optical characterizations of semiconductors jennifer weinberg-wolf september 7 th, 2005

Optical Characterizations of Semiconductors

Jennifer Weinberg-Wolf

September 7th, 2005

27 September 2005

► Inelastic scattering process that measures vibrational energies

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

Raman Spectroscopy

37 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 SWNTSiGe MOSFETs

Pressure Dep of 6T

Diamond Anvil Cell

Cs Intercalation of SWNT

47 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+

lase

r

DyeLaser

Sample

SpectrometerDetectorExperimental Setup

57 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)

67 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.

Presenta: Sony Corp.

Futurea: 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.

Presentb: IBM

Presentc: CDT

Presentc: Norelco

77 September 2005

Shaw, Seidler, IBM J. Res. & Dev. 45(1), 2001, 3-9.

Materials Development

Hol

e M

obili

ty c

m2 V

-1s-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

87 September 2005

RubreneMolecular Characteristics: Tetracene backbone C2h point group 102 active Raman modes HOMO/LUMO gap = 2.2 eV

Single Crystal Facts:o Physical Vapor Growtho Orthorhombic crystalo D2h symmetryo 4 molecules per unit cell (280 atoms)o Close packed/herringbone arrangemento 2.21 eV room temp band gapo Mobility as high as (anisotropic)

182hD

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 cm2V-1s-1

97 September 2005

Structural Information: Tetracene and Rubrene

Tetracene Rubrene

Single CrystalIsolated Molecule

107 September 2005

Raman of Rubrene – Single Crystal vs. Isolated Molecule

0

200

400

600

800

1000

1200

1400

1600

0 200 400 600 800 1000 1200 1400 1600Calculated Frequency (cm -1)

Slope = 0.998

R2 = 0.9997

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

117 September 2005







127 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 (cm2/V-s)

Highest measured hole mobilities (cm2/V-s)

1.0

2.1

1.3

2.2

Deng, et.al., J of Phys Chem B 108, 8614-8621, 2004.

~20 cm2/V-s

137 September 2005

Intermolecular Coupling

0 100 200 300 400 500

Calculated Modes

Observed Modes

Raman Shift (cm -1)50 100 150 200

300K20K

Raman Shift (cm-1)

< 4% max. change

50 100 150 200

Observed Modes Calculated Modes

Raman Shift (cm -1)

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

TetraceneRubrene

Weinberg-Wolf, McNeil, Liu and Kloc, submited to Phys. Rev B (April 2005).

147 September 2005







157 September 2005

• 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

Typical Scale mm

Alpha-Hexathiophene (T)

S

Thiol unit:

Crystal:Molecule:

PRB 59 10651, 1999.

167 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)

*

2

420

4

ˆ~

ˆ4

isS ee

cd

d

.~

~22

2

consti

Fme

o

and

o

o

o

o i

FmeF

i

me222

2

22

2 ~2

~~:o

:~F

:

:

:Electronic transition freq.Photon frequencyOscillator strength tensorWidth

Normal modes

177 September 2005

Resonant Raman Spectra at 33KOn 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.

187 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

(a)

(b)


197 September 2005

EnergeticsLowest Singlet Energy from literature: 2.3 eV*

Singlet-Triplet Energy Shift Other organic crystals ~0.5 eV, here ES-T=0.23 eV

Davydov splitting energies Singlet States: typically 100-1000’s cm-1

From literature: ED= 0.32 eV** equals ED= 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.

Frenkel Excitons

*: Frolov et al. PRB 63 2001, 205203**: J. Chem. Phys 109 10513, 1998.***: PRB 59 10651, 1999.


207 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

VT

PT

PT

V

V

1

TP

V

V

1 is the compressibility

Where is the expansivity

and

TP

-

Temperature (K)0 50 100 150 200 250 300

Pea

k Po

siti

on (

cm -1

)

255

265

275

285

1455

1465

1475

~0.3% drop

~0.3% drop

~1.6% drop

217 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

1

1

TkBen

0

2

, 00

2 0HHH0F si

nn nninni

ieRioneseR

iEEiEE

nnnn

energy state teintermedia:energy state initial:

phonon:photon scattered:

photonincident :

0

nn

s

i

EE

,

•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


18K

55K

227 September 2005







237 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-

photon

Thermalization

Continuum

Energy Level Diagram

Luminescence

exciton

247 September 2005

Photoluminescence

257 September 2005

Electronic States: Single Walled Carbon NanoTubes (SWNTs)

(0,0)

Ch = (10,5)

http://www.photon.t.u-tokyo.ac.jp/~maruyama

If n-m=3N, then the tube is metallic,otherwise it is semiconducting

C na m a n mh

1 2 ( , )

d a m m n nt c c ( ) ( )3

12

2 2 12

tan 1

123

2

m

n m

Rao et al., Science 275, 187 (1997).

267 September 2005

SWNTsAr+ 2.41 eV

Dye: 2.16 to1.95 eV

0=2.90 eV

E (

eV)

Kataura, et.al., Syn. Met. 103 2555, 1999.

SS

SM

metallic

semiconducting

277 September 2005







287 September 2005

Structure dependence on Hydrogen dilution ratio

Han, Lorentzen, Weinberg-Wolf and McNeil J. of Applied Phys, 94 2930, 2003

Crystalline volume fraction40%

Crystalline volume fraction65%

297 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

optical characterizations of semiconductors jennifer weinberg-wolf september 7 th, 2005

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