Basic Principles of Raman spectroscopy and its applications for semiconductor characterization

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Basic Principles of Raman spectroscopy and its
applications for semiconductor 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

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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 Ei 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

Hyper Raman effect
Raman effect
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Classical Theory

• Both pi and ?ij may change if the molecule
  vibrates
• Then pi and ?ij may be expanded as Taylor
  series(2)qn generalized co-ordinates of
  normal modes

• 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
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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
  scattering geometries
• 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 xz
  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 E2h 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 the ratio between the Stokes
  intensity and anti-Stokes intensity.
• In nanotechnology, a Raman microscope can be used
  to analyze nanowires 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
• ?0, ?, ? fitting parameters

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Crystal temperature

• Non-invasively temperature monitoring
  (T10-1275K) 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 cant
  be related to properties of the material
• More complex theoretical modeling provides this
  connection (Cui 96)
• ?0, A, B fitting parameters
• Diamond, GaNLiu, AlN
• E2(high) for AlGaN

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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 mW
  polarized vertically HeNe laser, spot size of
  about 1 µm
• Two switcheable gratings
• Scanning range 1800gr range0-950 nm ,600gr
  range0-2850 nm
• Accuracy In the range between 450 nm and 850 nm,
  the wavenumber 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
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Confocal microscopy

• very high spatial resolution the lateral and
  depth resolutions were 250 nm and 1.7 µm,
  respectively, using a confocal Raman
  microspectrometer 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 this reason 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 single
  molecules.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
  observed frequency shifts.10

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Other Raman techniques

• Hyper Raman - A non-linear effect in which the
  vibrational modes interact with the second
  harmonic of the excitation beam. This requires
  very high power, but allows the observation of
  vibrational modes which are normally "silent". It
  frequently relies on SERS-type enhancement to
  boost the sensitivity.11
• Spontaneous Raman Spectroscopy - Used to study
  the temperature dependence of the Raman spectra
  of molecules.
• Optical Tweezers Raman Spectroscopy (OTRS) - Used
  to study individual particles, and even
  biochemical processes in single cells trapped by
  optical tweezers.
• Spatially Offset Raman Spectroscopy (SORS) - The
  Raman scatter is collected from regions laterally
  offset away from the excitation laser spot,
  leading to significantly lower contributions from
  the surface layer than with traditional Raman
  spectroscopy.12 This technique allows highly
  accurate chemical analysis of objects beneath
  obscuring surfaces, such as tissue, coatings and
  bottles. Examples of uses include analysis of
  bone beneath skin,2 tablets inside plastic
  bottles,3 explosives inside containers4 and
  counterfeit tablets 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 approximately the size of
  the tip apex (20-30 nm). TERS has been shown to
  have sensitivity down to the single molecule
  level.

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Important relations