- Email: [email protected]
Radiation effects on hole drift mobility in polysilanes
Radiat. Phys. Chem. Vol. 49, No. 3, pp. 389-393, 1997
Pergamon
0969-806X(95)00105-0
© 1997 Publishedby ElsevierScienceLtd Printed in Great Britain. All rights reserved 0969-806X/97 $17.00+0.00
RADIATION EFFECTS ON HOLE DRIFT MOBILITY IN POLYSILANES SHU SEKI, 1'2 H I R O M I SHIBATA, 3 YOICHI YOSHIDA,' K E N K I C H I I S H I G U R E 2 and SEIICHI T A G A W A I t qnstitute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 2Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo and 3Research Center for Nuclear Science and Technology, University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki, Japan (Received 20 February 1995; accepted 2 May 1995)
Abstract--The radiation effects on hole drift mobility in polysilane derivatives were studied in the present paper. The values of hole drift mobility (about 10- 4cm2/V.s) obtained by the DC Time-of-Flight (TOF) measurement were improved by ion beam irradiation for poly(methylphenylsilane) (PMPS) and poly(di-n-hexylsilane) (PDHS). The irradiated PMPS showed five times higher values of hole drift mobility than the non irradiated one. Their low photo-induced carrier yield, one of the highest barrier to use polysilanes as photoconductors, was also improved by the irradiation, The mechanism of the mobility improvement will be discussed in relation to the model of changes in the silicon skeleton structure induced by the radiation. © 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION The hole transport is one of the most practical features of polysilane derivatives. Several studies have been carried out on this feature of polysilanes (Kepler et al., 1987; Fujino, 1987; Stolka et al., Abkowitz et al., 1987; Kepler et al., 1984a,b; Samuel et al., 1989; Phillpot, 1987; Abkowitz et al., 1990a,b), and reported the highest values of hole drift mobility in the amorphous polymer materials without any dopants. However the value of hole drift mobility in polysilane (about 10-4cm2/V-s) is not the highest value in comparison with molecularly doped carbon backbone polymers with n-conjugated system such as polyacetylene (Rice, 1979; S u e t al., 1979). Thus further improvement in the values is needed to use the polymers as real photoconducting materials, The silicon skeletons of polysilanes are made up entirely of Si-Si a-bond. The high values of hole drift mobility are considered to be due to the interesting electronic structure of the silicon skeleton: so-called a-conjugated system (Phillpot, 1987; Abkowitz, 1990a,b; Rice and Phillpot, 1987). Polysilanes have been regarded as 1-dimensional silicon materials: analogs of 3-dimensional amorphous and crystalline silicon. It is well known that those materials show higher values of electron and hole mobility (10- ' - 10j cm2/V.s) than polysilanes. The carrier transport in 3-D amorphous and crystalline silicon is explained by the band conduction model vs the Poole--Frenkel hopping model for the hole transport in I-D "J'Author to whom all correspondence should be addressed.
polysilanes. The values of hole drift mobility in the amorphus polysilanes may be determined by following two processes: the presence of intermolecular and intersegment hopping barriers. A polysilane molecule is divided into several a-conjugated segments along the silicon skeleton. Therefore, hole transport is possibly determined by the hopping barrier between a-conjugated segments as illustrated in Fig. 1. It is needed to elucidate whether intermolecular or intersegment hopping is the determining process to the hole transport in polysilanes. The improvement of low photo-induced carrier yield and high sensitivity for photodegradation is needed to use polysilanes as photoconductors. Linear polysilanes have no visible light absorption band, but UV light absorption bands. Charge carriers are generated only for UV light (300-400 nm) exposure. The materials also show main chain scission for UV light exposure with high quantum efficiency. Trefonas et al. (1983) reported that films of high molecular weight PDHS showed a UV absorption spectral shift and photodegradation with UV light. Seigler et al. (1985), Hofer et al. (1984) and Miller (1990) reported that photo volatilization was caused by excimer laser irradiation for alkyl substituted polysilanes. The experimental results suggested that UV light exposure causes not only charge carrier production, but polymer main chain scission. West et al. (1981) reported that crosslinked polysilanes showed increase of conductivity with the presence of an electron acceptor. Our previous studies also suggested that ion beam irradiation caused crosslinking of silicon skeletons leading to
389
390
Shu Seki et al. Au semitransparent electrode
lO~n Hole Fig. 2. The sandwich structure of electrodes and polysilane film.
Intermolecularhopping
Intersegmenthopping Fig. 1. Schematic models of intermolecular and intersegment hopping in polysilanes. gelation of these materials (Seki et al., to be published). The irradiated polysilanes grew into network silicon skeletons. The results suggested conversion from linear polysilane molecules to network polysilanes with higher dimensional silicon skeletons. Therefore the ion beam irradiation is expected to reduce the intermolecular hopping barrier, and also predicted to make holes highly mobile. It was also confirmed that ion beam irradiated polysilanes showed high resistivity to photodegradation by UV light because of their network silicon skeleton. Thus, ion beam irradiation can be an effective way to improve hole transport in polysilanes.
solutions of these polymers were transferred into separatory funnel with water, and poured into isopropylalcohol followed by reprecipitation using tetrahydrofran-methanol system. Molecular weight distributions of these polymers were measured by Toyosoda TSK-GEL gel permeation chromatography system. The obtained polymers have their weight average molecular weight was Mw = 7.1 × 104 for PMPS, and Mw = 8.5 × 105 for PDHS respectively. Polymers were dissolved into toluene at 1015wt.% and casted on aluminium substrates at 10/~m thickness. The remaining solvent was removed in vacuum oven during 5 h at 120°C, and the films were annealed. The sandwich structure was completed by evaporating a semitransparent Au top contact as shown in Fig. 2. The schematic diagram of Time-of-Flight carried mobility measurement system was shown in Fig. 3. Nihon-Koshuha LGO-2N-200A nitrogen laser (337 nm) was used as an excitation light source. The laser pulses with 2 ns width were strongly absorbed at the upper surface of polymer films through semitransparent Au top contacts, leading to plane-like charge packets. These carriers
1". O. F. measured
EXPERIMENTAL
Poly(methylphenylsilane) (PMBS) and poly(di-nhexylsilane) (PDHS) were synthesized by the Wultz condensation with a methylphenyldichlorosilane for PMPS and a di-n-hexyl-dichlorosilane for PDHS as monomers respectively. All monomers were prepared by Shinetsu Chemical Co. Ltd, and purified by distillation. Polymerization was carried out in Ar atmosphere and in 100ml undecane as a reflux solvent. PHDS was synthesized in 100 ml toluene as a reflux solvent. The monomer was added to the reaction vessel and stirred with melted sodium metal for 4 h. The reaction mixture containing PMPS and PDHS solution was poured into iso-propylalcohol after filtration to roughly eliminate NaCI, and the precipitate was dried under vacuum. Toluene
~ +
Trig
I
~
~ P o l y m e r
Film
][.Z....~d~-.[ II TerminateResistance ~ 1 ~ IOOQ- lOkQ
Di~ltizina Oscilloscope
Computer
Fig. 3. Flight time observation system for hole drift mobility in polysilanes.
Radiation effects on hole drift mobility were forced to move by the applied electric fields, and the transient current was observed as voltage signals over 1-10kf~ resistances. Transient current was observed by the Sony/Tektronics TDS350 and DSA601 digitizing oscilloscopes. Ion beam irradiation was carried out by Van de Graaff accelerator at Research Center for Nuclear Science and Technology, The University of Tokyo. 2 MeV H + beam irradiated the film specimens of PMPS and PDHS at the fluence of 0.05-0.5 /~C/cm 2. Energy deposition of 2 MeV H ÷ beam in these media (10/~m depth) was estimated to be max, 10% of incident particle energy, and the irradiation was considered to be homogeneous. RESULTS AND DISCUSSION Figure 4 shows temperature dependence of hole drift mobility in PMPS at the applied field of 4.0 × 104V/cm. Equation (1) is introduced to deconvolute field and temperature dependence, # = ~exp[-(E0 -
flE'/2)/kTefr]
(1)
where E denotes field, E0 denotes zero field limit activation energy, and 1/Te~= I/T-1~To, as proposed by Gill (1972), Pal (1970) and Chen and Slowick (1975) for hole transport in poly(vinylcarbazole). Figure 4 is obtained by the extrapolation of the field dependence of hole drift mobility in PMPS shown in Fig. 7. The slope of the Arrhenius plot corresponds to activation energy of zero field limit; E0 as shown in the equation (1). The result of E0 is estimated to be 0.39 eV in this PMPS media. Figure 5"displays the ion beam induced changes in the TOF current-mode transit pulses collected on glassy PMPS media. These transit pulse shapes are obtained by the specimens irradiated by 2 MeV H + with the fluence of 0.05, 0.2 and 0.5/~C/cm2, respectively. The lower current is obtained for the specimen before irradiation in comparison with these transit currents after irradiation. The values of transit time are estimated by the edges of transit pulse shapes. It is clear that the flight time is shortened by 10 -2
> 10-3
°~°
=l.
10-4 o
°~o
10-5 O
10-6 3.0
I 3.2
I 3.4
I 3.6
I 3.8
I 4.0
I 4.2
I 4.4
1000/T(K) Fig. 4. Arrhenius plot of hole drift mobility in poly(methylphenylsilane); P M P S films at electric field: E = 4.2 x 104V/cm.
391
•
0 . 5 I.t C / c m 2
.n
0
I
I
I
I
20
40
60
80 Its
I
I
I
I
100 120 140 160
Fig. 5. Changes in transient current pulse shapes of irradiated poly(methylphenylsilane)by 2 MeV H + ion beam at 298 K, 4.2 × l& V/cm.
the ion beam irradiation. Another attractive change is the increase of carrier yield with the irradiation. The yield of holes is estimated by the integration of transient current. Especially for the specimen irradiated at 0.5/~C/cm 2, the yield is two times larger than that of before irradiation as summarized in Table 1. (Normalized carrier yield means the ratio of carrier yield to the yield before irradiation.) Figure 6 shows changes with the irradiation in the transit pulse shapes collected on the crystalline PDHS media. The ion beam irradiation was carried out at the same condition as PMPS, and the irradiation fluence was varied from 0.05/~C/cm 2 to 3 gC/cm 2. The transit pulses in Fig. 6 are slightly distorted in comparison with the case of PMPS as shown in Fig. 5, and it is impossible to distinguish a sharpe edge that presents the value of the transit time. The mobility seems to consist of multiple components for this polymer. However, it should be noted that pulse signals after irradiation showed faster decay than the pulse before irradiation, resulting in the generation of fast components of charge carrier mobility. The yield of charge carrier also increases with the irradiation similarly to the irradiation for PMPS. The yield of holes is increased until the fluence reaches 0.20.5/~C/cm 2, and decreases with further irradiation as summarized in Table 1. In this case, the yield also becomes maximally two times larger than that of before irradiation. The values of hole drift mobility in polysilane derivatives have beeUn reported to depend on the square root of electric field in Poole-Frenkel hopping Table 1. Charge carrier yield changeswith 2 MeV H+ ion beam irradiation for poly(methlphenylsilane)and poly(di-n-hexysilane) Normalized carrier yield Fluence(pC/cm2) PMPS PDHS 0 1.00 1.00 0.05 1.21 1.35 0.2 1.36 2.33 0.5 2.05 2.03 3 -1.14
392
Shu Seki et al. the sites of branched structures leading to decline of the hole drift mobility (Seki et al., in p r e s s ) . Comparing the effects of ion beam irradiation to that of the chemically induced branched structure, the difference is in the dimensional change of silicon skeletons. Ion beam irradiation changes molecular structure of polysilanes into the 3-D Si network along incident ion tracks in spite of chemically induced branched structures meaning 2-D Si network structures (Furukawa et al., 1990; Bianconi and Weidman, 1988; Bianconi et al., 1989).
Before irradiation 0.05 ~C/cm 2 0.2 ItC/cm 2
/ ~. -- --
~it^\ "\ - - 0.SgC/cm2 I \ • ....... 3 ItCIcm2
~!,-\ ,,~
• o
;I :22.
lie"
°
",
~
\
~,~\
-.\
J 10
20
30
40 IXS
50
60
70 CONCLUSION
Fig. 6. Changes in transient current pulse shapes of irradiated poly(di-n-hexylsilane)by 2 MeV H ÷ ion beam at 298 K, 4.2 x 104V/era. model (Stolka et al., 1987; Abkowitz et al., 1987; Kepler et al., 1984a,b; S u e t al., 1979). The electric field dependence in the PMPS also appears to obey the equation (1) even after the irradiation as summarized in Fig. 7. The values of zero field limit mobility are almost the same for all irradiated samples in spite of the lower value observed for the specimen before irradiation. In this electric field range, the carriers are accelerated with the irradiation. The specimen irradiated at 0.5 ~C/cm 2 shows five times larger value of mobility than that before irradiation. The acceleration phenomenon of holes is fundamentally explained by a simple model of network structure generation as suggested in our previous work (Seki et al., to be published). Contrarily, lower values of hole drift mobility have been observed for structural defects induced polysilanes (chemically branched polysilanes) (Seki et al., unpublished results). Those structural defects induced polysilanes have also been investigated by means of the electron beam pulse radiolysis. The experimental results showed hole localization around
The values of hole drift mobility obtained in the present study corresponded to several previous data. The field dependence of hole drift mobility in PMPS was well explained by Poole-Frenkel type hopping model. The model was also effective to account for the field dependence even in the ion beam irradiated PMPS. The temperature and field dependence was deconvoluted for PMPS before irradiation, and the activation energy was estimated to be 0.39 eV. Ion beam irradiation increased the values of hole drift mobility in glassy PMPS and crystalline PDHS, and the yield of charge carriers increased with the irradiation to the specimens of the PMPS and the PDHS. The yield of charge carrier was increased until the fluence reached 0.2-0.5/~C/cm2 followed by decrease with further irradiation. The yield became two times larger than that before irradiation for both the PMPS and the PDHS. The acceleration was basically explained by a simple model of Si network structure generation induced by ion beam irradiation. Intermolecular hopping barrier was reduced by extended a-conjugated system. The model of ion beam induced Si network structures is concluded to be 3-D cylindrical Si network structures generated by ion beam irradiation along incident ion tracks. The model is quite different from that of chemically induced 2-D network structures with Si branching.
0.5p.C/cm2
\
1 x 10 -3 --
~.~
.~•'~"
0.2~tC/cm2
L'-
0.05 ~tC/cm2
~- = V2 ~ °'• - - - c'" i-" -. . . . .
~
1 x 10 -4
180
[
190
[
200
Acknowledgements--The authors acknowledge the useful advice of Dr N. Matsumoto, and Dr M. Fujiki, Dr K. Furukawa, Dr M. Fujino, Dr H. Teramae, and Dr K. Takeda at Basic Research Laboratory, Nippon Telephone and Telegraph Co. Ltd, and Mr M. Narui and Mr T. Omata at the University of Tokyo.
.
-o •
REFERENCES
Before irradiation
[
210
[
220
[
230
[
240
[
250
Et/2 (Vlcm)~/2 Fig. 7. Semi-logarithmicplot of hole drift mobility/1 vs Et/2 (V/cm)]/2 in ion beam irradiated polysilane films at 298 K.
Abkowitz M. A., Rice M. J. and Stolka M. (1990a) Phil. Mag. B 61, 25. Abkowitz M. A., Stolka M., WeagleyR. J., McGrane K. M. and Knier F. E. (1990b) In Silicon Based Polymer Science, Advances in Chemistry Series 224 (Edited by Zcigler J. M. and Fearon F. W.) American Chemical Society, Washington DC. Abkowitz M. A., Knier F. E., Yuh H. J., Weagley R. J. and Stolka M. (1987) Solid State Commun. 62, 547. Bianconi P. A. and Weidman T. W. (1988) J. Am. Chem. Soc. 110, 2342.
Radiation effects on hole drift mobility Bianconi P. A., Schilling F. C. and Weidman T, W. (1989) Macromolecules 22, 1697. Chert I. and Slowick J. H. 0975) Solid State Commun. 17, 783. Fujino M. (1987) Chem. Phys. Lett. 136, 451. Furukawa K., Fujino M. and Matsumoto N. (1990) Macromolecules 23, 3423. Gill W. D. (1972) J. Appl. Phys. 43, 5033. Hofer D. C., Miller R. D. and Willson C. G. (1984) Proc. SPIE 469, 16. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1984a) Bull. Am. Phys. Soc. 29, 509. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1984b) Bull. Am. Phys. Soc. 28, 362. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1987) Phys. Rev. B. 35, 2818. Miller R. D. (1990) Advanced in Chemistry Series 224. American Chemical Society, Washington DC. Pai D. M. (1970) J. Chem. Phys. 52, 2285. Phillpot S. R. (1987) Phys. Lett. 31, 43, Rice M. J. (1979) Phys. Lett. A. 71, 152.
393
Rice M. J. and Phillpot S. R. (1987) Phys. Rev. Lett. 58, 937. Samuel L. M., Sanda P. N. and Miller R. D. (1989) Chem. Phys. Lett. 159, 227. Seki S., Shibata H., Ban H., Ishigure K. and Tagawa S. Macromolecules, to be published. Seki S., Cromack K. R., Trifunac A. D., Tagawa S., Ishigure K., Yoshida Y. and Kozawa T. (1995) Radiat. Phys. Chem., in press. Stolka M., Yuh H. J., McGrane K. and Pai D. M. (1987) J. Polym. Sci., Polym. Chem. Ed. 25, 823. Su W. P., Schrieffer J. R. and Heeger A. J. (1979) Phys. Rev. Lett. 58, 937. Trefonas P., West R., Miller R. D. and Hofer D. (1983) J. Polym. Sci., Polym. Left. Ed. 21, 823. West R. D., Djurovic L. D., Stearley P. I., Srinvasan K. S. and Yu H. (1981) J. Am, Chem. Soc. 103, 7352. Zeigler J. M., Harrah L. A. and Johnson A. W. (1985) Proc. SPIE 539, 166.
Pergamon
0969-806X(95)00105-0
© 1997 Publishedby ElsevierScienceLtd Printed in Great Britain. All rights reserved 0969-806X/97 $17.00+0.00
RADIATION EFFECTS ON HOLE DRIFT MOBILITY IN POLYSILANES SHU SEKI, 1'2 H I R O M I SHIBATA, 3 YOICHI YOSHIDA,' K E N K I C H I I S H I G U R E 2 and SEIICHI T A G A W A I t qnstitute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 2Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo and 3Research Center for Nuclear Science and Technology, University of Tokyo, 2-22 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki, Japan (Received 20 February 1995; accepted 2 May 1995)
Abstract--The radiation effects on hole drift mobility in polysilane derivatives were studied in the present paper. The values of hole drift mobility (about 10- 4cm2/V.s) obtained by the DC Time-of-Flight (TOF) measurement were improved by ion beam irradiation for poly(methylphenylsilane) (PMPS) and poly(di-n-hexylsilane) (PDHS). The irradiated PMPS showed five times higher values of hole drift mobility than the non irradiated one. Their low photo-induced carrier yield, one of the highest barrier to use polysilanes as photoconductors, was also improved by the irradiation, The mechanism of the mobility improvement will be discussed in relation to the model of changes in the silicon skeleton structure induced by the radiation. © 1997 Elsevier Science Ltd. All rights reserved.
INTRODUCTION The hole transport is one of the most practical features of polysilane derivatives. Several studies have been carried out on this feature of polysilanes (Kepler et al., 1987; Fujino, 1987; Stolka et al., Abkowitz et al., 1987; Kepler et al., 1984a,b; Samuel et al., 1989; Phillpot, 1987; Abkowitz et al., 1990a,b), and reported the highest values of hole drift mobility in the amorphous polymer materials without any dopants. However the value of hole drift mobility in polysilane (about 10-4cm2/V-s) is not the highest value in comparison with molecularly doped carbon backbone polymers with n-conjugated system such as polyacetylene (Rice, 1979; S u e t al., 1979). Thus further improvement in the values is needed to use the polymers as real photoconducting materials, The silicon skeletons of polysilanes are made up entirely of Si-Si a-bond. The high values of hole drift mobility are considered to be due to the interesting electronic structure of the silicon skeleton: so-called a-conjugated system (Phillpot, 1987; Abkowitz, 1990a,b; Rice and Phillpot, 1987). Polysilanes have been regarded as 1-dimensional silicon materials: analogs of 3-dimensional amorphous and crystalline silicon. It is well known that those materials show higher values of electron and hole mobility (10- ' - 10j cm2/V.s) than polysilanes. The carrier transport in 3-D amorphous and crystalline silicon is explained by the band conduction model vs the Poole--Frenkel hopping model for the hole transport in I-D "J'Author to whom all correspondence should be addressed.
polysilanes. The values of hole drift mobility in the amorphus polysilanes may be determined by following two processes: the presence of intermolecular and intersegment hopping barriers. A polysilane molecule is divided into several a-conjugated segments along the silicon skeleton. Therefore, hole transport is possibly determined by the hopping barrier between a-conjugated segments as illustrated in Fig. 1. It is needed to elucidate whether intermolecular or intersegment hopping is the determining process to the hole transport in polysilanes. The improvement of low photo-induced carrier yield and high sensitivity for photodegradation is needed to use polysilanes as photoconductors. Linear polysilanes have no visible light absorption band, but UV light absorption bands. Charge carriers are generated only for UV light (300-400 nm) exposure. The materials also show main chain scission for UV light exposure with high quantum efficiency. Trefonas et al. (1983) reported that films of high molecular weight PDHS showed a UV absorption spectral shift and photodegradation with UV light. Seigler et al. (1985), Hofer et al. (1984) and Miller (1990) reported that photo volatilization was caused by excimer laser irradiation for alkyl substituted polysilanes. The experimental results suggested that UV light exposure causes not only charge carrier production, but polymer main chain scission. West et al. (1981) reported that crosslinked polysilanes showed increase of conductivity with the presence of an electron acceptor. Our previous studies also suggested that ion beam irradiation caused crosslinking of silicon skeletons leading to
389
390
Shu Seki et al. Au semitransparent electrode
lO~n Hole Fig. 2. The sandwich structure of electrodes and polysilane film.
Intermolecularhopping
Intersegmenthopping Fig. 1. Schematic models of intermolecular and intersegment hopping in polysilanes. gelation of these materials (Seki et al., to be published). The irradiated polysilanes grew into network silicon skeletons. The results suggested conversion from linear polysilane molecules to network polysilanes with higher dimensional silicon skeletons. Therefore the ion beam irradiation is expected to reduce the intermolecular hopping barrier, and also predicted to make holes highly mobile. It was also confirmed that ion beam irradiated polysilanes showed high resistivity to photodegradation by UV light because of their network silicon skeleton. Thus, ion beam irradiation can be an effective way to improve hole transport in polysilanes.
solutions of these polymers were transferred into separatory funnel with water, and poured into isopropylalcohol followed by reprecipitation using tetrahydrofran-methanol system. Molecular weight distributions of these polymers were measured by Toyosoda TSK-GEL gel permeation chromatography system. The obtained polymers have their weight average molecular weight was Mw = 7.1 × 104 for PMPS, and Mw = 8.5 × 105 for PDHS respectively. Polymers were dissolved into toluene at 1015wt.% and casted on aluminium substrates at 10/~m thickness. The remaining solvent was removed in vacuum oven during 5 h at 120°C, and the films were annealed. The sandwich structure was completed by evaporating a semitransparent Au top contact as shown in Fig. 2. The schematic diagram of Time-of-Flight carried mobility measurement system was shown in Fig. 3. Nihon-Koshuha LGO-2N-200A nitrogen laser (337 nm) was used as an excitation light source. The laser pulses with 2 ns width were strongly absorbed at the upper surface of polymer films through semitransparent Au top contacts, leading to plane-like charge packets. These carriers
1". O. F. measured
EXPERIMENTAL
Poly(methylphenylsilane) (PMBS) and poly(di-nhexylsilane) (PDHS) were synthesized by the Wultz condensation with a methylphenyldichlorosilane for PMPS and a di-n-hexyl-dichlorosilane for PDHS as monomers respectively. All monomers were prepared by Shinetsu Chemical Co. Ltd, and purified by distillation. Polymerization was carried out in Ar atmosphere and in 100ml undecane as a reflux solvent. PHDS was synthesized in 100 ml toluene as a reflux solvent. The monomer was added to the reaction vessel and stirred with melted sodium metal for 4 h. The reaction mixture containing PMPS and PDHS solution was poured into iso-propylalcohol after filtration to roughly eliminate NaCI, and the precipitate was dried under vacuum. Toluene
~ +
Trig
I
~
~ P o l y m e r
Film
][.Z....~d~-.[ II TerminateResistance ~ 1 ~ IOOQ- lOkQ
Di~ltizina Oscilloscope
Computer
Fig. 3. Flight time observation system for hole drift mobility in polysilanes.
Radiation effects on hole drift mobility were forced to move by the applied electric fields, and the transient current was observed as voltage signals over 1-10kf~ resistances. Transient current was observed by the Sony/Tektronics TDS350 and DSA601 digitizing oscilloscopes. Ion beam irradiation was carried out by Van de Graaff accelerator at Research Center for Nuclear Science and Technology, The University of Tokyo. 2 MeV H + beam irradiated the film specimens of PMPS and PDHS at the fluence of 0.05-0.5 /~C/cm 2. Energy deposition of 2 MeV H ÷ beam in these media (10/~m depth) was estimated to be max, 10% of incident particle energy, and the irradiation was considered to be homogeneous. RESULTS AND DISCUSSION Figure 4 shows temperature dependence of hole drift mobility in PMPS at the applied field of 4.0 × 104V/cm. Equation (1) is introduced to deconvolute field and temperature dependence, # = ~exp[-(E0 -
flE'/2)/kTefr]
(1)
where E denotes field, E0 denotes zero field limit activation energy, and 1/Te~= I/T-1~To, as proposed by Gill (1972), Pal (1970) and Chen and Slowick (1975) for hole transport in poly(vinylcarbazole). Figure 4 is obtained by the extrapolation of the field dependence of hole drift mobility in PMPS shown in Fig. 7. The slope of the Arrhenius plot corresponds to activation energy of zero field limit; E0 as shown in the equation (1). The result of E0 is estimated to be 0.39 eV in this PMPS media. Figure 5"displays the ion beam induced changes in the TOF current-mode transit pulses collected on glassy PMPS media. These transit pulse shapes are obtained by the specimens irradiated by 2 MeV H + with the fluence of 0.05, 0.2 and 0.5/~C/cm2, respectively. The lower current is obtained for the specimen before irradiation in comparison with these transit currents after irradiation. The values of transit time are estimated by the edges of transit pulse shapes. It is clear that the flight time is shortened by 10 -2
> 10-3
°~°
=l.
10-4 o
°~o
10-5 O
10-6 3.0
I 3.2
I 3.4
I 3.6
I 3.8
I 4.0
I 4.2
I 4.4
1000/T(K) Fig. 4. Arrhenius plot of hole drift mobility in poly(methylphenylsilane); P M P S films at electric field: E = 4.2 x 104V/cm.
391
•
0 . 5 I.t C / c m 2
.n
0
I
I
I
I
20
40
60
80 Its
I
I
I
I
100 120 140 160
Fig. 5. Changes in transient current pulse shapes of irradiated poly(methylphenylsilane)by 2 MeV H + ion beam at 298 K, 4.2 × l& V/cm.
the ion beam irradiation. Another attractive change is the increase of carrier yield with the irradiation. The yield of holes is estimated by the integration of transient current. Especially for the specimen irradiated at 0.5/~C/cm 2, the yield is two times larger than that of before irradiation as summarized in Table 1. (Normalized carrier yield means the ratio of carrier yield to the yield before irradiation.) Figure 6 shows changes with the irradiation in the transit pulse shapes collected on the crystalline PDHS media. The ion beam irradiation was carried out at the same condition as PMPS, and the irradiation fluence was varied from 0.05/~C/cm 2 to 3 gC/cm 2. The transit pulses in Fig. 6 are slightly distorted in comparison with the case of PMPS as shown in Fig. 5, and it is impossible to distinguish a sharpe edge that presents the value of the transit time. The mobility seems to consist of multiple components for this polymer. However, it should be noted that pulse signals after irradiation showed faster decay than the pulse before irradiation, resulting in the generation of fast components of charge carrier mobility. The yield of charge carrier also increases with the irradiation similarly to the irradiation for PMPS. The yield of holes is increased until the fluence reaches 0.20.5/~C/cm 2, and decreases with further irradiation as summarized in Table 1. In this case, the yield also becomes maximally two times larger than that of before irradiation. The values of hole drift mobility in polysilane derivatives have beeUn reported to depend on the square root of electric field in Poole-Frenkel hopping Table 1. Charge carrier yield changeswith 2 MeV H+ ion beam irradiation for poly(methlphenylsilane)and poly(di-n-hexysilane) Normalized carrier yield Fluence(pC/cm2) PMPS PDHS 0 1.00 1.00 0.05 1.21 1.35 0.2 1.36 2.33 0.5 2.05 2.03 3 -1.14
392
Shu Seki et al. the sites of branched structures leading to decline of the hole drift mobility (Seki et al., in p r e s s ) . Comparing the effects of ion beam irradiation to that of the chemically induced branched structure, the difference is in the dimensional change of silicon skeletons. Ion beam irradiation changes molecular structure of polysilanes into the 3-D Si network along incident ion tracks in spite of chemically induced branched structures meaning 2-D Si network structures (Furukawa et al., 1990; Bianconi and Weidman, 1988; Bianconi et al., 1989).
Before irradiation 0.05 ~C/cm 2 0.2 ItC/cm 2
/ ~. -- --
~it^\ "\ - - 0.SgC/cm2 I \ • ....... 3 ItCIcm2
~!,-\ ,,~
• o
;I :22.
lie"
°
",
~
\
~,~\
-.\
J 10
20
30
40 IXS
50
60
70 CONCLUSION
Fig. 6. Changes in transient current pulse shapes of irradiated poly(di-n-hexylsilane)by 2 MeV H ÷ ion beam at 298 K, 4.2 x 104V/era. model (Stolka et al., 1987; Abkowitz et al., 1987; Kepler et al., 1984a,b; S u e t al., 1979). The electric field dependence in the PMPS also appears to obey the equation (1) even after the irradiation as summarized in Fig. 7. The values of zero field limit mobility are almost the same for all irradiated samples in spite of the lower value observed for the specimen before irradiation. In this electric field range, the carriers are accelerated with the irradiation. The specimen irradiated at 0.5 ~C/cm 2 shows five times larger value of mobility than that before irradiation. The acceleration phenomenon of holes is fundamentally explained by a simple model of network structure generation as suggested in our previous work (Seki et al., to be published). Contrarily, lower values of hole drift mobility have been observed for structural defects induced polysilanes (chemically branched polysilanes) (Seki et al., unpublished results). Those structural defects induced polysilanes have also been investigated by means of the electron beam pulse radiolysis. The experimental results showed hole localization around
The values of hole drift mobility obtained in the present study corresponded to several previous data. The field dependence of hole drift mobility in PMPS was well explained by Poole-Frenkel type hopping model. The model was also effective to account for the field dependence even in the ion beam irradiated PMPS. The temperature and field dependence was deconvoluted for PMPS before irradiation, and the activation energy was estimated to be 0.39 eV. Ion beam irradiation increased the values of hole drift mobility in glassy PMPS and crystalline PDHS, and the yield of charge carriers increased with the irradiation to the specimens of the PMPS and the PDHS. The yield of charge carrier was increased until the fluence reached 0.2-0.5/~C/cm2 followed by decrease with further irradiation. The yield became two times larger than that before irradiation for both the PMPS and the PDHS. The acceleration was basically explained by a simple model of Si network structure generation induced by ion beam irradiation. Intermolecular hopping barrier was reduced by extended a-conjugated system. The model of ion beam induced Si network structures is concluded to be 3-D cylindrical Si network structures generated by ion beam irradiation along incident ion tracks. The model is quite different from that of chemically induced 2-D network structures with Si branching.
0.5p.C/cm2
\
1 x 10 -3 --
~.~
.~•'~"
0.2~tC/cm2
L'-
0.05 ~tC/cm2
~- = V2 ~ °'• - - - c'" i-" -. . . . .
~
1 x 10 -4
180
[
190
[
200
Acknowledgements--The authors acknowledge the useful advice of Dr N. Matsumoto, and Dr M. Fujiki, Dr K. Furukawa, Dr M. Fujino, Dr H. Teramae, and Dr K. Takeda at Basic Research Laboratory, Nippon Telephone and Telegraph Co. Ltd, and Mr M. Narui and Mr T. Omata at the University of Tokyo.
.
-o •
REFERENCES
Before irradiation
[
210
[
220
[
230
[
240
[
250
Et/2 (Vlcm)~/2 Fig. 7. Semi-logarithmicplot of hole drift mobility/1 vs Et/2 (V/cm)]/2 in ion beam irradiated polysilane films at 298 K.
Abkowitz M. A., Rice M. J. and Stolka M. (1990a) Phil. Mag. B 61, 25. Abkowitz M. A., Stolka M., WeagleyR. J., McGrane K. M. and Knier F. E. (1990b) In Silicon Based Polymer Science, Advances in Chemistry Series 224 (Edited by Zcigler J. M. and Fearon F. W.) American Chemical Society, Washington DC. Abkowitz M. A., Knier F. E., Yuh H. J., Weagley R. J. and Stolka M. (1987) Solid State Commun. 62, 547. Bianconi P. A. and Weidman T. W. (1988) J. Am. Chem. Soc. 110, 2342.
Radiation effects on hole drift mobility Bianconi P. A., Schilling F. C. and Weidman T, W. (1989) Macromolecules 22, 1697. Chert I. and Slowick J. H. 0975) Solid State Commun. 17, 783. Fujino M. (1987) Chem. Phys. Lett. 136, 451. Furukawa K., Fujino M. and Matsumoto N. (1990) Macromolecules 23, 3423. Gill W. D. (1972) J. Appl. Phys. 43, 5033. Hofer D. C., Miller R. D. and Willson C. G. (1984) Proc. SPIE 469, 16. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1984a) Bull. Am. Phys. Soc. 29, 509. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1984b) Bull. Am. Phys. Soc. 28, 362. Kepler R. G., Zeigler J. M., Harrah L. A. and Kurtz S. R. (1987) Phys. Rev. B. 35, 2818. Miller R. D. (1990) Advanced in Chemistry Series 224. American Chemical Society, Washington DC. Pai D. M. (1970) J. Chem. Phys. 52, 2285. Phillpot S. R. (1987) Phys. Lett. 31, 43, Rice M. J. (1979) Phys. Lett. A. 71, 152.
393
Rice M. J. and Phillpot S. R. (1987) Phys. Rev. Lett. 58, 937. Samuel L. M., Sanda P. N. and Miller R. D. (1989) Chem. Phys. Lett. 159, 227. Seki S., Shibata H., Ban H., Ishigure K. and Tagawa S. Macromolecules, to be published. Seki S., Cromack K. R., Trifunac A. D., Tagawa S., Ishigure K., Yoshida Y. and Kozawa T. (1995) Radiat. Phys. Chem., in press. Stolka M., Yuh H. J., McGrane K. and Pai D. M. (1987) J. Polym. Sci., Polym. Chem. Ed. 25, 823. Su W. P., Schrieffer J. R. and Heeger A. J. (1979) Phys. Rev. Lett. 58, 937. Trefonas P., West R., Miller R. D. and Hofer D. (1983) J. Polym. Sci., Polym. Left. Ed. 21, 823. West R. D., Djurovic L. D., Stearley P. I., Srinvasan K. S. and Yu H. (1981) J. Am, Chem. Soc. 103, 7352. Zeigler J. M., Harrah L. A. and Johnson A. W. (1985) Proc. SPIE 539, 166.