Chapter 32
UV
Photoelectron Spectroscopy of Polysilanes and Polygermanes 1
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Kazuhiko Seki , Akira Yuyama , Satoru Narioka , Hisao Ishii , Shinji Hasegawa, Hiroaki Isaka , Masaie Fujino , Michiya Fujiki , and Nobuo Matsumoto 3
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Department of Chemistry, Faculty of Science, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-01, Japan Institute for Molecular Science, Myodaiji, Okazaki 444, Japan ΝTΤ Basic Research Laboratories, Wakamiya, Morinosato, Atsugi 243-01, Japan 2
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UV photoelectron spectra were measured for ten polymers with main chains composed of Si and Ge atoms. The spectra of poly(dial kylsilane)s were described well as the over lap of the spectra of constituent parts, while polysilanes with aryl side groups showed the effect of σ -π mixing. The spectra of polygermanes and Si-Ge block copolymers are fairly similar to those of corresponding polysilanes, while Si-Ge random copolymers show indications of the effect of the presence of many Si-Ge bonds. Recently polysilanes and polygermanes have attracted attention due t o t h e i r unique properties. These are mainly due t o σ-conjugation along the main chain, with possible applications t o photoresists, conducting polymers, and i n i t i a t o r s of polymerization, etc. (1,2). Knowledge of t h e i r e l e c t r o n i c structures forms the basis of under standing these i n t e r e s t i n g properties. So f a r there have been r e ports on the valence e l e c t r o n i c structures of several polysilanes studied by UV photoelectron spectroscopy (UPS) (3-5), but they suf fered e i t h e r from low spectral q u a l i t y or from l i m i t e d coverage of the valence band. In t h i s work we report the whole valence electron i c structures of a series of polysilanes, polygermanes, and Si-Ge copolymers with UPS using synchrotron r a d i a t i o n , and analyze the r e s u l t s by comparison with the spectra of constituent parts. Experimental A l l the sample materials were synthesized at NTT. The samples for UPS measurements were prepared by spin-coating of 0.4 weight % toluene s o l u t i o n of each compound under N atmosphere. The samples 2
0097-6156/94/0579-0398$08.00/0 © 1994 American Chemical Society
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
32. SEKI ET AL.
UV Photoelectron Spectroscopy of Polysilanes
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prepared under these conditions showed no sign of charging during the UPS measurement. The spectra were measured under u l t r a h i g h vacuum on the order of 10*" Pa by using an angle-resolving photoe l e c t r o n spectrometer at the beamline 8B2 of the UVSOR f a c i l i t y at I n s t i t u t e f o r Molecular Science. Monochromatized l i g h t from a plane grating monochromator (5) impinged on the sample, and the energy d i s t r i b u t i o n of photoelectrons was analyzed with a hemispherical e l e c t r o s t a t i c energy analyzer. The o v e r a l l r e s o l u t i o n was about 0.2 eV as deduced from the measurement of the Fermi edge of gold. No detectable damage by photon i r r a d i a t i o n was observed. A l l the meas urements were c a r r i e d out at room temperature. 8
Polysilanes Polydialkylsilanes. In Figure 1(a) the spectrum of poly(methylprop y l s i l a n e ) (SiMePr)^ at hy= 40 eV i s shown. We can c l e a r l y see several f i n e structure peaks i n the upper valence region, i n con t r a s t to previously reported spectra (3), which suffered from severe sample charging and do not show f i n e structure i n t h i s region. For comparison, reported UPS and XPS spectra of methane (7,8) propane (8,9), and permethyltetrasilane (10) are shown i n Figure l ( b ) - ( d ) . They show good correspondence with the spectrum of (SiMePr) . From such a comparison and l i t e r a t u r e assignments (4,5,10), we can ascribe the features i n 5-6 eV region to Si3p-derived
