Optical Characterizations of Semiconductors

1
Optical Characterizations of Semiconductors

• Jennifer Weinberg-Wolf
• September 7th, 2005

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Raman Spectroscopy

• Inelastic scattering process that measures
  vibrational energies

• Probe phonon modes, electronic structure and the
  coupling of the e--phonon states

3
Raman Spectroscopy
Pressure Dep of a6T
SiGe MOSFETs
Cs Intercalation of SWNT
Temperature Dep of SWNT

• Learn about materials in a wide variety of
  environments
• Temperature
• Strain
• Pressure
• In-Situ Reactions
• Non-invasive, non-destructive probe
• Measure samples in many different forms
• Single crystal, polycrystalline, amorphous,
  powder, solution
• Multiphase samples

Diamond Anvil Cell
Lin, Öztürk, Misra, Weinberg-Wolf and McNeil, MRS
Spring 2005.
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Experimental Setup

• Raman Spectroscopy Single Crystals
• Spectra-Physics Ar pump laser
• Continuously tunable Spectra-Physics dye laser
• Kiton Red dye 608 to 655 nm (2.04 to 1.89 eV)
• Rhodamine 6G dye 590 to 640 nm (2.1 to 1.93 eV)
• Dilor XY Triple monochromator
• LN2 cooled CCD Detector
• Photoluminescence Spectroscopy Single Crystals
• Dilor 1403 double monochromator
• PMT detector
• Theoretical Simulations Single Molecule
• Software Gaussian 03 C02 SMP
• Machine SGI Origin 3800, 64 CPUs, 128 GB mem w/
  IRIX 6.5 OS
• Structure Optimization HF/6-31G9(d)
• Frequency Calculation DFT B3LYP/6-311G(d,p)

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Outline of talk

• Basic structural information
• Tetracene
• 5,6,11,12-tetraphenyl tetracene (Rubrene)
• Vibrational coupling
• Intermolecular Modes of Rubrene
• Electron-phonon coupling
• Alpha-hexathiophene resonance modes
• Investigation of Electronic States
• Organic Semiconductors (Rubrene)
• Single Walled Nanotubes
• Structural Disorder
• Solar cell materials (amorphous and mcrystalline
  Si)

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Why Organics?

• Cheap(er)
• Easily Processed
• Environmentally Friendly
• Flexible
• Low power consumption
• Chemically tailor molecules
• Tunable white light
• Some materials used
• Oligoacenes, Oligothiophenes, Polyphenylene
  Vinylene (PPV), etc.
• Devices made so far
• OFETS, OLEDS, Photovoltaic devices, etc.

a Forrest, Nature 428, 2004, 911-918. b
Dimitrakopoulos, IBM J. Res. Dev. 45(1), 2001,
11-27. c Borchardt, Materials Today, 7(9), 2004,
42-46.
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Vibrational spectra of organic semiconductors
Why use Raman?

• Fundamental understanding of the relationship
  between structural and electronic properties is
  limited by the availability of high quality
  single crystals
• Optical measurements can give insight into
  important materials properties
• Measured device characteristics may not reflect
  bulk material properties

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Rubrene

• Molecular Characteristics
• Tetracene backbone
• C2h point group
• 102 active Raman modes
• HOMO/LUMO gap 2.2 eV

• Devices
• 100 Photoluminescence Yield
• Common dopant in emitting and transport layers
  of current OLEDs

20 cm2V-1s-1
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Structural Information Tetracene and Rubrene
Tetracene
Rubrene
Single Crystal Isolated Molecule
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Raman of Rubrene Single Crystal vs. Isolated
Molecule

• 20 of the 25 highest-intensity modes from the
  single-molecule calculation appear in the
  measured crystal spectrum

• Only Ag and B2g modes are allowed in
  backscattering geometryunobserved modes
  presumably belong to different symmetry

• Higher-energy observed modes are all within 2
  of calculated frequencies

• Can use the calculated spectrum to describe the
  vibrations of the single crystal

http//www.physics.unc.edu/project/mcneil/jweinber
/anim.php
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Outline of talk

• Basic structural information
• Tetracene
• 5,6,11,12-tetraphenyl tetracene (Rubrene)
• Vibrational coupling
• Intermolecular Modes of Rubrene
• Electron-phonon coupling
• Alpha-hexathiophene resonance modes
• Investigation of Electronic States
• Organic Semiconductors (Rubrene)
• Single Walled Nanotubes
• Structural Disorder
• Solar cell materials (amorphous and mcrystalline
  Si)

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Raman of Rubrene Device Characteristics

• Most FET measurements complicated by possible
  surface layer (peroxide)
• Raman measures the bulk properties of the material

20 cm2/V-s
Calculated hole mobilities (cm2/V-s)
Highest measured hole mobilities (cm2/V-s)
Deng, et.al., J of Phys Chem B 108, 8614-8621,
2004.
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Intermolecular Coupling

• No observed intermolecular modes!!
• Raman at low temperature confirms this.
• Low intermolecular coupling makes origin of high
  mobility unclear
• Fewer intermolecular phonons to scatter carriers
• But low p-electron overlap (resulting from low
  packing density) usually leads to low mobility

Tetracene
Rubrene
Weinberg-Wolf, McNeil, Liu and Kloc, submited to
Phys. Rev B (April 2005).
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Outline of talk

• Basic structural information
• Tetracene
• 5,6,11,12-tetraphenyl tetracene (Rubrene)
• Vibrational coupling
• Intermolecular Modes of Rubrene
• Electron-phonon coupling
• Alpha-hexathiophene resonance modes
• Investigation of Electronic States
• Organic Semiconductors (Rubrene)
• Single Walled Nanotubes
• Structural Disorder
• Solar cell materials (amorphous and mcrystalline
  Si)

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Alpha-Hexathiophene (a6T)

• Monoclinic crystal
• C2h point group
• 4 molecules per unit cell
• Close packed/herringbone arrangement
• Rigid Rod with lt1 deviation from a plane
• 2.2 eV band gap

• Macroscopic single crystals from Lucent
  Technologies

PRB 59 10651, 1999.
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Electron-phonon Coupling Resonant Raman
Spectroscopy
and

• Coupling of the electronic and phonon states
• Electronic state has the same symmetry as the
  vibrational state
• Large enhancement of the vibrational term
• Also changes the lineshape of the Raman signal
  (no longer symmetric Lorentzian distribution)

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Resonant Raman Spectra at 33K
J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B
69 125202, March 2004.
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Exciton Identification

• Resonance peaks at excitation energies of
  2.066 eV and 2.068 eV.
• Each peak has a FWHM of 2 meV.

J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B
69 125202, March 2004.
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Frenkel Excitons

• Energetics
• Lowest Singlet Energy from literature 2.3 eV
• Singlet-Triplet Energy Shift
• Other organic crystals 0.5 eV, here DES-T0.23
  eV
• Davydov splitting energies
• Singlet States typically 100-1000s cm-1
• From literature DED 0.32 eV equals DED 2580
  cm-1
• Triplet States typically 10s cm-1
• In this experiment 2 meV equals ED16 cm-1
• Or two binding sites of a singlet exciton
• Singlet binding energy of 0.5 eV from in
  literature.

J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B
69 125202, March 2004.
Frolov et al. PRB 63 2001, 205203 J. Chem.
Phys 109 10513, 1998. PRB 59 10651, 1999.
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Temperature effects on Molecular Crystals
vibrations

• Explicit Effect
• First term change in phonon occupation numbers
• Implicit Effect
• Second term change in interatomic spacing with
  thermal expansion or contraction

-
Where
is the expansivity
and
is the compressibility
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18K
Electron-phonon Coupling Temperature effects
Increasing Temperature

• Quenching is direct link to the lifetime of the
  exciton
• Can measure the binding energy of the triplet
  exciton or the binding energy of the trap

55K
Width (lifetime) of exciton (intermediate states)
also temperature dependent!!
Temperature dependent probability of the crystal
being in the initial state
J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B
69 125202, March 2004.
22
Outline of talk

• Basic structural information
• Tetracene
• 5,6,11,12-tetraphenyl tetracene (Rubrene)
• Vibrational coupling
• Intermolecular Modes of Rubrene
• Electron-phonon coupling
• Alpha-hexathiophene resonance modes
• Investigation of Electronic States
• Organic Semiconductors (Rubrene)
• Single Walled Nanotubes
• Structural Disorder
• Solar cell materials (amorphous and mcrystalline
  Si)

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Photoluminescence Spectroscopy Direct measure of
electronic states

• Electrons are excited optically, relax and then
  return to their ground state by the emission of
  light
• Can probe low-lying electronic states and any
  associated vibronic side bands

exciton
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Photoluminescence
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Electronic States Single Walled Carbon NanoTubes
(SWNTs)
(0,0)
Ch (10,5)
If n-m3N, then the tube is metallic, otherwise
it is semiconducting
Rao et al., Science 275, 187 (1997).
http//www.photon.t.u-tokyo.ac.jp/maruyama
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SWNTs
Kataura, et.al., Syn. Met. 103 2555, 1999.
Ar 2.41 eV
Dye 2.16 to1.95 eV
DE (eV)
g02.90 eV
metallic
semiconducting
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Outline of talk

• Basic structural information
• Tetracene
• 5,6,11,12-tetraphenyl tetracene (Rubrene)
• Vibrational coupling
• Intermolecular Modes of Rubrene
• Electron-phonon coupling
• Alpha-hexathiophene resonance modes
• Investigation of Electronic States
• Organic Semiconductors (Rubrene)
• Single Walled Nanotubes
• Structural Disorder
• Solar cell materials (amorphous and mcrystalline
  Si)

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Structure dependence on Hydrogen dilution ratio
Crystalline volume fraction 40
Crystalline volume fraction 65
Han, Lorentzen, Weinberg-Wolf and McNeil J. of
Applied Phys, 94 2930, 2003
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Conclusions

• Can use optical techniques to answer a variety of
  questions
• Raman tells more than just the vibrational
  structure of a material
• Experiments in a variety of environments
• Samples in a variety of phases

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Notes
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Raman of Rubrene Crystal Quality

• Good check of growth process
• Multiple crystallites from a single growth run,
  multiple scans of a single crystallite
• All spectra are substantively the same

Yield is very pure, unstrained homogeneous
rubrene crystals
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Raman dependence on polarization

• Will identify the symmetry of the resonant
  vibrational modes
• Will identify symmetry of electronic excitations
  that are resonant with the vibrational modes
• Could confirm or refute other groups
  identification of non-resonant Raman lines (since
  their theory is not perfect in light of my data)

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Pressure Effects

• Pressure can cause
• frequency shifts of elementary excitations
• line-shape changes
• selection rule changes accompanying phase
  transitions
• pressure-tuned resonant Raman scattering
• Pressure selectively enhances effects that are
  specifically associated with interactions between
  molecules
• Effective probe of intermolecular interactions
• In benzene, 25 kbars of pressure doubles
  intermolecular mode frequencies
• Pressure effectively makes a molecular crystal
  less molecular because it closes the gap between
  intermolecular and intramolecular mode
  frequencies

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Grüneisen Parameter

• An exponent that tells how wi scales with volume.
• The mode-Grüneisen parameter connects the volume
  dilation with each fractional change in phonon
  frequency.
• In the Grüneisen approximation, all the mode
  parameters are assumed equal. ONLY for
  intermolecular modes
• In Si g0.98
• In solid noble gasses g2.5 to 2.7

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Pressure Experimental Details DAC

• The sample and a ruby chip for calibration are
  suspended in a 41 methanolethanol liquid.
• As the system is compressed, the diamonds
  compress the inconel gasket that, as it decreases
  the hole size, imparts pressure on the liquid
  medium.
• As long as the sample is not touching the sides
  of the gasket, the imparted pressure should be
  hydrostatic.
• Possible to attain pressures up to 70 kbar with
  this setup

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Structural Disorder Irradiated GeSi

• Previously shown by Birtcher, Grimsditch, and
  McNeil
• Dose of approximately 1014 ions/cm2 for 3.5-MeV
  Kr ions will cause crystalline germanium to
  become amorphous
• Dose of approximately 1016 ions/cm2 produces a
  second structural transformation
• Small (50-nm) cavities form in the Ge
• Amorphous Ge becomes a sponge-like material
• Second transformation requires higher doses of
  Kr in Si
• What happens to Kr irradiated crystalline
  Ge1-xSix?

Phys. Rev. B, 50, 8990 (1994)
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Structural Disorder Solar Cell Materials
where
Raman Intensity (arbitrary units)
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Raman and Brillouin Scattering
Raman
Brillouin
McNeil, et al., Phil. Mag. Lett 84, 93 (2004)