1. Electronic transport through Single Organic Crystals
Alberto Morpurgo
The Delft Team
* R.W.I. de Boer
* A. Stassen
* N. Iosad
Collaborations
* M.E Gershenson
* N.Karl
* T.T.M. Palstra

2. Outline Introduction organic thin-film transistors
time-of-flight on single crystals
dc Transport through organic single crystals
growth and characterization
FET fabrication
Transport through FETs
Conclusions

3. Field Effect Transistors
gate electrode
gate dielectric
source
Molecular material
drain
Conducting
layer
pentacene
thin film FET
Schoonveld et al
Nature 2000

4. Mobility of Charge Carriers
Thin film FETs
Polymers and oligomers
Dimitrakopoulos & Malenfant 2002

5. Best organic thin-film FETs
mobility
m = 1-3 cm2/Vs
Nelson 1998
Identically prepared
devices
behave differently
Go beyond thin-films

6. Intrinsic Transport Properties
Single crystals
Time-of-flight data
Zone-refined molecules
- m ~ 1 RT
- dm/dT < 0
- m anisotropy
N. Karl 85
m ~ 1 cm2/Vs

7. Molecular Crystals
Anthracene
Tetracene
Pentacene
Perylene
Rubrene

8. Crystal growth
Important:
Growth process also purifies the molecules

9. Purification by sublimation: Tetracene
1st Growth
Re-growing crystals => Less Impurities
2nd Growth

10. Tetracene single crystals
SCLC + TOF Characterization
* m ~ 1 room T
* dm/dT < 0
metallic-like T dependence
* Structural phase transition
at K
Consistent results

11. Electrostatic bonding
Compatible with any
insulating layer:
e.g., high-k dielectrics

12. Rubrene
Electrostatically bonded
Rubrene single crystal FET

13. Rubrene/ Tetracene crystal FETs
Drain
20 mm
Source
m = 6 cm2/Vs
Delft Single Organic Crystal FETs

14. Mobility-anisotropy in Rubrene FETs
First experimental
observation in
FETs
Impossible in thin films
Rogers/Gershenson
to appear in Science

15. FET fabrication on top of crystal
Gate electrode
Gate insulator
Drain
Source
Organic Crystal
FET Device
Interface
quality?

16. Gershenson 2003

17. Rubrene FETs with Parylene Gate Insulator
Gershenson 2003
RT up to 15 cm2/Vs

18. Insensitivity to processing
FETs fabricated on top of crystals with parylene
gate insulator
Pentacene m = 0.5 cm2/Vs
Rubrene m = 4 cm2/Vs
@RT
Delft
Limited by purity
contacts
m = 15 cm2/Vs
Rutgers

19. Metal/Organic interface
Tetracene crystal
Evaporated Contact
Bonded Contact
Substrate
Contact fabrication
Introduces surface traps
Extrinsic Effects

20. Overview of m(T) in single crystal FETs
Pentacene
Rubrene
150
200
250
300 8 9 10 m
(cm 2
/Vs) T
(K) 7
Tetracene
Non-monotonic behavior often
observed in single crystals

21. High mobility in pentacene
m from SCLC:
no high-m single crystal
FET yet
in-plane
Space charge limited current
I-V characteristics
O.D. Jurchescu
(Palstra group/Groningen)

22. Single crystal FETs seem suitable for fundamental studies
Conclusions
Technological advances
* Different single crystal FET fabrication techniques
* Reproducibility
Measurements through single crystals
* “record” mobilities
* signatures of intrinsic properties
* “new” relevant molecules
Upcoming work
* metal/organic interface
* chemical purity (zone refinement)
Rapid developments:
Single crystal FETs seem suitable for fundamental studies
of organic semiconductors

23. Rubrene vs Tetracene
Rubrene
Tetracene
Non-planar
side groups

24. p-orbital overlap Expected: Better in rubrene than in polyacenes
High m
Herringbone
structure
Low m
Crystal structure vs Polaronic effects ?
Still purity limited ?
m ~ 10 RT (Palstra last month)

25. Alkyne-substituted Pentacene
How can we check?
Alkyne-substituted Pentacene
Similar to Rubrene
Collaboration with J. Anthony
“Same” crystal structure