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

2. 2 7 September 2005 ► Inelastic scattering process that measures vibrational energies ► Probe phonon modes, electronic structure and the coupling of the e - -phonon states Raman Spectroscopy

3. 3 7 September 2005 Loi, et. Al., Syn. Met. 116 321 (2001). Raman Spectroscopy 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 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). Lin, Öztürk, Misra, Weinberg-Wolf and McNeil, MRS Spring 2005. Temperature Dep of SWNT SiGe MOSFETs Pressure Dep of  6T Diamond Anvil Cell Cs Intercalation of SWNT

4. 4 7 September 2005  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) Ar + laser Dye Laser Sample Spectrometer Detector Experimental Setup

5. 5 7 September 2005 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  crystalline Si)

6. 6 7 September 2005 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. Present a : Sony Corp. Future a : Universal Display Corp. 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. Present b : IBM Present c : CDT Present c : Norelco

7. 7 7 September 2005 Shaw, Seidler, IBM J. Res. & Dev. 45(1), 2001, 3-9. Materials Development Hole Mobility cm 2 V -1 s -1 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 rubrene

8. 8 7 September 2005 Rubrene Molecular Characteristics:  Tetracene backbone  C 2h point group  102 active Raman modes  HOMO/LUMO gap = 2.2 eV Single Crystal Facts: o Physical Vapor Growth o Orthorhombic crystal o D 2h symmetry o 4 molecules per unit cell (280 atoms) o Close packed/herringbone arrangement o 2.21 eV room temp band gap o Mobility as high as (anisotropic) a = 26.901 Å b = 7.1872 Å c = 14.43 Å ~4 Å Devices:  ~100% Photoluminescence Yield  Common dopant in emitting and transport layers of current OLEDs 20 cm 2 V -1 s -1

9. 9 7 September 2005 Structural Information: Tetracene and Rubrene Tetracene Rubrene Single Crystal Isolated Molecule

10. 10 7 September 2005 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 A g and B 2g 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. 11 7 September 2005 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  crystalline Si)

12. 12 7 September 2005 Raman of Rubrene – Device Characteristics Most FET measurements complicated by possible surface layer (peroxide) Raman measures the bulk properties of the material Naphthalene Anthracene Tetracene Pentacene 1.32 1.84 4.24 5.37 Calculated hole mobilities (cm 2 /V-s) Highest measured hole mobilities (cm 2 /V-s) 1.0 2.1 1.3 2.2 Deng, et.al., J of Phys Chem B 108, 8614-8621, 2004. ~20 cm 2 /V-s

13. 13 7 September 2005 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  -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. 14 7 September 2005 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  crystalline Si)

15. 15 7 September 2005 Monoclinic crystal C 2h 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 Typical Scale  mm Alpha-Hexathiophene (  T) Thiol unit: Crystal: Molecule: PRB 59 10651, 1999.

16. 16 7 September 2005 Electron-phonon Coupling: Resonant Raman Spectroscopy 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) and :  Electronic transition freq. Photon frequency Oscillator strength tensor Width Normal modes

17. 17 7 September 2005 Resonant Raman Spectra at 33K On Resonance ( ex = 599.43 nm, 2.0683 eV) Off Resonance ( ex = 602 nm, 2.059 eV) ** * * * * * * * ** * * : Resonant Lines * (a) (b) J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B 69 125202, March 2004.

18. 18 7 September 2005 Exciton Identification Resonance peaks at excitation energies of 2.066 eV and 2.068 eV. Each peak has a FWHM of 2 meV. Ratio of Resonant Raman to Non-Resonant Raman Peak Heights EE (a) (b) J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B 69 125202, March 2004.

19. 19 7 September 2005 Energetics  Lowest Singlet Energy from literature: 2.3 eV *  Singlet-Triplet Energy Shift Other organic crystals ~0.5 eV, here  E S-T =0.23 eV  Davydov splitting energies Singlet States: typically 100-1000’s cm -1  From literature:  E D = 0.32 eV ** equals  E D = 2580 cm -1 Triplet States: typically 10’s cm -1  In this experiment: 2 meV equals E D =16 cm -1 Or – two binding sites of a singlet exciton  Singlet binding energy of ~0.5 eV *** from in literature. Frenkel Excitons *: Frolov et al. PRB 63 2001, 205203 **: J. Chem. Phys 109 10513, 1998. ***: PRB 59 10651, 1999. J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B 69 125202, March 2004.

20. 20 7 September 2005 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 is the compressibility Whereis the expansivity and -

21. 21 7 September 2005 Electron-phonon Coupling: Temperature effects Width (lifetime) of exciton (intermediate states) also temperature dependent!! Temperature dependent probability of the crystal being in the initial state 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 Increasing Temperature J.R. Weinberg-Wolf and L.E. McNeil Phys. Rev. B 69 125202, March 2004. 18K 55K

22. 22 7 September 2005 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  crystalline Si)

23. 23 7 September 2005 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 Excited States e-e- photon Thermalization Continuum Energy Level Diagram Luminescence exciton

24. 24 7 September 2005 Photoluminescence

25. 25 7 September 2005 Electronic States: Single Walled Carbon NanoTubes (SWNTs) (0,0) C h = (10,5) http://www.photon.t.u-tokyo.ac.jp/~maruyama If n-m=3N, then the tube is metallic, otherwise it is semiconducting Rao et al., Science 275, 187 (1997).

26. 26 7 September 2005 SWNTs Ar + 2.41 eV Dye: 2.16 to1.95 eV  0 =2.90 eV  E (eV) Kataura, et.al., Syn. Met. 103 2555, 1999. S S S M metallic semiconducting

27. 27 7 September 2005 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  crystalline Si)

28. 28 7 September 2005 Structure dependence on Hydrogen dilution ratio Han, Lorentzen, Weinberg-Wolf and McNeil J. of Applied Phys, 94 2930, 2003 Crystalline volume fraction 40% Crystalline volume fraction 65%

29. 29 7 September 2005 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

30. 30 7 September 2005 Notes

31. 31 7 September 2005 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

32. 32 7 September 2005 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)

33. 33 7 September 2005 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

34. 34 7 September 2005 Grüneisen Parameter An exponent that tells how  i 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:  =0.98 In solid noble gasses:  =2.5 to 2.7

35. 35 7 September 2005 Pressure Experimental Details: DAC The sample and a ruby chip for calibration are suspended in a 4:1 methanol:ethanol 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

36. 36 7 September 2005 Structural Disorder: Irradiated GeSi Previously shown by Birtcher, Grimsditch, and McNeil *  Dose of approximately 10 14 ions/cm 2 for 3.5-MeV Kr + ions will cause crystalline germanium to become amorphous  Dose of approximately 10 16 ions/cm 2 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 Ge 1-x Si x ? *: Phys. Rev. B, 50, 8990 (1994)

37. 37 7 September 2005 Structural Disorder: Solar Cell Materials Crystalline Si Microcrystalline Si Amorphous Si where Raman Intensity (arbitrary units)

38. 38 7 September 2005 Longitudinal WaveShear Wave Raman and Brillouin Scattering Raman 0% 5% 2.3% Si Concentration 1E15 ions/cm 2 1E16 ions/cm 2 5E16 ions/cm 2 Kr + irradiation dose Brillouin McNeil, et al., Phil. Mag. Lett 84, 93 (2004)