Electronic transport through Single Organic Crystals

1
Electronic transport throughSingle Organic
Crystals

• Alberto Morpurgo

Collaborations M.E Gershenson N.Karl
T.T.M. Palstra
The Delft Team R.W.I. de Boer A. Stassen
N. Iosad
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
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 cm2/Vs _at_ RT
• - dm/dT lt 0
• - m anisotropy

N. Karl 85
m 1 cm2/Vs
7
Molecular Crystals
Tetracene
Pentacene
Anthracene
Perylene
Rubrene
8
Crystal growth
Important Growth process also purifies the
molecules
9
Purification by sublimation Tetracene
1st Growth
Re-growing crystals gt Less Impurities
2nd Growth
10
Tetracene single crystals
SCLC TOF Characterization
m 1 cm2/Vs _at_ room T dm/dT lt 0
metallic-like T dependence Structural phase
transition at 170-200 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
Mobility _at_ 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
_at_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
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
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 cm2/Vs _at_ RT (Palstra last month)
25
Alkyne-substituted Pentacene
How can we check?
Similar to Rubrene
Collaboration with J. Anthony
Same crystal structure