[ACS Symposium Series] Polymeric Materials for Microelectronic Applications Volume 579 (Science and Technology) || Electronic Properties of Polysilanes

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<ul><li><p>Chapter 31 </p><p>Electronic Properties of Polysilanes </p><p>R. G. Kepler1 and Z. G. Soos2 </p><p>1Sandia National Laboratories, Department 1704, M/S 0338, P.O. Box 5800, Albuquerque, NM 87187-0338 </p><p>2Department of Chemistry, Princeton University, Washington Road, Princeton, NJ 08544 </p><p>The results of recent studies of the electronic properties of polysilanes are reviewed. The electronic states can be described by the Hckel model if coulomb interactions are included using the Pariser-Parr-Pople approximation. The long polymer chains appear to be divided into random length, short, ordered segments by conformational defects, with the energy of the excited states depending on the length of the segments. In isolated polymer chains energy is transferred from high-energy, short segments to longer, lower energy segments but the distance and time during which transfers take place is very limited. In solid films the excitons become highly mobile and remain mobile throughout their lifetime, even at low temperatures. Holes are quite mobile in solid films and the characteristics of transport are the same as those of charge carrier transport in molecularly doped polymer films. </p><p>Soluble polysilanes are a relatively new class of polymers that have recently come under intense scrutiny for a variety of possible applications. They are excellent photoconductors (/), their third order nonlinear optical coefficients are quite large for materials which are transparent in the visible (2), potentially useful structures have been produced photochemically in thin films (3), and they may prove to be useful in photoresist applications (4), particularly those which tend to be environmentally benign and do not require high resolution. </p><p>Since polysilanes are -bonded polymers which contain no -bonds, it initially appears that their electronic states would be quite different from those in the more familiar -conjugated polymers. It has now been shown, however that the -electrons are delocalized and play the same role as the -electrons. The theory that can be used to describe the electronic states, as a first approximation, is the Huckel theory modified by the inclusion of coulomb interactions using the Pariser-Parr-Pople approximation (5-7). In both types of material the transfer integrals t alternate, t+ and t-, but they alternate for </p><p>0097-6156/94/0579-0387S08.00/0 1994 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>388 POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS </p><p>different reasons. In the -conjugated polysilanes one integral is between -orbitals on neighboring silicon atoms and the other between -orbitals on the same silicon atom. In -conjugated polymers the alternation is typically caused by different bond lengths. The magnitude of is generally larger in the polysilanes also. </p><p>This theory and the experimental evidence that supports it (8) points up a strong similarity between the states of solid polysilane films and those of molecular crystals. In Figure 1 we indicate the energy, relative to the ground state, of the various excited states of poly(di-n-hexylsilane), PDHS, films and of the same states in anthracene single crystals. </p><p>Solution Studies </p><p>The absorption and fluorescence spectra of poly(methylphenylsilane), PMPS, in a dilute cyclohexane solution are shown in Figure 2. Two noteworthy aspects of these spectra will be discussed. The absorption spectrum is broad and featureless and the fluorescence spectrum is narrower and on the low energy edge of the absorption spectrum. </p><p>The broad featureless absorption spectrum is believed to result from inhomogeneous broadening. The long polysilane molecule is thought to be divided into random length, short, ordered segments on the order of 20 silicon atoms long. The defects which divide the molecule have been left undefined but are presumed to be conformational defects. The energy of the excited state of these segments depends on the length, being higher for shorter lengths. Experimental evidence for this hypothesis is provided by studies of the temperature dependence of the absorption spectrum in dilute solutions. The energy at the peak of the absorption spectrum decreases as the temperature is lowered. The temperature dependence can be modeled quite well by assuming that it takes a specific amount of energy to create the conformational defects which divide the segments, leading to more defects and shorter segments at higher temperatures (9). Further evidence is provided by the fact that in dilute solutions of PDHS an order-disorder transition is observed below room temperature. Schweizer (9) has shown that the solvent dependence of the transition temperature can be explained by assuming that the transition results from a competition between entropy favoring shorter segments created by more defects and energetic stabilization provided by van der Waals interaction between the segments, the polarizability of which is greater for longer segments, and the polarizable solvent molecules. The transition temperature is observed to increase with the polarizability of the solvent molecules, as predicted by this model, and does not depend on the solvent dielectric constant. </p><p>The fact that the fluorescence spectrum is narrower than the absorption spectrum and is on its low energy edge is generally interpreted as evidence that the excited state moves from the shorter, higher energy segments to longer, lower energy segments before a fluorescence photon is emitted. Time-dependent fluorescence depolarization experiments (70) show that the amount of depolarization of the fluorescence spectrum depends on the energy of the exciting light, being greater for higher energies, and occurs only for a short time following excitation. These results have led to the conclusion that the </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>31. KEPLER AND SOOS Electronic Properties of Polysilanes 389 </p><p>PDHS Anthracene </p><p>- 5.2 evN^ Charge Carriers </p><p>4.3 eV\^ Two-Photon 4 , 1 e V </p><p>3.4eV\^ Singlet 3 5 e V 3 </p><p>*3.1 eV E9 </p><p>Triplet </p><p>1.8 eV </p><p>Figure 1. Threshold energies, in eV, for one-photon (Eg), two-photon (Ea), triplet () and charge carrier (E D ) excitations of PDHS films and anthracene crystals. (Reproduced with permission from reference 8) </p><p>Figure 2. The absorption and fluorescence spectra of PMPS in dilute cyclohexane solution. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>390 POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS </p><p>excited states quickly become trapped in the longer segments. Measurements of the absorption coefficient and the fluorescence lifetime have led to the conclusion that the fluorescing segments are 30-40 silicon atoms long. </p><p>Studies of the fluorescence spectra excited by light in the red edge of the absorption spectrum (11) provide additional evidence for the segment and exciton trapping model, and also show that the Stokes shift of the excited state is very small. If a nearly monochromatic beam of light with energy in the low-energy edge of the absorption band is used to excite PDHS molecules in 3-methylpentane at 1.4 K, the emission parallels the energy of the exciting light, as shown in Figure 3, indicating a Stokes shift of at most 10 cm" 1. If the energy of the exciting photons is higher, so that shorter segments are excited, the broad fluorescence spectrum is observed. These results also show that in isolated strands, the excitation energy is trapped in the longer, low-energy segments. </p><p>Two-photon absorption spectra can be observed in PDHS solutions by observing the fluorescence induced by two-photon transitions as a function of the energy of the exciting light (12). Such experiments locate the 2*A g state and the results are in agreement with the predictions of the theoretical model outlined above (13). </p><p>Solid State Studies </p><p>The absorption, fluorescence and two-photon absorption spectra of solid films of polysilanes are essentially the same as those in solution except for small solid-state energy shifts. Therefore, it is generally believed that in the solid state the absorption spectrum is still inhomogeneously broadened as it is in solution but it has been shown that excitons are no longer trapped in the longer segments, even at low temperature. In addition, photoconductivity studies have shown that positive charge carriers, holes, are quite mobile in a wide variety of polysilanes and the temperature and electric field dependence of the mobility is the same as the mobility of holes in molecularly doped polymers, suggesting that the holes are hopping among the segments discussed above. </p><p>Photoconductivity. Early time-of-flight studies of photoconductivity in polysilanes showed that at room temperature hole transport is nondispersive, i. e. the hole mobility is well defined, with very little trapping and that the mobility is strongly temperature dependent (1). Studies of the temperature and electric field dependence of the mobility (14) have shown that hole mobility in polysilanes can be described by the empirical relation </p><p> = 0 (1) </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>31. KEPLER AND SOOS Electronic Properties of Polysilanes 391 </p><p>where 6 is the activation energy for hopping, is the limiting value at high temperatures, l/Teff= 1/ + l/TQ with 0 being defined empirically and gives the field dependence. Such behavior was first noted by Gill (75) for hole transport in polyvinylcarbazole and has subsequently been observed in many molecularly doped polymers, where transport involves hopping among the dopant molecules. A theoretical description of this behavior still does not exist. However it is clear that it is characteristic of transport by hopping among molecular states and clearly suggests that hole transport in polysilanes is dominated by hopping among the ordered segments of the polysilane chains. </p><p>Carrier generation experiments in PMPS have shown that charge carriers are generated when photons with energies in the lowest energy, strong absorption band are absorbed (/ ). This observation was initially surprising since it is known that absorption in this band creates excitons, but it was shown that the carriers are generated by excitons diffusing to the film surface and injecting carriers from the electrode (1). It was concluded that the exciton diffusion length was 50 nm, in agreement with later results obtained in studies of exciton-exciton annihilation (7(5), to be discussed below. Measurements of carrier generation with much higher energy photons show a large increase in the quantum efficiency for carrier generation beginning at 4.6 eV which is interpreted as evidence for band-to-band transitions and a free carrier band gap of 4.6 eV (17). Similar measurements in PDHS suggest that the free carrier band gap in that material is 5.2 eV (18). </p><p>Electroabsorption experiments on PMPS films have supported these assignments (17). Some of die results are shown in Figure 4 where the change in transmission , induced by the application of an electric field, divided by the transmission T, is plotted versus photon energy. The strong signal, roughly proportional to the first derivative of the absorption band at 3.7 eV is indicative of a Stark shifted exciton band. The magnitude of the signal indicates a polarizability change of 1.8 10-22 cm3, in reasonable agreement with the PPP model (17). The weaker signal at 4.65 eV is clearly not proportional to the first derivative of the absorption spectrum and is presumably associated with the free carrier band gap. The shape of the feature suggests redistribution of oscillator strength away from the center which could result from electron transfer between segments (17). Tachibana et al. (19) have measured the electroabsorption of PDHS and observed a very large feature beginning near 5.2 eV which may be associated with the band gap and is, therefore, also in agreement with the photoconductivity results. </p><p>Exciton Transport. An early potential application of polysilanes was as a self-developing resist, one in which material was removed under deep UV radiation (4). Studies of the quantum efficiency for material removal by light indicated that it decreased at high light intensities. This observation led to die hypothesis that exciton-exciton annihilation was responsible for the decrease and to measurements of the exciton-exciton annihilation rate constant. </p><p>The rate equation for the concentration of excitons, , with exciton-exciton annihilation is </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>392 POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS </p><p>3400 3500 3600 WAVELENGTH () </p><p>Figure 3 Absorption (thin line) and emission (thick line) spectra of PDHS in 3-methylpentane at 1.4 K. The vertical line at 351.1 nm indicates the wavelength of the exciting light. (Reproduced with permission from reference 11) </p><p>3.5 4.0 4.5 5.0 PHOTON ENERGY (eV) </p><p>Figure 4. The electroabsorption spectrum at room temperature of PMPS (solid line). The absorption coefficient and its derivative are shown as the long- and short-dashed lines, respectively. The derivative of the absorption spectrum is plotted relative to the left ordinate and normalized to the magnitude of the electroabsorption spectrum at the maximum of the major peak. (Reproduced with permission from reference 17) </p><p>Dow</p><p>nloa</p><p>ded </p><p>by U</p><p>NIV</p><p> OF </p><p>CIN</p><p>CIN</p><p>NA</p><p>TI </p><p>on N</p><p>ovem</p><p>ber </p><p>11, 2</p><p>014 </p><p>| http</p><p>://pu</p><p>bs.a</p><p>cs.o</p><p>rg </p><p> Pub</p><p>licat</p><p>ion </p><p>Dat</p><p>e: M</p><p>ay 5</p><p>, 199</p><p>5 | d</p><p>oi: 1</p><p>0.10</p><p>21/b</p><p>k-19</p><p>94-0</p><p>579.</p><p>ch03</p><p>1</p><p>In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995. </p></li><li><p>31. KEPLER AND SOOS Electron...</p></li></ul>