
State
•Absorban e (AU)
*
1
0.8
0.6
0.4
0.2
0
300 350 400 450 500 550
Wavelength (nm)
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Experimental Techniques
M1
Reference
sample
Beams from
Monochrometer
And Chopper
Photomultiplier
Absorpotion spectrometer
detector
Beam
combiner
sample
source
reference
Source-dispersing element-detector
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•
Foundamental absorption in
semiconductor
Direct transitions:
 ~ h  E g 1 2 for h  E g
 ~0
•
for h  E g
Indirect transitions:
A(h  E g  E p ) 2
 a (h ) 
Ep


exp

1


kT


Eg
for h  E g  E p
A(h  E g  E p ) 2 for
h  E g  E p
 e (h ) 
Ep 

1  exp

kT 

•
Absorption tail :
No absorption is to be expected below Eg . However many timesα decreases much
more slowly below the fundamental edge than suggested by theory .
Urbach repoted that the tail obeys a relationship d ln  d h  1 kT
    
Pankove found in GaAs that the slop of the absorption tail
varies with impurity concentration
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Photoluminescence
•Photoluminescence (PL) is the optical radiation emitted by a physical
system from excitation to a non-equilibrium distribution of electronic state
by irradiation with light.
•Three distinct processes are involved:
Creation of electron-hole (e-h) pairs by absorption of the excited light.
Radiative recombination of the e-h pairs.
Escape of the recombination radiation from the solid being studied
Ec
Ev
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Several Electronic Transitions in A
Semiconductor
Conduction band to acceptor level
Excitonic recombinations
Donor level to valence band
Donor level to acceptor level
Conduction band to valence band
e e
e
e
Ec
B-B
h
(a)
D-V C-A
h
(b)
h
(c)
Exciton energy
band
D-A
h
(d)
Ev
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Basic Experimental Setup
Laser
Lock-in Amp.
Chopper
Detector
Monochromater
Cryostat
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Why Photoluminescence ?
•Direct relation between PL and radiative devices: LED,
CRT’s, solid state lasers…
•Simplicity of experimental apparatus
Easy data collection.
No sample preparation required.
Contact-less\ non-destructive technique.
•PL Can Efficiently Measure the Following in Semiconductors
 Band:
– Gap
– Band offset
 Lattice:
– Alloy composition
– Crystallinity
– Stress
 Impurity and defect:
– Presence and type
– Concentration
 Microstructure:
– Surface behavior
– Interface behavior
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Thermalization effects
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Raman spectroscopy
•
Raman spectroscopy is based on the scattering of light (or photons) by
molecules
•
Rayleigh scattering is the result of elastic collisions between the photon
and the molecule
- energy of the photon is unchanged during the collision
Raman scattering is the result of inelastic collisions between the particles
- energy of the photon and its direction, are changed during collision with
the molecule
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Sir C. V. Raman discovered that the spectrum of scattered lines not only
consisted of the Rayleigh lines but a pattern of lines of shifted frequency –
the Raman spectrum
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Stokes V.S. Anti-Stokes Lines
•Raman bands at frequency less than the incident frequency,
(i-vib) are referred to as Stokes bands
•Those at frequency higher than the incident frequency,
(i+vib) are referred to as anti-Stokes bands
•Raman band is not only characterized by its absolute
wavenumber but by the magnitude of its wavenumber shift |D|
from the incident wavenumber
•In general the population of a higher state is less than that
of a lower state. Therefore more transitions from a lower
state to a higher state are expected, thereby causing greater
intensities in Stokes lines
Rayleigh
Stokes
anti-Stokes
hν
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Theory
•
Since light possesses an electric field and molecules are made up of positive
and negative charges, we can expect the molecule to become polarized in the
light – an induced dipole moment.
Ei  E0 cos(2i t )
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The degree to which a dipole,P, is induced by the electric field, E, is described
by the molecular polarizability, 
P=  E
 is related to how readily the electrons will move under the influence of an
electric field with respect to the nuclei of the molecule.
If the molecule undergoes some internal motion, by rotating or vibrating, the
polarizability will change as well, either in direction or magnitude, resulting in
a change in the induced electric dipole.
   0   vib cos(2vibt )
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The rate at which the polarizability changes is a direct measure of the motion
in the molecule.
This allows us to use Raman spectroscopy to study rotations and vibrations in
molecules that have induced electric dipoles, not permanent ones.
 vib E0  2cos 2 ( i  vib )t  cos 2 ( i  vib )t 
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Experimental
•A monochromatic light source (i.e. laser) is shined on a sample and the
scattered light is detected.
•Most of the scattered light will be a result of Rayleigh scattering, but a
small amount will be a result of Raman scattering
•Using Different Excitation Wavelength to Eliminate Fluorescence
Interference
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Summary
Spectroscopic methods are widely used for the determination
of molecular structure, the characterization of unknown
compounds…..
It is powerful, easy, and fast
針對樣品特性選擇適當的光譜技術是必須的
光譜技術已是很成熟的技術，並且廣為應用，同時具有
發展空間
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Reference
• V. Swaminathan and A. T. Macrander in ”Materials Aspects of GaAs
and InP Based Structures” p.264~340
• Sideney Perkowitz in “Optical Characterization of Semiconductors:
Infrared’ Raman’ and Photoluminescence Spectroscopy”
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