-sexithiophene

pentacene Fig. 1 molecular organic semi- conductors studied in this work.

SS

SS

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論文要旨

Control of Organic Thin Film Growth via Modification of Substrate Surface

(基板表面修飾による有機薄膜の成長制御に関する研究)

金森 由男

Introduction

Organic semiconducting materials have recently attracted

considerable attention due to their reduced cost, flexibility, wide

applications, and low environmental impact. Over the last few years,

performance of organic light emitting diodes (OLEDs) and organic

photovoltaic cells (OPVCs) have been greatly improved by

designing organic molecules which have appropriate electronic

properties. With regard to organic thin film transistors (OTFTs),

however, device performance is still far from practical application.

What seems to be lacking is crystallinity of thin films rather than

their electronic properties. The subject of this study is to control the morphology of organic thin films by

applying appropriate growth condition or novel techniques. We studied organic film growth on electrode

metals and insulating dielectrics, via both physical and chemical modification approaches. The target

materials are -sexithiophene (6T) and pentacene, which are depicted in Fig.1.

1: Physical modification of dielectric substrate: Graphoepitaxy of a-sexithiophene on periodic grooves

We examined epitaxy on artificially patterned substrates to fabricate high orientated organic thin film.

This epitaxy is known as graphoepitaxy and is a promising technique for oriented growth on substrates with

an amorphous dielectric top layer. Periodic grooves were fabricated by electron lithography on Si wafers

with an oxide layer of 300 nm (Fig.2). The width (W) and depth (d) of the grooves were set at 100 nm and

10 nm, respectively. The periodicity L has various lengths (from 200 nm to 10 µm).

A typ

in Fig.3.

(FG), elo

should be

The c

their sha

of , whi

groove d

is depict

features;

direction

parallel t

tends to b

tends to b

is worth

extreme

Next

examined

GFGs) ar

number o

groove p

gradually

is surpris

nucleatio

of nuclea

determin

In this m

the exist

migrating

Therefor

became s

this mod

largest E

GFGs wo

Fig. 3 6T(L = 200(Right) T

pical AFM im

Three differ

ongated facet

e noted that m

crystallograph

apes. In-plane

ich it defined

direction. The

ted in Fig. 4

first, there a

n. It is likely

to grooves (th

be parallel to

be large (45 t

noting that

flatness of EG

, 6T morph

d. At first, de

re evaluated.

of grains per

periodicity L.

y obeying a p

sing that nEG

on decreased

ation around

ned by the cap

model deposite

ting island, w

g molecules

re, nucleation

small. The ca

del, we can e

EG with this m

ould be locate

T films on pe0nm). (Left)The illustratio

mage of 6T fil

rent types of

tted grain (E

most of GFGs

hic orientatio

e orientation

d as the angle

e histogram o

4. We can cle

are several p

y that these d

he assignmen

o the grooves

to 90). EGs w

connection o

G surface.

hology depe

ensities (n) of

The grain de

unit length n

. It can be s

power law in

GandEG remain

with L. This

EGs and FG

pture length o

ed 6T molecu

which result

and suppress

n-free region

apture length L

explain why

mechanism (s

ed at random

eriodic groove) An AFM topon of classifi

lms with a th

6T grains ar

G) and groov

s have EGs an

on of EGs and

of 6T grains

between b-ax

of in-plane or

early see the

peaks in histo

directions are

nt is shown i

for EGs. Sec

would be form

of these EGs

endence on

f three kinds

ensity was ob

normal to the

een in Fig. 5

the region of

ns almost con

will be attrib

Gs. The dens

of 6T molecu

ules will be i

s in decreas

sion of nucle

ns appeared

LC can be est

groove perio

see Fig.5(b)).

m positions an

e patterned supographic imed grains.

hickness of 15

re observed;

ve filling gra

nd FGs on th

d FGs can be

can be discu

xis of the 6T

rientation of

e following t

ogram, indic

e determined

in Fig.4). We

cond, of EG

med as a resu

s is perfect a

groove per

of gains (EG

tained by cal

groove, as a

5(a) that nGF

f L smaller th

nstant, while

buted to the

sity of EGs a

ule (Capture z

immediately

e of the den

eation around

and the film

timated to be

odicity L = 2

If L is smalle

nd their captur

ubstrate mage. Fig. 4 Hist

of the b-axi

5 nm is show

facetted grai

ain (GFGs). I

hem.

e estimated b

ussed in term

grains and th

EGs and FG

two importan

ating existen

d to make a

e can say tha

Gs tends to b

ult of FGs’ co

at the coalesc

riodicity is

Gs, FGs and

lculating the

function of

G decreases

han 2 m. It

the site of

suppression

and FGs are

zone model).

captured by

nsity of 6T

d the island.

m coverage

1 µm. With

2 µm gives

er than 2Lc,

re zones are

togram showis direction

0.1

0.1

1

10 (a)

Den

sity

(m

-1)

Groo

0.10

10

20

30

Ave

rage

leng

th o

f E

Gs

(m

)

Groo

(b)

Fig.5 (a) Gr

on groove p

(b) Average

variousove g

n

n

It

y

ms

he

Gs

nt

nce of prefere

low index d

t b-direction

e small (0 to

alescence on

cence bound

Fig.2 Sof a substrat100nm respecti

wing the diof 6T grain

1

ove periodicity(

1

ove periodicity (

rain density d

periodicity.

length of EG

groove period

ential crystal

direction of t

of 6T films

45), while th

GFGs with s

dary, judging

Schematic illugroove

te. W and and

ively.

W

L

d

terrace

wall

bottom

istribution

ns.

10

(m)

nEGandFG

nGFG

ngroove

10

(m)

dependence

Gs at

dicity.

llographic

the grains

generally

hat of FGs

small . It

from the

ustration pattened

d are 10nm,

overlapp

suppresse

graphoep

2: Chem

on SiO2

In bo

film grow

the electr

electrode

investiga

regions

probes o

applicabl

influence

attempted

without u

of freque

Two

alkyl ch

aromatic

fabricatio

the subst

and then

chamber.

is shown

increase

voltage.

semicond

the resul

samples.

off curre

in Fig. 7

regions d

voltage a

showed t

regions e

SAMs ha

3: A nov

Rece

which ar

novel die

device ap

were disp

ed. As a resu

ed. When L

pitaxy is not l

mical modific

substrate

ottom contact

wth on electro

rodes is likel

es. The s

ate the elec

is to fabri

on the subs

le since addit

e the film

d to invest

using any add

ency response

kinds of SAM

hain) and b

ring), were

on of Au ele

trates were ex

n pentacene

. Device perf

n in Fig. 6.

in mobility

Contact

ductor interfa

t of frequenc

Moreover, fr

ent regions ne

. Introduction

disappear co

and contact r

the presence

electronic sta

as the effect t

vel dielectric

ntly, graphen

re obtained as

electric of ult

pplication. G

persed in me

ult, each dom

is larger tha

likely to occu

cation of elec

t OTFTs, orga

odes is very d

ly to be influ

simplest ap

tric properti

icate additio

strates, but

tional electro

m morpholog

igate electri

ditional elect

e measuremen

Ms, octanethi

benzenethiol

used in thi

ectrodes on S

xposed in SA

was deposi

formance of

SAM treated

y and decre

resistances

aces are extra

cy response m

frequency resp

ear the electro

n of SAMs co

mpletely in

esistance) see

of morpholog

ates and carri

to prevent the

substrate fo

ne has attract

s an intermed

trathin thickn

GO was prepa

thanol. In ord

main could no

an 2Lc, on th

ur. The optimi

ctrodes: grow

anic semicon

different from

enced by the

pproach to

ies in these

onal electric

this is not

odes will also

gy. So we

c properties

trodes by use

nt and theoret

iol (OT : thio

(BT: thiol

s study. Afte

SiO2/Si subs

AM vapor for

ited in a va

fabricated sa

d samples sh

ease in thre

at elec

acted by anal

measurement.

ponse measu

odes. The spa

ould reduce t

BT-modified

ems to be sm

gical disorder

ier density ar

ese morpholo

r organic film

ted much atte

diate substanc

ness. We studi

ared from nat

der to obtain

ot incorporate

the other han

ized L for EG

wth of penta

nductors are d

m that of on d

e

o

e

c

t

o

e

s

e

etical analysis

ol with

with

er the

strates,

1 day,

acuum

amples

howed

eshold

ctrode-

lyzing

. Contact resi

urement also i

atial distribut

these unexpe

d samples de

maller than O

r in a bare sa

re different f

gical disorde

m growth: p

ention due to

ce during chem

ied growth o

tural graphite

a single or f

OT B0.0

0.1

0.2

0.3

0.4

0.5

Mob

ility

(cm

2 /Vs)

SA

Fig.6 De

TFTs. OcD

ensi

ty o

f of

f-cu

rren

t car

rier

(cm

-2)

Fig(rignea

e sufficient 6

nd, the grain

Gs growth is L

cene SAM-m

deposited on e

dielectric, the

s we have dev

istance dram

indicates that

tion of off-cu

cted regions.

spite the SA

OT-modified s

ample but not

from those o

rs.

entacene gro

o its unique p

mical fabrica

f pentacene f

e powder by

few layers GO

BT Bare

AMO

0

-2

-4

-6

-8

-10

-12

Thr

esho

ld V

olta

ge (

V)

evice perform

ctancethiol an

0 20 400

1x109

2x109

3x109

4x109

y(

)

Positi

g. 7 (left) Thght) AFM imar Au electrod

6T molecules

nucleation o

L~ 2Lc,≒2 µm

modified Au

electrodes an

film morpho

veloped.

atically decre

t there are hig

urrent carrier

Surprisingly

AMs effect (o

samples. AFM

t in a BT-mod

n dielectrics.

owth on grap

properties. G

ation process,

film on GO su

the Hummer

O film, a high

OT BT Bare

SAM

0

0

1

1

2

2

Con

tact

Res

ista

nce

(

)

mance of SA

nd benzenethi

60 80 100 120

ion (m)

OT BT Bare

e density of mages of orgdes.

s and the elon

on terrace oc

m.

electrodes fa

nd dielectric.

ology on diele

eases for SAM

gh resistivity

is extracted

y, these high-r

on mobility,

M images (Fi

dified sample

. We have sh

phene oxide

Graphene oxid

are also exp

ubstrates for

r’s method. G

hly doped Si

OT BT bare0.0

0.5

1.0

1.5

2.0

2.5

SAM

AMs treated

iol are also d

off-current cganic film d

O

on SiO2 on A

ngation is

ccurs and

abricated

When the

ectric near

M treated

y and high

as shown

resistivity

threshold

ig.7 right)

e. In these

hown that

des (GO),

ected as a

aiming at

GO flakes

substrate

pentacene

epicted.

carriers. disorder

BT

Bare

OT BT

SH SH

Au

was dipp

the SiO2

system (J

First

using co

The resu

electroni

function

the GO

the condu

emission

Pentacen

elongated

9(a). Fro

found th

elongated

pyramida

depositio

under th

occurren

was simu

found th

preferenc

surface.

elongated

Summar

A ser

to contro

from frag

organic m

practical

ped in the dis

2/Si substrate

JEOL JSPM-

the I-V char

onducting can

ults are show

c conductan

of F1/2, whe

sheet. Judgin

uction mecha

n.

FI exp

ne film depo

d islands and

om the micro

hat orientation

d islands wh

al layers. Th

on (Fig.9 (b)

he in-plane r

nce of transm

ulated. By c

hat pentacen

ce of the (02

These result

d islands.

ry

ries of studies

ol organic mo

gile nature of

molecules an

applications.

spersion liqui

e. Topograph

5200).

racteristics of

ntilever of A

wn in Fig.8

nce is well

ere F is electr

ng from this

anism is cons

kT

Fq 4/p

3

osited onto G

d 2D pyramid

Raman spect

n of pentace

hile pentacene

he RHEED

)). The diffr

rotation of t

mission diffrac

comparing w

ne film orie

21) or (-121)

ts showed th

s about organ

olecular orien

f organic ma

nd provide f

.

d and lifted.

ic and curren

f GO sheets

AFM measure

. For any G

described by

ric field appl

kind of field

sidered to be

(eq.1)

GO sheets co

dal layer as

troscopy mea

ene molecule

e molecules

images were

raction patte

the sample.

ction, the dif

with experime

entation is l

) planes para

he crystal st

nic film grow

ntation. Thes

terials. It sho

fundamental

This process

nt images of

are measure

ement system

GO thicknes

y exponentia

lied normal t

d dependence

Poole-Frenke

onsists of 3D

shown in Fig

asurement, it i

s are lying i

are standing

e taken duri

ern is invaria

Assuming t

ffraction patte

ental data, it

likely to ha

allel to the G

tructure of 3

wth on various

se results solv

ould be noted

knowledge o

s yielded GO

f GO were m

ed

m.

s,

al

to

e,

el

D

g.

is

in

in

ing

ant

the

ern

is

ave

GO

3D

sly modified

ved a part of

d that the pre

on the develo

Fig. 8 PThe incalculat

110-

10-

10-

10-

10-

10-

10-

I/F

(A

(V/c

m)-1

)

Fig. pentacon Gimage

sheets with v

measured wit

substrates we

f complexity

esent results a

opment of o

Poole Frenkelnset shows rted by (eq.1).

250 1500 1

8

7

6

5

4

3

2

GO thickness 8nm 15nm 20nm 25nm 50nm

F

9 (left) Thcene of thickn

GO sheets. (.

various thick

th SPM mea

ere conducted

in OTFTs o

are applicabl

organic electr

l plot for GO relative perm.

750 2000 22

F (V/cm)1/2

0 10 200

2

4

6

8

Rel

ativ

e Pe

rmit

tivi

ty

GO thick

he AFM imkness 10 nm (right) The

knesses on

asurement

d in order

riginating

e to other

ronics for

sheets. mittivity

250 250

30 40 50

kness (nm)

mage of deposited

RHEED