Structure/Majority Carrier Relationships in Ion-Implanted Polymer Films
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STRUCTURE/MAJORITY CARRIER RELATIONSHIPS IN ION-IMPLANTED POLYMER FILMS
G. E. WNEK, B. WASSERMANt AND I.-H. LOH Department of Materials Science and Engineering tDepartment of Physics Massachusetts Institute of Technology, Cambridge,
MA
02139,
U.S.A.
ABSTRACT Ion implantation of selected polymer films leads to the formation of semiconductive (a - 10-4 S/cm) derivatives, which are presumably partially carbonaceous products derived from gross structural rearrangements. In support of this contention we find that Br-implanted polyacrylonitrile, PAN, exhibits a narrow ESR signal with a g-value of 2.0033, consistent with the presence of free radicals delocalized within a u-electron system. Thermopower measurements reveal that the sign of the majority carrier is dependent upon the molecular structure of the parent polymer. For example, implantation of PAN with Br+ affords n-type derivatives while under similar conditions poly(p-phenylene sulfide), PPS, and poly(2,6-dimethylphenylene-oxide), PPO, yield p-type semiconductors. It is suggested that the majority carriers are the ions (carbenium ions or carbanions) which are best stabilized by the parent polymer structure, remnants of which presumably exist after implantation. Resonance and/or inductive effects are invoked to explain the data. For example, the rather electron-rich backbone of PPQ is expected to stabilize holes more effectively than electrons, and the observed p-type behavior is consistent with this prediction.
INTRODUCTION Ion implantation of several low molecular weight organic solids and high polymers results in rather large increases in electrical conductivity [1-4]. Of particular interest is the use of this technique for the fabrication of environmentally stable semiconductor devices from thin organic films. Thus, the ability to control the sign of the majority carriers is of great importance. In this article we present preliminary data concerning the majority carriers from thermopower measurements for ion-implanted poly(acrylonitrile), PAN, poly(p-phenylenesulfide), PPS, and poly(2,6-dimethylphenylene oxide), PPO, and suggest a simplistic (but potentially useful) rationalization of the data.
EXPERIMENTAL Thin films of PAN, PPS, and PPO (solvents: DMF, diphenyl ether and chloroform, respectively) were spun (0-2500 R.P.M.) onto -2 cm. glass squares cut from microscope slides. Electrical contacts were made with conductive epoxy (EPO- TEKH20E; cured at 1200 C). Ion implantation was performed on an Accelerators Incorporated 300 MP implanter using doses between 17 2 104 and 10 /cm , and an ion beam energy of 200 keV. The samples were im5 planted with 79Br and 7 As. The sample temperature during implantation was monitored by a thermocouple attached to the glass substrate. It was found not to exceed ca. 120%C and therefore the effects of implantation cannot be attributed to pyrolysis. EPR spectra were recorded on a Varian E-9 spectrometer. tions were determined [5J using a CuSO4 standard.
Spin concentra-
Mat. Res. Soc.Symp. Proc. Vol. 27 (1984) Published b
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