oligomers between metal islands

4 19 8

Synthetic Metals, 55-57 (1993) 4198-4203

FIELD-EFFECT TRANSPORT IN CONJUGATED POLYMERS/OLIGOMERS BETWEEN METAL ISLANDS

J. PALOHEIMO, H. STUBB and L. GRt)NBERG Semiconductor Laboratory, Technical Research Centre of Finland, Otakaari 7 Bi SF-02150 Espoo (Finland)

ABSTRACT A thin film field-effect transistor structure has been used to study the field-effect mobility in thin films of poly(3-alkylthiophenes) or quinquethiophene on discontinuous metal films of gold or copper. The field-effect was always p-type, but the mobilities were typically lower than in the reference samples without metal islands. The results seem to imply that the limiting step in the transport is polaronic hopping in the interisland gap of the polymer/oligomer film.

INTRODUCTION Conjugated organic polymers and oligomers are attractive materials for molecular electronics. In principle, the molecular structure of organic molecules can be tailored and complicated functional properties obtained. The charge carrier mobilities measured in polymer/oligomer thin film fieldeffect transistors (FETs) are low, typically of the order of 10 -4 cm2/Vs at room temperature (RT), due to the self-localized polaronic states and thermally activated hopping transport [1]. These FETs have still quite large dimensions: channel length is = 1...10 I.tm. FETs with 1 I,tm channels using spin-coated poly(3-hexylthiophene) (PHT) or Langmuir-Blodgett (LB) deposited PHT/arachidic acid (AA) films operate like the longer channel versions [2], Fig. 1. In this work we have studied the transport in channels with metal island films overcoated with PHT, poly(3-octylthiophene) (POT) or quinquethiophene (QT) thin films. The motivation was to study the transport over short distances, the metal islands acting here as very small injecting electrodes, and the originally micrometers long channel was speculated to be transformed into a network of short metal-polymer/oligomer-metal structures with gaps of a few nanometers. This approach was adopted because the real scaling down of the transistor channel to the interesting range L << 100 nm is difficult. Elsevier Sequoia

4199

TRANSISTOR EQUATIONS The drain c u r r e n t in the linear region for a p-type film is [3]

I d = - (Wl.tCox/L)[(Vg-Vt)Vds - Vds2/2]

(i)

Here Cox is the oxide capacitance per unit area, W and L the channel width and length, r e s p e c t i v e l y . Vg, V t and Vds are the gate, threshold and drain-source voltages, respectively. Eq. 1 defines the field-effect mobility i1. The average conductivity o or the c o r r e s p o n d i n g sheet c o n d u c t a n c e crA = od (d is the film thickness) is directly obtained for Vg equal to the flat-band voltage, which is r o u g h l y zero for most cases. We define the flat-band carrier c o n c e n t r a t i o n to be P0 = o/(el.t). Their concentration per unit area is P0,A = POd = ° A / ( e B ) . The carrier c o n c e n t r a t i o n per unit area (PA) in a p-type film in a FET is increased or decreased from P0,A for negative or p o s i t i v e Vg'S, respectively.

/

12 10 f<( ~"

8 6

73

I

4

5

-Vd

10

15

20

(v)

Fig. 1. C u r r e n t - v o l t a g e characteristics of a conventional 1 Bm long FET with a spin-coated PHT film of 70 nm. The curves correspond to gate voltages of +20 (lowest curve), 0, -20 and -40 V ( u p p e r m o s t curve). The oxide thickness is 300 nm.

EXPERIMENTAL The FET structure has been discussed earlier [1, 2]. Here we typically used p-type silicon wafers and 33 nm thick gate oxides. A thin oxide was chosen to maximize the ratio between the i n t e r i s l a n d gap and the oxide thickness. Highly doped n-type substrates and thicker oxides were o c c a s i o n a l l y used. The drain and source electrodes were of gold (20...30 nm) undercoated with c h r o m i u m (3...10 nm). L i f t - o f f technique or chemical etching was used for patterning. L was typically 5 Bm and W 8 cm. The t h i c k n e s s e s of the gold films were dme t = 2, 5, or I0 nm and those of copper 2 or 4 nm, all these films having a l o w - c o n d u c t i v i t y discontinuous structure. The substrates were typically held at RT during the deposition but the thickest gold films (dine t = I0 nm) were deposited at 200°C.

4 2 0 0

M o s t o f the f i l m s were d e p o s i t e d by e - b e a m e v a p o r a t i o n . The d e p o s i t i o n rate for the two t h i n n e s t gold f i l m s was a b o u t 1 ./k[s and h i g h e r for the o t h e r films. T h i c k e r metal f i l m s t u r n e d out to be too c o n d u c t i n g and t h u s p o s s i b l y c o n t i n u o u s . L a r g e i s l a n d s are a d v a n t a g e o u s in this case b e c a u s e o f t h e i r lower c h a r g i n g e n e r g y . T h e p o l y m e r / o l i g o m e r film was d e p o s i t e d on the m e t a l i s l a n d s . T h e PHT a n d P O T f i l m s were s p i n - c o a t e d or LB deposited. The LB f i l m s c o n t a i n e d 40 tool% AA or SA. V a c u u m e v a p o r a t i o n or LB d e p o s i t i o n were used for QT. The s t r u c t u r e of the m e t a l i s l a n d FET ( M I F E T ) is s h o w n in Fig. 2.

100

SOURCE DRAIN •-.~--..~ISLANDS' k _POLYMER_.F._ _

=

8O

< D

T rl

60 40

20

ISLAND GEOMETRIES

S

0

-Vds (V) Fig. 2. S t r u c t u r e o f a m e t a l island FET ( M I F E T ) . T h e c h a n n e l is c o v e r e d with a d i s c o n t i n u o u s m e t a l film and a p o l y m e r / o l i g o m e r film. R is the m e a n m e t a l i s l a n d r a d i u s and s the gap b e t w e e n two n e i g h b o u r i n g islands.

Fig, 3. C u r r e n t - v o l t a g e c h a r a c t e r i s t i c s o f a QT (1000 nm) r e f e r e n c e FET for Vg = 0 (lowest curve), -1, -2, -3, -4, -5 ( u p p e r m o s t curve) at RT, solid curve. The c o r r e s p o n d i n g c h a r a c t e r i s t i c s o f a An (10 nm) - QT (1000 nm) MIFET for Vg = 0 (lower d a s h e d c u r v e ) and Vg = -5 V (upper d a s h e d curve).

RESULTS AND DISCUSSION All the M I F E T s s t u d i e d b e h a v e d q u a l i t a t i v e l y as the r e f e r e n c e FETs w i t h o u t m e t a l i s l a n d s : the ratio I d / V d s i n c r e a s e d for Vg < 0 and d e c r e a s e d for Vg > 0. T h e f i l m s t h e r e f o r e a p p e a r to be ptype. T h e M I F E T s were a l w a y s n o r m a l l y - o n d e v i c e s (i.e. V t > 0) in c o n t r a s t to s o m e of the P O T / S A and QT r e f e r e n c e FETs, Fig. 3. No s a t u r a t i o n of I d could be d e t e c t e d in the M I F E T s even for the h i g h e s t Vds'S. F u r t h e r m o r e , Ix t y p i c a l l y d e c r e a s e d c o m p a r e d to the r e f e r e n c e F E T s , T a b l e 1. T h e t e m p e r a t u r e d e p e n d e n c e of s o m e MIFETs was s t u d i e d below and a b o v e RT. Fig. 4 s h o w s the s h e e t c o n d u c t a n c e and f i e l d - e f f e c t m o b i l i t y in a Au (5 nm) - PHT (70 nm) MIFET. At e l e v a t e d t e m p e r a t u r e s , a b o v e 3 6 0 . . . 3 7 0 K, Ix and ~A d e c r e a s e i r r e v e r s i b l y . The RT m o b i l i t y d e c r e a s e d from 8 x i 0 - 5 to 3 x 1 0 -5 c m 2 / V s . In the n o r m a l l y - o n type r e f e r e n c e FET the d e c r e a s e was larger. T y p i c a l l y the c o n d u c t a n c e o f bare metal f i l m s was m u c h lower than that in M I F E T s . The c u r r e n t a n d c o n d u c t i v i t y of bare Au (rime t = 10 nm) on SiO 2 e.g. s h o w e d an e x p o n e n t i a l d e p e n d e n c e on Vds 1/2. We use this d e p e n d e n c e to e s t i m a t e the island g e o m e t r i e s . The s h a p e of the barrier is

4201

m o d i f i e d by the a p p l i e d field Fex t = V d s / L thereby a f f e c t i n g the a c t i v a t i o n e n e r g y and g i v i n g the c o n d u c t a n c e the form c A c t exp[[3(gFext)l/2/kT] [4]. Here the factor I~ = 2[e3/(4rce)] 1/2 (the p r e f a c t o r 2 d e p e n d s on the a s s u m p t i o n s m a d e ) is a f u n c t i o n of the dielectric c o n s t a n t of the m e d i u m , e. The v o l t a g e drop a c r o s s the island can be n e g l e c t e d and the a m p l i f i c a t i o n of the field is t a k e n into a c c o u n t by a g e o m e t r i c a l factor g = (2R+s)/s (R is the island r a d i u s , s the gap). The field in the gap then b e c o m e s Fgap = gFex t. A s s u m i n g the transport to take p l a c e t h r o u g h the v a c u u m (e = e0) w o u l d give a value g = 3. If the transport is via the SiO 2 s u b s t r a t e (e = 3.9e0), g = 10. T h e i n t e r i s l a n d gap s is therefore s m a l l e r than the island radius 2R. It is o b v i o u s that R is at l e a s t of the order of d m e t, and s is probably a few n a n o m e t e r s . T h e s e v a l u e s give s o m e idea a b o u t the s t r u c t u r e of the d i s c o n t i n u o u s metal film. TABLE 1 F i e l d - e f f e c t m o b i l i t y (IX), s h e e t c o n d u c t a n c e (~A), and carrier c o n c e n t r a t i o n per unit area (P0,A = CA/(eix)) in s o m e of the s t u d i e d MIFETs and c o r r e s p o n d i n g r e f e r e n c e FETs at RT. rime t and d are the a v e r a g e m e t a l and p o l y m e r / o l i g o m e r film t h i c k n e s s , r e s p e c t i v e l y . # d e n o t e s n o r m a l l y - o f f type behaviour.

sample

method

metal

dmet/d (nm) It (cm2fVs) ffA (S)

P0,A (cm'2)

PHT PHT

spin

Au Au

2/70 5/70

3.2x10"5 5,2x10 -5

7.7x10-10 1.5x10 -9

1.5x1014 1.8x1014

Au

10170 -/70

2.8x10-5 3.1x10-3

7.7x10 -10 1.7x10 -9

1.7x1014 3.4x1012

PHT PHT ref. PHT PHT ref.

spin

Cu

2.3/70 -/70

3.6x10-5 1.2x10 -3

9.1x10-1l 5.9x10-10

1.6x1013 3.1x1012

POT/SA POT/SA

1 layerLB

Au Au

2/3...4 5/3...4

2x10-7 7x10-7

3x10-12 6x10-12

7x1013 5x1013

POT/SA ref.

-/3...4

2x10-6

very low #

POT/SA 2 layers LB Au POT/SA Au POT/SA ref.

2/7 5/7 -/7

2x10-7 5x10-7 2x10-5

4x10-12 6x10-12 lxl0 -12

lxl014 7x1013 4x1011

QT QT ref.

vac. evap.

Au

10/1000 -/1000

3.5x10-5 4.9x10-3

2.1x10-10 < 10-11 #

3.7x1013

QT QT ref.

vac. evap.

Cu

2.3/1000 -/1000

1.3x10 -5 6.9x10-4

1.2x10 -11 2.2x10-11

5.8x1012 2.0x1011

A r e c e n t s t u d y [5] of NOPF 6 d o p e d

PHT thin films r e v e a l e d that such doped f i l m s c o n s i s t of

h i g h l y c o n d u c t i n g i s l a n d s , with the island and barrier widths t o g e t h e r a m o u n t i n g to a b o u t 4...6 nm. T h e t r a n s p o r t is by c h a r g i n g - e n e r g y - l i m i t e d t u n n e l i n g (CELT) t h r o u g h the barriers g i v i n g a A = ~ 0 , A e x p [ - ( T 0 / T ) 1/2] [6]. The NOPF 6 doping i n c r e a s e s Ix, in c o n t r a s t to the e f f e c t of the m e t a l i s l a n d s . The s h e e t c o n d u c t a n c e of our Au (5 n m ) - PHT (70 nm) MIFET, Fig. 4 (a), a l t h o u g h p l o t t e d in a T -1 scale, can be w e l l - f i t t e d to the T -1/2 d e p e n d e n c e in the r a n g e T = 80...360 K. The fit gives: ~ 0 , A = 0.55 mS and T O = 4.7x104 K. This T O is not too far from t h o s e of the NOPF 6

4202 d o p e d f i l m s [5]. T h e f i e l d - e f f e c t m o b i l i t y in Fig. 4 (b) has roughly the same t e m p e r a t u r e d e p e n d e n c e as ¢rA. H o w e v e r , we reject the C E L T model here. T h e a s s u m p t i o n s on the r e l a t i o n b e t w e e n the c h a r g i n g e n e r g y and the gap in the C E L T m o d e l are less w e l l - s u i t e d to 2 - d i m e n s i o n a l m e t a l i s l a n d s y s t e m s . F u r t h e r m o r e , ~ A o f the Au (5 nm) - PHT (70 nm) M I F E T is not i n c r e a s e d by the i s l a n d s t r u c t u r e at R T c o m p a r e d to a A o f the n o r m a l l y - o n r e f e r e n c e FET, and their t e m p e r a t u r e d e p e n d e n c e s are n e a r l y e q u a l [1]. We t h i n k that the c o n d u c t i v i t y of the Au (5 nm) - PHT (70 nm) M I F E T is d o m i n a t e d by the m u c h t h i c k e r bulk at t e m p e r a t u r e s a b o v e a b o u t 100 K.

1E-08 1E-09 ~

0.001 0.0001

1E-1C

1 E-O.=

i 1E-11_:

1E-OE

1E-1£

_.1 n"l

0

1E-11 1E-1~

1E-151 o

o.~os o.bl lfr

O.OlS o.b2 o.o25

1 E-O;

1E-O~ 1E-O. c 1E-1(

(l/K)

0.605 oh1

0.615 o.b2 0.022

1/T(1/K)

Fig. 4. Au (5 n m ) - P H T (70 rim) MIFET. (a) Sheet c o n d u c t a n c e vs t e m p e r a t u r e . (b) F i e l d - e f f e c t m o b i l i t y for Vg = 0 -> -5 V (upper curve) and Vg = +5 -> 0 V (lower curve) vs t e m p e r a t u r e . T h e t e m p e r a t u r e c y c l e was: RT -> 40K, RT -> 4 0 0 K -> RT.

T h e f i e l d - e f f e c t m o b i l i t y c h a r a c t e r i z e s the oxide i n t e r f a c e region w h e r e the m e t a l i s l a n d s r e s i d e in M I F E T s . T h e C E L T m o d e l is in d i s a g r e e m e n t with the e x p e r i m e n t a l It, too. In the C E L T m o d e l the f i e l d - e f f e c t r e s u l t s from the i n c r e a s e of the n u m b e r of c h a r g e d i s l a n d s , i.e. c h a r g e carriers [7]. T h e n u m b e r o f c h a r g e d i s l a n d s for f l a t - b a n d s would be a b o u t N 0 e x p [ - E 1 / ( k T ) ] . N O is the d e n s i t y o f i s l a n d s , p r o b a b l y _= 1 0 1 1 . . . 1 0 1 2 cm -2 for A t . T h e c h a r g i n g e n e r g y E 1 is a b o u t e 2 / 8 ~ r

(r is the

r a d i u s o f a " m e t a l ball" in a m e d i u m e). For e = 3.75e 0 (an a v e r a g e b e t w e e n e(SiO2) = 3.9e 0 and E(PHT) _= 3.6~0) and for r e a s o n a b l e r -~ 10...2 nm, E 1 b e c o m e s 20...100 m e V , and would g i v e a c a r r i e r c o n c e n t r a t i o n per unit area not m u c h lower than the d e n s i t y of i s l a n d s . The r e a s o n a b l e e s t i m a t e s of N O are still m u c h lower than the m e a s u r e d P0,A'S, T a b l e 1, s t r e s s i n g the M I F E T s t r u c t u r e s with n o r m a l l y - o f f type r e f e r e n c e FETs. F u r t h e r m o r e , a Vg o f only 0 . 1 5 . . . 1 . 5 V a c r o s s the 33 nm t h i c k o x i d e is n e e d e d to c h a n g e PA by an a m o u n t equal to N O. A s s u m i n g that I d a (N O PA)PA w o u l d g i v e a d e c r e a s i n g c u r r e n t for PA a p p r o a c h i n g N O [7]. T h e r e is no clear e x p e r i m e n t a l e v i d e n c e o f s u c h an e f f e c t . M o r e o v e r , the e x p e r i m e n t a l t e m p e r a t u r e d e p e n d e n c e o f It in A t (5 nm)

4203

- PHT (70 rim) M I F E T s (Fig. 4 (b)) is very s i m i l a r to that of n o r m a l PHT f i l m s [1], and the a c t i v a t i o n e n e r g y h i g h e r than 0.1 eV above 100 K e x c e e d s the e s t i m a t e d E 1 . T h e r e f o r e we t h i n k that the t r a n s p o r t b e t w e e n the m e t a l i s l a n d s is l i m i t e d by p o l a r o n i c h o p p i n g , as in the r e f e r e n c e f i l m s . T h e d e c r e a s e of Ix and t~A above 360...370 K, Fig. 4, can be c o n n e c t e d to the i n c r e a s i n g n u m b e r of d i s o r d e r and a d e c r e a s i n g n u m b e r of u n i n t e n t i o n a l d o p a n t s , s i n c e a c o r r e s p o n d i n g m a x i m u m was o b s e r v e d at 350 K in the r e f e r e n c e s a m p l e where the o r d e r - d i s o r d e r t r a n s i t i o n is k n o w n to o c c u r [1]. T h i s f u r t h e r s u p p o r t s the fact that the transport m e c h a n i s m s are m u c h the s a m e , a l t h o u g h the gap is only of the order of a typical h o p p i n g length. T h e i s l a n d s t u r n e d the n o r m a l l y - o f f type FETs ( m a r k e d with # in Table 1) into n o r m a l l y - o n type t h u s i n c r e a s i n g ~A- The i s l a n d s probably give extra h o l e s to the p o l y m e r / o l i g o m e r in the i n t e r i s l a n d gap for f l a t - b a n d s (i.e. Vg --- 0), m u c h like in n o r m a l o h m i c c o n t a c t s , b e c a u s e the work f u n c t i o n o f the m e t a l is high, the barrier h e i g h t is reduced by the i m a g e - f o r c e p o t e n t i a l s , and the c h a r g i n g e n e r g y is r e l a t i v e l y low, thus r e s u l t i n g in n o r m a l l y - o n type b e h a v i o u r . T h e s m a l l IX in the M I F E T s can be u n d e r s t o o d to be due to the u n e v e n l y d i s t r i b u t e d h o l e s in the a c c u m u l a t i o n or d e p l e t i o n b e c a u s e only the h o l e s in the gaps are e f f e c t i v e c h a r g e carriers. The i s l a n d s can take n e a r l y all e x t r a h o l e s i n d u c e d by the gate b e c a u s e of the s m a l l g e o m e t r i c a l ratio s / d o x ~ 0.1 (dox is the o x i d e t h i c k n e s s ) , and t h e r e f o r e the g a t e - i n d u c e d c h a n g e of PA is lower in the gaps than in i s l a n d s . A t h i n n e r oxide would be f a v o u r a b l e in this respect. F u r t h e r m o r e , the M I F E T m o b i l i t y v a l u e s o b t a i n e d s h o u l d be d i v i d e d by the factor g to get the i n t e r i s l a n d f i e l d - e f f e c t m o b i l i t y . No s i g n s of easy t r a n s p o r t of ( b i ) p o l a r o n s along single c h a i n s b e t w e e n metal i s l a n d s t h r o u g h the p e r c o l a t i o n r o u t e could be seen. Our r e s u l t s are in c o n t r a s t to the t r a n s i e n t p h o t o c o n d u c t i v i t y w h e r e the p h o t o g e n e r a t e d carriers m a i n t a i n a large t e m p e r a t u r e i n d e p e n d e n t m o b i l i t y over a d i s t a n c e o f s o m e tens of n a n o m e t e r s before b e c o m e trapped [8].

ACKNOWLEDGEMENTS T h i s work was part of our c o n d u c t i n g p o l y m e r r e s e a r c h s p o n s o r e d by the T e c h n o l o g y D e v e l o p m e n t C e n t r e , F i n l a n d (TEKES). We thank Dr. E. Punkka for critical c o m m e n t s on the m a n u s c r i p t . The p o l y ( 3 - a l k y l t h i o p h e n e s ) were obtained from Drs J. L a a k s o and J.-E. O s t e r h o l m ( N e s t e Ltd). T h e q u i n q u e t h i o p h e n e was s y n t h e t i z e d by E. V u o r i m a a .

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