Stanford Synchrotron Radiation Lightsource

Yet More Thin Film X-ray Scattering (Highly Textured) plus

X-ray ReflectivityMike Toney, SSRL

1. Introduction (real space – reciprocal space)2. Highly textured film: pentacene3. X-ray Reflectivity4. Summary

• how do you get diffraction data from thin films• how to choose what to do (what beam line & scans)• what do you learn

Real and Reciprocal Space

“powder” film(polycrystalline)

spheres

textured film

rings width = few degs

Highly Textured FilmReal space Reciprocal space

ringswidth < 1 deg

continuum in texture• distinguish highly textured (small molecules) from textured (metals, oxides, polymers)

Thin Film Scattering: what do I do?

•what beam line? (2-1, 7-2, 11-3)• area vs point detector, nature of sample• energy & flux

•what scans? (“where” in reciprocal space)• what do you want to learn?• what kind of sample (epitaxial, textured)?

phase identificationlattice parametersdefectstexture crystallite sizeatomic structure

Thin Film Scattering

2θ

Q

specular (θ/2θ): 2-1gixs (high resolution): 7-2gixs (area –low res): 11-3

2-1 11-3 7-2

Organic Thin Film TransistorsOrganic FET

Sandra Fritz,C. Daniel Frisbie,Mike Ward,Chemical Engineering and Materials ScienceUniversity of Minnesota

Organic Thin Films

Structure (molecular packing; defects) => Transport

Organic FET

• charge transport in first few layers• what is crystal structure (packing of molecules)?

• changes in structure with thickness• crystallite size & defects

Organic EvaporatorBell Jar

Heating/CoolingSubstrate

Shadow MasksShutter

Source Material

Vacuum Pump

Pentacene on a-SiO2• thermal evaporation at ca 50 C

Ruiz, et al. Appl. Phys. Lett. 85, 4926 (2004)Yang, et al. JACS 127, 11542 (2005)Mayer, et al. PRB 73, 205307 (2006)

Bulk Pentacene

• Bulk Single Crystal Structure: triclinica = 6.266(1) Åb = 7.775(1) Åc = 14.530(1) Åα = 76.475(4)o

β = 87.684(4)o

γ = 84.684(4)o

d001 = 14.1 Å• Charge transport is highly anisotropic

D. Holmes et al. Chem. Eur. J. 1999, 5, 3399C. C. Mattheus et al. Acta Cryst. C 2001, C57, 939T. Siegrist et al. Angew. Chem. 2001, 40, 1732R. B. Campbell and J. M. Robertson Acta Cryst. 1962, 15, 289R. B. Campbell et al. Acta Cryst. 1961, 14, 705

Specular X-ray ScatteringSpecular: probes planes orientedparallel to surface

Beam line 2-12-circle diffractometer

2θ

incident k scatteredk’

Q

Q = k’ – k = (4π/λ) sin θperpendicular to surface

incidentscattered

sample

Q

Pentacene Thin FilmsPentacene on a-SiO2• substrate induced “thin-film” and

“bulk-like” phases• film is highly textured (aligned out-of

plane (001) and powder in-plane)

rocking curve width <0.005o

=> highly textured films

“bulk” phased001 = 14.4Å

“thin film” phased001 = 15.4Å

thin film

bulk-like

Specular (2-1): 65 nm film

300 nm SiO2

Si

1.5 – 60 nm pentacene

Grazing Incidence Scattering

Q = k’ – k

• α = small, limits penetration into substrate

• β = large, Qz not small

α β

k’Q

nk

QxyQz

Qxy

Qz

SiO2

pentacene

300 nm SiO2

Si

pentacene

Grazing Incidence Diffraction

X-rays

scattered X-rays

Beam line 11-3image plate detection

Beam line 7-2point detection

Q = k’ – k

Pentacene Thin Films

b*

c*

a*

(-200)

Qxy

(200)(-110)

(-1-10)

(110)

(1-10)

b*

c*

a*

(-2 0 0)

} c*Qz

Qxy

(2 0 0)

(2 0 1)

(2 0 2)

(-2 0 1)

(-2 0 2)

(-2 0 3)

2a*-2a*

} c*Qz

Qxy

(2 0 0)

(2 0 1)

(2 0 2)

(-2 0 1)

(-2 0 2)

(-2 0 3)

2a*

Qz

Qxy

(-2 0 1)

(-2 0 2)

} c*

(-2 0 0)

Pentacene Thin Films

b*

c*

a*

Qz

Qxy

(1-1 0)

(1-1 1)

(1-1 2)

(-11 1)

(-11 2)

(-11 3)

-a* + b*a* - b*

} c*Qz

Qxy

(-1-1 0)

(-1-1 1)

(-1-1 2)

(111)

(112)

(113)

a* + b* -a* - b*

Qz

Qxy

(11 L)&

(1-1 L) } c*

(0 -2 L)

(12 L)&

(1-2 L)

Pentacene Thin Films

Q

20 nm film

• quick• complete diffraction pattern => index peaks• rough lattice parameters • misses details (poor resolution)• weak peaks hard to detect Qxy

Qz

(1-10)(-1-10)

(-111)

(1-11)

(-1-11)

(021)(0-20)

(0-21)(0-22)

(121)(-200)

(-121)

Pentacene Thin Films (20 nm)

(0-20)(021)

(022) (0-21)

c* = 0.408 Å-1

Qxy

Qz

specular

c* = 0.408 Å-1

20 nm

10x

(11L)

(02L)

(12L)

(20L)

(21L)

(13L)(22L)

(14L)

(23L)(31L)

(24L)

Pentacene Thin Film (20 nm)Scan Qxy at Qz peak position for (1 ±1 L)

splitting for (11L) & (1-1L)splitting for (12L) & (1-2L)no splitting for (02L) & (20L)γ ≠ 90 in thin-film phasecell is triclinic

Qxy

Qz

Fritz et al., unpublished

(1-10)(-111)(-112)

(-1-10)(-1-11)

(1-10)(-1-10)

(-111)

(1-11)

(-1-11)

(000)(10L)

(0 -1L)

(-10L)

(0 -2L)

a*

b*(1 -1L)(-1 -1L)

Pentacene Thin Films (20 nm)Scan Qz at Qxy peaks

Qxy

Qz

00.10.20.30.40.50.60.70.80.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Qz (Å-1)

Inte

nsity

(1 -1 L)(-1 -1 L)

a = 5.920 Å, b = 7.556 Å, c = 15.54 Åα = 81.6 deg, β = 87.2 deg, γ = 89.84 deg

Thin film: triclinic unit cell:

b

c

aβ α

γsee Merlo et al. J. Am. Chem. Soc. 127,3997 (2005) for details.

Pentacene Thin Films (20 nm)

Thin Film: herringbone motif: molecules tilted ≈ 10 degherringbone angle ≈ 50-60 deg

a

b

γ

a = 5.920 Åb = 7.556 Åc = 15.54 Åα = 81.6 degβ = 87.2 degγ = 89.84 deg

Pentacene Peak Widths

0.007 Å-1

width

20 nm – ca 13 layers

5 μm

=> Crystallite size is large (D > 80 nm)

Deposition temperature ≈ melting temperature

20 nm

Grazing Incidence Scattering

area detection

point detection

area detection (image plate, CCD):fast & great for indexing, but positions NOT accurate & limited dynamic range

Summary: highly textured films

• Pentacene films are crystalline, but distinct from bulk• Thin Film: triclinic cell with herringbone motif and molecules tilted 10-20

degs with large domain size (> 80 nm)• Monolayer: rectangular cell with herringbone motif and molecules vertical• Gradual transition from monolayer to thin film

Pentacene films:lattice parameters structure determinationcrystallite size

• Scan choice requires knowledge of reciprocal space & what you want to learn

• Use different beam lines

X-ray Reflectivity

Q = (4π/λ) sin θnormal to surface

R= reflectivity= ratio of reflected to incident= |r1|2, where r1 is reflectance

Q < Qc : R ≈ 1Q >> Qc: R ≈ (Qc/Q)4

Q >> Qc: R ≈ (Qc/Q)4 exp –(Qσ)2

Qc ≈ √ρe- , electron densityσ = rms roughness

incident reflectedQθ θ

• Lu, Lee, Thomas, Acta Cryst. A52, 11-41 (1996).• Tolan, “X-ray Scattering from Soft-Matter: Materials

Science and Basic Research”, Springer (1998).

Qc

X-ray ReflectivityQ = (4π/λ) sin θ

r1 (r2 ) = reflectance ateach interface

R ≈ | r1 + r2 exp (iQD)|2

incident reflectedQ

D

θ θ

• Lu, Lee, Thomas, Acta Cryst. A52, 11-41 (1996).• Tolan, “X-ray Scattering from Soft-Matter: Materials

Science and Basic Research”, Springer (1998).

Qc

2π/D

r1r2

ρe- & σ

X-ray Reflectivity on 2-1

incidentreflected

sample

Q

2θincident

Q reflected

θ

Lubricant Films

How thick is the lubricant?

CR =

F

F

C

H

H

OH 4700 g/mol

CR =

F

F

F 4600 g/mol

Z-Dol

random co-polymer

FomblinDisk Lubricants:

Z

What we did:use reflectivity to measure various thicknessescompare with ‘established’ methods

Lubricant Films: Thickness

film thickness (accurate!)film densityfilm roughness

What can you learn?

• Toney, Mate, Pocker, IEEE Trans. Magn. 34, 1774 (1998)• Toney, Mate, Leach, Pocker, J. Coll. Inter. Sci. 225, 119 (2000)

hydrocarbons

2π/D R ≈ | r1 + r2 exp (iQD)|2

Reflectivity Modeling

R ≈ | r1 + r2 exp (iQD)|2parratt32:http://www.hmi.de/bensc/instrumentation/instrumente/v6/refl/parratt_en.htm

Lubricant Films: Thickness

organics

ellipsometry and ESCA can provide accurate thickness

organicslubricant

Slope = 1.0

ellipsometry ≡ 0

• Toney, Mate, Pocker, IEEE Trans. Magn. 34, 1774 (1998)• Toney, Mate, Leach, Pocker, J. Coll. Inter. Sci. 225, 119 (2000)

ellipsometry

reflectivity

Lubricant Films: Roughness

Toney, Mate, Leach, Appl. Phys Lett. 77, 3296 (2000)

• lubricant smoothes carbon surface

• for thick films, roughness approaches limit due to molecular nature of lubricant molecule

Diffuse X-ray Reflectivity

lateral length scale of roughness

X-ray Reflectivity: Summary

What you can learn:accurate film thickness

(Å resolution)film densityfilm roughnesssurface morphologysingle and multiple layers