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Basic Principles of Raman

spectroscopy and its applications forsemiconductor characterization

 Kiril Kirilov

 Faculty of Physics, Sofia University, 5. blvd. J.Bourchier, 1164 Sofia,Bulgaria

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the Raman effect

predicted by Adolf Smekal in 1923

named after one of its discoverers

in 1928, the Indian scientist Sir 

Chandrasekhara Venkata Raman Raman scattering is the inelastic

scattering of a photon – change in

photon energy

By nature weak effect (approximately 1

in 107 photons)

Sir C. V. Raman

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 A Typical Setup (simplified)

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Schematic Raman spectrum

Rayleigh line – elastic scattering

Raman Stokes line – scattered

photon give up energy

Raman Anti-stokes line – scattered photon gain energy

   L   i  g   h   t   i  n   t  e  n

  s   i   t  y

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Schematic diagram of the process

Schematic diagram of the Raman scattering processThe vertical direction represents energy

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Classical Theory 

The electric field E i of the light wave acts on the charges in the

material

Interaction of light with a single molecule considered

Induced dipole moment Pi of a molecule (vector)

(1) 

 pi    – induced permanent dipole moment 

 ij    – polarizability (tensor)

i , j , k , l   – subscripts running over directions x , y , z 

Raman effect Hyper Raman effect

... k  jijk  jijii E  E  E  p P    

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Classical Theory 

Both pi and  ij maychange if the moleculevibrates

Then pi and  ij may beexpanded as Taylor series

(2)

qn  – generalized co-ordinates of normalmodes

 Assuming small atomic

displacements qn, we can

approximate the time

dependence

(3)

 n , L  – frequency of displac-

ement and electric field

These expressions can be

substituted into a linear version

of (1) … 

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Classical Theory 

IR absorptionRayleigh scattering

Raman scattering (Stokes & anti-Stokes)

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Sample Geometry 

“Brewster” 

Setup

Back 

Scattering

Forward

Scattering

Right-angle

Scattering

transparent

sample

non-transparent

sample

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Porto notation

Convention of representing experimental scatteringgeometries

Example: x(zx)y 

excitation light incident on the sample along x  axis,polarized along z direction

scattered light was detected along y axis,

polarized along x direction Useful notation if the axes are defined with respect

to the crystal axes of symmetry,then  x…z  relate to Raman tensor components

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In solid state physics 

spontaneous Raman spectroscopy is used to among other things,characterize materials:

measure temperature.

find the crystallographic orientation of a sample.

Determine Crystal stress: through E2

h mode

Determine carrier concentration: through A1(LO) mode and LPP- 

can be used to observe other low frequency excitations of the solid,such as plasmons, magnons, and superconducting gap excitations.

Obtain information on the population of a given phonon mode in theratio between the Stokes intensity and anti-Stokes intensity.

In nanotechnology, a Raman microscope can be used to analyzenanowires to better understand the composition of the structures.

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Determine Crystal stress: through E2h mode

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Crystal tension

Commonly employed for the analysis of the pressure

dependence of phonon modes is quadratic

relationship

ω0, ω’, ω” fitting parameters 

20 "' P  P      

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Crystal temperature

Non-invasively temperature monitoring (T=10-1275K) during growth, processing,

high power electronic devices It is used simple empirical relation to describe temperature dependence of the

phonon frequencies

Simple but accurate

Unfortunately the parameters from fitting can’t be related to properties of thematerial

More complex theoretical modeling provides this connection (Cui [96])

ω0, A, B – fitting parameters

Diamond, GaN[Liu], AlN E2(high) for AlGaN

1)/exp()(

0

0

T k  Bhc

 AT 

 B   

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Carriers concentration

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Crystal direction

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Raman setups at our faculty 

Micro-Raman, LabRam HR spectrometer , 20 mWpolarized vertically HeNe laser, spot size of about 1μm 

Two switcheable gratings Scanning range: 1800gr range:0-950 nm ,600gr range:0-

2850 nm

Accuracy: In the range between 450 nm and 850 nm, thewavenumber accuracy is± 1 cm-1 with 1800 l/mm grating

Objectives – x10, x20, x50, x100 Peltier cooled CCD1024x256

(T=-70oC)

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Doping and Plasmon-phonon coupled

modes

Наблюдаването на L+ и

L- модовете може да се

използва за определяне

на концентрацията на

свободните носители.

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Confocal microscopy  

very high spatial resolution: the lateral and depth resolutionswere 250 nm and 1.7 µm, respectively, using a confocal Ramanmicrospectrometer with the 632.8 nm line from a He-Ne laser  with a pinhole of 100 µm diameter 

much higher resulting photon flux than achieved in conventional

Raman setups. benefit of enhanced fluorescence quenching.

high photon flux can also cause sample degradation, and for thisreason some setups require a thermally conducting substrate.

Water does not generally interfere with Raman spectral

analysis.

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Surface enhanced Raman spectroscopy

(SERS)

a surface sensitive technique that

results in the enhancement of 

Raman scattering by molecules 

adsorbed on rough metal surfaces.

The enhancement factor can be as

much as 1014-1015, which allows

the technique to be sensitive

enough to detect singlemolecules.[1](Wikipedia)

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Resonance Raman spectroscopy 

The excitation wavelength is matched to an

electronic transition of the molecule or crystal, so

that vibrational modes associated with the excited

electronic state are greatly enhanced.

This is useful for studying large molecules such as

polypeptides, which might show hundreds of bands

in "conventional" Raman spectra. It is also useful for 

associating normal modes with their observedfrequency shifts.[10] 

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Other Raman techniques

Hyper Raman - A non-linear effect in which the vibrational modes interact with the secondharmonic of the excitation beam. This requires very high power, but allows the observationof vibrational modes which are normally "silent". It frequently relies on SERS-typeenhancement to boost the sensitivity.[11] 

Spontaneous Raman Spectroscopy - Used to study the temperature dependence of theRaman spectra of molecules.

Optical Tweezers Raman Spectroscopy (OTRS) - Used to study individual particles, andeven biochemical processes in single cells trapped by optical tweezers.

Spatially Offset Raman Spectroscopy (SORS) - The Raman scatter is collected fromregions laterally offset away from the excitation laser spot, leading to significantly lower contributions from the surface layer than with traditional Raman spectroscopy.[12] Thistechnique allows highly accurate chemical analysis of objects beneath obscuring surfaces,such as tissue, coatings and bottles. Examples of uses include analysis of: bone beneathskin,[2] tablets inside plastic bottles,[3] explosives inside containers[4] and counterfeittablets inside blister packs.

Tip-Enhanced Raman Spectroscopy (TERS) - Uses a silver or gold tip to enhance the

Raman signals of molecules situated in its vicinity. The spatial resolution is approximatelythe size of the tip apex (20-30 nm). TERS has been shown to have sensitivity down to thesingle molecule level.

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Important relations

][*77,8067][

][10][

1

71

eV  E cm

nmcm