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Title: Stanford%20Synchrotron%20Radiation%20Laboratory


1
Stanford Synchrotron Radiation Laboratory
Yet More Thin Film X-ray Scattering (Highly
Textured) plus X-ray Reflectivity
Mike Toney, SSRL
  • Introduction (real space reciprocal space)
  • Highly textured film pentacene
  • X-ray Reflectivity
  • Summary
  • how do you get diffraction data from thin films
  • how to choose what to do (what beam line scans)
  • what do you learn

2
Real and Reciprocal Space
powder film (polycrystalline)
spheres
textured film
width few degs
rings
3
Highly Textured Film
Reciprocal space
Real space
width lt 1 deg
rings
  • continuum in texture
  • distinguish highly textured (small molecules)
    from textured (metals, oxides)

4
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 identification
  • lattice parameters
  • defects
  • texture
  • crystallite size
  • atomic structure

5
Thin Film Scattering
specular (?/2?) 2-1 gixs (high resolution)
7-2 gixs (area low res) 11-3
11-3
7-2
2-1
6
Organic Thin Film Transistors
Organic FET
Sandra Fritz, C. Daniel Frisbie, Mike
Ward, Chemical Engineering and Materials
Science University of Minnesota
7
Organic Thin Films
Organic FET
  • Pentacene on a-SiO2
  • thermal evaporation at ca 50 C
  • charge transport in first few layers
  • what is crystal structure (packing of
    molecules)?
  • changes in structure with thickness
  • crystallite size defects

Structure (molecular packing defects) gt
Transport
Ruiz, et al. Appl. Phys. Lett. 85, 4926
(2004) Yang, et al. JACS 127, 11542 (2005) Mayer,
et al. PRB 73, 205307 (2006)
8
Bulk Pentacene
  • Bulk Single Crystal Structure triclinic
  • a 6.266(1) Å
  • b 7.775(1) Å
  • c 14.530(1) Å
  • a 76.475(4)o
  • b 87.684(4)o
  • 84.684(4)o
  • d001 14.1 Å
  • Charge transport is highly anisotropic

D. Holmes et al. Chem. Eur. J. 1999, 5, 3399 C.
C. Mattheus et al. Acta Cryst. C 2001, C57,
939 T. Siegrist et al. Angew. Chem. 2001, 40,
1732 R. B. Campbell and J. M. Robertson Acta
Cryst. 1962, 15, 289 R. B. Campbell et al. Acta
Cryst. 1961, 14, 705
9
Specular X-ray Scattering
Specular probes planes oriented parallel to
surface
scattered k
Q
incident k
Q
incident
scattered
Q k k (4p/?) sin ? perpendicular to surface
sample
Beam line 2-1 2-circle diffractometer
10
Pentacene Thin Films
  • Pentacene on a-SiO2
  • substrate induced thin-film and bulk-like
    phases
  • film is highly textured (aligned out-of plane
    (001) and powder in-plane)

Specular (2-1) 65 nm film
rocking curve width lt0.005o gt highly textured
films
11
Grazing Incidence Scattering
k
n
k
Q
Qz
a
Qxy
b
Q k k
  • a small, limits penetration into substrate
  • ß large, Qz not small

12
Grazing Incidence Diffraction
Q k k
13
Pentacene Thin Films
14
Pentacene Thin Films
15
Pentacene Thin Films
20 nm film
Q
  • quick
  • complete diffraction pattern gt index peaks
  • rough lattice parameters
  • misses details (poor resolution)
  • weak peaks hard to detect

16
Pentacene Thin Films (20 nm)
Qz
Qxy
17
Pentacene Thin Film (20 nm)
Scan Qxy at Qz peak position for (1 1 L)
(1-10) (-111) (-112)
(-1-10) (-1-11)
(-1-11)
(1-11)
Qz
(-111)
(-1-10)
(1-10)
Qxy
  • splitting for (11L) (1-1L)
  • splitting for (12L) (1-2L)
  • no splitting for (02L) (20L)
  • ? ? 90 in thin-film phase
  • cell is triclinic

Fritz et al., unpublished
18
Pentacene Thin Films (20 nm)
see Merlo et al. J. Am. Chem. Soc. 127, 3997
(2005) for details.
19
Pentacene Thin Films (20 nm)
Thin Film herringbone motif molecules tilted
10-15 deg herringbone angle 50-60 deg
a 5.920 Å b 7.556 Å c 15.54 Å a 81.6
deg ß 87.2 deg ? 89.84 deg
Structure refinement still in progress
20
Pentacene Peak Widths
20 nm
gt Crystallite size is large (D gt 80 nm)
Deposition temperature melting temperature
20 nm ca 13 layers
21
Grazing Incidence Scattering
area detection (image plate, CCD) fast great
for indexing, but positions NOT accurate
limited dynamic range
22
Summary highly textured films
  • Pentacene films
  • lattice parameters
  • structure determination
  • crystallite size
  • Scan choice requires knowledge of reciprocal
    space what you want to learn
  • Use different beam lines
  • 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 (gt 80 nm)
  • Monolayer rectangular cell with herringbone
    motif and molecules vertical
  • Gradual transition from monolayer to thin film

23
X-ray Reflectivity
incident
Q (4p/?) sin ? normal to surface
Q
reflected
?
?
R reflectivity ratio of reflected to
incident r12, where r1 is reflectance Q lt
Qc R 1 Q gtgt Qc R (Qc/Q)4 Q gtgt Qc R
(Qc/Q)4 exp (Qs)2
Qc
Qc v?e- , electron density s rms roughness
  • Lu, Lee, Thomas, Acta Cryst. A52, 11-41 (1996).
  • Tolan, X-ray Scattering from Soft-Matter
    Materials Science and Basic Research, Springer
    (1998).

24
X-ray Reflectivity
incident
Q (4p/?) sin ?
reflected
Q
?
?
r1
r2
Qc
R r1 r2 exp (iQD)2
r1 (r2 ) reflectance at each interface
?e- s
2p/D
  • Lu, Lee, Thomas, Acta Cryst. A52, 11-41 (1996).
  • Tolan, X-ray Scattering from Soft-Matter
    Materials Science and Basic Research, Springer
    (1998).

25
X-ray Reflectivity on 2-1
26
Lubricant Films
Fomblin Disk Lubricants
random co-polymer
Z-Dol
How thick is the lubricant?
  • What we did
  • use reflectivity to measure various thicknesses
  • compare with established methods

Z
27
Lubricant Films Thickness
2p/D
R r1 r2 exp (iQD)2
hydrocarbons
  • Toney, Mate, Pocker, IEEE Trans. Magn. 34, 1774
    (1998)
  • Toney, Mate, Leach, Pocker, J. Coll. Inter. Sci.
    225, 119 (2000)

28
Reflectivity Modeling
parratt32 http//www.hmi.de/bensc/instrumentation
/instrumente/v6/refl/parratt_en.htm
R r1 r2 exp (iQD)2
29
Lubricant Films Thickness
organics
ellipsometry 0
ellipsometry
Slope 1.0
lubricant
organics
reflectivity
ellipsometry and ESCA can provide accurate
thickness
  • Toney, Mate, Pocker, IEEE Trans. Magn. 34, 1774
    (1998)
  • Toney, Mate, Leach, Pocker, J. Coll. Inter. Sci.
    225, 119 (2000)

30
Lubricant Films Roughness
  • lubricant smoothes carbon surface
  • for thick films, roughness approaches limit due
    to molecular nature of lubricant molecule

Toney, Mate, Leach, Appl. Phys Lett. 77, 3296
(2000)
31
Diffuse X-ray Reflectivity
lateral length scale of roughness
32
X-ray Reflectivity Summary
  • What you can learn
  • accurate film thickness (Å resolution)
  • film density
  • film roughness
  • surface morphology
  • single and multiple layers

33
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