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Optical Characterizations of Semiconductors

Optical Characterizations of Semiconductors. Jennifer Weinberg-Wolf September 7 th , 2005. Raman Spectroscopy. Inelastic scattering process that measures vibrational energies. Probe phonon modes, electronic structure and the coupling of the e - -phonon states.

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Optical Characterizations of Semiconductors

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  1. Optical Characterizations of Semiconductors Jennifer Weinberg-Wolf September 7th, 2005

  2. Raman Spectroscopy • Inelastic scattering process that measures vibrational energies • Probe phonon modes, electronic structure and the coupling of the e--phonon states

  3. E.C.T. Harley and L.E. McNeil, J. Phys. Chem. Solids 65, 1711 (2004). L.E. McNeil et.al., J. Ap. Phys. 96 9, 5158 (2004). Loi, et. Al., Syn. Met. 116 321 (2001). 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.

  4. Spectrometer Detector Sample Dye Laser Ar+ laser 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/6-311+G(d,p)

  5. 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)

  6. Presenta: Sony Corp. Presentc: Norelco Futurea: Universal Display Corp. Presentb: IBM Presentc: CDT 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.

  7. Materials Development Hole Mobility cm2V-1s-1 rubrene Shaw, Seidler, IBM J. Res. & Dev. 45(1), 2001, 3-9. 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

  8. Single Crystal Facts: • Physical Vapor Growth • Orthorhombic crystal • D2h symmetry • 4 molecules per unit cell (280 atoms) • Close packed/herringbone arrangement • 2.21 eV room temp band gap • Mobility as high as (anisotropic) ~4 Å a = 26.901 Å b = 7.1872 Å c = 14.43 Å 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

  9. Structural Information: Tetracene and Rubrene Tetracene Rubrene Single Crystal Isolated Molecule

  10. 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 geometry—unobserved 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

  11. 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)

  12. 1.32 1.0 Anthracene Tetracene Naphthalene 1.84 2.1 4.24 1.3 Pentacene 5.37 2.2 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.

  13. 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).

  14. 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)

  15. Crystal: Molecule: Thiol unit: Typical Scale  mm Alpha-Hexathiophene (a6T) • Monoclinic crystal • C2h point group • 4 molecules per unit cell • Close packed/herringbone arrangement • Rigid Rod with <1° deviation from a plane • ~2.2 eV band gap • Macroscopic single crystals from Lucent Technologies PRB 59 10651, 1999.

  16. x Normal modes : Electronic transition freq. Photon frequency Oscillator strength tensor Width 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)

  17. * On Resonance ( l = 599.43 nm, 2.0683 eV) ex (b) * (a) * * * * * * * * * * * Off Resonance ( l = 602 nm, 2.059 eV) ex : Resonant Lines * Resonant Raman Spectra at 33K J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B 69 125202, March 2004.

  18. Ratio of Resonant Raman to Non-Resonant Raman Peak Heights (a) (b) DE 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.

  19. Frenkel Excitons • Energetics • Lowest Singlet Energy from literature: 2.3 eV* • Singlet-Triplet Energy Shift • Other organic crystals ~0.5 eV, here DES-T=0.23 eV • Davydov splitting energies • Singlet States: typically 100-1000’s cm-1 • From literature: DED= 0.32 eV** equals DED= 2580 cm-1 • Triplet States: typically 10’s cm-1 • In this experiment: 2 meV equals ED=16 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.

  20. 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

  21. 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 Temperature dependent probability of the crystal being in the initial state Width (lifetime) of exciton (intermediate states) also temperature dependent!! 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)

  23. Energy Level Diagram Excited States Continuum e- Luminescence Thermalization photon 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

  24. Photoluminescence

  25. Electronic States: Single Walled Carbon NanoTubes (SWNTs) (0,0) Ch = (10,5) If n-m=3N, 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

  26. S M S S SWNTs Kataura, et.al., Syn. Met. 103 2555, 1999. Ar+ 2.41 eV Dye: 2.16 to1.95 eV DE (eV) g0=2.90 eV metallic semiconducting

  27. 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)

  28. 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

  29. 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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