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Electrical properties of ion implanted polythiophene
Synthetic Metals, 28 (1989) C305-C310
C305
ELECTRICAL PROPERTIES OF ION IMPLANTED POLYTHIOPHENE
H. ISOTALO and H. STUBB Semiconductor Laboratory, Technical Research Centre of Finland, Espoo (Finland) P. KUIVALAINEN Electron Physics Laboratory, Faculty of Electrical Engineering, Helsinki University of Technology, Espoo (Finland)
ABSTRACT Polythiophene as a free standing film has been ion implanted with 25 keY F+ ions. Rutherford backscattering measurements showed that the surface of the film was modified to a depth of 0.4 ~n. The dc-conductivity increase was more than 7 orders of magnitude, the highest value being 1.3.10 -2 Q-Icm-I for a dose of 2.1017 F+/cm2. The increased conductivity, which was stable with time was accompanied by a drastic decrease in thermoelectric power which showed m e t a l l i c - l i k e behaviour. The temperature dependence of conductivity and thermoelectric power points to variable range hopping between localized states caused by the implantation. Electron spin resonance data support the concept of localized charge carriers.
INTRODUCTION Ion implantation is widely used in semiconductor processing as a standard way of introducing controlled amounts of acceptor or donor impurities. The technology has been used also for increasing the conductivity in conjugated polymers. Here this spatially selective modification has been studied with microelectronics applications in mind ranging from high density packaging interconnections [ I ] to pn-juctions [2]. The conductivity increase associated with low energy (< lO keV) ion bombardment is found to produce chemically doped N
material, creating for example the pn-junctions. As the other extreme, high qr
Present address: Institute for Polymers and Organic Solids, University of California at Santa Barbara (USA) 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C306
implantation energy (> 100 keY) results in carbonization of the bombarded surface [3]. Another variable is the ion used for implanting. Potentially active dopants such as F, As, or Na have been used, but also inactive ions l i k e He, N, Ar or Kr give high conductivity to the target surface. The conduction type - p or n - of these layers determined by thermoelectric power (TEP) measurements depends on the target used [4]. In this work we study the effects of low energy (25 keV F+ bombardment of free standing polythiophene thin films. The work continues an e a r l i e r study on the same combination [5]. EXPERIMENTAL Electrochemically synthesized polythiophene (PT) [6] thin films were implanted with 25 keV F+ ions with doses 7.1015, 7.1016 and 2.1017 F+/cm2. Figure 1 depicts the implantation set-up and resulting layer. The samples were firmly attached between aluminium substrates (not shown in the figure) and aluminium masks. The implanted area was of the order of 2 x 1 cm2. The part of a sample which was shadowed by the mask and not implanted is labelled "masked". This part was also carefully examined. Two heavily implanted samples were made: one with liquid nitrogen cooling labelled "N2" and the other without cooling. The current in all runs was as low as possible considering practical time ~ limitations [5].
F+
Pl
I
I
2S
|1
keU
I |
I
AI I~oo
Fig. 1. Principle of ion implantation of PT. Calculated depth and distribution of F+ ions are also shown. Rutherford backscattering (RBS) spectra were measured as in reference [5] with 2 MeV He+ ions using low doses ~ 1014 He+/cm2 to avoid damaging the samples [7].
C307
Dc-conductivity (Odc) measurements were done in the temperature range 4 350 K in N2. Measurements were mainly performed by using a 2-point method, however, 4-point checks were made whenever possible. Contacts were made with graphite paste. A TEP measuring apparatus was constructed of two separately heated single crystal quartz blocks the sample being attached with graphite paste between the blocks [6]. The available temperature range was 4- 300 K, although only measurements near room temperature were possible due to the high resistance of the samples. Electron spin resonance (ESR) measurements were made at 9.1GHz (X-band) as described in reference [8]. In the estimations of the spin density the total volume of the samples was used although the implantation creates new unpaired electrons only near the surface (Fig. I ) . RESULTS AND DISCUSSION The s t u d i e d samples are shown in t a b l e i . The Odc values are c a l c u l a t e d using 400 ~ [ 5 ] (see also Fig. i )
as the e f f e c t i v e
thickness of the o t h e r w i s e
170 #n t h i c k samples. For samples behind the mask and the unimplanted one the total
thickness 170 pm was used. ~dc at room temperature increases by more than
7 orders of magnitude due to i m p l a n t a t i o n . This increase was not due to heating e f f e c t s because: (a) a h i g h l y conducting FeCl3-doped PT sample known to w i t h s t a n d 150°C w i t h o u t
losing its
conductivity
[ 6 ] was not a f f e c t e d upon
i m p l a n t a t i o n , (b) samples behind the mask had same magnitude of c o n d u c t i v i t y and a c t i v a t i o n energy as those unimplanted and (c) the c o n d u c t i v i t y and a c t i v a t i o n energy remained the same in s e p a r a t e l y heat t r e a t e d samples; 2h in 54, 106, 150, 188 and 208°C in He. TABLE 1 Samples s t u d i e d . Values are given at room temperature.
Sample D o s e
Effective
Dc-conduc-
Thermoel. Activation
Spin
thickness
tivity Q-Icm-I
power ~V/K
energy meV
density cm-3
547 504 322 244 249 142
6.10 17 2.10 18 3.1018 1.3,1018 1.6.10 le 1.7.1018
F+/cm2 1 2 3 4 5 6
0 masked 7.10 zs 7.1016 2.1017(N 2) 2.1017
170 170 400 400 400 400
~m pm ~ ~ ~ ~
5.9,10 -10 1.9.10 - 9 1.4.10 -5 4.2.10 -5 5.8.10 -5 1.3.10 -2
thickness 170 ~m in all samples
420 410 50 5
C308 Figures 2 and 3 more clearly show that the temperature dependenceof the conductivity has changed in every implanted sample, including sample 3 where, however, the absolute value of the square r e s i s t i v i t y is practically the same as the one of unimplanted samples or masked regions. This lead us to use the calculated thickness 400 ~ for the implanted samples. The temperature dependencies of the conductivities in figures 2 and 3 correspond to two and three dimensional variable range hopping (VRH) mechanisms, respectively. The 3-dimensional VRH seems to give a s l i g h t l y better f i t , especially in the highest dose sample 6.
100
/
1014~
no
+o
I
°
3++
,+,+++
1~
0
,,
N" ~+ ,-4 t~
++ +.+
16 2 .
5 4 ~ AA AA~d'~A~AAA~AAA A Aid
? bAA A
+t
° b A~
AA
,-4F"
~+++~ d] 0 " ~ o
168
at,~ ~ Al,~l'a 6
108
"+-2
<'+
AA C~
O~
%++ 4.,
16 6 . AA
t-' 0
i\ t06 0,1/+
,
, 0,16
,
, 0,18
,
,
0,2 TEMPERATURE-1/3 (K-1/3)
Fig. 2. Square r e s i s t i v i t y of 25 keyi(+ ion implanted PT as a function of T" ~ . Sample numbers are from table I.
1~12' 0,23
'
i
i
i
i
0,31 ~½5 ' 0,27 0,~ TEMPERATURE-1/4 (K-1/4)
Fig. 3. Dc conductivity. Qf 25 keV F+ ion implanted PT vs. T-1/4. Sample numbers are from table 1.
In low dose samples TEP could be measured only at room temperature where i t was positive and of the order of a few 100 pV/K (Table 1). In the highest dose samples TEP was small increasing l i n e a r l y with temperature, i . e . a metallic like behaviour. The l a t t e r is also in agreementwith the VRH mechanism as shown in reference [g]. RBS spectra shown in figure 5 clearly indicate how the - 0.4 ~m deep surface layer is modified in implanted samples. This value i f used in the calculation of ~dc gives an order of magnitude lower values compared to those shown in table 1. The lowest dose, 7.10Is F+/cm2, does not change the spectrum. The ratio of S and C corresponds to that of the chemical structure; C4H2S. At higher doses a minimum in S concentration is found below the surface. This
C309
phenomenon is different from our results obtained e a r l i e r [5] where the S concentration decreased without forming any minimum for materials synthesized and implanted under the same conditions. Another differing feature is the small 0 increase seen in RBS spectra. The F concentration on the other hand is too small to be seen. The ion implantation induced features found in RBS point to a modification of the polythiophene surface, viz. the formation of a p a r t i a l l y carbonized layer, beyond a possible doping.
z (~ LJ
50C
30C IOC
CHANNELS Fig. 4. RBS spectra of ion implanted polythiophene. Spectrum of the unimplanted sample is in absolute scale, others have been shifted upwards. Direct proof in favour of doping or carbonization cannot be obtained from Odc' TEP or RBS measurements. However, one may point out an analogy with amorphous silicon [10] where the large increase in unpaired spins and the localized states in the VRH conduction are related to dangling bonds. The implantation induced surface modification could give rise to similar defects in the polymer. Thin films of some 100 A would eliminate the interpretation ambiguities connected with the unimplanted region in the present samples. The development of processible PT derivatives [11] w i l l f a c i l i t a t e this.
C310
CONCLUSIONS Low energy ion implantation of PT strongly increases ~dc in a thin layer near the surface of the sample. RBS measurements showed that the implantation causes a loss of sulphur and consequently p a r t i a l carbonization in a ~ 0.4 ~m surface layer. Odc and TEP are described by VRH between l o c a l i z e d states. The l a t t e r are also seen in ESR measurements but t h e i r o r i g i n - carbonization and/or doping - remains a subject for f u r ther study. ACKNOWLEDGEMENTS We are indebted to J.E. Osterholm from Neste Oy for polythiophene samples and to J. S a a r i l a h t i for performing the RBS analysis. REFERENCES 1
S.E. Jenekhe and S.J. Tibbets, J. Polym. Sci. B-Polym. Phys., 26 (1988) 201-209.
2
T. Wada, A. Takeno, M. lwaki, H. Sasabe and Y. Kobayashi, J. Chem. Soco
3
T. Venkatesan, Nucl. Instrum. Meth. Phys. Res., B7/8 (1985) 461-467.
Chem. Commun. (1985) 1194-1195. 4
B. Wasserman, M.S. Dresselhaus, G. Braunstein, G. E. Wnek and G. Roth, J. Electron. Mat. 14 (1985) 157-170.
5
H. I s o t a l o , H. Stubb and J. S a a r i l a h t i , in H. Kuzmany, M. Mehring and S. Roth (eds.) Springer Series in Solid State Sciences 76 (1987) 285-290.
6
J.-E. Osterholm, P. Passiniemi, H. Is o talo and H. Stubb, §ynth. Met. 18
7
F. Namavar and J . l .
8
P. Kuivalainen, H. Stubb, H. I s o t a l o , P. Y l i - L a h t i and C. Holmstr6m, Phys.
(1987) 213-218. Budnik, Nucl. I n s t r . and Meth. BI5, (1986) 285.
Rev. B 31 (1985) 7900-7909. 9
P. Kuivalainen, H. I s o t a l o and H. Stubb, phys. s t at . sol. (b) 122(1984) 791-799.
i0 P. Kuivalainen, J. Heleskivi, M. Leppihalme, U. Gyllenberg-G~strin and H. I s o t a l o , Phys. Rev. B 26 (1982) 2041-2055 i i J.-E. Osterholm, J. Laakso, P. Nyholm, H. I s o t a l o , H. Stubb, O. Ingan~s and W.R. Salaneck, Synth. Met., 28 (1989) C435 (these Proceedings).
C305
ELECTRICAL PROPERTIES OF ION IMPLANTED POLYTHIOPHENE
H. ISOTALO and H. STUBB Semiconductor Laboratory, Technical Research Centre of Finland, Espoo (Finland) P. KUIVALAINEN Electron Physics Laboratory, Faculty of Electrical Engineering, Helsinki University of Technology, Espoo (Finland)
ABSTRACT Polythiophene as a free standing film has been ion implanted with 25 keY F+ ions. Rutherford backscattering measurements showed that the surface of the film was modified to a depth of 0.4 ~n. The dc-conductivity increase was more than 7 orders of magnitude, the highest value being 1.3.10 -2 Q-Icm-I for a dose of 2.1017 F+/cm2. The increased conductivity, which was stable with time was accompanied by a drastic decrease in thermoelectric power which showed m e t a l l i c - l i k e behaviour. The temperature dependence of conductivity and thermoelectric power points to variable range hopping between localized states caused by the implantation. Electron spin resonance data support the concept of localized charge carriers.
INTRODUCTION Ion implantation is widely used in semiconductor processing as a standard way of introducing controlled amounts of acceptor or donor impurities. The technology has been used also for increasing the conductivity in conjugated polymers. Here this spatially selective modification has been studied with microelectronics applications in mind ranging from high density packaging interconnections [ I ] to pn-juctions [2]. The conductivity increase associated with low energy (< lO keV) ion bombardment is found to produce chemically doped N
material, creating for example the pn-junctions. As the other extreme, high qr
Present address: Institute for Polymers and Organic Solids, University of California at Santa Barbara (USA) 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C306
implantation energy (> 100 keY) results in carbonization of the bombarded surface [3]. Another variable is the ion used for implanting. Potentially active dopants such as F, As, or Na have been used, but also inactive ions l i k e He, N, Ar or Kr give high conductivity to the target surface. The conduction type - p or n - of these layers determined by thermoelectric power (TEP) measurements depends on the target used [4]. In this work we study the effects of low energy (25 keV F+ bombardment of free standing polythiophene thin films. The work continues an e a r l i e r study on the same combination [5]. EXPERIMENTAL Electrochemically synthesized polythiophene (PT) [6] thin films were implanted with 25 keV F+ ions with doses 7.1015, 7.1016 and 2.1017 F+/cm2. Figure 1 depicts the implantation set-up and resulting layer. The samples were firmly attached between aluminium substrates (not shown in the figure) and aluminium masks. The implanted area was of the order of 2 x 1 cm2. The part of a sample which was shadowed by the mask and not implanted is labelled "masked". This part was also carefully examined. Two heavily implanted samples were made: one with liquid nitrogen cooling labelled "N2" and the other without cooling. The current in all runs was as low as possible considering practical time ~ limitations [5].
F+
Pl
I
I
2S
|1
keU
I |
I
AI I~oo
Fig. 1. Principle of ion implantation of PT. Calculated depth and distribution of F+ ions are also shown. Rutherford backscattering (RBS) spectra were measured as in reference [5] with 2 MeV He+ ions using low doses ~ 1014 He+/cm2 to avoid damaging the samples [7].
C307
Dc-conductivity (Odc) measurements were done in the temperature range 4 350 K in N2. Measurements were mainly performed by using a 2-point method, however, 4-point checks were made whenever possible. Contacts were made with graphite paste. A TEP measuring apparatus was constructed of two separately heated single crystal quartz blocks the sample being attached with graphite paste between the blocks [6]. The available temperature range was 4- 300 K, although only measurements near room temperature were possible due to the high resistance of the samples. Electron spin resonance (ESR) measurements were made at 9.1GHz (X-band) as described in reference [8]. In the estimations of the spin density the total volume of the samples was used although the implantation creates new unpaired electrons only near the surface (Fig. I ) . RESULTS AND DISCUSSION The s t u d i e d samples are shown in t a b l e i . The Odc values are c a l c u l a t e d using 400 ~ [ 5 ] (see also Fig. i )
as the e f f e c t i v e
thickness of the o t h e r w i s e
170 #n t h i c k samples. For samples behind the mask and the unimplanted one the total
thickness 170 pm was used. ~dc at room temperature increases by more than
7 orders of magnitude due to i m p l a n t a t i o n . This increase was not due to heating e f f e c t s because: (a) a h i g h l y conducting FeCl3-doped PT sample known to w i t h s t a n d 150°C w i t h o u t
losing its
conductivity
[ 6 ] was not a f f e c t e d upon
i m p l a n t a t i o n , (b) samples behind the mask had same magnitude of c o n d u c t i v i t y and a c t i v a t i o n energy as those unimplanted and (c) the c o n d u c t i v i t y and a c t i v a t i o n energy remained the same in s e p a r a t e l y heat t r e a t e d samples; 2h in 54, 106, 150, 188 and 208°C in He. TABLE 1 Samples s t u d i e d . Values are given at room temperature.
Sample D o s e
Effective
Dc-conduc-
Thermoel. Activation
Spin
thickness
tivity Q-Icm-I
power ~V/K
energy meV
density cm-3
547 504 322 244 249 142
6.10 17 2.10 18 3.1018 1.3,1018 1.6.10 le 1.7.1018
F+/cm2 1 2 3 4 5 6
0 masked 7.10 zs 7.1016 2.1017(N 2) 2.1017
170 170 400 400 400 400
~m pm ~ ~ ~ ~
5.9,10 -10 1.9.10 - 9 1.4.10 -5 4.2.10 -5 5.8.10 -5 1.3.10 -2
thickness 170 ~m in all samples
420 410 50 5
C308 Figures 2 and 3 more clearly show that the temperature dependenceof the conductivity has changed in every implanted sample, including sample 3 where, however, the absolute value of the square r e s i s t i v i t y is practically the same as the one of unimplanted samples or masked regions. This lead us to use the calculated thickness 400 ~ for the implanted samples. The temperature dependencies of the conductivities in figures 2 and 3 correspond to two and three dimensional variable range hopping (VRH) mechanisms, respectively. The 3-dimensional VRH seems to give a s l i g h t l y better f i t , especially in the highest dose sample 6.
100
/
1014~
no
+o
I
°
3++
,+,+++
1~
0
,,
N" ~+ ,-4 t~
++ +.+
16 2 .
5 4 ~ AA AA~d'~A~AAA~AAA A Aid
? bAA A
+t
° b A~
AA
,-4F"
~+++~ d] 0 " ~ o
168
at,~ ~ Al,~l'a 6
108
"+-2
<'+
AA C~
O~
%++ 4.,
16 6 . AA
t-' 0
i\ t06 0,1/+
,
, 0,16
,
, 0,18
,
,
0,2 TEMPERATURE-1/3 (K-1/3)
Fig. 2. Square r e s i s t i v i t y of 25 keyi(+ ion implanted PT as a function of T" ~ . Sample numbers are from table I.
1~12' 0,23
'
i
i
i
i
0,31 ~½5 ' 0,27 0,~ TEMPERATURE-1/4 (K-1/4)
Fig. 3. Dc conductivity. Qf 25 keV F+ ion implanted PT vs. T-1/4. Sample numbers are from table 1.
In low dose samples TEP could be measured only at room temperature where i t was positive and of the order of a few 100 pV/K (Table 1). In the highest dose samples TEP was small increasing l i n e a r l y with temperature, i . e . a metallic like behaviour. The l a t t e r is also in agreementwith the VRH mechanism as shown in reference [g]. RBS spectra shown in figure 5 clearly indicate how the - 0.4 ~m deep surface layer is modified in implanted samples. This value i f used in the calculation of ~dc gives an order of magnitude lower values compared to those shown in table 1. The lowest dose, 7.10Is F+/cm2, does not change the spectrum. The ratio of S and C corresponds to that of the chemical structure; C4H2S. At higher doses a minimum in S concentration is found below the surface. This
C309
phenomenon is different from our results obtained e a r l i e r [5] where the S concentration decreased without forming any minimum for materials synthesized and implanted under the same conditions. Another differing feature is the small 0 increase seen in RBS spectra. The F concentration on the other hand is too small to be seen. The ion implantation induced features found in RBS point to a modification of the polythiophene surface, viz. the formation of a p a r t i a l l y carbonized layer, beyond a possible doping.
z (~ LJ
50C
30C IOC
CHANNELS Fig. 4. RBS spectra of ion implanted polythiophene. Spectrum of the unimplanted sample is in absolute scale, others have been shifted upwards. Direct proof in favour of doping or carbonization cannot be obtained from Odc' TEP or RBS measurements. However, one may point out an analogy with amorphous silicon [10] where the large increase in unpaired spins and the localized states in the VRH conduction are related to dangling bonds. The implantation induced surface modification could give rise to similar defects in the polymer. Thin films of some 100 A would eliminate the interpretation ambiguities connected with the unimplanted region in the present samples. The development of processible PT derivatives [11] w i l l f a c i l i t a t e this.
C310
CONCLUSIONS Low energy ion implantation of PT strongly increases ~dc in a thin layer near the surface of the sample. RBS measurements showed that the implantation causes a loss of sulphur and consequently p a r t i a l carbonization in a ~ 0.4 ~m surface layer. Odc and TEP are described by VRH between l o c a l i z e d states. The l a t t e r are also seen in ESR measurements but t h e i r o r i g i n - carbonization and/or doping - remains a subject for f u r ther study. ACKNOWLEDGEMENTS We are indebted to J.E. Osterholm from Neste Oy for polythiophene samples and to J. S a a r i l a h t i for performing the RBS analysis. REFERENCES 1
S.E. Jenekhe and S.J. Tibbets, J. Polym. Sci. B-Polym. Phys., 26 (1988) 201-209.
2
T. Wada, A. Takeno, M. lwaki, H. Sasabe and Y. Kobayashi, J. Chem. Soco
3
T. Venkatesan, Nucl. Instrum. Meth. Phys. Res., B7/8 (1985) 461-467.
Chem. Commun. (1985) 1194-1195. 4
B. Wasserman, M.S. Dresselhaus, G. Braunstein, G. E. Wnek and G. Roth, J. Electron. Mat. 14 (1985) 157-170.
5
H. I s o t a l o , H. Stubb and J. S a a r i l a h t i , in H. Kuzmany, M. Mehring and S. Roth (eds.) Springer Series in Solid State Sciences 76 (1987) 285-290.
6
J.-E. Osterholm, P. Passiniemi, H. Is o talo and H. Stubb, §ynth. Met. 18
7
F. Namavar and J . l .
8
P. Kuivalainen, H. Stubb, H. I s o t a l o , P. Y l i - L a h t i and C. Holmstr6m, Phys.
(1987) 213-218. Budnik, Nucl. I n s t r . and Meth. BI5, (1986) 285.
Rev. B 31 (1985) 7900-7909. 9
P. Kuivalainen, H. I s o t a l o and H. Stubb, phys. s t at . sol. (b) 122(1984) 791-799.
i0 P. Kuivalainen, J. Heleskivi, M. Leppihalme, U. Gyllenberg-G~strin and H. I s o t a l o , Phys. Rev. B 26 (1982) 2041-2055 i i J.-E. Osterholm, J. Laakso, P. Nyholm, H. I s o t a l o , H. Stubb, O. Ingan~s and W.R. Salaneck, Synth. Met., 28 (1989) C435 (these Proceedings).