1. Introduction to laser spectroscopy techniques
黃建智
Department of Physics
Nation Taiwan University

2. Outline What is spectroscopy ? Absorption Photoluminescense
Raman spectroscopy
Summary
Reference

3. Extreme Ultraviolet (EUV)
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 UV
VUV
Extreme Ultraviolet (EUV)
Soft X-rays (SXR)
Hard X-rays
Photon energy
Wavelength
1eV
10eV
100eV
1keV
10keV
10-2eV
10-4eV
10-6eV
MICROWAVE
RADIO 1m
100nm
10nm
1nm
100m
100cm
10m IR
Molecul-ar
Vibrations
Finestructure
Outer
Electron
Trans
0.1nm

4. What Instruments Are Needed?
Radiation source :
Analyzer
Detector

5. Radiation Source and Detector
Infrared Nernst filament
(black body irradiation)
Visible Tungsten-halogen lamp
(350 – 5000 nm)
UV Deuterium discharge lamp
(190 – 350 nm)
Xenon discharge lamp
(< 190 nm)
Lasers UV-Vis-NIR
Infrared :Semiconductors
(HgCdTe, InSb, PbS, InGaAs)
Visible & UV :
Photomultiplier: PMT
Photodiode: Si
Diode array detector
Charge coupled device CCD

6. Optics of Spectrometer
Constructive interference:
n = 1, 2, … is the diffraction order
d is distance between the grooves
Resolution depends on :
1.grooves/mm in relation to λ
2. Length of monochromator
3. Slit width

7. Reflection\ Scattering
Types of Interaction
Reflection\ Scattering
Absorption Po P d
rayleigh
Luminescence

8. Absorption Ground State Excited State
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.
Ground State
Excited State * hv  E
when E = hv
Wavelength (nm)
300
350
400
450
500
550
Absorban e (AU)
0.2
0.4
0.6
0.8 1

9. Experimental Techniques
Beams from
Monochrometer
And Chopper
Reference
sample
Photomultiplier
Absorpotion spectrometer
detector
Beam combiner
sample
source
reference
Source-dispersing element-detector

10. Foundamental absorption in semiconductor
Direct transitions: Eg
for
Indirect transitions:
for
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
Pankove found in GaAs that the slop of the absorption tail varies with impurity concentration

11. Photoluminescence P/NSTL@NTU
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

12. Several Electronic Transitions in A Semiconductor
Conduction band to acceptor level
Donor level to valence band
Donor level to acceptor level
Conduction band to valence band
Excitonic recombinations
Exciton energy band
B-B
D-V
C-A
D-A Ec Ev h e
(a)
(b)
(d)
(c)

13. Basic Experimental Setup
Laser
Lock-in Amp.
Chopper
Detector
Monochromater
Cryostat

14. 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

15. Thermalization effects

16. Raman spectroscopy P/NSTL@NTU
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
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

17. Stokes V.S. Anti-Stokes Lines
Raman bands at frequency less than the incident frequency, (ni-nvib) are referred to as Stokes bands
Those at frequency higher than the incident frequency, (ni+nvib) 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 |Dn| 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

18. 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.
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.
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.

19. Experimental P/NSTL@NTU
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

20. 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

21. 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”