ORGANIC THIN FILM TRANSISTORS

by

Wen Wei ZHU

Department of Electrical & Computer Engineering

McGill University, Montreal

July, 2003

A thesis submitted to McGill University in partial fulfillment of the

requirements for the degree of Master of Engineering

Copyright 2003, by Wen Wei ZHU. All rights reserved.

Abstract

Organic thin film transistors (OTFTs) have been fabricated using four different

semiconducting polymers: poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene

vinylene] (MEH-PPV), polyhedral oligomeric silsesquioxanes (POSS) poly (2-methoxy-

5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV-POSS), poly[N-(3-

methylphenyl)-N,N-diphenylamine-4,4'-diyl] (poly-TPD), and polyhedral oligomeric

silsesquioxanes (POSS) poly (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine (poly-

TPD-POSS). These OTFTs were fabricated on heavily doped «-type silicon wafers with

thermally grown silicon dioxide layer was used as gate insulator. Except for MEH-PPV,

the OTFTs studied in this work are the first for the above organic semiconductor

materials.

From results of current-voltage measurements, it was observed that the present

OTFTs showed I-V characteristics of typical /^-channel thin film transistors. Some of the

fabricated OTFTs showed performance with relatively large field-effect mobilities (>10~4

cm2 V"1 s"1). The mobility of semiconducting polymer with polyhedral oligomeric

silsesquioxanes (POSS) was at least one order of magnitude larger than that of parent

polymer without the POSS. The largest mobility value was obtained on poly-TPD-POSS

(4.34 x 10"4 cm2 V"1 s"1) in room atmosphere and at room temperature.

Thermal annealing under different conditions was carried out on the polymers and

the effects on carrier field-effect mobilities were examined. The thermal annealing can

increase slightly the field-effect mobilities of the polymers without POSS. However, no

significant effect was observed on the field-effect mobilities of the polymers with POSS.

TI

Resume

Les meilleurs transistors organiques (OTFT — Organic Thin Film Transistor) ont

ete fabriques a base de quatre polymeres semi-conducteurs: poly[2-methoxy-5-(2'-ethyl-

hexyloxy)-l,4-phenylene vinylene] (MEH-PPV), polyhedral oligomeric silsesquioxanes

(POSS) poly (2-methoxy-5-(2'-ethyl-hexyloxy)-l ,4-phenylene vinylene) (MEH-PPV-

POSS), poly[N-(3-methylphenyl)-N,N-diphenylamine-4,4'-diyl] (poly-TPD), et

polyhedral oligomeric silsesquioxanes (POSS) poly (N,N'-bis(4-butylphenyl)-N,N'-

bis(phenyl)benzidine (poly-TPD-POSS). Ces OTFTs ont ete fabriques sur les wafers de

silicium type n avec le Si02 thermique comme l'isolateur de la grille. Dans cette these, les

polymeres sauf le MEH-PPV sont la premiere fois pour la fabrication des transistors

organiques.

Les resultats des caracteristiques electriques (I-V) presentent les comportements

typiques des transistors sur le film mince de semi-conducteur inorganique type p. Une

partie des OTFTs fabrique a montre l'execution avec les bon mobilites relatives des

porteurs de charge (>10~4 cm2 V"1 s"1). La mobilite des porteurs de charge de la polymere

semi-conducteur avec le polyhedral oligomeric silsesquioxanes (POSS) etait au moins un

ordre de grandeur plus grande que cela de polymere de parent sans le POSS. La meilleur

valeur de la mobilite des porteurs de charge a ete obtenu par l'OTFT a base du poly-TPD-

POSS (4.34 x 10"4 cm2 V"1 s"1) dans l'atmosphere ambiante et a la temperature ambiante.

TTI

Les recuires thermiques avec les conditions differentes ont ete executees sur les

polymeres et les effets sur les mobilites des porteurs de charge ont ete examinees. La

recuire thermique peut augmenter legerement les mobilites de charge des polymeres sans

POSS. Cependant, aucun effet significatif a ete observe sur les mobilites des porteurs de

charge des polymeres avec POSS.

TV

Acknowledgements

The author wishes to express her sincere gratitude to her supervisor, Dr. I.Shih for

his guidance and assistance throughout this study.

Appreciations are also due to Dr. Steven Xiao of American Dye Source Inc. for

providing the polymers.

Appreciations are also due to Defence Research and Development Canada -

Valcartier for preparing reactively sputtered silicon nitride films.

Thanks are also due to Dr. Philips Laou for technical advising.

The author is especially grateful to her families for their endless love and support.

V

Table of Contents

Page

Abstract

Resume

Acknowledgements

Table of Contents

Chapter 1

Chapter 2

2.1

2.2

2.3

2.4

2.5

5

3.1

3.2

3.3

3.4

I

4.1

4.2

4.3

4.4

4.4.1

Chapter 3

Chapter 4

I

Ill

V

VI

Introduction 1

Introduction to Organic Semiconductors 4

Overview 4

Comparisons Between Non-conjugated and

Conjugated Polymers 6

Electronic Energy Structure of Conjugated

Polymers 7

Carrier Mobility 9

Conclusions 11

Organic Thin film Transistors 14

Overview 14

Principles of Operation 15

Characteristics of Organic Thin Film

Transistors 20

Conclusions 22

Fabrication and Measurement Setup 29

Overview 29

Fabrication procedure for OTFTs on Silicon 31

Thermal annealing Process 33

Measurement Setup 35

Current-Voltage Measurements 35

VT

4.4.2 Capacitance-Voltage Measurements 35

4.5 Conclusions 36

Chapter 5 Results and Discussions 41

5.1 Overview 41

5.2 Gate Dielectric 42

5.3 Metal Contact 45

5.4 Device Performance 47

5.4.1 MEH-PPV Organic Thin Film Transistors 47

5.4.1.1 Without Any Treatment 47

5.4.1.2 Effect of POSS 48

5.4.1.3 Effect of Thermal Annealing 51

5.4.2 Poly-TPD Organic Thin Film Transistors 52

5.4.2.1 Without Any Treatment 52

5.4.2.2 Effect of POSS 54

5.4.2.3 Effect of Thermal Annealing 55

5.5 Conclusions 57

Chapter 6 Conclusions 69

References 71

VTT

Chapter 1

Introduction

A transistor is a semiconductor device where the flow of electrical current between

two terminals is controlled by an applied voltage to a third terminal. William Shockley,

Walter Brattain, and John Bardeen discovered the transistor effect and demonstrated the

point-contact transistor at Bell Labs in 1947 [1.3] and received the 1956 Nobel prizes in

physics. The transistors rapidly replaced vacuum tubes, which are bulky and high in

power consumption, and became the basic elements of integrated circuits. The transistors

consist of diffused drain and source regions in bulk-semiconducting materials. In 1962,

the first successful thin film transistor (TFT) was reported by Paul K. Weimer [1.1].

Weimer's investigations showed that the characteristics and performance of TFT made of

microcrystalline layers of cadmium sulfide (CdS) approached that of commercial

transistors made by bulk, monocrystalline silicon. Essentially all of today's transistors,

including TFTs, are made from inorganic semiconductors, such as silicon (Si) and

gallium arsenide (GaAs). However, the discovery and development of conductive

polymers introduce a new world for microelectronic. Alan J. Heeger, Alan G.

MacDiarmid, and Hideki Shirakawa shared the 2000 Nobel prizes in chemistry for the

work on conductive polymers [1.3]. Following the developments of conducting and

semiconducting polymers, organic electronics has attracted more and more research

interest.

The organic thin film transistors (OTFTs) based on conjugated polymers, oligomers

or other organic molecules are very similar to the metal oxide semiconductor field-effect

transistors (MOSFETs) based on silicon technology. The disadvantages of organic

semiconductors, such as low charge carrier mobility at room temperatures compared to

their inorganic counterparts, limit the performance of OTFTs. On the other hand, the

fabrication of organic thin films does not require high vacuum systems and can be carried

out exclusively at room temperatures. Therefore, OTFTs are attractive for applications

requiring low speed, large area coverage, flexible structure at low manufacturing cost,

such as the switching elements for flat-panel displays (FPDs) based on liquid crystals or

organic light emitting diodes (OLEDs).

It is highly desirable to integrate the OLEDs and OTFTs using the same organic

material as active layer. However, few polymers are suitable for both OLEDs and OTFTs.

One of the requirements is that, conductivity of the polymer channel at a given gate

voltage must be high enough so that sufficient drain current can be obtained. This may be

accomplished by increasing the carrier mobility and by reducing the interface barrier

between conjugated polymer and electrodes using end-capping polymer chains with

polyhedral oligomeric silsequioxanes (POSS) [5.2].

The concentration of this thesis is on the development of fabrication processes and

characterization of TFTs based on organic semiconducting materials, which are usually

used for OLEDs. Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene] (MEH-

PPV), and for the first time, polyhedral oligomeric silsesquioxanes (POSS) poly (2-

methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV-POSS), poly[N-(3-

methylphenyl)-N,N-diphenylamine-4,4'-diyl] (Poly-TPD), and polyhedral oligomeric

silsesquioxanes (POSS) poly (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine (Poly-

TPD-POSS) were used as the active layers of OTFTs in TFT fabrication. Usually, Poly-

TPD is used for OLED with blue light emission and MEH-PPV is used for OLED with

red light. These OLEDs were successfully fabricated in our laboratory. We found the

chosen polymers can also be used for the fabrication of OTFTs. The main objectives of

the research work in this thesis are to study the field-effect carrier mobility of the chosen

polymers and the methods to increase the field-effect mobility.

The arrangement of the thesis is as follows. A brief introduction to semiconducting

polymers is given in chapter 2. The basic theory and operation principles of thin film

transistors are addressed in chapter 3. In chapter 4, the experimental procedures of

fabrication and thermal annealing of organic thin film transistors in this work are

presented. Results of the characterization of organic thin film transistors are presented in

chapter 5. Finally, conclusions and recommendations for further study are given in

chapter 6.

Chapter 2

Introduction to

Organic Semiconductors

2.1. Overview

Polymers are large molecules constructed from small repeatable units, which are

called monomers. Polymers such as proteins exit naturally. Today, most of the polymers

including those used in this thesis are synthesized. Polymers were traditionally considered

as insulator. However, doping of polyacetylene to achieve relatively high conductivity

was reported in 1977 [2.1]. This discovery opened a new world for organic electronic.

The energy band gap between the occupied and unoccupied states determines the

electronic properties of polymers. There are four major classes of semiconducting

polymers, which are filled polymers, ionically conducting polymers, charge transfer

polymers, and conjugated polymers. [2.2]

Filled polymers are loaded with conductive fillers, such as metal particles. This

type of semiconducting polymers was invented in 1930 and has the broadest applications

in electronic devices. The first use of filled polymers was for preventing corona discharge

in advanced printed circuitries. This inhomogeneous polymer system causes many

problems, such as weak dielectric strength and steep percolation threshold in

conductivity. Ionically conducting polymers are widely used in commercial applications

such as rechargeable batteries and fuel cells although it is sensitive to humidity. In

organic semiconductors, charge transfer polymers and conjugated polymers are two

important subgroups. IBM introduced the first commercial organic photoreceptor based

on charge transfer polymer in 1972 [1.2]. Furthermore, charge transport polymers, the

combination of an oxidant and charge transfer polymer, are being used in today's multi­

layer photoreceptors. Conjugated polymers have been the dominant in the era of plastic

electronics since electroluminesecence in poly(para-phenylene vinylene) (PPV) was

discovered in 1990. Conjugated polymers act as active semiconducting materials for

many semiconductor devices, such as light-emitting diodes (LEDs), photo-diodes,

transistors, and plastic lasers.

In this work, conjugated semiconducting polymers were used to fabricate organic

thin film transistors (OTFTs). Four different kinds of conjugated polymers were selected

for the fabrication. These polymers are poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-

phenylene vinylene] (MEH-PPV), polyhedral oligomeric silsesquioxanes (POSS) poly (2-

methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV-POSS), poly[N-(3-

methylphenyl)-N,N-diphenylamine-4,4'-diyl] (Poly-TPD), and polyhedral oligomeric

silsesquioxanes (POSS) poly (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine (Poly-

TPD-POSS). Figure 2.1 shows the schematic molecular-structure of these examined

polymers.

2.2 Comparisons between non-conjugated and conjugated poiymers

As mentions in the former section, the polymers are constructed from repeating

units (monomers). The monomers couple with each other to form a polymer chain.

Carbon and hydrogen atoms are two basic elements in those monomers of majority

polymers, including non-conjugated and conjugated polymers. Non-conjugated and

conjugated polymers have the same mechanical properties, however, they have different

electronic properties because of different structures of chemical bonds.

As shown in Figure 2.2(a), the chemical bonds of non-conjugated polymers are

formed by sp3 hybrids on the carbon atoms and Is functions on the hydrogen atoms. It

requires a large energy for an electron moving from one bonding orbital to the equivalent

antibonding orbital. This means that a large energy gap appears between occupied and

unoccupied bands. This large energy gap of the non-conjugated polymer results in the

electrical properties of insulator or plastic.

The chemical bond structures of conjugated polymer are different from those of

non-conjugated polymer. A "backbone" structure consisting of alternating single and

double carbon bonds resulting in a "71-conjugated network" leads to relatively small

energy gap [2.3]. As shown in Figure 2.2(b), parts of chemical bonds are formed by sp2

hybrids with the carbon atoms and Is function with the hydrogen atoms, the other parts of

chemical bonds are formed by p function with the carbon atoms. Hence, the latter leads to

n bonds between the carbon atoms. These bonds require less energy for an electron to

move from a TT bond to an anti-bonding one. The energy gap between occupied and

unoccupied bands is relatively smaller than that of non-conjugated polymer. The typical

gaps are in the range of 1-3 eV [2.2]. For example, the energy gap of MEH-PPV is 2.17

eV[2.10].

2.3 Eiectronic energy structure of conjugated polymers

As it was mentioned in section 2.2, the alternate single and double carbon bonds in

a conjugated polymer lead to electron delocalisation. These electrons form a band

structure, which exhibits semiconducting or metallic properties. The delocalised electrons

will also act as charge carriers and move along the polymer chain.

One can explain the intramolecular electronic state by using polyacetylene ((CH)n),

the simplest conjugated polymers, as the example. The energy levels depend on the

number of n. The energy gap, which is between the occupied n and unoccupied n* states,

decreases with n. The fact that the presence of the infinite n and a uniform bond order

results in every orbital to be occupied by one electron, the polymer should have the same

energy structure as an intrinsic metal. The bond dimerization occurs as the result of

Peierls distortion when the band is half-filled. An energy gap is opened at the Fermi

surface with approximately 1.5 eV. The system then becomes a semiconductor [2.1]. The

valence effective Hamiltonian (VEH) method can be used to calculate the electronic

energy structure for conjugated polymers. The basic parameters include the energy gap,

the bandwidth, ionization potential, and electron affinity. Using the dimensionless

characteristic parameter A, = 2a2/^t0Kand bandwidth W = 4t0, the band gap (Eg = 2A0)

is given by

A0 = 2Wexp I 2X)

(2.1)

Where to is the inter carbon transfer integral for 7t-electron, K is the effective spring

constant, and a is the lattice constant.

Unlike polyacetylene, most other polymers such as PPV do not have the symmetry

of bond alternation. These polymers have a nondegenerate ground state of bond

alternation. Polarons or bipolarons are defined as the charged excitations of a

nondegenerate ground state polymer. Two gap states produced by these additional states

displace the mid-gap. If electrons do not occupy these levels, a positive bipolaron occurs.

These levels can be occupied by 1, 2, or 4 electrons, namely positive polaron, negative

polaron and negative bipolaron, respectively. Figure 2.3 depicts the band structures of the

presence of a positive polaron and a positive bipolaron.

The electrons and holes traverse the delocalised system until they combine to form

a bound electron-hole pair under Coulombic force. This bound electron-hole pair is called

an exciton. If the exciton locates on one molecular unit, it is called Frenkel exciton. If the

exciton extends over molecular units, it is called Mott-transfer exciton. [2.1] The exciton

is neutral. The Mott-transfer exciton is a weaker bound state then the Frenkel exciton

because the spatial extent of Mott-transfer exciton is larger than the lattice constant. The

exciton has a lower energy state than electron or hole. The decomposition of the exciton

releases a photon, and therefore, the semiconducting polymer may have photon emission

property. Movable excitons affect the photo-physics of conjugated polymers.

The latter will limit the intramolecular charge transport. Intermolecular charge

transport is important for the low conductivity in organic materials. This transport occurs

by intermolecular hopping of charges. Isoenergetic and energy-dependent mechanisms

are related to charge hopping. Both of these two mechanisms are temperature dependent.

Several models have been established to describe the hopping transports. The temperature

dependent mobility will increase with temperature [2.4].

2.4 Carrier mobility

To day, it is not completely understood of charge transport in organic

semiconductors. In inorganic semiconductors, such as silicon, strong covalent bonds

occur between atoms and charge carriers move within delocalised levels. In this band

transport, the scattering of carriers by lattice vibrations limits the carrier mobility. Thus,

the mobility increases with decreasing temperature.

However, the band transport may not be appropriate for organic semiconductors.

With well agreements, carrier transports take place by hopping between localized states in

organic semiconductors at room temperature [2.11, 2.12]. Temperature-dependent and

gate-bias-dependent measurements have been used to study the carrier transport in

organic semiconductors used in field-effect transistors. The vibrational energies of the

molecules are much lower than the intermolecular bond energies when temperatures are

below 250 K. In this case, the band transport mechanism dominates the carrier

transportations and a relatively high mobility will exhibit. However, at or above room

temperatures, hopping between localized states is the major transport mechanism because

the phonon scatting becomes so high that the band transport becomes small. The mobility

increases with increasing temperature due to the hopping becomes easier with increasing

temperature. The weak intermolecular interaction forces in organic semiconductors may

limit the mobilities at and above room temperature. The time-of-flight experiments were

used to determine the upper limits of microscopic mobilities of organic molecular crystals

in the range of 1 to 10 cm2 V"1 s'l at 300 K [2.7]. Experimental and theoretical researches

have addressed that the carrier mobility depends on the applied gate voltage as well as on

the temperature [2.7, 2.8, 2.9, 2.11] in organic thin film transistor. An accumulation layer

of charge carrier will occur in the interface of organic semiconductor film and insulator in

an organic field-effect transistor. As the appropriated gate bias increasing, more charge

carriers will inject into the accumulation layer. The additional charges require less energy

to hop between different states. This results higher carrier mobility. Some researches had

indicated that high mobility might not be obtained at high electric field [2.9, 2.11]. It may

due to the higher charge carrier concentration in the accumulation layer at high electric

field.

10

2.5 Conclusions

The semiconducting polymers are synthesized by organic chemistry. Their

electronic structures and electronic properties have been studied, and many models have

been established. However, these models are complex due to the molecular nature of

organic semiconductors. The basic ideas of organic semiconductor properties are given in

this chapter. These organic semiconductors present the similar electrical properties as the

inorganic semiconductors. Organic semiconductors are the potential materials for low

cost electronic devices. The following chapter will explain the operation principles of

organic thin film transistors.

11

-K>vOt

HXO

(a)

R \

R 0-81 ' V s^-o>

ro 0.

y

\ / 0 ' SL

•8k

"sJ-R0

•or, -Si'

\

-R

O HXO

(c)

(b)

R

'°'" " * W o ^ /

o

-"n

0 \ R

-R

Where R =

si 0 b i ^ R 0

=\^Sk ^ S i ^ °

Figure 2.1 Schematic diagrams of molecular-structures of (a) MEH-PPV, (b)

Poly-TPD, (c) MEH-PPV-POSS and (d) Poly-TPD-POSS.

12

H

\ c / c \ c /

(a)

I I H H

/ H H

- c c -\—*s H y \

H

(b)

Figure 2.2 Schematic diagrams of chemical bond structures of (a) non-

conjugated polymers and (b) conjugated polymers.

Conduction Band

Eg/2

Valence Band

Polaron"1" ni„rti,Krt„2+ Bipolaron Bipolaron Band

Figure 2.3 Schematic diagrams of band structure of the presence of a positive

polaron and a positive bipolaron.

13

Chapter 3

Organic Thin Film Transistors

3.1. Overview

Compared with transistors and thin film transistors (TFTs) based on monocrystalline

and polycrystalline inorganic semiconductors, organic thin film transistors (OTFTs) have

low carrier field-effect mobilities, which prevent the OTFTs from being widely used.

Pentacene-based OTFTs has the highest carrier field-effect mobility, which is about 1.0

cm2 V"1 s"1. This value is three orders of magnitudes lower than that of transistors made

by monocrystalline silicon. On the one hand, OTFTs cannot be used in high-speed

applications [2.11]. Nevertheless, due to the simple fabrication processing, structural

flexibility, and low cost, OTFTs have potential applications such as the switching

elements of flat panel display and smart cards [2.12].

Semiconducting polymers have been extensively studied for low cost organic light

emitting diodes (OLEDs). In such application, it is highly desirable to integrate the

OLEDs and OTFTs using the same polymer as active layer. However, few polymers are

suitable for making LEDs and TFTs. One of the requirements is that, conductivity of the

polymer channel at a given gate voltage must be high enough to allow sufficient drain

14

current. The testing polymers studied in this work were used to make LEDs. For example,

Poly-TPD-based LED yields blue colour. It is a challenge to make reasonable OTFTs

with these polymers and integrate them with LEDs.

The device configurations of TFTs are suitable for low conductivity, organic

semiconductor. Three possible configurations of OTFTs are shown in Figure 3.1. Due to

the low metallic contact effect [3.1] and simple fabrication process, the bottom contact

configuration ( Figure 3.1(a)) was chosen in this work.

3.2 Principles of Operation

Thin film transistor (TFT) was first introduced by Weimer in 1962. It is one of

many configurations of metal-oxide-semiconductor field-effect transistor (MOSFET).

The operation of a TFT is similar to a conventional MOSFET except that TFT only

operates in accumulation regime because of the absence of a depletion layer. If a p-typt

semiconductor is used as the active layer of a TFT, holes are induced in the

semiconductor to form an accumulation layer when a negative gate voltage is applied. On

the other hand, if an «-type semiconductor is used, electrons are induced in to form an

accumulation layer when a positive gate voltage is applied. Since most of today's

semiconducting polymers are p-type, the following explanation of principles of operation

is based on/?-type semiconductor.

15

The metal-insulator-semiconductor (MIS) diode can be used to explain the general

operation of a MOSFET. Figure 3.2 represents the device configuration of a MIS diode.

Work function is the minimum energy for an electron to escape into vacuum level from

the Fermi level. An ideal MIS diode is a diode with zero metal-semiconductor work

function difference at a zero-bias voltage. Figure 3.3 shows the energy band diagram of

an ideal MIS diode under thermal equilibrium, indicating the metal-semiconductor work

function difference (<|)ms). This is given in the Equation 3.1.

d> = < b T ms T m

X + T ^ + K forp-type (3.1) . 2q J

Where <j)m is the metal work function, which is defined as the potential required to inject

an electron from the metal into the conduction band of the oxide; x is the electron

affinity; Eg is the energy band gap; and cpfP is the difference between the intrinsic Fermi

level (Efj) and the Fermi level ofp-type semiconductor (Ef).

As shown in Figure 3.4(a), with a negative bias voltage applied to the metal gate,

an accumulation of majority carrier (holes) in the p-type semiconductor is formed due to

the bending of valence-band edge towards the Fermi level at the oxide-semiconductor

interface. On the other hand, when a positive voltage is applied to the metal gate, the

conduction-band edge bends towards the Fermi level as shown in Figure 3.4(b). A space

charge region is then induced. If this positive voltage is large enough, an inversion layer

of electrons will be created at the oxide-semiconductor interface. Flat-band voltage (Vfb)

is defined as the applied gate voltage required to create a situation without the band

bending in the semiconductor. Figure 3.3 also indicates the band diagram of MIS

16

structure at flat-band condition. In other words, the flat-band voltage accounts for the

work function difference between the semiconductor and the metal gate.

A typical configuration of TFT is shown in Figure 3.5, where L is the channel

length and W is the channel width. Ifp-type semiconductor is used as the active layer, the

source and drain are connected electrically by a conducting surface /7-channel through

which a current is flowing when a sufficient negative bias is applied to the gate.

Assuming the unit capacitance of the insulating layer (Q) is constant throughout the

channel, the charge induced in the accumulation layer is equal to Cj x [Vg-Vft,-V(y)],

where Vg is the applied gate voltage and the V(y) is the potential difference of the

semiconductor at a point y from the source electrode. The total charge consists of the

charge induced in the accumulation layer and the intrinsic bulk charge Qo (electrons or

holes) in the semiconductor. Qo is defined as ± qnot, where q is the elemental charge, no is

the density of free carriers, and t is the thickness of the semiconductor. The drain current

can be computed using Equation 3.2.

Id = Wii[C, (V, - Vft - V(y)) + n 0 q t ] ^ £ (3.2)

Where W is the channel width and JJ. is the charge carrier mobility. Integrating Equation

3.2 from source (y=0, V=0) to the drain (y=L, V=Vd) yields

v w Id = ^ J[Ci(Vg - Vft - V(y)) + n0qt]dV _ _ ( 3 . 3)

17

Where Vd is the drain voltage. By defining (± -J—2- + Vft) as the value of threshold

voltage (VT) and assuming a constant mobility, Equation 3.3 becomes

W (v g -vT )vd - V, (3.4)

Equation 3.4 corresponds to 0 < Vd < (Vg - VT), which is the linear regime. When the

drain voltage is increased to Vd = Vg - VT, the number of charge carriers at the drain will

saturate and a pinch-off state is established. The drain voltage at this point is defined as

pinch-off voltage (Vp). Beyond this point, the drain current is assumed to be constant. The

voltage range where Vd > Vg - VT is called the saturation regime. Equation 3.5 gives the

saturation drain current.

1dsat ~ T V v e v j ) 2L

-(3.5)

The channel conductance (gd) and the transconductance (gm) are two important

parameters. In the linear region, gd and gm are given by

8c dl

dVA V. =const

w = r ^iC i (V g -V T ) -(3.6)

dl. o m

dVn

W

= r ^ v d Vd =const

-(3.7)

The transconductance in the saturation region is given by Equation 3.8.

g m = ^ C l ( V * " V T ) -(3.8)

18

The characteristics of drain current and voltage of a TFT based on inorganic

semiconductor is shown in Figure 3.6. By plotting the square root of drain current (^jld )

versus gate voltage at certain drain voltage (Figure 3.7), one can estimate the threshold

voltage value for a transistor. In this work, the threshold voltages of fabricated OTFT will

be estimated by this method.

Most of organic semiconductors, including conjugated polymers, exhibit p-type

behaviours. The operation of organic thin film transistors can be similarly described by

using the operation models of inorganic thin film transistors. As a result, the equations

described above may be used to analyze organic thin film transistors. In this work,

Equation 3.5 is used to estimate the carrier field-effect mobility of organic

semiconductors. The method described according to Figure 3.7 is used to estimate the

threshold voltage of organic thin film transistors.

Charge carrier field-effect mobility concerns the performance of an organic thin

film transistor. Thus, improving the field-effect mobility is one of the major issues in

most of researching works on organic thin film transistors. Many factors can affect the

mobility, such as contact resistance, film morphology, and so on. Besides carrier field-

effect mobility, ON/OFF current ratio, threshold voltage, and leakage current are the

basic parameters of a transistor. Depending on the nature of molecular structure, a great

diversity of organic semiconductors exhibits low carrier mobilities [1.2, 2.11]. The

bottom contact structure indicated higher field-effect mobility than the top contact

19

structure did [2.112]. Different kinds of treatments including thermal annealing have been

done to improve the device performances by reducing the contact resistance and

improving the molecular ordering. A polymer field-effect transistor based on poly-

hexylthiophene (P3HT) with field-effect mobility of 0.05 - 0.1 cm2 V"1 s"1 and high

ON/OFF ratio of 106 - 107 had been fabricated by careful device processing in nitrogen

atmosphere in Reference [3.7]. Chemically modified source and drain electrodes of

pentacene-based organic thin film transistors can improve the linear field-effect mobility

greater than 0.5 cm2 V"1 s"1 and ON/OFF ration greater than 107 [3.8]. The main objective

of the thesis work is to study the effects of different treatment methods on the polymer-

based OTFTs for possible improvement of charge carrier field-effect mobility of the

choosing polymers used in this thesis. The details will be discussed in the following

sections and chapters.

3.3 Characteristics of organic thin film transistors

Today, most of the organic semiconductors are p-type. All of the examined

polymers in this work are p-type. It is necessary to construct the energy band diagram for

the characterization and performance prediction of the organic thin film transistors. Using

MEH-PPV, a band diagram is plotted as follow. Heavily doped w-type silicon wafer was

used as the gate contact and the thermally grown silicon dioxide (Si02) on the wafer was

used as the gate insulator. The electron affinity of Si02 (qXox), the work function and

electron affinity of n+-Si (q<J>si and qxsi) are 0.95 eV, 4.14 eV and 4.05 eV, respectively

20

[3.5]. The electron affinity and energy band gap of MEH-PPV (qxsi and Eg) are 2.97 eV

and 2.17 eV, respectively [2.10]. When an MIS system is formed by n+-Si, Si02, and

MEH-PPV, the potential difference between Fermi level and intrinsic Fermi level of

MEH-PPV ((j)fp) is 0.09 eV according to the flat-band condition. When an «+-Si is used as

a gate electrode, the band diagram will be modified as shown in Figure 3.8. Band

diagrams of other polymers, including MEH-PPV-POSS, Poly-TPD, and Poly-TPD-

POSS, can be established in a similar manner.

Normal operation of a TFT requires a source contact having low impedance for

injection of majority carriers. When a low electrical resistance contact is connected to the

source, high current density can be drawn from the polymer film despite the high

resistance of the polymers. The difference of the work functions between the metal and

semiconductor should be as small as possible. Since metals usually have work function

less than 5 eV, such as aluminium (Al), a Schottky contact will be formed at the interface

and this affects the performance of the device. Gold (Au) is chosen as the metal of ohmic

contacts of OTFT. According to the energy band diagram of Au and MEH-PPV (Figure

3.9), there is a potential barrier at the metal-semiconductor interface for holes because the

work function of Au is higher than the valence band edge of MEH-PPV. A majority

carrier accumulation layer is then formed on the semiconductor side. This layer may

cause a low contact resistance. However, leakage current will be resulted due to the free

movement of charge carriers between Au and polymers with the absence of the potential

barrier.

21

3.4 Conclusions

In this chapter, the basic theory and operation principles of organic thin film

transistors have been described. The energy band diagrams have been given with MEH-

PPV as the active semiconducting layer of OTFTs. The charge carrier field-effect

mobility is the most important parameter for OTFT to evaluate its performance. The

operation principles for OTFTs are similar to that of TFTs and therefore, theories of TFTs

were used to describe and estimate the performance of OTFTs. However, organic

semiconductors are micro molecular materials and not the single crystals. Thus, the

estimated results for OTFTs will have slight but acceptable deviation from the true

characteristics.

22

Metal contact Polymer Metal contact

Si02

n+-Si

(a) Bottom metal contact configuration

Metal contact Metal contact Polymer

Si02

n+-Si

(b) Top metal contact configuration

Metal gate contact Gate insulator

Metal contact Polymer Metal contact

Glass

(c) Top gate configuration

Figure 3.1 Schematic diagrams of three OTFTs configurations.

23

Metal

Insulator

Semiconductor 6 7\

0

Figure 3.2 A schematic diagram of the metal-insulator-semiconductor (MIS)

diode.

Vacuum Level A

q * m

• • • '

* 77777777

Metal Insulator

I T

qx

i ± q*fp

- E f i

••' 7777777777777777— E„

P-type semiconductor

Figure 3.3 Energy band diagram of a MIS diode at flat band condition.

24

V<0 V®. r:-<J O Er

% 0 0 0 0 0 0 E«

.Efi

:EI

®

(a)

Et^ Y>0

QO G Ec

/ ^ - • . - - - • • - - . - - - . - - • • - r : =

/® © 5 0 0 E

Efi Ef

(b)

Figure 3.4 Energy band diagrams of an MIS diode with p-type semiconductor

under (a) a negative gate bias and (b) a positive gate bias.

25

Source Semiconductor Drain

Insulator

v Gate

Vg

1 Vd

£ Figure 3.5 A schematic diagram showing the 3D-structure of a thin film transistor

(TFT).

1.0 .

E,

_P 0 6 .

0.2 .

[

Vg: = 25V

20 V

/y 15V

J/^^ 10V ] 5 10 15 20 25 3

Vd (V)

Figure 3.6 Drain current (ld) and drain voltage (Vd) characteristics of a typical

inorganic-based TFT.

26

Vds (V)

Figure 3.7 The square root of drain current {-jTd) versus gate voltage in the

saturation region of a typical inorganic-based TFT.

Vacuum Level

qXsi =4.05 eV

q&i = 4.14 eV

Eg/2 = 0.56 eV

: v

n+-Si

qJk^OSSeV

qX=2.97eV

EcjL E,/2 = 1.08eV

Ea = 8 eV

v SiO:

| - - - | V a 0 9 e V

Ev

MEH-PPV

Figure 3.8 Energy levels in three separated components that form an MIS

system: n+-Si, thermally grown Si02, and MEH-PPV.

27

Vacuum Level A

qX=2 97eV

Ec" ±. Eg/2 = 1.08 eV

T : v

MEH-PPV

c 'Xm=5eV

v

Au

Figure 3.9 Energy levels in two separated components that form a drain/source

contact: gold (Au) and MEH-PPV.

28

Chapter 4

Fabrication and Measurement Setup

4.1 Overview

Most of semiconductor devices are fabricated on monocrystalline silicon substrates.

The facilities involved in the industrial processes are usually high in cost of ownership

(COO). These facilities include high grade cleanrooms, ultra high vacuum systems for

metals and dielectric deposition, furnaces for impurity diffusion, oxidation and heat

treatment, autonomous processing systems for photolithography and etching, etc. The

basic process of metal-oxide-silicon field-effect transistor (MOSFET) fabrication on

silicon starts with the creation of source and drain regions. Then the gate region, usually

consists of thermally grown or deposited silicon dioxide, is created in the channel of the

MOSFET between the source and drain regions. At the end of the fabrication, metal is

deposited onto the silicon substrate; and electrodes, including drain, source, gate, and

ground electrodes, are formed by photolithography.

Up to date, bottom gate contact configuration is the most widely used configuration

of organic thin film transistors (OTFTs). If a silicon substrate is used, the gate electrode,

29

gate insulator layer and drain/source electrodes are subsequently fabricated. A thin film of

organic semiconductor material will then be either vacuum deposited or applied by spin

coating on the substrate.

Depending on the types of organic materials, the formation of the semiconducting

layer can be obtained by either vacuum vapour deposition or solution-based application

[2.11]. Since some conjugated polymers have limited solubility in organic solvents, such

as pentacene and a-Dihexyltetrathienyl (DH-a-4T), vacuum deposition is the preferred

fabrication method. Pentacene-based organic thin film transistors have yielded the highest

field-effect mobility among the organic thin film transistors reported. The highest

mobility of pentacene-based OTFT ( 2.7 cm2 V"1 s"1 [2.12]) reported to date is in the same

9 1

order of magnitude as thin film transistor fabricated using amorphous silicon (~ 1 cm V"

s"1). Vacuum-deposited polymer film usually has a better surface morphology than

solution-processed polymer film [2.11]. Therefore, it is not surprising that vacuum-

deposited organic films result in organic thin film transistors with mobilities higher than

the solution-processed films. This is due to the fact that the carrier mobility is strongly

dependent on film morphology. Nevertheless, solution-processed organic thin film

transistors have substantial potential, especially in industries, for their simple fabrication

process and low COO of facilities. Spin coating, solution casting, and direct printing are

the most commercially viable techniques. Different solvents and processing conditions

were found to yield devices with different performance. For example, with a mixture of

poly(3-hexylthiophene) and chloroform, the mobility of the spun film is about 1 to 2 x

10"4 cm2 V"1 s"1 which is about two orders of magnitudes higher than those prepared using

30

other solvents [2.11]. This preparation also enhances the ON/OFF current ratio of poly(3-

hexylthiophene) field-effect devices.

Our research work has focused on alternate fabrication procedure and different

treatments to achieve the mobilities of the polymers as high as possible for organic thin

film transistor applications. It is noted that the organic materials studied in this work are

rarely used for OTFTs before. Silicon substrates with a <100>-orientation are used in our

experiments due to low interface defect density between silicon dioxide and silicon. The

procedure of fabrication, conditions of treatment and the measurement setup are presented

in the following sections in this chapter.

4.2 Fabrication procedure for OTFTs on Silicon

In the procedure shown in Figure 4.1, an n+-Si (100) substrate with a resistivity of

0.02 - 0.06 Q-cm is first thermally oxidized to form an isolation layer between the drain

and source regions and the silicon substrate (Figure 4.1(a)). After gate window opening

(Figure 4.1(b)), a thin layer of gate oxide is grown by thermal oxidation (Figure 4.1(c)).

Gold (Au) is then evaporated on the entire surface and patterned to form the drain and

source contacts by photolithography (Figure 4.1(d)). Organic semiconducting material is

then spun on the finished substrate (Figure 4.1(e)). At last, a heat treatment step is carried

out. Figure 4.2 shows the patterns of the photo masks used for the photolithography. The

detailed TFT-fabrication procedure on Si substrate is described as follow:

31

1. Cut n+-type Si wafer into appropriate size using diamond scriber

2. Carry out a cleaning process for Si substrate

3. Perform wet oxidation process at 1100 °C for 180 minutes with constantly flowing

oxygen (flowing rate is 1.5 units per second) (the thickness of this thermally grown

Si02 layer is about 1 p.m)

4. Carry out the first photolithography process using mask #1 for gate window opening:

a) Spin Shipley AZ-1827 photoresist at 3000 rpm on the entire wafer

b) Prebake the wafer at 90°C for 15 minutes

c) Align and expose the wafer to UV light (Oriel Corp. Model 87100 , Timer control,

77510 Dual Fiber Optical Illuminator, 68810 ARC-Lamp power supply 200-500

Watt Hg, Wavelength 436 nm G-line, power output at 300 W) for 180 seconds

d) Develop the wafer using Shipley developer

e) Postbake the wafer at 115°C for 15 minutes

f) Carry out a window opening process by immersing the wafer in buffered HF

solution to remove unwanted Si02

g) Remove photoresist using acetone

5. Perform dry thermal oxidation process for 30 minutes to form gate oxide layer

(furnace temperature is 1050°C, the oxygen is passed through TCE, and flowing rate

of oxygen is 1.5 units per second) (the thickness of this thermally grown Si02 is about

0.1 um)

6. Evaporate Au on the entire wafer in vacuum system

32

7. Carry out the second photolithography process using mask #2 for drain and source

contacts:

a) Spin Shipley AZ-1827 photoresist at 3000 rpm on the entire wafer

b) Prebake the wafer at 90°C for 15 minutes

c) Align and expose the wafer to UV light for 180 seconds

d) Develop the wafer with Shipley developer

e) Carry out a wet etching process for drain and source contacts by immersing the

wafer in KI + I2 solution to remove unwanted Au

f) Remove photoresist using acetone

8. Spin polymer solution at 1500 rpm on entire wafer

9. Remove polymer thin film on the drain and source contacts using acetone

10. Carry out heat treatment (details in section 4.3)

Figure 4.3 shows a top view of a finished OTFT fabricated in this work. The

channel width of the OTFT is either 75 um or 180 |am and the channel length is in the

range of 3 - 8 |im.

4.3 Thermal annealing process

As mentioned in Section 4.1, different heat treatment conditions were found to

affect the characteristics of OTFTs. These conditions include heat treatment in vacuum

and dry nitrogen, and treatment at different ambient temperatures for different durations,

33

and treatment of the gate insulator with Hexamethydisilizane (HMDS). It is believed that

HMDS can improve the adhesion of photoresist to the silicon surface. Vacuum and

nitrogen ambient are the most popular choices for thermal annealing of organic films. It

was found that the OFF-state currents of poly(3-hexylthiophene)-based OTFTs can be

lowered after the thermal annealing under N2 at 100 °C for 5 minutes. However, the

mobility is lowered dramatically at an elevated temperature of 150 °C [4.2]. The field-

effect mobilities of pentacene-based OTFT increases from 0.0006 cm2 V"1 s"1 without

treatment to 0.0013 cm2 V"1 s"1 with an annealing in N2 ambient at 120 °C for 60 minutes

[4.3]. The temperature of heat treatment depends on materials. It was concluded that 90 ~

120 °C is acceptable for heat treatment of the polymers after examining the film qualities.

Considering the simplification of processes, the treatments in this thesis focus on

thermal annealing in vary conditions. Samples in this work were divided into two major

groups, with and without thermal annealing. For MEH-PPV and MEH-PPV-POSS

samples with the treatment, the conditions are (1) in vacuum at 90 °C for 30 minutes; (2)

in N2 at 90°C for 30minutes after (1); (3) in N2 at 90 °C for 60 minutes; and (4) in N2 at

120°C for 60minutes. For Poly-TPD and Poly-TPD-POSS samples, the treatment

conditions are (1) in vacuum at 90 °C for 30 minutes; (2) in N2 at 90 °C for 30 minutes;

and (3) in N2 at 90 °C for 60 minutes.

34

4.4 Measurement setup

4.4.1 Current-Voltage Measurements

Electrical characteristics of the fabricated OTFTs were measured using an HP-

4145A semiconductor parameter analyzer. The drain voltage was swept from 0 to -25 V

at various gate voltages from 0 to -20 V with a long integration time. The results will be

described in the following sections. Gold (Au) wires were used to connect the chips to

external testing circuit by soldering with Wood's alloy. The testing circuit was protected

by a metal box to reduce electrical noise during the measurements. Although it was

proven that the characteristics and more specifically the performance of OTFTs are

different and superior under vacuum and lower temperature conditions, all OTFTs in this

work were characterized in atmospheric ambient at room temperature conditions under

which the OTFTs are expected to be functioned in order to be viable for future

commercialization. A schematic diagram of the measurement setup is shown in Figure

4.4.

4.4.2 Capacitance-Voltage Measurements

The capacitance-voltage (C-V) measurements can be used to determine the

capacitance of gate oxide. The MOS capacitor as shown in Figure 4.5 is used. The C-V

measurements are carried out using an HP 4274A LCR meter, which is controlled by an

IBM-PC computer. The dc bias voltage is monitored using an HP 3478A multimeter. The

schematic diagram of the C-V measurement setup is shown in Figure 4.6. To perform the

35

C-V measurements, an MOS configuration of an «+-Si/Si02/Al was used. Al was

deposited as an electrode on Si02 by evaporating through a shadow mask. Area of the

electrode was about 0.02 cm . The C-V measurements were done at dc bias voltage from

-25 V to 25 V at two different frequencies, 10 kHz and 100 KHz, respectively. The

complex impedance of the capacitor is a function of the applied dc bias voltage and the

capacitance can be extracted from the imaginary part of the impedance. The average

value of gate oxide capacitance was about 600 pF.

Using a similar configuration, an n+-Si-gate/Si02/polymer/Al capacitor structure

was used to estimate the thickness of polymer film at zero dc bias voltage. The thickness

of polymer film was examined and confirmed by using a scanning electron microscope

(SEM). The average thickness of the spun polymer films is about 0.2 (im.

4.5 Conclusions

In this chapter, the fabrication procedure of OTFTs on silicon substrate, the

thermal annealing, and measurement setup were described and explained in details. It is

worthwhile to note that gate oxide with low leakage current is required for transistor

operation. Therefore, the cleaning process after gate window opening and the dry

oxidation process for gate oxide formation will affect the quality of the resulted transistor.

The performance of the fabricated OTFTs will be presented and discussed in the next

chapter.

36

SiO-

Gate SiO?

i SiO-

n+-Si

Au

SiO. SiO-

n+-Si

Si02

Polymer

Si02

n+-Si

(a)

(b)

(c)

(d)

(e)

Figure 4.1 Schematic diagrams showing the fabrication procedure of OTFTs

based on silicon substrate.

37

Mask#1 Mask #2

Figure 4.2 Diagrams of photo mask patterns.

Channel of transistor

i Gate oxide

Channel of transistor

Gate oxide

Figure 4.3 Top views of an OTFT.

38

Drain

SiO:

Polymer I SOL

Si02

Source

n+-Si

Gate / / / i i Glass Stage

L L L L I

O

O

o

Metal Box

HP4145A

Semiconductor

Parameter

Analyzer

Figure 4.4 A schematic diagram of the measurement setup.

I Metal contact

Gate dielectric

n+-Si gate

I

G

Figure 4.5 A schematic diagram of the MOS capacitor.

39

IBM-PC Computer

IEEE 488 Bus

L A

HP 3478A multimeter

HP 4274A LCR meter

© ©

Sample

Figure 4.6 A schematic diagram of the measurement setup.

40

Chapter 5

Results and Discussions

5.1 Introduction

In this work, 51 OTFT samples were fabricated and studied. Results were obtained

from more than half of these samples, with eight transistors on each of them. The results

of selected samples are reported in this thesis. Those results indicated that all of the tested

polymers, MEH-PPV, MEH-PPV-POSS, Poly-TPD, and Poly-TPD-POSS, could be used

as the active semiconducting layers of organic thin film transistors (OTFTs).

In order to examine the effects of polyhedral oligomeric silsesquioxanes (POSS) on

MEH-PPV and Poly-TPD, POSS was physically mixed with these polymers, respectively.

It is noted that the results of all OTFTs obtained in this work show typical drain current

and drain voltage characteristics of ̂ -channel field-effect transistors.

As stated before, the main objective of this work is to study the effects of different

treatment methods on the polymer-based OTFTs for possible improvement of charge

carrier field-effect mobility. The field-effect mobility is a significant parameter of OTFTs

because it determines the channel conductance and transconductance, which should be as

high as possible. A high quality thin film transistor also requires low leakage current

41

between drain and source in the OFF state, large ratio of ON and OFF currents, and small

threshold voltage. The details of the performances of the fabricated devices will be

presented in subsequent sections. The highest mobility of pentacene-based OTFT (2.7

2 1 1

cm V" s" [2.12]) reported to date is in the same order of magnitude as thin film

transistor fabricated using amorphous silicon (~ 1 cm2 V"1 s"1). From the results of this

work, the optimal field-effect mobilities of polymer-based OTFTs can be increased at

lease one order of magnitude.

5.2 Gate Dielectric

A high quality organic thin film transistor requires a reliable gate dielectric material

with high dielectric strength. For example, the typical value of dielectric strength for

reliable silicon dioxide is 107 V/cm [3.5]. Both organic and inorganic dielectric can be

used as gate insulators. For example, a gate dielectric polymer, poly-vinylphenol (PVP),

has been used to fabricate the organic thin film transistors (OTFTs) [5.6]. Silicon nitride

or silicon oxide is the common choice of gate materials for OTFTs fabricated on silicon.

In our research works, reactively sputtered silicon nitride and thermally grown silicon

dioxide on heavily doped «-type single-crystal silicon were used. These were

characterized using capacitance-voltage (C-V) measurements, which were introduced in

Chapter 4. The extracted capacitance is indicated to the movement of charges in the

materials.

42

Silicon nitride films were prepared by Defence Research and Development Canada

- Valcartier (DRDC-Valcartier) using a reactive ion-beam sputtering method. The films

were sputtered under the following conditions: a heavily doped «-type silicon wafer acted

as the target and its temperature was maintained at 148°C; RF power was set at 80 watts;

cathode bias voltage set at -340 V; and the nitrogen flow rate was set so that a chamber

pressure of 3 mtorr was obtained. The silicon nitride is deposited on the silicon surface.

The average thickness of silicon nitride films is 0.2 jim after 60 minutes. When the C-V

measurements were carried out on the MOS capacitors with those silicon nitride films as

dielectric layers, the leakage currents exceeded the tolerance of the equipments. The

undesirable charges and the pit of the films may have caused the leakage currents through

the MOS capacitors. On the other hand, those silicon nitride films have poor adhesion on

the silicon surface causing weak tolerance on chemicals and cleaning process with de-

ionized (Dl) water, thus it is not suitable for the fabrication procedures of OTFTs after

deposition of the gate dielectric. Such phenomenon indicated that the sputtered silicon

nitride films prepared by DRDC-Valcartier under the above conditions could not be used

as the gate dielectric of OTFTs for the thesis research.

Thermally grown silicon dioxide films usually have high reliability and quality and

are used in the semiconductor industries. Thermally grown silicon dioxide films were

prepared in our lab. The fabrication process was carried out in a furnace at atmospheric

pressure and at about 1050°C using heavily doped n-type silicon wafers. Oxygen was

allowed to pass through the furnace tube at 1.5 units per second through trichloroehylene

43

(TCE) in a small bubbler. TCE can reduce ionic impurities in oxygen. The average

thickness of the silicon dioxide films is about 0.1 urn after an oxidation for 30 minutes.

Compared the silicon nitride films prepared by DRDC-Valcartier and the silicon

dioxide (Si02) films prepared in our lab, the silicon dioxide films show acceptable

electrical properties and superior tolerance in chemicals. The dielectric strength of these

silicon dioxide films is in the range of 106 to 107 V/cm. MOS capacitors were fabricated

on the thermally grown oxide for C-V measurements. Figure 5.1 shows the C-V

characteristics obtained on the MOS capacitors at a frequency of 10 kHz. The film

thickness is about 0.1 urn and the area of the aluminum (Al) contact is about 0.02 cm . By

using the following equation, the thickness of the Si02 layer can be obtained from the

capacitance value measured (690 pF).

r - e°*A

^ox ~ t (5.1)

ox

14 Where eox is the permittivity of Si02, which equals to 3.9 x 8.85 x 10" F/cm, A is the

area of the aluminum contact and tox is the thickness of Si02 film.

When a positive voltage is applied to the aluminum contact and a negative voltage

to the heavily doped «-type silicon substrate, a large density of mobile electrons will be

attracted to the Si/Si02 interface forming an accumulated layer of electrons near the

interface. This accumulation layer acts as a negatively charged capacitor plate opposite to

the positively charged aluminum contact. The thickness of the dielectric layer in the

capacitor in accumulation region is identical to the thickness to the oxide layer. However,

44

from Figure 5.1, it is noted that the measured capacitance in the accumulation region,

which is about 600 pF, is different from the theoretical value of 690 pF assuming an

oxide thickness of 0.1 urn. This may be due to variation of the actual contact area and

oxide thickness between different devices. Other possible reasons also include the fix and

positive charges in the oxide. A depletion layer will be formed when a negative voltage is

applied to the aluminum contact and a positive voltage is applied to the heavily doped n-

type silicon substrate. The thickness of the depletion layer will increase until the

maximum width is reached. In this case, the thickness of the capacitor equals to the sum

of the thicknesses of two parallel capacitors. One is due to the oxide and the other due to

the width of the depletion region. In this case, a minimum capacitance occurred with the

value smaller than the oxide capacitance mention before. This is because the substrate is

heavily doped n-type silicon with electrical resistivity in the range of 0.02 - 0.06 Q-cm.

The unit capacitance of Si02 film in this work is about 30 nF/cm with the average

capacitance and contact area equating to 600 pF and 0.02 cm respectively. In summary,

the thermally grown silicon dioxide films prepared in our lab are at least fair.

5.3 Metal Contact

In the fabricated OTFTs, gold (Au) was deposited for the drain and source contacts.

In the early experiments, we used aluminum and silver for the drain and source contacts.

However, both of these two metals were found to be not suitable for the formation of

ohmic contacts with the polymer layer due to high contact resistances. According to Table

5.1, gold is a reliable choice among the metals because its work function is 5.1 eV, and

45

can form ohmic contact with the polymer layer. Gold can be readily evaporated to form a

smooth film on the silicon substrate with gate oxide. The shapes of contacts can be

defined by photolithography and etching Au in KI+I2 solution at room temperature.

Table 5.1 Work functions of some metals [3.2]

Metal

Silver (Ag)

Aluminium (Al)

Gold (Au)

Chromium (Cr)

Work Function

4.26

4.28

5.10

4.50

46

5.4 Device Performance

5.4.1 MEH-PPV Organic Thin Film Transistors

5.4.1.1 Without any treatment

Following the procedures described in chapter 4, the MEH-PPV-based organic thin

film transistors were fabricated as shown in Figure 5.2. It was found that sample #48

showed the best performance in the group of MEH-PPV-based OTFTs without any

treatments. Figure 5.3 shows output characteristics of a MEH-PPV-based OTFT (sample

#48) without any treatment. Due to the small current flowing through the channel, the I-V

characteristics in the saturation region are very sensitive to noise and often show large

fluctuation. At a given applied drain-source voltage (VdS = -10 V), the leakage current or

OFF-state current flowing between the drain and source is about -1.55 pA and the ON-

state current is about -11 pA at a Vg = -10 V. According to these values, the ratio of

ON/OFF currents is about 7. The ratio between ON and OFF currents in the present

devices is thus too small for efficient switching applications. It is noted that the small

current ratio is due to the large leakage current. This large leakage current also causes the

I-V curves to shift with different applied gate voltages. From the transfer characteristics

shown in Figure 5.4, the extracted threshold voltage is about 2.6 V. The field-effect

mobility was estimated using Equation 3.5 and a value of 3.66xl0"6 cm2 V"1 s"1 was

obtained in the saturation region (Vds = -10 V, Vg = - 2 V). The average field-effect

mobilities of the samples are in the range of 10"7 to 10"6 cm2 V"1 s"1. All of the data here

shows that the MEH-PPV-based OTFTs without any treatments have poor performance.

Although the curves of output and transfer characteristics of MEH-PPV-based OTFTs

47

without any treatment follow the basic characteristics of a thin film transistor, these

curves show significant fluctuations. For the devices based on MEH-PPV, the contact

resistance with gold is large. This large contact resistance causes large contact noises. In

addition, the MEH-PPV used was synthesized for organic light emitting diodes and not

optimized for organic thin film transistors. Thus the molecular properties of the MEH-

PPV materials used did not yield OTFTs with high mobilities and high currents.

In view of the above results, it was decided to study annealing of the polymer thin

films in an attempt to improve these films. Furthermore, research work was carried out to

fabricate OTFTs using MEH-PPV containing POSS units. The results obtained are

described in the following sections.

5.4.1.2 Effects of POSS

As mentioned in the previous chapters, conductivity of the polymer channel at a

given gate voltage must be high enough so that sufficient drain currents can be obtained.

This can be accomplished by reducing the interface barrier between conjugated polymer

and electrodes, by using end-capping polymer chains with polyhedral oligomeric

silsequioxanes (POSS). There are other research works reported on hybrid organic-

inorganic polymers containing segments of POSS showing multifunctional and improved

mechanical and physical properties, such as high thermal stability in air [5.2]. Except the

synthesized POSS anchored polymer, polymer also is physically mixed polymer with

POSS in order to examine the effects of POSS. POSS powder is first dissolved in toluene,

then dropped in polymer solution. Ultra sonic bath is used to mix the solutions.

48

All the results to be described in this section are obtained from the samples without

any annealing steps. By comparing Figure 5.5 and Figure 5.3, it is noted that the

performance of an OTFT (sample #47) can be improved dramatically with the

synthesized POSS unit in MEH-PPV-POSS. The ON current reaches -1.2 nA at an

applied voltage of Vds = -10 V. Furthermore, the leakage current of MEH-PPV-based

OTFT remains reasonably low, about -11 pA. The ratio of ON/OFF currents increases to

over 100 from a value of 7 for the OTFTs without POSS. By comparing Figure 5.6 and

Figure 5.4, the threshold voltage decreases from 2.6 V to 0.4 V. The resulted leakage

current may be due to the free movement of charge carriers between Au and polymers

with the absence of the potential barrier. A reasonably small leakage current is still

present and causes the nonzero OFF-state current and a shift of the I-V curves in the

output characteristics. Since the polyhedral oligomeric silsequioxanes (POSS) anchored

semiconducting polymer, it is believed that the decrease in contact resistances and noises

are due to improvement in the thermal stability and adhesion of the polymers.

Table 5.2 shows mobility results obtained in this work for MEH-PPV, MEH-PPV-

POSS and physically mixed MEH-PPV with POSS. It is noted that there is a large

increase of field-effect mobilities of OTFT, by two orders of magnitude, with the addition

of polyhedral oligomeric silsequioxanes (POSS). For MEH-PPV and MEH-PPV-POSS,

the field effect mobilities are estimated to be about 10"7- 10"6 cm2 V"1 s"1 and 10"4 cm2 V"1

s"\ respectively. According to Table 5.2, it is also noted that one may not obtain an

improvement of field-effect mobility on the OTFT simply by mixing the parent polymer

and POSS. However, the mobilities can be improved by using the synthesized POSS

49

anchored semiconducting polymers as the active layer of the OTFT. It is believed that the

molecular properties of MEH-PPV with the synthesized POSS units have improved.

When the MEH-PPV-POSS solution is spun onto the substrate, the molecular ordering

may be pronounced compared to that for conventional MEH-PPV. Hence, the carrier

mobility is higher for devices fabricated using MEH-PPV-POSS.

Table 5.2 A comparison of the mobilities of MEH-PPV, MEH-PPV-POSS and

physically mixed MEH-PPV with POSS.

Mobilities at saturation regime (Vd—10 V, Vg=-2V) (cm V s )

Without any treatment

MEH-PPV

MEH-PPV-POSS

Physically mixed MEH-PPV with POSS

3.66x10-6

5x10-5 **

1.16x10-4

7.4x10-5

** Value for the same material reported by other research group [3.1].

50

5.4.1.3 Effects of thermal annealing

It is interesting to determine whether the performance of OTFTs can be further

improved by thermal annealing in different conditions. In general, thermal annealing can

improve the mobilities of semiconducting polymers [5.3]. To carry out these experiments,

several OTFTs were fabricated under the same conditions. These devices were then

treated in vacuum or N2 at different temperatures and for different durations. The mobility

results obtained for these devices are summarized in Table 5.3 (for MEH-PPV). In the

early experiments in this work, the heat treatments were carried out at temperatures

between 45 °C and 85 °C in an air ambient. It was found that the mobility values were

smaller than those without any treatments. It was then decided to raise the treatment

temperature to 90°C [2.11, 3.1] either in vacuum or nitrogen ambient. It was noted that

the field-effect mobilities have dramatically increased after the above thermal annealing.

The best mobility result, with a magnitude of the order of 10"4 cm2 V"1 s"1, was

observed on a device heat treated in nitrogen at 90 °C for 60 minutes. It is noted in Table

5.3 that this best value of mobility is two orders of magnitude higher than that of mobility

of the device without any treatment. This mobility improvement may be due to the

change of morphology of the polymer film. From the Figure 5.7, after the heat treatment

in vacuum at 90 °C for 30 minutes, no obvious defects can be observed on the MEH-PPV

(sample #30) under scanning electron microscope (SEM). The polymer films may re-

crystallize under certain conditions of thermal annealing, resulting in films with less

defects. Fewer defects of the films mean a longer lifetime of the carriers in the channel.

Thus, the carrier mobility will increase after heat treatments with proper conditions.

51

Table 5.3 A comparison of the mobilities of MEH-PPV under different thermal

annealing conditions.

Mobilities at saturation regime (Vd=-10V, Vg=-2V) (cmVY 1 )

Without any treatment

Vacuum heat treatment at 90 °C for 30 min.

After a vacuum heat treatment and N2 heat treatment at 90 °C for 30 min.

N2 heat treatment at 90 °C for 60 min.

N2 heat treatment at 120 °C for 60 min.

3.66 xlO"6

2.28 xlO-5

1.36 xlO"5

2.20 xlO"4

1.92 xlO"5

5.4.2 Poly-TPD Organic Thin Film Transistors

5.4.2.1 Without Any treatment

The manufacturer of the poly-TPD used in the present work has stated this material

is a polymer with high carrier mobility. Hence, it was anticipated that OTFTs fabricated

using the poly-TPD to have better performance than the OTFTs from MEH-PPV. Several

OTFTs were fabricated using the Poly-TPD using the procedure described in Chapter 4.

Figure 5.8 shows an optical micrograph of one of the fabricated OTFTs. Current-voltage

characteristics were measured on these devices and the results for device No. 51 are

52

shown in Figure 5.9. It is noted that the results shown for the OTFT without any treatment

are generally better than that for a MEH-PPV-based OTFT. The ON-state current exceeds

-1.5 nA when the applied voltage is -10 V. Comparing with the ON-state current, the

OFF-state (leakage) current is rather small, about -12 pA. The ON/OFF current ratio for

the OTFT with poly-TPD is about 127. From Figure 5.10, a threshold voltage of 0.5 V

was obtained. With this small threshold voltage and large ON-state currents, the field-

effect mobility in saturation region (Vds = -10 V, Vg = - 2 V) is 4.18 x 10"5 cm2 V"1 s'1.

This mobility value is large enough for many applications of TFTs. Since the spin coating

conditions for OTFTs involving poly-TPD and MEH-PPV thin films are the same, the

difference in performance of the organic thin film transistors must be due to differences in

molecular structures. The present experimental results also confirmed that poly-TPD is

more suitable for the fabrication of thin film transistors than MEH-PPV. The leakage

current between drain and source of Poly-TPD-based OTFTs may due to the absence of

potential barrier between gold and Polymer. In the former sections, it was found that the

addition of POSS units and thermal annealing have led to improved performance of the

MEH-PPV-based TFTs. Thus, these two methods may improve further the performance

of poly-TPD-based thin film transistors.

53

5.4.2.2 Effects of POSS

Figure 5.11 shows I-V characteristic of an OTFT with poly-TPD-POSS channel

(sample #45). It is noted that the drain currents are even larger than those of poly-TPD-

based OTFT at the same applied voltage, described in the previous section. These results

show that the synthesized POSS units are advantageous to the electronic characteristics in

poly-TPD, resulting in an OTFT with improved performance.

From Table 5.4, it is evident that there is a large increase of field-effect mobilities

of the OTFTs fabricated on poly-TPD with the addition of polyhedral oligomeric

silsequioxanes (POSS). For devices fabricated on poly-TPD and poly-TPD-POSS, the

estimated field-effect mobilities are about 10"6 - 10"5 cm2/V-s and 10"5 - 10"4 cm2/V-s,

respectively. In Table 5.3, it is further noted that improved field-effect mobility may not

be obtained simply by mixing the parent polymer and POSS. However, the mobilities can

be increased by using the synthesized POSS anchored poly-TPD (poly-TPD-POSS). The

presence of synthesized POSS units assures the improvement in thermal stability of the

polymers in air. The adhesion of polymers to both the metal contacts and substrate may

also have improved. The non-synthesized POSS units will not offer such improvement.

This is due to a pronounced molecular ordering in organic film with the synthesized

POSS units. As mentioned in the previous sections, the improved adhesion reduces both

contact resistance and noises. Thus, the channel conductance and transconductance can be

increased.

54

Table 5.4 A comparison of the mobilities of poly-TPD, poly-TPD-POSS, and

physically mixed poly-TPD with POSS

Mobilities in saturation regime (Vd—10V, Vg=-2V) (cm V s )

Without any treatment

Poly-TPD

Poly-TPD-POSS

Physically mixed Poly-TPD with POSS

4.18x10-5

4.34x10-4

2.86x10-5

5.4.2.3 Effects of thermal annealing

Usually, a suitable thermal annealing can improve the performance of thin film

transistors. This has been demonstrated in the studies of OTFTs based on MEH-PPV in

the present work. It is interesting to determine whether the performance of TFTs on poly-

TPD and poly-TPD-POSS can be further improved by thermal annealing. Several organic

thin film transistors were fabricated under same conditions. These transistor devices were

then treated in vacuum and N2 at different temperatures and for different durations.

55

The mobility results obtained from these devices are summarized in Table 5.5. It is

noted that only the TFTs fabricated using the polymers without POSS have shown an

increase in mobility after the annealing, possibly due to the improvement of the

morphology. As show in Figure 5.12, after treating in vacuum 90 °C for 30 minutes, it is

difficult to find any defects of poly-TPD film under scanning electron microscope (SEM).

The thermal annealing may not improve the mobility of poly-TPD-POSS. This may be

due to the fact that there is a substantial improvement to the mobility of the TFT with

POSS, resulting from the improvement in the adhesion between the polymer thin film, the

drain and source contacts, and substrate. Further heat treatment only leads to moderate

improvement in the performance. For the treatment in nitrogen of poly-TPD, the

following conditions are found to be the best: temperature at 90 °C for a duration of 30

minutes.

It is thus clear that the effect of annealing is less significant among these devices.

However, we have observed that the field effect mobilities of POSS anchored poly-TPD

decrease after the annealing. The thermal annealing may degrade the adhesion between

the polymer film and substrate. The relatively poor adhesion will result in larger contact

resistance. Contrarily, the synthesized POSS units exhibit reasonably high field effect

mobilities in thin film transistor without any heat treatments.

56

Table 5.5 A comparison of the mobilities of poly-TPD and poly-TPD-POSS with

different thermal annealing conditions

Mobilities in saturation regime (Vd=-10V, Vg=-2V) (cm V s )

Without any treatment

Vacuum heat treatment at 90 °C for 30 min.

N2 heat treatment at 90 °C for 30 min.

N2 heat treatment at 90 °C for 60 min.

Poly-TPD

4.18 xlO*5

1.85 xlO"4

2.04 xlO"4

3.44 xlO-5

Poly-TPD-POSS

4.34 xl0~4

3.34 xlO"4

2.90 xlO"4

1.38 xlO"4

5.5 Conclusions

The electrical characteristics of organic thin film transistors based on different

semiconducting polymers were measured and described in this chapter. For the fabricated

organic thin film transistors in this work, the output and transfer characteristics are similar

to that from inorganic thin film transistors. Comparing the performance of OTFTs based

on MEH-PPV and poly-TPD, it is clear that the ones with poly-TPD show better

electrical results. Since the carrier field-effect mobility is a function of the channel

conductance, the effects of treatment conditions on the performance improvement can be

57

studied by examining the mobility. It was found that the semiconducting polymers

containing segments of polyhedral oligomeric silsesquioxanes (POSS) yield large

improvement on the field effect mobility. The overall high field effect mobility leads to

relatively good device performance. For MEH-PPV organic thin film transistors, the best

heat treatment conditions are at 90 °C in N2 for 60 minutes. For poly-TPD organic thin

film transistors, the conditions are at 90 °C in N2 for 30 minutes. It is noted that the

thermal annealing may not improve the performance of devices based on POSS anchored

semiconducting polymers. In the present work, Au was used as the electrode material and

leakage currents may not be avoided due to the absence of potential barrier between

polymers and Au.

58

620

610 -

600

3 590 * - » <J

I 580 u

570

560

550

— r " ^ 1 1 1 WWWtttt

Al contact Si02

n+-Si gate ©

-25 -20 -15 -10 -5 0 5

Bias Voltage (V)

10 15 20 25

Figure 5.1 Graph of capacitance-voltage of thermal silicon dioxide (negative sign

of bias voltage indicates the positive bias to silicon substrate).

Figure 5.2 Optical micrograph of a MEH-PPV-based OTFT without any

treatment.

59

V ds (V)

Figure 5.3 Characteristics of drain current (lds) vs. drain voltage (Vds) at various

gate voltages for an MEH-PPV-based OTFT without any treatment (Sample No.

48, W«76p.m, L « 7 urn).

60

I

O O

re

v t = 2.6 V

Figure 5.4 A plot of square root of drain current (Ids) vs. gate voltage (Vg) at a

drain voltage = - 5 V for the MEH-PPV-based OTFT without any treatment

(Sample No. 48, W « 76jim, L « 7 jim).

61

< c

T3

-2D

-1.6

-1.2

-0.8

-0.4

-

-

-

-*-Vg=0V -^Vg=-2V -*-Vg=-4 V — Vg=-6V -^Vg=-8V -^Vg=-10V

I 1

1

m* ' ^ ̂ t — » < > —i . i 1 . « « » * » • « » - t ' * * * * * * * f -5 -10 -15

V ds (V)

Figure 5.5 Characteristics of drain current vs. drain voltage at various gate

voltages for the MEH-PPV-POSS-based OTFT without any treatment (Sample

No. 47, WV73 um, L* 7 um).

62

V t = 0.4V

-6 -7

Vg(V)

10 -11 -12 -13 -14 -15

Figure 5.6 Plot of square root of drain current (Ids) vs. gate voltage (Vg) at drain

voltage = - 5 V for the MEH-PPV-POSS-based OTFT without any treatment

(Sample No. 47, W~73 um, L* 7 um).

63

Figure 5.7 SEM graph of a MEH-PPV-based OTFT treated in vacuum at 90 °C

for 30 minutes (sample #30)

Figure 5.8 Optical micrograph of a poly-TPD-based OTFT without any treatment.

64

-2J0

-1.5

<

J -1.0

-0.5

-

- ^Vg=0V — Vg=-2V -*-Vg=-4V -^Vg=-6V ^-Vg=-8V -—Vg=-10V

/ / -m****

* T ^ 1 1 » » t « » ' 1 « t • t » « —t— t ' =*= —t— —t— —• »— » » — > — »

— « — i

•

-10 -15

V ds (V)

Figure 5.9 Characteristics of drain current vs. drain voltage at various gate

voltages for a poly-TPD-based OTFT without any treatment (Sample No. 51, W *

180 um, L ~4 um).

65

< Q.

-a i—i

root

of

Squ

are

50

45

40

35

30

25

20

15

10

5 -

m+ i i i i i i i i i i n n i

0 -1 v t = 0.5V

-2 -3 -4 -5 -6 -7

Vg(V) -9 -10 -11 -12 -13 -14 -15

Figure 5.10 A plot of square root of drain current (Ids) vs. gate voltage (Vg) at a

drain voltage = - 5 V for the poly-TPD-based OTFT without any treatment

(Sample No. 51, W « 180 um, L ~ 4 um).

66

-2

< c

T3

-

-^Vg=OV

— Vg=-2V

-*-Vg=-4V

— Vg=-6V

— Vg=-8V

— Vg=-10V

V ds (V) -10 -15

Figure 5.11 Characteristics of drain current vs. drain voltage at various gate

voltages for the poly-TPD-POSS-based OTFT without any treatment (Sample No.

45, W*73 um, L* 7 um).

67

Figure 5.12 SEM graph of a poly-TPD-based OTFT treated in vacuum at 90 °C

for 30 minutes (sample #36)

68

Chapter 6

Conclusions

The development of organic microelectronics based on organic semiconductors is

an interesting and important subject. The main goal of this work is to develop a low-cost

and simple fabrication process for organic thin film transistors (OTFTs). In this work,

OTFTs were fabricated and characterized using materials that are developed for organic

light emitting diodes so as to study the possibility of integrating OTFTs and OLEDs on

the same active substrate. These materials are poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-

phenylene vinylene] (MEH-PPV), polyhedral oligomeric silsesquioxanes (POSS) poly (2-

methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV-POSS), poly[N-(3-

methylphenyl)-N,N-diphenylamine-4,4'-diyl] (Poly-TPD), and polyhedral oligomeric

silsesquioxanes (POSS) poly (N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine (Poly-

TPD-POSS). Poly-TPD and poly-TPD-POSS are used for the first time to fabricate

OTFTs. Functional OTFTs with satisfactory mobilities were obtained in this work. In the

following, the significant achievements of this work are summarized.

Firstly, the fabrication process developed yielded OTFTs on thermally grown

silicon dioxide with acceptable performance. The best value of carrier mobilities of most

of the polymers are about 10"4 cm2 V"1 s"1 at a channel width in the range of 70 to 180 um

69

and a channel length in the range of 4 to 8 um. Although the mobilities are inferior to

those of inorganic semiconductors, the values are acceptable for many applications and

are comparable to the large molecular organic semiconductors.

Secondly, the study has demonstrated that the polyhedral oligomeric

silsesquioxanes (POSS) anchored semiconducting polymers offer promising results. The

addition of POSS improves the carrier field effect mobilities of organic semiconductors

by at least one order of magnitude. On the other hand, thermal annealing did not have

significant improvement on the mobility of these polymers.

Finally, it was observed that the thermal annealing has improved the performance

of OTFTs without the POSS. The optimized thermal annealing improves the carrier field

effect mobilities of the examined polymers by one to two orders of magnitude. The

optimal annealing conditions for poly-TPD are 90 °C for 30 minutes in N2 and are 90 °C

for 60 minutes in N2 for the MEH-PPV.

As a result of the present research work, several suggestions are given here for

future improvements. It is recommended to carry out heat treatment under different

conditions or even different methods on the solution-processed semiconducting polymers

thin films in order to enhance further the performance of OTFTs. In addition, it is

recommended to fabricate OTFTs on different substrates, such as glass substrate, in order

to meet the requirements for low cost and large-scale applications.

70

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