First-Principles Study of Photoexcited Defects in Polysilane Chains
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FIRST-PRINCIPLES STUDY OF PHOTOEXCITED DEFECTS IN POLYSILANE CHAINS
J. W. MINTMIRE, R. C. MOWREY, D. W. BRENNER, B. I. DUNLAP, and C. T. WHITE Theoretical Chemistry Section, Naval Research Laboratory, Washington, DC 20375-5000
ABSTRACT Organopolysilane materials have recently demonstrated potential technological importance as positive photoresists, photoconductors, and nonlinear optical materials. Many of the technological applications of these materials depend intimately on the photoexcitation process in these materials, possibly resulting in either bond scission or the creation of mobile charge carriers. Herein we present some preliminary results of a model simulation of the photoexcitation process in oligomeric polysilane chains using a recently developed first-principles local-density functional method for the calculation of electronic structures, total energies, and gradients of the total energy with respect to nuclear coordinates.
INTRODUCTION The organopolysilanes are linear high polymers comprised of a chain of silicon nuclei a-bonded together, with two organic substituents attached to each backbone silicon 1 . Polysilanes were originally synthesized in the early part of this century, with poly(diphenylsilane) possibly synthesized by Kipping' in 1924, and with polydimethylsi3 lane (PDMS) definitely synthesized by Burkhard in 1949. The insolubility and general intractability of PDMS resulted in the neglect of the field for several decades, until Yajima et al.' found that PDMS could be used as a precursor for silicon carbide. More recent synthetic efforts have demonstrated the technological potential of these polymers 5 as electronic device materials, with specific capabilities as photoresist materials ` and as 9 photoconductorss' for potential imaging applications. Their photoresist capabilities result from a combination of favorable optical properties. First, the- polysilanes absorb in the UV from 300 to 400 nm with photoscission the dominant mode of degradation, although cross-linking has been noted in the arylsubstituted polysilanes. Second, these materials optically bleach, with shorter-length oligomers having higher-energy (and longer wavelength) absorption thresholds. Finally, under reactive ion etching the polysilanes degrade to an extremely inert Si0 2 layer, allowing high-resolution lithography. Similarly, these materials exhibit nearly dispersionless transport of hole defects, leading to possible use of polysilanes in xerographic processes. Most of the technological applications of these materials depend intimately on their electronic structure and resulting optical properties, and a better understanding of these properties will be essential to guide synthetic efforts towards the goal of more technologically useful polysilane materials. The primary channels for photoscission of a substituted -(SiRRK)- chain"0 are believed to be either the extrusion of a silylene radical, RRfSi: via the reaction
-SiRR'-(SiRkR),-SiRt'- --
-SiRR'.--SiRR'-SiRR'- + RI:! Si:
Mat. Res. Soc. Symp. Proc. Vol. 209. @19
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