ORGANIC TRANSISTORS FOR FLEXIBLE ELECTRONICS: …gr337ss0936... · 2011-11-28 · organic...

ORGANIC TRANSISTORS FOR FLEXIBLE ELECTRONICS:

FABRICATION AND DEVICE PHYSICS

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Yoonyoung Chung

November 2011

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/gr337ss0936

© 2011 by Yoonyoung Chung. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

ii



http://purl.stanford.edu/gr337ss0936

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Zhenan Bao, Primary Adviser


Boris Murmann, Co-Adviser


Yoshio Nishi

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.

iii

iv

Abstract

Organic semiconductors have shown promising potentials in flexible

electronics. Because the materials can be directly fabricated on flexible plastic

substrates at low temperatures, researchers envision development of novel flexible

applications, such as flexible displays, flexible circuits, and conformal sensors. In

order to develop these practical devices, organic transistor technologies should provide

reliable fabrication processes and mature techniques to control the electrical

characteristics of each device. In this dissertation, I will discuss several methods to

overcome these issues. First, I have demonstrated high-capacitance gate dielectric

using atomic layer deposition. This low-temperature process was used to fabricate

low-voltage and high-performance flexible organic transistors and inverters. I will

also present controlling current-voltage characteristics of organic transistors from the

device physics point of view.

v

Acknowledgements

First of all, I would like to express my heartfelt gratitude to Professor Zhenan

Bao for advising my graduate study. Since the day when I first met her, she has been

always supportive. When I joined the group, I was a pure electrical engineering

student who did not know anything about organic materials. During the first few

months, we regularly discussed several papers together, and I learned essential

background knowledge directly from her. Based on her previous experimental

experience in Bell Labs, she also taught several experimental techniques, which were

very helpful. I learned a lot from her in details about developing new ideas, doing

experiments, writing papers, communicating with other researchers, and so on. I was

lucky to work with her.

I thank my co-advisor, Professor Boris Murmann, for his help and advice.

When I focused on tiny problems in my experiment, he reminded me of the ultimate

goal of my project as well as other ways to proceed. He always helped me have a big

picture in my research. He also provided useful information about ethics in research,

logical thinking, English writing, and so on. He was a great mentor during my five

years at Stanford.

vi

I thank Professor Yoshio Nishi for all his help and advice throughout my

graduate study. Whenever I had questions or unsolved problems, he always welcomed

me, enjoyed discussions, and provided clear directions to solve them. He not only

helped me in research-related issues, but also gave sincere advice for my future. I

believe that he will be always my role model in my life.

I also thank Professor Alberto Salleo for his help. Although we did not write

any paper together yet, he helped me more than anyone else. He was always available

in his office or through emails whenever I had questions. I was so glad when he

agreed to chair my PhD oral defense. Grazie mille!

Atomic layer deposition (ALD) was one of the most important processes in my

research. I am indebted to my colleagues who worked on the ALD machine: Dr.

James McVittie, Dr. Jenny Hu, Yasuhiro Oshima, and Dr. Albert Lin. I am also

indebted to staff members of the Stanford Nanofabrication Facility who helped my

device fabrications and characterizations: Dr. James McVittie, Dr. Jim Kruger, Dr.

Mary Tang, Dr. Michael Deal, Ed Myers, Mario Vilanova, Mahnaz Mansourpour,

Jeannie Perez, Maurice Stevens, Nancy Latta, and Uli Thumser. I thank Dr. Arturas

Vailionis of the Geballe Laboratory for Advanced Materials and Dr. Yun Sun of the

Stanford Synchrotron Radiation Lightsource for characterizing my samples.

Many of my friends and former/present group members shared ideas with me

or helped my experiments. None of my work would have been possible without them.

Bao Group Members: Dr. Jungkyu Lee, Prof. Joon Hak Oh, Dr. Sangwon Ko, Dr.

Peng Wei, Prof. Mark Roberts, Dr. Sanghyun Hong, Jin Jeon, Ade Johnson, Dr.

Daniel Kaefer, Steve Park, Dr. Tony Sokolov, Prof. Hector Becerril, Dr. Eric

vii

Verploegen, Dr. Shuhong Liu, Gino Giri, Sondra Hellstrom, Ying Jiang, Dr. Hanying

Li, Dr. Darren Lipomi, Satoshi Morishita, Dr. Guihua Yu, Dr. Hylke Akkerman, Prof.

Chris Bettinger, Dr. Melbs LeMieux, Dr. Stefan Mannsfeld, Dr. Ming Lee Tang, Dr.

Atsushi Tatami, and Dr. Ajay Virkar.

Murmann Group Members: Dr. Wei Xiong, Alex Guo, Donghyun Kim, Noam Dolev,

Alex Omid-Zohoor, Ross Walker, Martin Kramer, Vaibhav Tripathi, Dr. Clay Daigle,

and Dr. Manar El-Chammas.

Nishi Group Members: Dr. Masaharu Kobayashi, Dr. Seong-Geon Park, Hye-Ryoung

Lee, Dr. Blanka Magyari-Köpe, and Prof. Baylor Triplett.

Other Group Members: Dr. Seunghwa Ryu, Wonseok Shin, Dr. Jeong-hee Ha, Dr.

Donghyun Kim, Dr. Sangbum Kim, Jung Woo Choe, Hyunki Kim, Young Moon Kim,

Dr. Kyung Hoae Koo, Dr. Saeroonter Oh, Dr. Wanki Kim, Young Min Park, and Prof.

Jin-Hong Park.

Finally, I would like to express my deepest gratitude to my parents, my brother,

and my fiancée Kangah.

viii

Table of Contents

Abstract.............................................................................................iv

Acknowledgements...........................................................................v

Table of Contents...........................................................................viii

List of Tables..................................................................................xii

List of Figures.................................................................................xiii

1 Introduction.............................................................................1

1.1 Motivation..........................................................................1

1.2 Organic Field-Effect Transistors........................................4

1.3 Challenges and Research Goals.........................................6

1.4 Organization of This Dissertation....................................10

References........................................................................11

2 Low-Voltage, Short-Channel, Top-Contact Organic

Transistors.......................................................................................15

2.1 Introduction......................................................................15

2.1.1 Ozone-Assisted Atomic Layer Deposition of Aluminum

Oxide for High-Capacitance Gate Dielectric...................16

ix

2.1.2 Parylene-C Shadow Mask for Sub-10-μm Channel

Lengths between Source and Drain Electrodes................19

2.2 Device Fabrication...........................................................21

2.3 Electrical Measurement....................................................23

2.4 Conclusion.......................................................................27

2.5 Comments........................................................................28

References........................................................................29

3 Controlling Dipoles in the Gate Dielectric of Organic

Transistors................................................................................31

3.1 Introduction......................................................................31

3.1.1 Self-Assembled Monolayers for Controlling Dipoles.....32



3.3.1 Current-Voltage Characteristics of Organic Transistors..37

3.3.2 Pentacene Metal–Insulator–Semiconductor Capacitors...40

3.4 Physical Structure of Self-Assembled Monolayers.........41

3.4.1 Grazing Incidence X-Ray Diffraction..............................42

3.4.2 X-Ray Reflectivity...........................................................43

3.5 Effects of the Dipoles Studied by Work Function

Measurement....................................................................46

x

3.6 Air-Stable n-Channel Organic Transistors.......................51

3.7 Morphology of Organic Semiconducting Layers.............58

3.8 Conclusion.......................................................................64

References........................................................................65

4 Engineering Metal Gate Electrodes for Organic

Transistors.......................................................................................69

4.1 Introduction......................................................................69

4.2 Dual-Metal Gates.............................................................75

4.2.1 Device Fabrication...........................................................76

4.2.2 Electrical Measurement....................................................77

4.2.3 Work Function Measurement on Metal Gates.................79

4.2.4 Morphology of Organic Semiconducting Layers.............81

4.3 Bilayer Metal Gates.........................................................82

4.3.1 Introduction......................................................................82

4.3.2 Device Fabrication...........................................................84

4.3.3 Electrical Measurement....................................................86

4.3.4 X-Ray Photoelectron Spectroscopy on Metal Gates........87

4.4 Conclusion.......................................................................89

xi

References........................................................................90

5 Complementary Flexible Organic Inverters.......................91

5.1 Introduction......................................................................91



5.4 Conclusion.....................................................................100

References......................................................................102

6 Conclusion............................................................................105

6.1 Summary of This Dissertation.......................................105

6.2 Future Work...................................................................107

6.2.1 Stability of Organic Semiconductors.............................107

6.2.2 Metal–Semiconductor Junction Resistance...................107

6.2.3 “Exciting” Applications.................................................108

References......................................................................109

xii

List of Tables

Table 1. Comparison between organic transistors and inorganic transistors................4

Table 2. Thickness and packing density data of OPA and OTS SAMs from XRR....45

Table 3. VON data of pentacene OFETs (saturation mode) with different AlOX

thickness, measured in a nitrogen atmosphere..............................................................50

Table 4. Integrated peak intensity of selected GIXD from pentacene layers. In the

magnified GIXD images from Figure 39, the blue and red boxes indicate the

diffraction peaks from the thin-film and bulk phases, respectively. A meaningful

figure of merit is the ratio of the two peaks, which is significantly higher for the

pentacene deposited on OPA compared to OTS. These ratios indicate that the

pentacene films deposited on OPA have a greater relative fraction of the thin-film

phase, and a lower relative fraction of the bulk phase, compared to the pentacene films

on OTS..........................................................................................................................62

Table 5. Mobility data of OFETs (saturation mode) on OPA/AlOX and OTS/AlOX,

measured in a nitrogen atmosphere. Equation (1) was used to extract the mobilities.

Values in parenthesis refer to standard deviations........................................................63

Table 6. Device parameters of the OFETs measured inside a nitrogen atmosphere.

FET and VTH data were extracted by fitting the measured data in equation (4)............79

Table 7. XPS results at the surface of bilayer metal gate electrodes...........................88

Table 8. Device parameters of the flexible OFETs (saturation mode) measured in air.

The ION/IOFF ratio was defined as ID (|VGS| = VDD) / ID (VGS = 0 V)..............................96

xiii

List of Figures

Figure 1. Flexible electronic applications using organic semiconductors. Flexible

displays from Samsung Electronics, Universal Display, and Plastic Logic, and flexible

electronic skin from Prof. Takao Someya’s group at the University of Tokyo..............2

Figure 2. Conceptual roll-to-roll processing by PolyIC................................................3

Figure 3. OFETs with (left) top-contact S/D electrodes and (right) bottom-contact

S/D electrodes. Much larger contact resistance exists in the bottom-contact structure

due to a discontinuity in semiconductor morphology, as shown in the right figure.......5

Figure 4. Schematic of bottom-gate OFETs with top-contact S/D electrodes..............6

Figure 5. Mobility improvement of organic semiconductors........................................7

Figure 6. (Left) Delaminated layer of organic semiconductor during fabrication of

OFETs and (right) decrease in FET values of OFETs while exposed to air...................8

Figure 7. Overview of my PhD research.......................................................................9

Figure 8. (Left) Illustration of organic circuits embedded with organic sensors on a

flexible substrate and (right) photograph of flexible organic inverters introduced in

Chapter 5.........................................................................................................................9

Figure 9. Comparison between ozone- and water-assisted ALD layer of AlOX (as-

deposited). The solid and dotted arrows represent forward and backward sweeps,

respectively. [Collaboration with Dr. Jenny Hu at Stanford University].....................18

Figure 10. Shadow masking process for patterning of S/D electrodes in OFETs.......19

Figure 11. Comparison between parylene-C and metal shadow masks. The metal

shadow mask does not have a good adhesion on most surfaces, so the minimum

xiv

feature size of the metal mask is about an order of magnitude higher than the parylene-

C mask..........................................................................................................................20

Figure 12. Structure of dielectrics made of AlOX and OPA/AlOX.............................21

Figure 13. (Top) Schematic of OFETs and (down) microscope images of S/D

electrodes patterned by parylene-C shadow masks.......................................................22

Figure 14. Drain current vs. gate-source voltage curves in the saturation regime (VDS

= -2 V)..........................................................................................................................24

Figure 15. Extracted device parameters from the pentacene OFETs in saturation

mode..............................................................................................................................25

Figure 16. Contact resistance data extracted using transmission line method. The red

dot is the data from top-contact OFETs of this work, and the blue dot is from previous

work [14] using bottom-contact structure.....................................................................26

Figure 17. Formation of SAM on a hydroxyl-terminated substrate and investigation

of the SAM structure using GIXD................................................................................33

Figure 18. Chemical structure of OPA and OTS molecules.......................................34

Figure 19. Schematic of OPA and OTS SAMs in the gate dielectric of OFETs. Both

SAMs have nearly identical interface to the semiconducting layer, so the charge

transport on the semiconductor channel is not affected by a choice of SAM

materials........................................................................................................................35

Figure 20. Chemical structures of organic semiconductors used to make OFETs.....36

Figure 21. Schematic of OFETs..................................................................................37

xv

Figure 22. ID-VGS data of OFETs (saturation mode) measured in a nitrogen

atmosphere. The red lines represent the data from OPA/AlOX, and the blue lines are

the data from OTS/AlOX...............................................................................................38

Figure 23. Average VON data of OFETs on OPA/AlOX and OTS/AlOX. The error

bars represent standard deviations................................................................................39

Figure 24. C-V data of pentacene MIS structure measured in a nitrogen atmosphere.

The schematic is the MIS structure used in the measurements, and the OPA and OTS

SAMs located between pentacene and AlOX................................................................40

Figure 25. GIXD images of OPA and OTS SAMs on AlOX. The Bragg rods for both

SAMs are located at Qxy = 1.49 (±0.012) Å-1

, and the full width at half maximum

(FWHM) of the peaks is 0.11 ± 0.007 Å-1

. This feature is indicative of a crystalline

monolayer with a 4.2 Å hexagonal lattice constant [3].................................................43

Figure 26. Application of XRR to measure thickness and density of SAM...............44

Figure 27. XRR data of OPA and OTS SAMs on AlOX in arbitrary units. The solid

black lines are fitted data, and the green and blue dots are measured data...................45

Figure 28. Schematic of how the VSAM is generated...................................................46

Figure 29. Effects of SAM on measured work function of metal...............................47

Figure 30. Work function measurement for dipoles from OPA/AlO X and

OTS/AlOX.....................................................................................................................48

Figure 31. Effects of SAM dipoles in the MIS structure. In this analysis it is assumed

that the flatband voltage without the dipoles is zero.....................................................49

xvi

Figure 32. Simplified energy band diagram of p- and n-channel OFETs and the

effects of electron-trapping species, which diffuse into the transistor channel through

the grain boundaries of organic semiconducting layer.................................................52

Figure 33. Lowering semiconductor energy levels to improve air stability of n-

channel OFETs..............................................................................................................52

Figure 34. Equivalent circuit model that describes the effect of SAM dipoles between

OPA and OTS in OFETs...............................................................................................53

Figure 35. Long-term stability of C60 OFETs in air. The mobility and the threshold

voltage parameters were extracted from equation (1)...................................................55

Figure 36. Long-term stability of PTCDI-C13 OFETs in air. The mobility and the

threshold voltage parameters were extracted from equation (1)...................................56

Figure 37. Qualitative description of energy band diagrams (a) in the gate-to-channel

direction and (b) in the horizontal direction from the source to the drain electrodes.

Due to the different dipole moments, the LUMO and the HOMO levels are different at

the dielectric-semiconductor interface between OPA/AlOX and OTS/AlOX................57

Figure 38. GIXD and AFM images of C60 and PTCDI-C13 on OPA and OTS

SAMs............................................................................................................................59

Figure 39. GIXD and AFM images of pentacene on OPA and OTS SAMs...............61

Figure 40. Description of how different metals affect the characteristics of electronic

devices. In this example Metal 1 has a lower work function (WF) than Metal 2.........70

Figure 41. Energy band diagrams of FET in the gate-to-channel direction. The WF of

the gate electrode is chosen to be less than that of the semiconductor. No voltage is

applied to the gate electrodes........................................................................................71

xvii

Figure 42. Controlling VTH of FETs using the gate electrodes with different WF. No

voltage is applied to the gate electrodes........................................................................72

Figure 43. Controlling VTH of transistors to maximize their performance..................73

Figure 44. An example of patterned SAMs on a substrate to achieve both negative

and positive VTH control................................................................................................73

Figure 45. Importance of high gate capacitance (Cg) for VTH control using different

gate WF. The WF difference is assumed to be 0.5 eV in this example. Because the

range of VTH control is limited, FETs with low Cg cannot achieve enough modulation

in their drain current......................................................................................................74

Figure 46. Schematic of OFETs with metal gate electrode (Ti or Pt)..........................77

Figure 47. (Drain current)0.5

vs. gate-source voltage (VGS) curves for pentacene and

C60 OFETs on Ti (red lines) and Pt (blue lines) gate electrodes. VDS refers to the

drain-source voltage, and the dotted black lines are linear fitting data for VTH

extraction.......................................................................................................................78

Figure 48. AFM images of pentacene and C60 layers (40 nm) on Ti and Pt gate

electrodes......................................................................................................................81

Figure 49. Controlling current-voltage (I-V) characteristics of OFETs by engineering

the gate electrodes. (Left) Dual-metal gate electrodes cause a discrete control, and

(right) bilayer metal gates provide a continuous control..............................................82

Figure 50. Structure of the bilayer metal gate electrodes in this study. The thickness

of the top Pt layer controls the WF at the “effective region” at the surface...................83

Figure 51. Schematic of the OFETs with bilayer metal gate......................................85

xviii

Figure 52. ID vs. VGS curves of the C60 OFETs with different thickness of top Pt

layer in saturation mode................................................................................................86

Figure 53. VON data of the C60 OFETs with different thickness of top Pt layer in

saturation mode.............................................................................................................87

Figure 54. Schematic model of how aluminum atoms diffuse into the top platinum

layer...............................................................................................................................89

Figure 55. Schematic and photograph of the flexible complementary inverters........94

Figure 56. Leakage current through the gate dielectric. The schematic represents the

structure of the measured devices.................................................................................95

Figure 57. Drain current vs. VGS curves of p-channel (pentacene) and n-channel (C60)

OFETs in saturation mode. The devices were measured in ambient air......................96

Figure 58. Drain current vs. VDS curves of p-channel (pentacene) and n-channel (C60)

OFETs measured in air. Both OFETs had the W/L ratio of 10....................................98

Figure 59. Transfer curve and small-signal gain of the complementary inverter (VDD

= 3.5 V) on a flexible substrate, measured in air..........................................................99

1

Chapter 1

Introduction

1.1 Motivation

Since the invention of organic field-effect transistors (OFETs) [1], organic

light-emitting diodes (OLEDs) [2], and organic photovoltaics (OPV) [3] in the late

1980s, organic semiconductors have attracted much attention in large-area and flexible

electronics [4-8]. They have several unique properties that are not available in

conventional inorganic semiconductors (e.g., silicon, germanium, and III-V

semiconductors). One of the most promising advantages is that these organic

materials can be processed at low temperatures of less than 100 °C, which makes them

2

suitable for direct processing on flexible plastic substrates. This low temperature

processing provides opportunities to develop flexible electronic applications as shown

in Figure 1.

Figure 1. Flexible electronic applications using organic semiconductors. Flexible

displays from Samsung Electronics, Universal Display, and Plastic Logic, and flexible

electronic skin from Prof. Takao Someya’s group at the University of Tokyo.

Another advantage is that organic devices can be potentially made by roll-to-roll

processing on plastics. Because organic semiconductors are dissolved in several

solvents at low temperatures [9-11], flexible electronic devices can be printed out as

shown in Figure 2.

3

Figure 2. Conceptual roll-to-roll processing by PolyIC.

Although this process is not commercialized yet, roll-to-roll processing technology on

plastics can provide an easy and potentially simple way to make electronic devices.

Material tunability is another advantage of organic semiconductors. Their chemical

structure can be changed by new material designs and chemical synthesis. Therefore,

organic semiconductors can be modified to have chemical specificity and be used to

detect specific molecules [12, 13].

Among the various applications of organic semiconductors, I have worked on

OFETs. For controlling electrical signals and driving input/output components,

transistors and circuits are essential to every electronic device. Baseline technologies

for OFETs can be widely used in flexible electronics.

4

1.2 Organic Field-Effect Transistors

OFETs employ similar device structures as field-effect transistors (FETs) made

of inorganic materials. However, the unique properties of organic semiconductors

result in several distinct characteristics between organic and inorganic transistors.

Table 1 shows a brief comparison between the two types of transistors.

Organic Transistor Inorganic Transistor

Semiconductor

material Organic molecules Si, Ge, III-V, etc.

Structure Polycrystal or amorphous Single crystal

Element

interaction “Weak” van der Waals force “Strong” covalent bonding

Mobility 10-2

– 101 cm

2/V·s 10

2 – 10

4 cm

2/V·s

Min. lateral

feature size ~ 1 m ~ 10 nm

Processing

technology

Vacuum evaporation, roll-to-

roll processing, spin coating…

Crystal growth, crystallization,

epitaxial growth…

Processing

temperature ~ 100 °C > 800 °C

Table 1. Comparison between organic transistors and inorganic transistors.

Because formation of high-quality gate dielectric on organic semiconducting layer is

challenging, bottom-gate structures have been widely used in OFETs. In the bottom-

gate structure, depending on the order of the fabrication process, OFETs have either

5

top-contact source and drain (S/D) electrodes or bottom-contact S/D electrodes.

Figure 3 shows these two types of S/D electrodes.

Figure 3. OFETs with (left) top-contact S/D electrodes and (right) bottom-contact

S/D electrodes. Much larger contact resistance exists in the bottom-contact structure

due to a discontinuity in semiconductor morphology, as shown in the right figure.

The S/D electrodes of the bottom-contact structure can be patterned by

photolithography, which can reduce the channel length, defined as the distance

between S/D electrodes, below 100 nm. In the bottom-contact structure, however, the

organic semiconducting layer contains a significant discontinuity in its morphology

due to the different surfaces between the S/D electrodes and the gate dielectric, as

shown in Figure 3. Thus, the advantages of using a short channel [14] is generally

diminished by the large contact resistance [8]. Conversely, the S/D electrodes of top-

contact structure are mostly made by shadow masking process on the organic

6

semiconducting layer, which does not cause the problem of such a large contact

resistance. For these reasons, I chose the bottom-gate and top-contact-S/D structure in

my research. Figure 4 shows the schematic of a bottom-gate OFET with top-contact

S/D electrodes.

Figure 4. Schematic of bottom-gate OFETs with top-contact S/D electrodes.

More details about OFETs can be found in several edited books [6, 8].

1.3 Challenges and Research Goals

The performance of organic semiconductors, usually quantified by their

mobility values, has been improved much. As shown in Figure 5, the mobility values

of organic semiconductors approached high enough values for implementing displays

[15-17], sensors [12, 13, 18], and even low-speed integrated circuits (ICs) [19-24].

7

Figure 5. Mobility improvement of organic semiconductors.

Despite of this advancement in mobility, there are several challenges in OFETs that

need to be solved for practical applications.

First, organic semiconducting layers can be easily delaminated and damaged

because they are bound by weak van der Waals forces. Also, device performance,

such as field-effect mobility (FET) of OFETs, can be degraded much when exposed to

air [25, 26]. This degradation is generally caused by trapped charges in the transistor

channel [27-29] as well as oxidation of organic semiconductors [30, 31].

8

Figure 6. (Left) Delaminated layer of organic semiconductor during fabrication of

OFETs and (right) decrease in FET values of OFETs while exposed to air.

Controlling the threshold voltage (VTH) of OFETs is another challenge. Some

electronic circuits may need low VTH for high driving current and high speed while the

others may require high VTH low power consumption. The VTH is also used to

implement desired circuit functions. Yet, there is no mature technology to control the

intrinsic properties of organic semiconductors (cf. ion implantation in silicon

technology) and hence the VTH.

Also, fabrication of high-quality gate dielectric is difficult. The gate dielectric

is one of the most important elements in FETs because it determines how efficiently

the transistors turn on and off the current flow in the channel [32]. For utilizing

OFETs in flexible applications, the dielectric fabrication must be done at low

temperatures less than the melting point of flexible substrates. This temperature

limitation makes it difficult to fabricate a high-quality gate dielectric layer with low

defect states.

0 1 2 3 410

-3

10-2

10-1

100

Days

F

ET (

cm

2/V

s)

9

In my PhD research, I aimed to study the fabrication of high-performance

flexible OFETs and the control of their current-voltage characteristics, as summarized

in Figure 7.

Figure 7. Overview of my PhD research.

I set a final goal to demonstrate high-performance and air-stable flexible OFETs and

inverter circuits, which can be potentially utilized in future flexible electronics as

shown in Figure 8.

Figure 8. (Left) Illustration of organic circuits embedded with organic sensors on a

flexible substrate and (right) photograph of flexible organic inverters introduced in

Chapter 5.

10

1.4 Organization of This Dissertation

This dissertation consists of 6 chapters. The motivation and research goals of

my research are introduced in Chapter 1. In Chapter 2, I describe basic fabrication

methods for OFETs. I discuss atomic layer deposition (ALD) of aluminum oxide

(AlOX) and parylene-C shadow masks for making the gate dielectric and for patterning

sub-10-m channel length between S/D electrodes, respectively. After these

fabrication processes, I describe methods to control current-voltage characteristics of

OFETs in Chapter 3 and Chapter 4. In Chapter 3, I introduce self-assembled

monolayers (SAMs) with different anchor groups to change electric dipoles in the gate

dielectric. For engineering the gate electrodes of OFETs, I discuss dual-metal gate

and bilayer metal gate electrodes in Chapter 4. Chapter 5 introduces complementary

organic inverter circuits on a flexible substrate, where I combine fabrication methods

and knowledge from the previous chapters. I summarize and conclude this

dissertation in Chapter 6.

11

References

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Physics Letters 51, 913 (1987).

[3] C. W. Tang, "Two-layer organic photovoltaic cell," Applied Physics Letters 48,

183 (1986).

[4] P. Peumans et al., "Small molecular weight organic thin-film photodetectors and

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[8] Z. Bao and J. Locklin Organic field-effect transistors. (CRC Press, 2007).

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12

[12] A. N. Sokolov et al., "Fabrication of low-cost electronic biosensors," Materials

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[15] G. H. Gelinck et al., "Flexible active-matrix displays and shift registers based on

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[17] I. Nausieda et al., "An organic active-matrix imager," Ieee Transactions on

Electron Devices 55, 527 (2008).

[18] D. D. He et al., "An Integrated Organic Circuit Array for Flexible Large-Area

Temperature Sensing," ISSCC Digest of Technical Papers, 142 (2010).

[19] W. Xiong et al., "A 3-V, 6-Bit C-2C Digital-to-Analog Converter Using

Complementary Organic Thin-Film Transistors on Glass," IEEE Journal of

Solid-State Circuits 45, 1380 (2010).

[20] R. Blache et al., "Organic CMOS Circuits for RFID Applications," ISSCC

Digest of Technical Papers, 208 (2009).

13

[21] H. Marien et al., "An Analog Organic First-Order CT ΔΣ ADC on a Flexible

Plastic Substrate with 26.5dB Precision," ISSCC Digest of Technical Papers,

136 (2010).

[22] K. Myny et al., "An 8b Organic Microprocessor on Plastic Foil," ISSCC Digest

of Technical Papers, 322 (2011).

[23] W. Xiong et al., "A 3V 6b Successive-Approximation ADC Using

Complementary Organic Thin-Film Transistors on Glass," ISSCC Digest of

Technical Papers, 134 (2010).

[24] W. Zhang et al., "A 1V Printed Organic DRAM Cell Based on Ion-Gel Gated

Transistors with a Sub-10nW-per-Cell Refresh Power," ISSCC Digest of


[25] U. Zschieschang et al., "Flexible Low-Voltage Organic Transistors and Circuits

Based on a High-Mobility Organic Semiconductor with Good Air Stability,"

Advanced Materials 22, 982 (2010).

[26] Y. Chung et al., "Controlling Electric Dipoles in Nanodielectrics and Its

Applications for Enabling Air-Stable n-Channel Organic Transistors," Nano

Letters 11, 1161 (2011).

[27] R. A. Street et al., "Extended time bias stress effects in polymer transistors,"

Journal of Applied Physics 100, 114518 (2006).

[28] S. G. J. Mathijssen et al., "Dynamics of threshold voltage shifts in organic and

amorphous silicon field-effect transistors," Advanced Materials 19, 2785 (2007).

[29] R. A. Street, "Bias-induced change in effective mobility observed in polymer

transistors," Physical Review B 77, 165311 (2008).

14

[30] T. Yamamoto and K. Takimiya, "Facile Synthesis of Highly π-Extended

Heteroarenes, Dinaphtho[2,3-b:2',3'-f]chalcogenopheno[3,2-b]chalcogenophenes,

and Their Application to Field-Effect Transistors," Journal of the American

Chemical Society 129, 2224 (2007).

[31] T. Yokoyama et al., "Oxygen-Related Degradation Mechanisms for On- and

Off-States of Perfluoropentacene Thin-Film Transistors," Japanese Journal of

Applied Physics 47, 3643 (2008).

[32] S. M. Sze and K. K. Ng Physics of Semiconductor Devices. 3rd edn (Wiley-

Interscience, 2007).

15

Chapter 2

Low-Voltage, Short-Channel, Top-

Contact Organic Transistors

2.1 Introduction

A nanometer-scale gate dielectric with high capacitance is required for making

field-effect transistors (FETs) that can operate at low voltages suitable for portable

electronic applications. Previously, a variety of methods were used to achieve high-

capacitance gate dielectrics for organic field-effect transistors (OFETs): oxidized

aluminum [1], solution-processed hafnium oxide [2], polyvinyl alcohol [3], and ion-

16

gel dielectrics [4]. However, problems had still remained such as the control of

dielectric thickness and the existence of trap states. Atomic layer deposition (ALD) [5]

of aluminum oxide (AlOX) is a good choice for OFETs because the thickness of the

deposited dielectric can be precisely controlled and defect-free dielectric films can be

grown without a high-temperature annealing process. Moreover, it can be done on a

variety of metals, which allows engineering of gate electrodes discussed in Chapter 4.

ALD of AlOX was previously used in pentacene OFETs [6, 7], but those OFETs used

thick dielectric films on the order of hundreds of nanometers, which resulted in high

operating voltages of more than 10 V.

The channel length between source and drain (S/D) electrodes affects a large

number of device parameters, such as drain current, gate delay, and power dissipation,

and FET performance is maximized with a shorter channel length [8]. For high-

performance OFETs with top-contact S/D electrodes we utilized parylene-C shadow

masks. These parylene-C masks successfully patterned small channel length as low as

5 m with shadow masking process.

The following 2 subsections introduce more details about ALD and parylene-C

shadow masks.

2.1.1 Ozone-Assisted Atomic Layer Deposition of Aluminum

Oxide for High-Capacitance Gate Dielectric

An ALD process uses repeated pulses of source materials whose reaction is

self-limited. In case of ALD of AlOX, compounds containing aluminum and oxygen

17

atoms are sequentially injected on a target substrate. The resulting film consists of

multilayers of AlOX, and the film thickness is proportional to the number of pulse

cycles. Trimethylaluminum (TMA, Al(CH3)3) is dominantly used for the source of

aluminum, and ozone (O3) or water (H2O) is used for the oxidant. The overall

reactions of ozone- and water-assisted ALD of AlOX are described as below:

3 3 2 3 2 632Al CH +O Al O +3C H

3 2 2 3 432Al CH +3H O Al O +6CH

The by-products of each process (ethane for ozone oxidant and methane for water

oxidant) are pumped out from an ALD reactor during the process. [Note: In theory

stoichiometric aluminum oxide has a chemical formula of Al2O3. However, X-ray

photoelectron spectroscopy (XPS) showed that our ALD-deposited aluminum oxide

film had an atomic ratio of Al:O ≈ 1:1.86, instead of 1:1.5, probably because there

may be other side reactions. Therefore, I express aluminum oxide as AlOX, instead of

Al2O3, in this dissertation.]

Our ALD machine was purchased from SVT Associates. Because ozone is

known to result in less defect states inside AlOX ALD film due to its higher reactivity

than water [9], we installed an ozone generator from Toshiba Mitsubishi-Electric

Industrial Systems Corporation to the ALD machine. We first tested metal–insulator–

semiconductor (MIS) capacitors on p-type silicon wafers in order to check the

superiority of ozone-assisted process. The insulator (AlOX) was deposited at 350 °C

in the ALD machine with ozone or water oxidant, and the number of ALD cycles was

fixed to 70 for both films. Capacitance-voltage measurements were performed

without any post annealing process for AlOX, as shown in Figure 9.

18

Figure 9. Comparison between ozone- and water-assisted ALD layer of AlOX (as-

deposited). The solid and dotted arrows represent forward and backward sweeps,

respectively. [Collaboration with Dr. Jenny Hu at Stanford University].

The ozone-assisted AlOX film had a negligible hysteresis while the water-assisted

AlOX film showed a significant voltage shift between the forward and the backward

sweeps. Although the hysteresis can be significantly reduced at annealing process

above ~ 400 °C for AlOX, flexible substrates are not compatible with such high

temperature. Therefore, ozone-assisted ALD of AlOX is a more suitable method for

flexible organic transistors and circuits. Further details about ALD can be found in

previous review articles [5, 10].

19

2.1.2 Parylene-C Shadow Mask for Sub-10-μm Channel

Lengths between Source and Drain Electrodes

Top-contact OFETs were typically fabricated via evaporation through metal

shadow masks as shown in Figure 10.

Figure 10. Shadow masking process for patterning of S/D electrodes in OFETs.

However, the metal shadow masks have several limitations, such as minimum feature

sizes greater than 20–30 μm, rigidness, and difficulty in alignment. Instead of using

the metal, we used parylene-C shadow masks. The parylene-C masks were recently

developed from Prof. Mehmet Dokmeci’s group at Northeastern University for

patterning of biomolecules on polystyrene, glass, and PDMS substrates [11]. Due to

their good adhesion on a variety of surfaces, patterns with features as small as 2 μm

20

have been demonstrated with high reproducibility. The parylene-C shadow mask is

described in Figure 11.

Figure 11. Comparison between parylene-C and metal shadow masks. The metal

shadow mask does not have a good adhesion on most surfaces, so the minimum

feature size of the metal mask is about an order of magnitude higher than the parylene-

C mask.

Although photolithography could be used to pattern metal electrodes on organic

semiconductor [12], chemical solvents for photolithography degrade the

semiconducting layer. Therefore, we used parylene-C shadow masks in order to

demonstrate sub-10-m channel lengths in organic transistors with top-contact S/D

electrodes.

21

2.2 Device Fabrication

A heavily doped n-type silicon wafer (<0.005 Ω∙cm) was used as a bottom-gate

electrode and a substrate. TMA (Air Liquide Co.) and ozone were used as the source

materials in ALD of AlOX, where 45 cycles were repeated at 350 °C. After the ALD

process on the wafer, it was immersed into an octadecylphosphonic acid (OPA,

CH3(CH2)17PO(OH)2, Alfa Aesar Co.) solution (3 mM in anhydrous ethanol from

Sigma Aldrich Co.) to form a densely packed alkane-terminated self-assembled

monolayer (SAM). This SAM is known to reduce interfacial trap states and to

increase the crystallinity of organic semiconductor layer. [Note: More details about

the effects of SAMs on the performance of OFETs and their characterization results

are discussed in Chapter 3.] For capacitance measurement of the dielectrics 100-nm-

thick gold electrodes (on the order of 10-4

cm2) were thermally evaporated on AlOX

and OPA/AlOX. These test capacitors are shown in Figure 12.

Figure 12. Structure of dielectrics made of AlOX and OPA/AlOX.

On the OPA/AlOX/Si sample, pentacene (C22H14, Sigma Aldrich Co.)

molecules were thermally evaporated with a substrate heating at 60 °C in a vacuum

chamber. The thickness of the pentacene layer was 45 nm, measured with a quartz

22

crystal monitor. Finally, gold S/D electrodes (40 nm) were thermally evaporated and

patterned on the pentacene layer using 10-μm-thick parylene-C shadow masks. The

fabrication of the parylene-C masks was described in a previous publication [11]. The

gold electrodes on the pentacene layer had channel lengths of L = 5, 10, and 20 μm,

and the ratio of channel width to length was fixed at 20. As shown in Figure 13, all

channel lengths were clearly defined due to a good adhesion of the parylene-C on the

pentacene layer.

Figure 13. (Top) Schematic of OFETs and (down) microscope images of S/D

electrodes patterned by parylene-C shadow masks.

23

The dielectric layers of OPA/AlOX and AlOX on silicon were characterized by

contact angle and ellipsometry measurements. The water contact angle (static) of

AlOX was 9° while that of the OPA layer was 106°. This high contact angle is

indicative of a densely packed OPA SAM on AlOX. An ellipsometer with He-Ne laser

(excitation wavelength of 6328 Å ) was used to measure the thickness of each layer.

The thickness of AlOX was 45.7 (±0.2) Å , and that of the OPA SAM was 18.7 (±0.4)

Å .

2.3 Electrical Measurement

The maximum leakage current through the OPA/AlOX dielectric was 0.47

A/cm2 when 3 V was applied, indicative of a pin-hole-free film. This value is

comparable to the data from previously reported high-capacitance and low-leakage

gate dielectrics for organic transistors [1, 2]. The capacitance values of the AlOX and

OPA/AlOX samples were 0.80 and 0.49 F/cm2, respectively.

Sixteen transistors were tested for each channel length in the saturation regime

(VDS = -2 V) in air. Figure 14 shows the drain current as a function of gate-source

voltage (VGS) in the pentacene OFETs with L = 5, 10, and 20 μm.

24

Figure 14. Drain current vs. gate-source voltage curves in the saturation regime (VDS

= -2 V).

The average field-effect-mobility (FET) of our devices was 1.14 (±0.08) cm2/V·s for

all the channel lengths, and this value is 2–3 times higher than the previous results

from oxidized aluminum [1] and plasma-enhanced ALD of AlOX on titanium [6].

25

Figure 15. Extracted device parameters from the pentacene OFETs in saturation

mode.

The device parameters from each channel length are summarized in Figure 15.

The FET values had a similar distribution in each channel length while the IMAX/IMIN

ratio and the threshold voltage (VTH) showed channel length dependence. As depicted

in Figure 14, the IMIN increased at L = 5 m. The magnitude of the VTH in Figure 15

gradually decreases as the channel length decreases. When the lateral electric field

0 5 10 15 20 250.6

0.8

11.2

1.41.6

F

ET (

cm

2/V

s)

0 5 10 15 20 2510

6

108

1010

I MA

X /

IM

IN

0 5 10 15 20 25

1

1.5

Channel Length (m)

-VT

H (

V)

26

increases, the injection of charge carriers becomes easier between S/D electrodes and

the channel. Therefore, as the channel length shrinks, the sub-threshold current

increases, and the required gate voltage to turn on the transistors decreases. With the

device structure in this work, the vertical electric field was much stronger than the

lateral electric field, so a large variation of the threshold voltage with different channel

lengths, observed in nanometer-scale silicon metal–oxide–semiconductor field-effect

transistors (MOSFETs), did not occur.

The contact resistance, extracted by the transmission line method [13], was 733

ohm-cm as shown in Figure 16.

Figure 16. Contact resistance data extracted using transmission line method. The red

dot is the data from top-contact OFETs of this work, and the blue dot is from previous

work [14] using bottom-contact structure.

0 5 10 15 200

5

10

15

20

Channel Length (m)

Tota

l R

esis

tance (

k

-cm

)

27

This value is approximately 24 times smaller than the value reported with bottom-

contact S/D electrodes [14]. Previously, bottom-contact structure with

photolithography process had to be used to fabricate sub-10-m channel lengths in

OFETs. This structure, unfortunately, inevitably increases the contact resistance,

which reduces the overall device performance. By using parylene-C shadow masks,

which allow achieving m-size patterns with shadow masking process, we have

successfully demonstrated top-contact OFETs with sub-10-m channel lengths.

From the analysis of contact resistance, the limit of the channel length where

the contact resistance becomes larger than the channel resistance was estimated

between 1 and 2 μm. These high mobility, low contact resistance, and small limit of

the channel length can be explained by the defect-free and ultra-smooth OPA/AlOX

gate dielectric, whose root-mean-square surface roughness was 0.2 nm, measured by

AFM, and the use of top-contact electrodes.

2.4 Conclusion

We have demonstrated short-channel pentacene OFETs with 2.5 V operating

voltage by utilizing an ultra-thin gate dielectric and flexible parylene-C shadow masks.

The gate dielectric was made by a reproducible ALD process, allowing 2.5 V

operations so that the OFETs can be used in portable applications. Rather than using

photolithography, parylene-C shadow masks were used to pattern sub-10 μm channel

lengths. The top-contact structure and shadow-mask process resulted in low contact

resistance and high field-effect mobility. Our OFETs showed approximately 3.6 times

28

higher field-effect mobility than previously reported short-channel and top-contact

pentacene transistors fabricated by photolithography[12] and 1.4 times higher mobility

than bottom-contact pentacene transistors[15].

The fabrication process for making low-voltage and top-contact OFETs,

described in this chapter, is used in all the following Chapters 3–5.

2.5 Comments

Although the parylene-C shadow masks have several advantages as described

in the previous sections, it can be too flexible and difficult to align. To date, the

parylene-C mask is more suitable for laboratory experiments where one-layer shadow

masking is required rather than multilayer processes.

In December 2010, Dr. Hagen Klauk and his colleagues presented a new type

of silicon shadow mask that can pattern sub-1-m dimensions in OFETs [16].

29

References

[1] H. Klauk et al., "Ultralow-power organic complementary circuits," Nature 445,

745 (2007).

[2] O. Acton et al., "Low-voltage high-performance C60 thin film transistors via

low-surface-energy phosphonic acid monolayer/hafnium oxide hybrid

dielectric," Applied Physics Letters 93, 083302 (2008).

[3] Y. Jang et al., "Low-voltage and high-field-effect mobility organic transistors

with a polymer insulator," Applied Physics Letters 88, 072101 (2006).

[4] J. H. Cho et al., "High-capacitance ion gel gate dielectrics with faster

polarization response times for organic thin film transistors," Advanced

Materials 20, 686 (2008).

[5] S. M. George, "Atomic Layer Deposition: An Overview," Chemical Reviews

110, 111 (2010).

[6] J. B. Koo et al., "Pentacene thin-film transistors and inverters with plasma-

enhanced atomic-layer-deposited Al2O3 gate dielectric," Thin Solid Films 515,

3132 (2007).

[7] X.-H. Zhang et al., "High-performance pentacene field-effect transistors using

Al2O3 gate dielectrics prepared by atomic layer deposition (ALD)," Organic

Electronics 8, 718 (2007).

[8] Y. Taur and T. H. Ning Fundamentals of modern VLSI devices. 2nd edn

(Cambridge University Press, 2009).

30

[9] J. B. Kim et al., "Improvement in Al2O3 dielectric behavior by using ozone as

an oxidant for the atomic layer deposition technique," Journal of Applied

Physics 92, 6739 (2002).

[10] R. L. Puurunen, "Surface chemistry of atomic layer deposition: A case study for

the trimethylaluminum/water process," Journal of Applied Physics 97, 121301

(2005).

[11] S. Selvarasah et al., "A reusable high aspect ratio parylene-C shadow mask

technology for diverse micropatterning applications," Sensors and Actuators A

145, 306 (2008).

[12] C.-C. Kuo and T. N. Jackson, "Direct lithographic top contacts for pentacene

organic thin-film transistors," Applied Physics Letters 94, 053304 (2009).

[13] G. Horowitz et al., "Extracting parameters from the current-voltage

characteristics of field-effect transistors," Advanced Functional Materials 14,

1069 (2004).

[14] D. Kumaki et al., "Reducing the contact resistance of bottom-contact pentacene

thin-film transistors by employing a MoO(x) carrier injection layer," Applied


[15] D. J. Gundlach et al., "Pentacene TFT with improved linear region

characteristics using chemically modified source and drain electrodes," IEEE

Electron Device Letters 22, 571 (2001).

[16] F. Ante et al., "Submicron Low-Voltage Organic Transistors and Circuits

Enabled by High-Resolution Silicon Stencil Masks," IEDM Technical Digest,

21.6.1 (2010).

31

Chapter 3

Controlling Dipoles in the Gate

Dielectric of Organic Transistors

3.1 Introduction

Manipulating the channel charge density of transistors is important to

maximize their functionality. In conventional silicon, germanium, and III-V

semiconductor devices, the amount of charge carriers can be precisely controlled

through doping [1]; however, the development of a suitable doping method for

accurate control of charge carriers has proven to be relatively difficult for novel

32

semiconductors such as -conjugated organic semiconductors, carbon nanotubes, and

graphene. We introduce a method to control electric dipoles using self-assembled

monolayers (SAMs) with different anchor groups. We utilized the SAM dipoles in the

gate dielectric of organic field-effect transistors (OFETs) and successfully adjusted the

energy levels of electrons in the channel, which in turn resulted in a significant change

in the transistor turn-on voltage (VON). Moreover, this tuning of electron energy levels

can significantly improve the performance of n-channel (electron-conducting) OFETs,

which are generally not stable in air. These changes in the electrical properties of the

transistors were made without alteration to the desirable head group of the SAMs that

is crucial for optimal growth of the organic semiconductors [2, 3]. We believe that the

findings here can also be applied to controlling the electrical properties of other

nanoelectronic devices.

3.1.1 Self-Assembled Monolayers for Controlling Dipoles

Self-assembly is described as the autonomous organization of components into

patterns or structures without human intervention [4]. A SAM is formed on a target

substrate when the substrate is soaked in a solution containing SAM molecules or

exposed to a vapor containing the molecules as shown in Figure 17.

33

Figure 17. Formation of SAM on a hydroxyl-terminated substrate and investigation

of the SAM structure using GIXD.

When appropriate conditions are maintained, the SAM molecules form a highly

ordered and densely packed structure [5]. These conditions include temperature,

solvent, pH, vapor pressure, concentration, and reaction time. Because the thickness

of SAMs is only few nanometers, grazing incidence X-ray diffraction (GIXD), where

an incident X-ray is almost horizontal to a target surface, is used to investigate their

packing structure [6].

Previously, several research groups demonstrated the use of SAMs to control

the threshold voltage (VTH) of OFETs [7-11]. SAMs have also been used to control

the Schottky barrier between organic semiconductors and metals [12, 13]. In these

previous studies, the common approach to control the electrical properties has been to

use electron-withdrawing or -donating head groups of the SAMs. However, it was

shown that the mobility of organic transistors can vary by more than one order of

magnitude with different head groups [7, 8]. The chemical moieties at the surface of

34

the SAMs can significantly impact the morphology and the charge transport of the

subsequently deposited semiconductor. Moreover, some head groups may transfer

charges to the semiconducting layer [7, 14], which can cause a significant change in

the VTH of OFETs [15, 16]. Thus, the SAM-induced dipoles are insufficient to explain

the voltage shifts in these previous studies although several groups have attempted to

interpret the shifts using the dipoles [7-9]. To date, no experimental demonstration

has been reported to explain accurately the voltage shifts caused by the SAMs.

In this study, we utilized octadecylphosphonic acid (OPA) and octadecylsilane

(OTS) to form the SAM, as shown in Figure 18.

Figure 18. Chemical structure of OPA and OTS molecules.

These two molecules have the same methyl head group and alkyl chain. Therefore,

the OPA and OTS SAMs are appropriate to study the influence of SAM-induced

electric dipoles without the secondary effects mentioned above. Previous works have

already demonstrated the usage of both SAMs in the gate dielectric of OFETs [2, 3,

35

17-19]. However, we found that the OPA and OTS SAMs on aluminum oxide (AlOX)

generated different dipole moments and built-in voltages, entirely due to the difference

of their anchor groups, while maintaining the same interface between the SAMs and

organic semiconductor. Our approach is described in Figure 19.

Figure 19. Schematic of OPA and OTS SAMs in the gate dielectric of OFETs. Both

SAMs have nearly identical interface to the semiconducting layer, so the charge

transport on the semiconductor channel is not affected by a choice of SAM materials.


An AlOX layer was grown on arsenic-doped silicon wafers (< 0.005 Ω·cm) in

an atomic layer deposition (ALD) chamber. The ALD process ran at 350 ˚C for 70

cycles using ozone as the oxidizer and trimethylaluminum (TMA, Al(CH3)3, Air

Liquide Co.) as the aluminum source. For the formation of the OPA SAM, the AlOX

wafer was immersed in an ethanol (anhydrous grade from Sigma Aldrich Co.) solution

containing 3 mM of OPA (Alfa Aesar Co.) for 20 hours in air. The OTS SAM was

made by immersing the AlOX wafer in a trichloroethylene solution containing 5 mM

36

of octadecyltrichlorosilane (TCI America Co.) for 30 minutes in a nitrogen

atmosphere. The OPA and OTS samples were rinsed with acetone and sonicated in

acetone and toluene for 5 minutes. For capacitance measurements, 100-nm-thick

circular gold electrodes (approximately 5×10-4

cm2) were thermally evaporated

through shadow masks on the samples.

Three organic semiconductors are used in this study: buckminsterfullerene

(C60, C60, Alfa Aesar Co.), N,N'-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide

(PTCDI-C13, C50H62N2O4, Sigma Aldrich Co.), and pentacene (C22H14, Sigma Aldrich

Co.). The chemical structures of these materials are in Figure 20.

Figure 20. Chemical structures of organic semiconductors used to make OFETs.

The semiconducting molecules were thermally evaporated on the OPA/AlOX and

OTS/AlOX gate dielectrics. During the evaporation, a quartz crystal monitor

37

controlled the thickness of each layer to be 40 nm (at a rate of 0.2 Å /s), and C60,

PTCDI-C13, and pentacene layers were heated at 90, 120, and 60 ˚C, respectively.

Source/drain electrodes of gold were thermally evaporated and patterned on the

semiconducting layers using shadow masks. The electrodes had a channel length of L

= 110 m and a channel width of W = 970 m. For capacitance measurements of

pentacene metal–insulator–semiconductor (MIS) devices, 100-nm-thick circular gold

electrodes (approximately 5×10-4

cm2) were thermally evaporated through shadow

masks on the pentacene layer. The OFET structure used in this study is shown in

Figure 21.

Figure 21. Schematic of OFETs.


3.3.1 Current-Voltage Characteristics of Organic Transistors

The capacitance values of the OPA/AlOX and OTS/AlOX dielectrics were

measured to be 0.455 (±0.004) F/cm2 and 0.465 (±0.001) F/cm

2, respectively. The

current-voltage characteristics of the OFETs were first measured in a nitrogen

38

atmosphere. The drain current (ID) vs. gate-source voltage (VGS) curves in saturation

mode are shown in Figure 22.

Figure 22. ID-VGS data of OFETs (saturation mode) measured in a nitrogen

atmosphere. The red lines represent the data from OPA/AlOX, and the blue lines are

the data from OTS/AlOX.

For all three types of OFETs similar amounts of negative voltage shifts were observed

in the ID-VGS curves for the OPA/AlOX gate dielectric compared to the OTS/AlOX case.

In order to quantify the voltage shifts, we compared the VON, which we define as the

VGS where the first derivative of ID-VGS curve is zero. [Note: The VTH, defined as a

fitting parameter in equation (1), depends on the mobility (FET) and does not have a

clear physical meaning in organic transistors [20]. Instead, the VON was measured

because it indicates when the conducting channel starts to be induced in OFETs.]

39

2

D,SAT FET g GS TH (1)2

WI C V V

L

(W: channel width, L: channel length, and Cg: gate capacitance)

The VON values for OPA/AlOX were always measured to be more negative than those

for OTS/AlOX, as summarized in Figure 23: C60 (-0.35 V), PTCDI-C13 (-0.35 V),

and pentacene (-0.38 V). These consistent voltage shifts imply that the OPA and OTS

SAMs have certain general effects on the OFETs, not depending on the nature of the

semiconductor material.

Figure 23. Average VON data of OFETs on OPA/AlOX and OTS/AlOX. The error

bars represent standard deviations.

C60 PTCDI-C13 Pentacene-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

VO

N (

V)

40

3.3.2 Pentacene Metal–Insulator–Semiconductor Capacitors

Capacitance-voltage (C-V) measurement on MIS devices provides another

experimental evidence of the voltage shifts. In order to compare the C-V profiles

between OPA/AlOX and OTS/AlOX, we extracted the flatband voltage (VFB) [21] of

pentacene MIS capacitors. [Note: The VFB is defined as the gate voltage where no

charge exists at the dielectric-semiconductor interface.] As shown in Figure 24, the

average difference in VFB between OPA/AlOX and OTS/AlOX was 0.41 V, which is

close to the VON differences.

Figure 24. C-V data of pentacene MIS structure measured in a nitrogen atmosphere.

The schematic is the MIS structure used in the measurements, and the OPA and OTS

SAMs located between pentacene and AlOX.

41

This VFB result and the voltage shifts in the ID-VGS curves imply that the electron

energy levels in the channel region for OPA/AlOX are lower than those for OTS/AlOX.

For all the devices the gate electrode and the semiconducting layer remained the same,

and only the SAM layer on AlOX was modified. Also, the OPA and OTS SAMs are

assumed to have the same amount of charge transfer to the semiconducting layer, if

there is any, due to the same headgroup. Therefore, we attributed these voltage-shift

phenomena to a built-in potential inside the gate dielectric and verified it with the

following experiments.

3.4 Physical Structure of Self-Assembled Monolayers

Before studying the built-in potential inside the gate dielectric, we investigated

the physical structure of the OPA and OTS SAMs on AlOX because morphological

difference on the substrate may result in different electrical characteristics of organic

semiconductor deposited above. Simple measurements were first performed to

measure contact angle and surface roughness. The static contact angle of water for

both SAMs was 106° while that of the AlOX was only 9°. The root-mean-square

roughness of the AlOX and both SAMs was measured to be 0.2 nm with an atomic

force microscopy (AFM). These results showed that surface energy and surface

roughness of both SAMs are essentially the same and are not the key factors that

affected the semiconducting layer.

42

3.4.1 Grazing Incidence X-Ray Diffraction

We conducted GIXD measurements in order to see whether the OPA and OTS

SAMs had different packing structures or not. If they did, they could result in

different molecular parking and morphology on the organic semiconducting layer.

These measurements were performed at the Stanford Synchrotron Radiation

Lightsource (SSRL) using beamline 11-3 with a photon wavelength of 0.09758 nm.

The scattering intensity was detected on a 2-D image plate with a pixel size of 150 μm

(2300 × 2300 pixels). The detector was located at a distance of 399.8 mm from the

sample center. The incidence angle was chosen in the range of 0.10–0.12° to optimize

the signal-to-background ratio.

GIXD revealed that the OPA and OTS SAMs were crystalline with nearly

identical packing structure, as evidenced by the presence of a Bragg rod at Qxy = 1.49

Å-1

for both SAMs in Figure 25.

43

OPA SAM OTS SAM

Figure 25. GIXD images of OPA and OTS SAMs on AlOX. The Bragg rods for both

SAMs are located at Qxy = 1.49 (±0.012) Å-1

, and the full width at half maximum

(FWHM) of the peaks is 0.11 ± 0.007 Å-1

. This feature is indicative of a crystalline

monolayer with a 4.2 Å hexagonal lattice constant [3].

3.4.2 X-Ray Reflectivity

X-ray reflectivity (XRR) measurement provides thickness and density

information of multilayers using a reflected X-ray beam on a flat surface. This

technique is useful to estimate the thickness and density of a SAM, as shown in Figure

26.

44

Figure 26. Application of XRR to measure thickness and density of SAM.

XRR were carried out at the Stanford Nanocharacterization Laboratory using a

PANalytical's X'Pert Materials Research Diffractometer equipped with a sealed-tube

source of copper and a multilayer X-ray mirror. We performed the fitting of simulated

profiles to the experimental data using the X'Pert Reflectivity simulation software

based on the Parratt formalism [22] and estimated the density and the thickness of

each layer. The packing densities of the OPA and OTS SAMs were calculated from

equation (2).

2

3

A

(molecules/nm )

(g/nm ) (nm) (molecules/mol)= (2)

(g/mol)

packing density

density thickness N

molar mass

(NA: Avogadro constant)

The XRR data from the OPA/AlOX and OTS/AlOX samples are shown in Figure 27.

45

Figure 27. XRR data of OPA and OTS SAMs on AlOX in arbitrary units. The solid

black lines are fitted data, and the green and blue dots are measured data.

Using the measured data, we estimated thicknesses and packing densities of the OPA

and OTS SAMs in Table 2.

OPA SAM OTS SAM

Thickness (nm) 2.42 (±0.04) 2.69 (±0.07)

Density (molecules/nm2) 4.55 (±0.16) 5.36 (±0.64)

Table 2. Thickness and packing density data of OPA and OTS SAMs from XRR.

0 2 4 6 810

0

105

1010

2 (deg)

Inte

nsity (

arb

itra

ry u

nits)

46

All the measurement methods that we used (water contact angle, AFM, GIXD, and

XRR) indicate there is no measurable difference in physical structure between the

OPA and OTS SAMs on AlOX.

3.5 Effects of the Dipoles Studied by Work Function Measurement

From the above experiments for the physical structure of the OPA and OTS

SAMs in the previous sections, we confirmed that they did not show any sufficient

evidence account to the observed voltage shifts in the ID-VGS and C-V curves. After

eliminating those factors, we focused on the built-in potential generated by the SAMs

(VSAM). The VSAM is related with two types of dipole moment as shown in Figure 28:

molecular dipole and bonding dipole.

Figure 28. Schematic of how the VSAM is generated.

Because each atom in a SAM molecule has different amount of electronegativity, the

molecule itself contains electric dipoles. When the SAM molecule forms a chemical

bonding on a substrate, bonding dipoles are generated.

47

We measured the work function of OPA- and OTS-treated metals in order to

confirm that the voltage shifts are due to the SAM dipoles. As Figure 29 shows,

SAMs can modify intrinsic work function of metal depending on their dipole moments.

Figure 29. Effects of SAM on measured work function of metal.

Because the surface of aluminum rapidly oxidizes once exposed to air, the influence of

the SAM dipoles on aluminum is nearly identical to the influence on AlOX. So, the

difference of VSAM between OPA/AlOX and OTS/AlOX can be measured directly

through modification of aluminum work function as Figure 30 shows.

48

Figure 30. Work function measurement for dipoles from OPA/AlOX and OTS/AlOX.

For this experiment another set of samples was prepared. A 25-nm-thick

aluminum was deposited on silicon wafers in an e-beam evaporator, followed by the

OPA and OTS SAM treatment described earlier. The work functions of the samples

were measured at beamline 8-1 of SSRL. A bias of -9.87 V was applied to the

samples, and the photoemission spectra were collected by a hemispheric analyzer of

electron energy. We used the low-energy cutoff of the photoemission spectra to

determine the work functions.

We observed that the work function of the OPA-treated aluminum was 0.50 eV

lower than that of the OTS-treated aluminum. This result suggests that the surface

potential of the OPA/AlOX is 0.50 V higher than the OTS/AlOX, which is comparable

to the voltage shifts in the ID-VGS and C-V curves. The only parts of the structures that

differentiate OPA and OTS are their anchor groups, which bind to the surface. Thus,

our work function data indicate that the chemical bonds between the SAMs and the

oxide surface can have a significant effect on the overall dipoles. Our observations are

consistent with previous theoretical calculation that showed that the bond dipoles

between a thiolate SAM and metals were dependent on the metal [23].

49

The effects of SAM dipoles in the MIS structure are described in Figure 31,

where the built-in potential generated by the dipoles (VSAM) is modeled as a DC

battery.

Figure 31. Effects of SAM dipoles in the MIS structure. In this analysis it is assumed

that the flatband voltage without the dipoles is zero.

With the polarity of dipole in this example, the SAM dipoles accumulate electrons at

the dielectric-semiconductor interface by lowering the electron energy levels of the

50

semiconductor. In order to remove the accumulated charges, an external voltage of

the same magnitude as the VSAM with the opposite polarity needs to be applied at the

gate. When this voltage (-VSAM) is applied, the accumulated charges are removed, and

this state is defined as the flatband condition [15]. Using the same analogy, the

voltage shifts in the ID-VGS and C-V curves due to the SAM dipoles can be estimated

as -VSAM. In order to check the validity of the explanation, we varied the

thickness of the AlOX layer and measured the VON difference of pentacene OFETs

between the OPA/AlOX and OTS/AlOX gate dielectrics in a nitrogen atmosphere. As

shown in Table 3, the difference of VON between OPA/AlOX and OTS/AlOX did not

depend on the thickness of the gate dielectric.

Gate Dielectric 4.5 nm AlOX 9.5 nm AlOX

OPA SAM OTS SAM OPA SAM OTS SAM

Cg (F/cm2) 0.455 (±0.004) 0.465 (±0.001) 0.328 (±0.002) 0.334 (±0.007)

VON (V) -0.38 (±0.05) -0.03 (±0.04) -0.55 (±0.06) -0.18 (±0.03)

VON (V) 0.35 0.37

Table 3. VON data of pentacene OFETs (saturation mode) with different AlOX

thickness, measured in a nitrogen atmosphere.

In previous publications, a different interpretation about the voltage shifts by

SAM dipoles was offered [7-9]. The authors asserted that an electric field created by a

SAM should be eliminated by external voltage in order to compensate the voltage shift

from the dipoles: -ESAM × d (ESAM: built-in electric field of SAM and d: gate dielectric

51

thickness). However, as Figure 31 shows, the accumulated charges due to the SAM

dipoles are removed simply by applying the opposite voltage of the built-in potential

of the SAM (-VSAM), not by removing the electric field.

3.6 Air-Stable n-Channel Organic Transistors

In general n-channel OFETs do not show stable operation in air due to the

presence of electron-trapping species, or oxidants, such as water and oxygen [24-28].

As most thin-film organic semiconductors are sensitive to chemicals and high

temperature, and are mechanically weak, the bottom-gate geometry is generally used

in OFETs. Because the oxidants diffuse into the transistor channel region through the

grain boundaries of organic semiconducting layer, electrons in n-channel OFETs can

be quenched by such oxidants in air. However, such charge trapping does not occur in

p-channel OFETs, because the current conduction is carried by holes, instead of

electrons. This mechanism is qualitatively illustrated in Figure 32.

52

Figure 32. Simplified energy band diagram of p- and n-channel OFETs and the

effects of electron-trapping species, which diffuse into the transistor channel through

the grain boundaries of organic semiconducting layer.

In spite of a great deal of effort devoted to increasing the electron affinity of organic

semiconductors, as shown in Figure 33, so that they are not affected by the oxidation,

only few materials showed mobilities greater than 1 cm2/V·s in n-channel OFETs

measured in air [29].

Figure 33. Lowering semiconductor energy levels to improve air stability of n-

channel OFETs.

53

This mechanism of poor performance on n-channel OFETs can be related with the

built-in potential inside the gate dielectric described in the previous section.

According to our experimental results, given the same applied VGS, OFETs with

OPA/AlOX are affected by more positive electric potential compared to OTS/AlOX, as

shown in Figure 34.

Figure 34. Equivalent circuit model that describes the effect of SAM dipoles between

OPA and OTS in OFETs.

Because of this effect, n-channel OFETs with the OPA SAM have lower electron

energy levels in the channel region, and thus the air stability can be more improved

than n-channel OFETs with the OTS SAM.

In order to prove this theory, we monitored the stability of C60 and PTCDI-

C13 OFETs on both types of gate dielectrics over an extended period in air (see Figure

35 and Figure 36). While the C60 and PTCDI-C13 OFETs were tested in air, the

transistors were covered with aluminum foil between each measurement, and the

temperature and the relative humidity of the laboratory were maintained to be

approximately 20 °C and 49 %, respectively. The n-channel OFETs on the OPA/AlOX

54

gate dielectric showed much less degradation than those on the OTS/AlOX gate

dielectric. For the transistors on the OTS/AlOX gate dielectric, the mobility was

observed to drop by more than one order of magnitude after only two days in air. The

decrease was accompanied by an increase in the corresponding VTH. However, for the

OPA/AlOX gate dielectric, the mobility of C60 OFETs was observed to change only

slightly from 1.69 (±0.14) to 1.65 (±0.11) cm2/V·s after 24 hours and maintained at

0.73 (±0.06) cm2/V·s even after one week in air. The mobility of PTCDI-C13 OFETs

on the OPA/AlOX gate dielectric was measured to be still 0.09 (±0.03) cm2/V·s after

one week exposure in air. Whereas, the C60 and PTCDI-C13 OFETs on OTS/AlOX

did not turn on after 1-week and 6-week exposure in air, respectively.

55

Figure 35. Long-term stability of C60 OFETs in air. The mobility and the threshold

voltage parameters were extracted from equation (1).

0 20 40 60 80 100 12010

-4

10-3

10-2

10-1

100

101

Days

Mobili

ty (

cm

2/V

s)

0 20 40 60 80 100 1200

0.5

1

1.5

2

2.5

Days

Thre

shold

Voltage (

V)

56

Figure 36. Long-term stability of PTCDI-C13 OFETs in air. The mobility and the

threshold voltage parameters were extracted from equation (1).

0 20 40 60 80 10010

-5

10-4

10-3

10-2

10-1

100

101

Days

Mobili

ty (

cm

2/V

s)

0 20 40 60 80 1000

0.5

1

1.5

2

2.5

Days

Thre

shold

Voltage (

V)

57

We have shown that the n-channel OFETs on OPA/AlOX have better air

stability than the OFETs on OTS/AlOX, due to the lowered electron energy levels in

the channel [24] (see Figure 37).

Figure 37. Qualitative description of energy band diagrams (a) in the gate-to-channel

direction and (b) in the horizontal direction from the source to the drain electrodes.

Due to the different dipole moments, the LUMO and the HOMO levels are different at

the dielectric-semiconductor interface between OPA/AlOX and OTS/AlOX.

58

This lowering of the energy levels by the OPA dipoles is equivalent to the increase in

the electron affinities of organic semiconductors, which enables air-stable n-channel

operation [25, 27]. The performance of n-channel OFETs made of C60 and PTCDI-

C13 has been known to rapidly degrade in air due to their low electron affinities [26,

30]. Previously, C60 and PTCDI-C13 OFETs with mobility values of greater than 1

cm2/V·s have only been reported in an inert environment [3, 31, 32]. However, by

using the OPA/AlOX gate dielectric, the energy levels of electrons in the channel are

lowered by the dipoles. Thus, the driving force for oxidizing the channel region is

weakened, and the air stability of n-channel organic transistors is significantly

improved.

3.7 Morphology of Organic Semiconducting Layers

We examined the morphologies of organic thin films on OPA and OTS using

GIXD and AFM to determine if morphological differences could contribute to

improved air stability when using the OPA/AlOX gate dielectric. GIXD and AFM

results indicate that the morphologies of C60 and PTCDI-C13 thin films are nearly

identical on the OPA and OTS SAMs. The GIXD images of the C60 thin films

displayed predominantly isotropic crystallite orientations, and GIXD of the PTCDI-

C13 thin films showed well-ordered crystalline morphologies. The GIXD and AFM

measurement results of C60 and PTCDI-C13 films are in Figure 38.

59

OPA SAM OTS SAM

C60

(40 nm)

PTCDI-C13

(40 nm)

Figure 38. GIXD and AFM images of C60 and PTCDI-C13 on OPA and OTS SAMs.

60

In contrast to C60 and PTCDI-13, there were significant morphological

differences between pentacene films deposited on the OPA and OTS SAMs as shown

in Figure 39. For all of the pentacene films (40-nm- and 5-nm-thick layers on both

SAMs) GIXD revealed that there is an increased preference for the ―thin-film‖ phase

when pentacene is deposited on OPA, and pentacene deposited on OTS displayed a

greater relative fraction of the ―bulk‖ phase (see Table 4) [3, 33]. This result indicates

that the surface of the OPA SAM promotes a 2-dimensional growth compared to the

OTS surface. AFM images of the pentacene films corroborated the conclusion in

Figure 39; the pentacene films exhibited rougher surface morphologies on OTS than

the films deposited on OPA. The different morphologies of the pentacene films on

OPA and OTS were due to a difference in the relative binding energy of pentacene to

the respective SAMs. As the thickness of the OPA and OTS SAMs was measured to

be 2.42 (±0.04) nm and 2.69 (±0.07) nm, respectively, from XRR measurements, it is

possible that different orientations of the top methyl groups between the two SAMs

may result in the different binding energies.

61

OPA SAM OTS SAM

Pentacene

(40 nm)

Pentacene

(5 nm)

Figure 39. GIXD and AFM images of pentacene on OPA and OTS SAMs.

62

Magnified

GIXD Image

Thin-film Peak

(arbitrary units)

Bulk Peak

(arbitrary units)

Ratio

(Thin film/Bulk)

Pentacene

(40 nm)

On OPA

3.4 × 106 4.6 × 10

5 7.39

On OTS

3.9 × 105 1.1 × 10

6 0.35

Pentacene

(5 nm)

On OPA

3.0 × 104 2.2 × 10

3 13.64

On OTS

4.4 × 104 8.6 × 10

3 5.12

Table 4. Integrated peak intensity of selected GIXD from pentacene layers. In the

magnified GIXD images from Figure 39, the blue and red boxes indicate the

diffraction peaks from the thin-film and bulk phases, respectively. A meaningful

figure of merit is the ratio of the two peaks, which is significantly higher for the

pentacene deposited on OPA compared to OTS. These ratios indicate that the

pentacene films deposited on OPA have a greater relative fraction of the thin-film

phase, and a lower relative fraction of the bulk phase, compared to the pentacene films

on OTS.

63

In the case of C60, the possibilities for alteration of the crystal structure were reduced

due to the high-symmetry structure of the molecule. Most likely the alkyl chains at

the end of the PTCDI-C13 core minimized the influence of the SAM on the

crystallization behavior. However, the pentacene core was in direct contact with the

SAM interface, and the SAM can be used to alter the pentacene morphology.

The difference in the pentacene morphologies resulted in a larger variations in

mobility values of OFETs than observed for C60 and PTCDI-C13, as shown in Table

5. Compared with the OTS/AlOX gate dielectric, the mobility of pentacene OFETs on

the OPA/AlOX gate dielectric was higher due to larger pentacene grains on the OPA

SAM as shown in Figure 39. However, this variation is much smaller than previously

reported pentacene OFETs on SAMs with different head groups [7-11].

Organic semiconductors

in OFETs

FET (cm2/V·s)

OPA SAM OTS SAM

C60 1.64 (±0.02) 1.59 (±0.06)

PTCDI-C13 0.65 (±0.08) 0.54 (±0.04)

Pentacene 1.25 (±0.05) 0.89 (±0.05)

Table 5. Mobility data of OFETs (saturation mode) on OPA/AlOX and OTS/AlOX,

measured in a nitrogen atmosphere. Equation (1) was used to extract the mobilities.

Values in parenthesis refer to standard deviations.

64

3.8 Conclusion

We manipulated the electric dipoles in the gate dielectric by using SAMs with

different anchor groups. The different dipoles between the OPA and OTS SAMs

resulted in significant voltage shifts in the ID-VGS and C-V curves of organic transistors.

Through the effects of dipoles, we suppressed the trapping of electrons due to air

exposure and greatly enhanced the air stability of existing organic semiconductors in

n-channel OFET applications. The potential difference caused by the OPA and OTS

SAMs on AlOX was measured to be 0.41–0.50 V. In nanoelectronics, where low

voltage is generally used, such potential difference can greatly modify the electrical

characteristics of the devices. Therefore, the OPA and OTS SAMs on AlOX can be

utilized in nanoelectronic devices composed of other semiconductor materials—for

which a precise method of doping does not exist—to induce different electric

potentials while maintaining nearly identical interface between the SAMs and the

semiconducting layer.

65

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69

Chapter 4

Engineering Metal Gate Electrodes

for Organic Transistors

4.1 Introduction

In Chapter 3 electric dipoles inside the gate dielectric are manipulated using

self-assembled monolayers (SAMs) in organic field-effect transistors (OFETs). These

dipoles generate a built-in potential across the SAMs and change the threshold voltage

(VTH) of OFET. In addition to using the SAMs to control the VTH, engineering the

work function of the gate electrode can result in the same effect in theory.

70

A simple mechanism of how metals affect the characteristics of electronic

devices is shown in Figure 40.

Figure 40. Description of how different metals affect the characteristics of electronic

devices. In this example Metal 1 has a lower work function (WF) than Metal 2.

Comparing Metal 1 and Metal 2 in Figure 40, they have different levels of the highest

electron energy. When both metals are used in electronic devices, the difference in

electron energy results in different electric voltages. We can utilize such different

voltages to control the characteristics of the devices.

More specifically, the effect of metal gate electrode on the VTH of field-effect

transistors (FETs) is described in Figure 41, where the WF of the gate is assumed to be

less than that of the semiconductor.

71

Figure 41. Energy band diagrams of FET in the gate-to-channel direction. The WF of

the gate electrode is chosen to be less than that of the semiconductor. No voltage is

applied to the gate electrodes.

When the electrodes are not electrically connected, the energy levels remain constant

as shown in the left diagram of Figure 41. Once they are electrically connected,

electrons move from the gate electrode to the dielectric-semiconductor interface in

order to align the Fermi levels. As a result, the device shows similar conditions of

having a positive voltage at the gate electrode, compared to a device where the WF of

gate and semiconductor are the same. This effect can be generalized that lowering the

WF of the gate electrode results in a negative shift in the VTH of FETs, and vice versa.

72

Figure 42. Controlling VTH of FETs using the gate electrodes with different WF. No

voltage is applied to the gate electrodes.

As summarized in Figure 42, the gate electrode of high-WF metal (EF,M > EF,S) results

in a positive VTH shift, and the gate electrode of low-WF metal (EF,M < EF,S) results in a

negative VTH shift. If two FETs are made of the same device structure and materials

except their gate electrodes, the difference on the VTH is the same as the WF difference

between the two gate electrodes as described in equation (3) [1].

TH_1 TH_2 F_1 F_2 (3)V V W W

Engineering the gate electrode has advantages over the SAM method of

Chapter 3 in a circuit fabrication. In circuit designs the VTH of each transistor needs be

controlled in order to maximize the circuit performance as depicted in Figure 43.

73

Figure 43. Controlling VTH of transistors to maximize their performance.

Because some transistors may need more positive VTH while the others may need more

negative VTH, the SAM method requires that different SAMs are to be patterned on a

substrate as shown in Figure 44.

Figure 44. An example of patterned SAMs on a substrate to achieve both negative

and positive VTH control.

Yet, the patterning of SAMs is more challenging than that of metals because most

high-quality SAMs are made by dipping in a solution or using a vapor treatment.

Photolithography and etching processes can be used, but these processes damage or

74

contaminate the SAMs. Because the quality of semiconductor-dielectric interface

greatly affects the device performance, the SAM method is not a good choice for

controlling the VTH in organic circuits. However, metals can be easily patterned by

shadow masking or photolithography process with a minimum impact on the

semiconductor-dielectric interface.

High-capacitance gate dielectric is essential in order to realize the VTH control

by modifying the WF of the gate electrode. The difference of WF is up to

approximately 1.5 eV among the metals used in semiconductor device processing.

When low-capacitance gate dielectric is used, where a supply voltage (VDD) of more

than 10 V is needed, VTH shifts by different metal gate electrodes can be negligible.

Comparison between high- and low-capacitance gate dielectrics is shown in Figure 45.

Figure 45. Importance of high gate capacitance (Cg) for VTH control using different

gate WF. The WF difference is assumed to be 0.5 eV in this example. Because the

range of VTH control is limited, FETs with low Cg cannot achieve enough modulation

in their drain current.

75

As shown in the figure above, the effect of VTH control with low Cg is less significant

compared to high Cg.

The control of VTH using different metal gates of OFETs was previously

demonstrates by Nausieda et al. using aluminum (Al) and platinum (Pt) electrodes [2].

However, they used low-capacitance gate dielectric made of parylene and a high VDD

of 10 V. At such a high VDD, relatively small VTH control, approximately 0.5 V, is not

effective. For overcoming this problem, we used aluminum oxide (AlOX) fabricated

by atomic layer deposition (ALD), described in Chapter 2, and achieved a high Cg of

approximately 0.4 F/cm2. This capacitance value is 18 times higher than that of

previous work [2].

We utilized two types of gate electrode structures to control the VTH of OFETs:

dual-metal gates and bilayer metal gates. Detailed designs and their operations are

discussed below.

4.2 Dual-Metal Gates

Titanium (Ti) and Pt were used as gate electrode materials for low WF and high

WF, respectively. There are several reasons for choosing this material combination.

Their as-deposited samples have shown quite large WF difference of more than 1 eV

[3]. Also, both metals are compatible with our fabrication process of OFETs such as

ALD of AlOX and a formation of SAM for passivating the traps on the AlOX surface.

76

4.2.1 Device Fabrication

A highly doped silicon wafer (As-doped, <0.005 Ω-cm) with a thermally

grown 300-nm-thick silicon dioxide (SiO2) layer was used as a substrate. For the

formation of the gate electrodes, Ti (25 nm) and Pt/Ti (20/5 nm) layers were deposited

in an e-beam evaporator at a rate of 0.2 Å /s. An AlOX layer was deposited by ALD on

both metal layers for making the gate dielectric. The ALD process used

trimethylaluminum (TMA, Al(CH3)3, Air Liquide Co.) and ozone as the source

materials and repeated 70 cycles with a substrate heating at 50 ˚C. The samples were

then immersed into a tetradecylphosphonic acid (TPA, CH3(CH2)13PO(OH)2, Alfa

Aesar Co.) solution (5 mM in anhydrous ethanol from Sigma Aldrich Co.) for 1 day to

form a SAM on AlOX. This SAM is known to passivate the surface traps on AlOX and

to improve the morphology of organic semiconductor deposited above.

On the TPA/AlOX/Ti and TPA/AlOX/Pt/Ti samples, an organic

semiconducting layer (40 nm) was deposited with a sample heating at 50 °C in a

thermal evaporator. The rate of evaporation was monitored by a quartz crystal and

maintained at 0.2 Å /s. We used pentacene (C22H14, Sigma Aldrich Co.) and

buckminsterfullerene (C60, C60, Alfa Aesar Co.) molecules for n- and p-channel

OFETs, respectively. Finally, we thermally evaporated a gold layer (40 nm) of source

and drain (S/D) electrodes with shadow masks. The electrodes had a channel length of

L = 110 m and a channel width of W = 970 m, measured by optical microscopy. A

schematic of the OFETs is shown in Figure 46.

77

Figure 46. Schematic of OFETs with metal gate electrode (Ti or Pt).

For capacitance measurement of the gate dielectric, gold electrodes (100 nm) were

thermally evaporated onto the TPA SAM without the semiconducting layer.

4.2.2 Electrical Measurement

All electrical measurements were performed in a nitrogen glovebox in order to

remove undesirable effects from ambient oxidants such as water and oxygen. The Cg

on both gate electrodes was measured to be 0.40 F/cm2. The OFETs were measured

in saturation mode as shown in Figure 47.

78

Figure 47. (Drain current)0.5

vs. gate-source voltage (VGS) curves for pentacene and

C60 OFETs on Ti (red lines) and Pt (blue lines) gate electrodes. VDS refers to the

drain-source voltage, and the dotted black lines are linear fitting data for VTH

extraction.

For both pentacene and C60 OFETs, the two types of gate electrodes resulted in a VTH

shift of 0.5 V. While changing the VTH, the gate electrodes barely affected the field-

effect mobility (FET) of the OFETs. For maximizing current output from the OFETs,

Pt and Ti gates are preferred for p-channel pentacene and n-channel C60 OFETs,

respectively. Comparing the maximum overdrive voltage, defined as (VGS-VTH)MAX,

the Pt gate resulted in a 41.6% higher value than Ti for the p-channel, and the Ti gate

resulted in 33.6% higher value than Pt for the n-channel OFETs. Detailed device

parameters are summarized in Table 6.

79

Pentacene OFETs C60 OFETs

Ti Gate Pt Gate Ti Gate Pt Gate

FET (cm2/V·s) 0.60 (±0.02) 0.67 (±0.04) 1.94 (±0.08) 2.09 (±0.08)

VTH (V) -1.25 (±0.04) -0.73 (±0.04) 0.51 (±0.04) 1.01 (±0.02)

VTH (V) 0.52 0.50

IMAX/IMIN (×106) 1.12 (±1.21) 0.91 (±0.75) 0.61 (±0.51) 1.69 (±1.50)

Table 6. Device parameters of the OFETs measured inside a nitrogen atmosphere.

FET and VTH data were extracted by fitting the measured data in equation (4).

2


WI C V V

L

(W: channel width and L: channel length)

The amount of VTH shifts (VTH ≈ 0.5 V) in our OFETs is almost identical to a result

from a previous study [2]; however, the VDD in this study is 4 times smaller than the

previous work (2.5 V vs. 10 V). Thus, the effect of VTH, which can be simply

quantified by VTH/VDD, is more significant in our work.

4.2.3 Work Function Measurement on Metal Gates

The VTH caused by the Ti and Pt gates, ~ 0.5 V, was smaller than the well-

known WF difference (WF) between the two metals (Ti: 4.33 eV; Pt: 5.65 eV) [3].

We attribute this difference to surface oxidations on the metal gate electrodes.

80

Previously, other researchers have found that the WF of oxidized metals were quite

different from their intrinsic values due to charged states between the metal and the

oxide layers [4, 5]. Especially, Ti is known to be easily oxidized absorbing water and

oxygen, which resulted in an abrupt increase of WF when it is exposed to oxygen [6].

During the fabrication of OFETs, the Ti and Pt gate electrodes were exposed to air

when transferring from the e-beam evaporator to the ALD machine. Therefore, the

surface of the metal gates in our OFETs was oxidized, and the WF values were

deviated from their intrinsic values. Different bonding dipoles between Ti-AlOX and

Pt-AlOX can also change the VTH. However, a thorough study on the oxidation state

and the bonding dipoles were beyond the scope of our research.

We performed WF measurements on the Ti and Pt gate electrodes using a

photo-electron spectrometer in order to get experimental evidence of the oxidation

mentioned above. The metal samples were also exposed to air while transferring from

the e-beam evaporator to the spectrometer. The WF of the Ti and Pt gates were

measured to be 4.89 (±0.01) eV and 5.39 (±0.02) eV, respectively, and their difference

was only 0.5 eV. Because there was no etching process in the WF measurements, we

measured the WF of oxidized Ti and Pt, instead of their intrinsic material property.

This measurement result confirms that the gate electrodes were oxidized and thus the

VTH caused by the Ti and Pt gates can be different from the WF between intrinsic Ti

and Pt.

81

4.2.4 Morphology of Organic Semiconducting Layers

Morphology of the pentacene and C60 semiconducting layers was examined

by atomic force microscopy (AFM) in order to check if a choice of the gate electrode

materials, Ti and Pt, can affect any property of the OFETs other than the VTH shifts.

The AFM results are shown in Figure 48.

Ti Gate Pt Gate

Pentacene

C60

Figure 48. AFM images of pentacene and C60 layers (40 nm) on Ti and Pt gate

electrodes.

82

As shown in the figure above, there is no difference in the surface morphology of the

semiconductors. This almost identical morphology is consistent with the similar FET

values between the two metal gate electrodes.

4.3 Bilayer Metal Gates

4.3.1 Introduction

In the previous section, we utilized the dual-metal gate electrodes made of Ti

and Pt in order to control the VTH of OFETs. However, this approach has a limitation

that the control of the VTH is discrete. To overcome this issue, we used a bilayer metal

structure to achieve continuous control of transistor current-voltage characteristics, as

shown in the right graph of Figure 49.

Figure 49. Controlling current-voltage (I-V) characteristics of OFETs by engineering

the gate electrodes. (Left) Dual-metal gate electrodes cause a discrete control, and

(right) bilayer metal gates provide a continuous control.

83

The structure of the bilayer metal gates used in this study is shown in Figure

50.

Figure 50. Structure of the bilayer metal gate electrodes in this study. The thickness

of the top Pt layer controls the WF at the “effective region” at the surface.

As the thickness of the top metal layer varies, the WF at the surface can be changed.

Thus, by changing the thickness of the top layer with few nanometers, much finer

control of the VTH can be achievable compared to the dual-metal gate electrodes.

These bilayer metal gate electrodes have been previously studied in silicon metal–

oxide–semiconductor field-effect transistors (MOSFETs) [7, 8]. [Note: Silicon

MOSFETs have a top-gate structure while our OFETs have a bottom-gate structure.

Thus, the top metal layer in this study is equivalent to the bottom metal layer in the

previous studies of silicon MOSFETs.] Among several proposed explanations, metal-

metal interdiffusion is one of the most promising mechanisms [8, 9]. The authors

suggested that diffusion of the bottom layer metal through the top layer to the

84

dielectric/metal interface can modify the gate WF and that the thickness of the top

layer can control the extent of the diffusion and hence the change in WF.

Previous study of bilayer metal gates on silicon MOSFETs showed that

forming gas annealing (FGA) at 400 °C on the gate electrode enhanced the metal-

metal interdiffusion [8]. This enhancement resulted in more gradual change in the VTH

with different metal thicknesses, compared to the devices without thermal annealing.

However, plastic substrates, for making flexible organic transistors and circuits,

cannot withstand such high temperature conditions. For this reason we utilized as-

deposited bilayer metal gates without thermal annealing although rigid silicon wafers

were used as substrates for a proof of concept. Pt and Al were used as the top and the

bottom metal layers, respectively. The reasons for this material combination are 1) Al

and Pt have a large WF difference (Al: 4.28 eV; Pt: 5.65 eV) [3] and 2) metal-metal

interdiffusion can be precipitated due to the low melting point of Al .

4.3.2 Device Fabrication

We used highly doped silicon wafers (As-doped, <0.005 Ω-cm) for a substrate

and a gate contact. After a removal of native oxide on the wafers by dipping into a 2%

hydrofluoric acid solution, bottom-gate bilayer metal electrodes that consisted of

bottom Al and top Pt were sequentially deposited in an e-beam evaporator. The

thickness of the top platinum layer was varied in 1, 1.5, 2, 2.5, 3, and 10 nm while the

thickness of the bilayers was fixed to 35 nm in order to minimize the roughness

variations on the surface of the gate electrodes. The gate dielectric was made of AlOX,

85

and its surface was passivated by a SAM. We used ALD, repeated 70 cycles and

assisted by ozone oxidant, to deposit AlOX on the gate electrodes at 50 ˚C. The

samples were then immersed into a TPA solution (5 mM in anhydrous ethanol from

Sigma Aldrich Co.) for 1 day to form the SAM. This SAM is known to passivate the

surface traps on AlOX and to improve the crystallinity of organic semiconductor

deposited above. We deposited an organic semiconducting layer (40 nm) of C60 (C60,

Alfa Aesar Co.) on the gate dielectric with a sample heating of 50 °C in a thermal

evaporator. The rate of evaporation was monitored by a quartz crystal and maintained

at 0.2 Å /s. Finally, we thermally evaporated a gold layer (40 nm) as the S/D

electrodes through shadow masks. The electrodes had a channel length of L = 110 m

and a channel width of W = 970 m. Figure 51 shows a schematic of the transistors.

For the characterization of the gate dielectric, gold electrodes (100 nm) were thermally

evaporated directly onto the gate dielectric without the semiconducting layer.

Figure 51. Schematic of the OFETs with bilayer metal gate.

86

4.3.3 Electrical Measurement

All electrical measurements were performed in a nitrogen glovebox in order to

remove undesirable effects from ambient oxidants such as water and oxygen. The Cg

was measured to be 0.39 F/cm2, except on the gate electrode with 1-nm-thick Pt.

Since the 1-nm-thick Pt could not form a continuous layer, a surface oxidation in the

bottom Al resulted in a lower capacitance of 0.36 F/cm2.

The C60 OFETs were tested in saturation mode (VDS = 2.5 V), and their FET

values were extracted to be 1.3 cm2/V·s for all devices. As shown in Figure 52, the

drain current (ID) vs. gate-source voltage (VGS) curves are shifted as the thickness of

the top Pt layer varies.

Figure 52. ID vs. VGS curves of the C60 OFETs with different thickness of top Pt

layer in saturation mode.

87

We use the turn-on voltage (VON), which we define as the VGS where the first

derivative of ID-VGS curve is zero, in order to quantify this shift. The relationship

between the VON and the top Pt thickness is summarized in Figure 53.

Figure 53. VON data of the C60 OFETs with different thickness of top Pt layer in

saturation mode.

From the figure above, it is clear that varying the thickness of the top Pt layer resulted

in a continuous control on the VON values.

4.3.4 X-Ray Photoelectron Spectroscopy on Metal Gates

As discussed earlier, the WF of the gate electrodes changes due to the diffusion

of the Al bottom layer through the Pt top layer. We performed X-ray photoelectron

0 2 4 6 8 10

0

0.2

0.4

0.6

Thickness of Pt, x (nm)

VO

N (

V)

88

spectroscopy (XPS) on the gate electrodes to study this diffusion. The angle between

the photoelectron detector and the substrate was lowered to be 3° in order not to

penetrate from the surface of the samples. In Table 7 the atomic concentration ratio of

Al increases as the thickness of the Pt layer decreases although all of the gate

electrodes were covered by Pt.

Ratio of atomic concentration at surface

Pt Al

1.5 nm Pt on Al 1.0 5.6

2.5 nm Pt on Al 1.0 0.7

10 nm Pt on Al 1.0 0

Table 7. XPS results at the surface of bilayer metal gate electrodes.

This result proves that Al atoms diffused into the surface of the Pt layer during the

deposition process without any intentional thermal treatment, as depicted in Figure 54.

The amount of diffusion was controlled by the thickness of Pt. As the ratio of Al

increases, the VON of the transistors decreases due to lowered gate WF.

89

Figure 54. Schematic model of how aluminum atoms diffuse into the top platinum

layer.

4.4 Conclusion

We have successfully demonstrated VTH control of the OFETs by engineering

the metal gate electrodes. Two methods were used to change the WF of the gate

electrodes: dual-metal gates and bilayer metal gates. In the first method Ti and Pt

gates resulted in discrete voltage shifts in the transistor ID-VGS curves by 0.5 V. The

second method with the bilayer metal gates achieved continuous control of the voltage

shifts over 0.6 V. Neither methods affect the charge transport of the organic

semiconducting layer and can be easily utilized in the fabrication of more complex

circuits. Our approaches are also potentially applicable for other classes of

semiconducting materials whose intrinsic properties are difficult to modify. Moreover,

the maximum substrate temperature during the entire fabrication was no more than 50

˚C, which is compatible with most flexible substrates having relatively large thermal

expansion coefficients and low melting temperature.

90

References



[2] I. Nausieda et al., "Dual Threshold Voltage Organic Thin-Film Transistor

Technology," IEEE Transactions on Electron Devices 57, 3027 (2010).

[3] H. B. Michaelson, "Relation between an Atomic Electronegativity Scale and

Work Function," IBM Journal of Research and Development 22, 72 (1978).

[4] Y. I. Semov, "Work Function of Oxidized Metal Surfaces and Estimation of

Al2O3 Film Band Structure Parameters," Physica Status Solidi 32, K41 (1969).

[5] M. Uda, "Open Counter for Low-Energy Electron Detection," Japanese Journal

of Applied Physics Part 1-Regular Papers Short Notes & Review Papers 24, 284

(1985).

[6] T. Smith, "Oxidation of Titanium between 25 Degrees C and 400 Degrees C,"

Surface Science 38, 292 (1973).

[7] I. S. Jeon et al., "A Novel Methodology on Tuning Work Function of Metal Gate

Using Stacking bi-metal layers," IEDM Technical Digest, 303 (2004).

[8] C.-H. Lu et al., "Bilayer metal gate electrodes with tunable work function:

Mechanism and proposed model," Journal of Applied Physics 107, 063710

(2010).

[9] C.-H. Lu, "Bilayer metal gate electrodes with tunable work function: behavior,

mechanism, and device characteristics," PhD Thesis, Stanford University (2007).

91

Chapter 5

Complementary Flexible Organic

Inverters

5.1 Introduction

In Chapter 2 to Chapter 4, I describe several methods for making high-

performance organic field-effect transistors (OFETs) and manipulating their

characteristics: 1) ozone-assisted atomic layer deposition (ALD) of aluminum oxide

(AlOX) for high-capacitance gate dielectric; 2) parylene-C shadow masks for short-

channel source and drain (S/D) electrodes; 3) electric dipoles from self-assembled

92

monolayers (SAMs) inside the gate dielectric for controlling the threshold voltage

(VTH) and enhancing the air stability of n-channel OFETs; and 4) dual-metal gate and

bilayer metal gate electrodes for controlling the VTH. By combining some of the

methods and the knowledge from the studies above, I demonstrate high-performance

inverters, which are basic building blocks for digital circuits, on a flexible substrate in

this chapter.

Although practical electronic circuits, including radio-frequency identification

(RFID) and data converters, have been demonstrated with OFETs [1-5], the low

performance of n-channel OFETs has limited the overall circuit performance. The n-

channel OFETs in the complementary organic inverters described below showed the

highest mobility and transconductance to date on a flexible substrate in air.


We used a 125-m-thick polycarbonate (PC) sheet (Teijin Co.) as a flexible

substrate. This PC substrate had a very smooth surface, whose root-mean-square

(RMS) roughness value was almost close to silicon wafers [6]. It is well known that a

rough interface between the gate dielectric and the transistor channel can significantly

degrade the field-effect mobility (FET) of OFETs [7]. Thus, the PC sheet was chosen

as the substrate in order to minimize the degradation of FET from a rough surface of

flexible plastic substrate. Gate electrodes were made by an e-beam evaporation of

platinum (25 nm) with a 5-nm-thick adhesion layer of titanium. ALD of AlOX was

used to make a 16-nm-thick dielectric on the platinum layer with a substrate heating at

93

50 ˚C. The sample was then immersed in an ethanol (anhydrous grade from Sigma

Aldrich Co.) solution containing 5 mM of tetradecylphosphonic acid (TPA,

CH3(CH2)13PO(OH)2, Alfa Aesar Co.) for 20 hours to form a SAM. Detailed

information about the effects of the SAM on OFETs is described in Chapter 3. After

the immersion, the sample was thoroughly cleaned by spraying with ethanol, acetone,

isopropyl alcohol, and deionized water. This TPA/AlOX layer was used as the gate

dielectric of the OFETs. Semiconducting layers (40 nm) were then formed by thermal

evaporation of organic molecules onto the gate dielectric with a sample heating of

50 °C in a vacuum chamber. The rate of evaporation was monitored by a quartz

crystal and maintained at 0.2 Å /s. Pentacene (C22H14, Sigma Aldrich Co.) and

buckminsterfullerene (C60, C60, Alfa Aesar Co.) molecules were used in p- and n-

channel layers, respectively. Finally, we thermally evaporated the gold layer (40 nm)

of S/D electrodes and interconnect lines. The inverters consisted of p- and n-channel

OFETs with the width to length (W/L) ratios of 3 and 10, respectively. The channel

length was 100 m for all the OFETs. For the characterization of the gate dielectric,

gold electrodes (100 nm) were thermally evaporated onto the gate dielectric without

the semiconducting organic layers. Each layer was patterned by a shadow masks.

Figure 55 shows a schematic and a photograph of the flexible inverters.

94

Figure 55. Schematic and photograph of the flexible complementary inverters.


All electrical measurements were performed in ambient air. The schematic in

Figure 56 represents the structure used to characterize the gate dielectric.

95

Figure 56. Leakage current through the gate dielectric. The schematic represents the

structure of the measured devices.

As Figure 56 shows, the maximum leakage current of the gate dielectric was only 5.11

nA/cm2 at a VDD of 3.5 V. This value is approximately two orders of magnitude lower,

compared to the previous gate dielectric of OFETs made of oxidized aluminum on a

flexible substrate [8]. The gate capacitance was measured to be 0.25 F/cm2.

The p- and n-channel OFETs were tested in saturation mode, followed by an

extraction of FET and VTH values using equation (5).

2


WI C V V

L

(W: channel width, L: channel length, Cg: gate capacitance, VGS: gate-source voltage)

96

Table 8 summarizes the extracted device parameters.

p-channel

Pentacene OFETs

n-channel

C60OFETs

FET (cm2/V·s) 0.35 (±0.04) 0.56 (±0.04)

VTH (V) -1.07 (±0.10) 0.99 (±0.24)

ION / IOFF 6.98 (±2.81) × 105 2.65 (±0.95) × 10

5

Table 8. Device parameters of the flexible OFETs (saturation mode) measured in air.

The ION/IOFF ratio was defined as ID (|VGS| = VDD) / ID (VGS = 0 V).

As shown in Table 8 and Figure 57, the current-voltage characteristics of the n-

channel OFETs were similar to that of the p-channel OFETs.

Figure 57. Drain current vs. VGS curves of p-channel (pentacene) and n-channel (C60)

OFETs in saturation mode. The devices were measured in ambient air.

-3 -2 -1 0 1 2 310

-12

10-10

10-8

10-6

VGS

(V)

Dra

in C

urr

ent

(A)

n-channel OFETp-channel OFET

97

The n-channel OFETs showed a high FET of 0.56 (±0.04) cm2/V·s and a maximum

transconductance of 3.8 nS/m. These mobility and transconductance values are an

order of magnitude higher than recently reported n-channel OFETs on a flexible

substrate [8]. Previously, hexadecafluorocopperphthalocyanine (F16CuPc, C32CuF16N8)

has been widely used for semiconducting material of n-channel OFETs in air [8, 9],

but their FET have been one or two orders of magnitude lower than p-channel OFETs.

As I discuss in Chapter 3, the built-in potential generated by the interface between

alkylphosphonic acid and AlOX drastically enhanced the FET of n-channel organic

transistors [10]. By utilizing this built-in potential and the very smooth PC substrate,

we successfully improved the flexible n-channel OFETs, and their performance was

similar to the p-channel counterparts. In Figure 58, the drain current vs. VDS curves

show that the p- and n-channel transistors exhibited both linear and saturation

operations, where VDS refers to the drain-source voltage.

98

Figure 58. Drain current vs. VDS curves of p-channel (pentacene) and n-channel (C60)

OFETs measured in air. Both OFETs had the W/L ratio of 10.

99

The inverter, depicted in Figure 55, was tested at VDD. The transfer curve of the

inverter in Figure 59 shows a sharp transition between high and low states. The

maximum small-signal gain was measured to be 135, which is more than 3 times

higher compared to the previous flexible organic inverters measured in air [8].

Figure 59. Transfer curve and small-signal gain of the complementary inverter (VDD

= 3.5 V) on a flexible substrate, measured in air.

0 1 2 30

1

2

3

VIN

(V)

VO

UT (

V)

0 1 2 30

50

100

150

VIN

(V)

Gain

100

While the output voltage is flat at low input voltage, the output slightly increases at

high input in Figure 59. We attribute this increase to high contact resistance between

S/D electrodes and the channel region in the n-channel OFETs. Also, this contact

resistance lowered the drain current vs. VDS curves of n-channel OFET at low VDS in

Figure 58. As a previous study using ultraviolet photoelectron spectroscopy (UPS)

and X-ray photoelectron spectroscopy (XPS) shows, the Schottky barrier between gold

and C60 (n-channel) was 1.4 eV whereas the Schottky barrier between gold and

pentacene (p-channel) was only 0.56 eV [11]. For improving the performance of the

n-channel OFETs and the inverters, it is desirable to reduce the contact resistance by

lowering the Schottky barrier between the S/D electrodes and the organic

semiconductor.

5.4 Conclusion

We have demonstrated complementary organic inverters on a flexible substrate

with a maximum small-signal gain of 135. Improving the mobility of the n-channel

OFETs, we have made the p- and n-channel flexible organic transistors operate with

similar performance. Both OFETs have low off current in the order of 1 pA at VGS = 0

V, and the maximum leakage current of the gate dielectric was only 5.11 nA/cm2 at

VDD. The maximum temperature used in the fabrication process was only 50 ˚C,

which is compatible with most flexible substrates having relatively large thermal

expansions. Beyond the inverter circuits demonstrated here, our fabrication method

101

can be potentially used for making more complex organic circuits on flexible

substrates.

102

References

[1] R. Blache et al., "Organic CMOS Circuits for RFID Applications," ISSCC

Digest of Technical Papers, 208 (2009).

[2] K. Myny et al., "A 128b Organic RFID Transponder Chip, including Manchester

Encoding and ALOHA Anti-Collision Protocol, Operating with a Data Rate of

1529b/s," ISSCC Digest of Technical Papers, 206 (2009).

[3] W. Xiong et al., "A 3-V, 6-bit C-2C digital-to-analog converter using

complementary organic thin-film transistors on glass," Proceedings of ESSCIRC,

212 (2009).

[4] W. Xiong et al., "A 3-V, 6-Bit C-2C Digital-to-Analog Converter Using

Complementary Organic Thin-Film Transistors on Glass," IEEE Journal of

Solid-State Circuits 45, 1380 (2010).

[5] W. Xiong et al., "A 3V 6b Successive-Approximation ADC Using

Complementary Organic Thin-Film Transistors on Glass," ISSCC Digest of


[6] Unpublished data from Teijin Co.

[7] S. Steudel et al., "Influence of the dielectric roughness on the performance of

pentacene transistors," Applied Physics Letters 85, 4400 (2004).

[8] T. Sekitani et al., "Flexible organic transistors and circuits with extreme bending

stability," Nature Materials 9, 1015 (2010).

[9] Z. Bao et al., "New air-stable n-channel organic thin film transistors," Journal of

the American Chemical Society 120, 207 (1998).

103



Letters 11, 1161 (2011).

[11] S. J. Kang et al., "Energy level diagrams of C-60/pentacene/Au and

pentacene/C-60/Au," Synthetic Metals 156, 32 (2006).

105

Chapter 6

Conclusion

6.1 Summary of This Dissertation

I summarize each topic that I discussed in the previous chapters.

Atomic layer deposition (ALD) of aluminum oxide (AlOX) was utilized for

making a high-capacitance gate dielectric for organic field-effect transistors

(OFETs). Highly reactive ozone oxidant, used in ALD, resulted in low leakage

current through the dielectric without any high-temperature annealing process.

A supply voltage (VDD) of 2.5 V was enough to achieve the transistor ION/IOFF

106

ratio of more than 106 due to a high gate capacitance (Cg) of more than 0.4

F/cm2 (Chapter 2).

Parylene-C shadow masks were utilized to pattern short channel lengths of less

than less than 10 m between source and drain (S/D) electrodes in OFETs.

These masks are flexible, transparent, and attached very well on a variety of

surfaces (Chapter 2).

Self-assembled monolayers (SAMs) were used to control electric dipoles in the

gate dielectric of OFETs. We utilized octadecylphosphonic acid (OPA) and

octadecylsilane (OTS) to form the SAMs. Due to the difference in

chemisorption on AlOX between OPA and OTS, the threshold voltage (VTH) of

OFETs was significantly changed depending on a choice of the SAM

molecules. Moreover, we found that these dipoles in the gate dielectric can be

used to improve the air stability of n-channel OFETs (Chapter 3).

The VTH of OFETs was controlled by engineering the gate electrodes. Two

methods were used: dual-metal gates and bilayer metal gates. For dual-metal

gate electrodes we utilized titanium (Ti) and platinum (Pt). Due to the

difference in work functions (WF) between the two metals, the VTH was

changed by 0.5 V. As opposed to the dual-metal gates, bilayer metal gates can

continuously control the VTH by varying the thickness of top metal layer.

107

Utilizing top Pt and bottom aluminum (Al) layers, the VTH of OFETs was

continuously changed up to 0.6 V (Chapter 4).

Combining some of the methods discussed above, complementary organic

inverters were fabricated on a flexible substrate. The inverters had a maximum

small-signal gain of 135 at a VDD of 3.5 V and operated well in ambient air.

Flexible n-channel OFETs used in the inverters showed high field-effect

mobility (FET) of 0.56 cm2/V·s in air, which is more than an order of

magnitude higher than previously reported devices (Chapter 5).

6.2 Future Work

6.2.1 Stability of Organic Semiconductors

Stability of organic semiconductors needs to be further improved in order to be

useful in practical applications. Because organic molecules can be easily oxidized,

FET values of OFETs tend to be degraded much when exposed to air [1, 2]. Possible

solutions will be 1) developing new organic semiconductors that are less oxidized in

air [3] and 2) encapsulating organic semiconductors with robust materials [4, 5].

6.2.2 Metal-Semiconductor Junction Resistance

A Schottky barrier between metal and semiconductor causes a significant

junction resistance and results in an inefficient charge transport [6]. In conventional

108

inorganic semiconductors, such as silicon, germanium, and III-V semiconductors, this

problem can be overcome by doping the semiconductor to reduce the barrier width [6]

or by inserting an insulating layer to deactivate Fermi-level pinning [7]. Although

silicon nitride (SiNX) was used to reduce the junction resistance between metal and

organic semiconductors [8], a formation of SiNX layer can damage the organic

semiconductors. By using a new interfacial layer that is compatible with organic

semiconductor processing, this problem can be solved. One promising candidate will

be using organic molecules with a very high bandgap.

6.2.3 “Exciting” Applications

As discussed earlier, organic transistors and circuits are well suited in flexible

form factors, which are difficult to be realized by conventional silicon electronics.

Future applications of organic transistors may include flexible displays, flexible

smartphones, flexible chemical/pressure sensors, and so on. I believe that developing

exciting applications for organic electronics is the most important remaining task.

109

References

[1] U. Zschieschang et al., "Flexible Low-Voltage Organic Transistors and Circuits

Based on a High-Mobility Organic Semiconductor with Good Air Stability,"




Letters 11, 1161 (2011).

[3] T. Yamamoto and K. Takimiya, "Facile Synthesis of Highly π-Extended

Heteroarenes, Dinaphtho[2,3-b:2',3'-f]chalcogenopheno[3,2-b]chalcogenophenes,

and Their Application to Field-Effect Transistors," Journal of the American


[4] I. Kymissis et al., "A Lithographic Process for Integrated Organic Field-Effect

Transistors," Journal of Display Technology 1, 289 (2005).

[5] H. Jia et al., "Patterning effects on poly (3-hexylthiophene) organic thin film

transistors using photolithographic processes," Organic Electronics 8, 44 (2007).



[7] M. Kobayashi et al., "Fermi level depinning in metal/Ge Schottky junction for

metal source/drain Ge metal-oxide-semiconductor field-effect-transistor

application," Journal of Applied Physics 105, 023702 (2009).

[8] Z. Liu et al., "Contact engineering for organic semiconductor devices via Fermi

level depinning at the metal-organic interface," Physical Review B 82, 035311

(2010).

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