Fabrication and characterization of nanometer scale organic electronic devices: application to field-effect transistors [Elektronische Ressource] / von Celio Enrique Clavijo Cedeño

FABRICATION AND CHARACTERIZATION OF
NANOMETER SCALE ORGANIC ELECTRONIC
DEVICES:
APPLICATION TO FIELD-EFFECT TRANSISTORS
Im Fachbereich Elektrotechnik, Informationstechnik und
Medientechnik der Bergischen Universit˜at Wuppertal
zur Erlangung des akademischen Grades eines
Doktor-Ingenieurs
genehmigte Dissertation
von
Celio Enrique Clavijo Cedeno~ M.Sc. (Phys.)
aus Esmeraldas, Ecuador
Wuppertal 2004Referent: Prof. Dr. rer. nat. C.M. Sotomayor Torres
Korreferent: Prof. Dr. rer. nat. L.J. Balk
Tag der mundlic˜ hen Prufung:˜ 25.06.2004
iiHiermit versichere ich, die vorliegende Arbeit selbst˜andig verfa…t und
nur die angegebenen Quellen und Hilfsmittel verwendet zu haben.
Celio Enrique Clavijo Cedeno~
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iiiTo Marilu¶ Cedeno.~
ivTable of Contents
Table of Contents v
List of Tables viii
List of Figures ix
Abstract xv
Acknowledgements xvii
1 Introduction 1
1.1 Organic Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Electron Delocalization in …-Systems . . . . . . . . . . . . . . . 5
1.1.2 Conjugated Polymers . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 Short Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.1.4 Transport in organic semiconductors . . . . . . . . . . . . . . . 13
1.2 Organic Electronic Devices . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.2 Organic Light Emitting Diodes . . . . . . . . . . . . . . . . . . 18
1.2.3 Field-Efiect Transistors: Basic Structure and Operation . . . . . 19
1.2.4 Organic Field-Efiect Transistors . . . . . . . . . . . . . . . . . . 23
1.2.5 Electrical Contacts . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.2.6 Nanometer Channel Length Devices . . . . . . . . . . . . . . . . 41
1.2.7 Molecular Electronic Devices . . . . . . . . . . . . . . . . . . . . 43
1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2 Fabrication and Microscopic Characterization of Organic NanoFETs 48
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2 Electron-Beam Lithography . . . . . . . . . . . . . . . . . . . . . . . . 50
2.2.1 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.2.2 Proximity Efiect . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.2.3 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.2.4 Bilayer Resist . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
v2.3 Nanoimprint Lithography . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.3.1 Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.3.2 Stamp Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.3.3 Imprinting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2.3.4 Residual Layer Removal . . . . . . . . . . . . . . . . . . . . . . 64
2.4 Metal Nanoelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.4.1 Choice of Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
2.4.2 Results for Linewidths and Spacings Down to 100 nm . . . . . . 70
2.4.3 Challenges Below Approximately 50 nm . . . . . . . . . . . . . 73
2.5 Microscopic Characterization. . . . . . . . . . . . . . . . . . . . . . . . 75
2.5.1 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . 77
2.5.2 Atomic Force Microscopy. . . . . . . . . . . . . . . . . . . . . . 79
2.6 Deposition Methods for Organic-Semiconductors and Resulting Mor-
phology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
2.6.1 Surface Treatment of Substrates . . . . . . . . . . . . . . . . . . 82
2.6.2 Thermal Evaporation . . . . . . . . . . . . . . . . . . . . . . . . 82
2.6.3 Solution Process . . . . . . . . . . . . . . . . . . . . . . . . . . 83
2.7 Fabrication of Molecular Electronic Devices . . . . . . . . . . . . . . . 87
2.7.1 Adjustable Contacts . . . . . . . . . . . . . . . . . . . . . . . . 87
2.7.2 Fixed Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
2.7.3 Combined Approach . . . . . . . . . . . . . . . . . . . . . . . . 90
2.7.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
2.8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
2.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
3 Electrical Characterization 98
3.1 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.2 Current-voltage characteristics of DH4T OFETs . . . . . . . . . . . . . 100
3.3 Gated Self-Assembled Layer Devices . . . . . . . . . . . . . . . . . . . 104
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4 Models for Transport and Current-Voltage Characteristics 113
4.1 Models for Charge Transport in fllms of organic semiconductors . . . . 114
4.1.1 Monte Carlo Simulations of Hopping Transport in a Disordered
System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.1.2 The Small Polaron . . . . . . . . . . . . . . . . . . . . . . . . . 118
4.1.3 Poole-Frenkel Model . . . . . . . . . . . . . . . . . . . . . . . . 120
4.1.4 Multiple Trapping and Release . . . . . . . . . . . . . . . . . . 121
4.1.5 Variable Range Hopping . . . . . . . . . . . . . . . . . . . . . . 122
4.2 Application of the Transport Models to Organic Field-Efiect Transistors 123
4.2.1 Temperature Dependence . . . . . . . . . . . . . . . . . . . . . 123
4.2.2 Electric-Field Dep . . . . . . . . . . . . . . . . . . . . . 126
vi4.3 Model for the output characteristics of OFETs . . . . . . . . . . . . . . 129
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5 Discussion 135
5.1 Surface Treatment and Morphology . . . . . . . . . . . . . . . . . . . . 136
5.2 Current-Voltage Characteristics . . . . . . . . . . . . . . . . . . . . . . 137
5.3 Field-Efiect Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
5.4 Channel Length Dependence of the Field-Efiect Mobility . . . . . . . . 147
5.5 Self-Assembled Layer Devices . . . . . . . . . . . . . . . . . . . . . . . 149
6 Conclusions and Perspectives 151
Publications 156
References 158
viiList of Tables
1.1 Field-efiectmobilityofthemostcommonlyusedorganicsemiconductors
in OFET fabrication. In the deposition method column: s denotes spin
coating,c dropcastingandv vacuumevaporation. Alsoshownsomenew
n-type organic semiconductors DFH-nT (per uorohexyl-substituted n-
thiophenes [27]).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.2 Summaryofthemainsimilaritiesanddifierencesbetweentheproperties
of organic and inorganic semiconductors and fleld-efiect transistors. . . 46
2.1 NIL parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.1 Channel dimensions of the devices. . . . . . . . . . . . . . . . . . . . . 101
5.1 Channeldimensionsoftenrepresentativedevicesandflttingparameters
of the respective I ¡ V curves using Equations 4.26 and 1.6. Theds ds
average mobility for V between -2 and -16 V is given. The thresholdg
voltage and the contact and bulk resistances are given at V = -2 V.g
The averaged relative fltting error and the device type are given. The
flrst letter under the type column indicates drop casting (c) or spin
coating (s) device while the second gives the type (I or II) of output
⁄characteristics. Devices marked with showed non-Ohmic features at
low V and forjV j‚ 10 and 14 V for devices 7 and 8 respectively. . . 141ds g
viiiList of Figures
1.1 Alternation of single and double bond in trans-polyacetylene. This
causes bond length alternation along the chain. Both conflgurations
shown are isoenergetic (degenerate). . . . . . . . . . . . . . . . . . . . . 7
1.2 Energy band diagram for inorganic a) n-type a) p-type semiconductors.
The terms in blue refer to the analogous in organic
In inorganic crystalline semiconductors the bands represent delocalized
charge carriers along the crystal. In the case of organic semiconductors
the term \band" refers only to an energetic interval. . . . . . . . . . . 8
1.3 Chemicalstructureofpoly(3-hexylthiophene)(P3HT)andpoly(2,5-thienyl-
enevinylene)(PTV),themostfrequentlyusedpolymersfororganicfleld-
efiect transistors; poly(2,5-thienylene vinylene) (PPV) used in organic
light-emitting diodes is also shown . . . . . . . . . . . . . . . . . . . . 8
1.4 Oxidation of polythiophene. Polaron and bipolaron states are created
in the process. For materials standing high doping levels, the possibility
exists for the creation of bipolaron bands that may eventually merge
the oxidation and reduction bands giving the possibility of metallic-like
conduction. Adapted for polythiophene from Ref. [6