Implantation-Induced Conductivity of Polymers
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IMPLANTATION-INDUCED CONDUCTIVITY OF POLYMERS
B. WASSERMAN, *+ G. BRAUNSTEIN, *+ M.S. DRESSELHAUS,*+# AND G.E. WNEK*0 Center for Materials Science and Engineering; +Department of Physics; #Department of Electrical Engineering and Computer Science; Department of Materials Science and Engineering; Massachusetts Institute of Technology, Cambridge, MA 02139, USA
ABSTRACT
Ion implantation causes an increase by -14 orders of magnitude in the electrical conductivity of normally insulating polymers such as polyacrylonitrile (PAN), poly(2,6 dimethyl phenylene-oxide) (PPO), and poly (pphenylene sulfide) (PPS), after ion implantation with Br 5 2 at fluences of 3 x 10 ions/cm . The temperature dependence of the dc conductivity was measured in the range 23 < T < 293 K and results show an exponential law S - exp(-To/T)a) for PAN, PPO and PPS with a = :, suggesting a one-dimensional hopping mechanism. The temperature dependence of the thermoelectric power (TEP) identifies the sign of the dominant carrier type. The TEP exhibits linear metallic behavior, with small magnitudes (-3pV/K), and shows Br implantation to yield p-type material in PPS and n-type material in PAN with extremely 2 low values of mobility (< 10-3 cm /V sec) and correspondingly very high values of the carrier concentration estimated to be 5 x 1022 cm . Results are also reported for the frequency dependence of the AC conductivity and of similarly implanted PPS and PAN samples.
INTRODUCTION In this work we present the results of studies on transport properties of three ion implanted polymers; polyacrylonitrile (PAN), poly(2,6 dimethyl phenylene-oxide) (PPO), and poly (p-phenylene sulfide) (PPS). The observed tremendous (-14 orders of magnitude) increase of conductivity through implantation leads to the possibility of creating easily processed, low cost materials with selective conductivity, interconnections for 2D and 3D integrated circuits, and of constructing p-n junctions.
EXPERIMENTAL To achieve easily measurable conductivity for typical ion penetration depths (J 3000 X), thin polymer films were used for the host matrix. These thin polymer film (200OX to Ipm) samples were prepared by a spinning technique, using a thin microscope slide as a substrate. The spinning rate varied from 0 to 2500 R.P.M. Chloroform was used as a solvent for PPO and NN-dimethylformamide was used to dissolve PAN. The best films 0 were obtained using solution temperatures Ts between 100 < T < 130 C. The films were cured in a vacuum oven for a few hours at 100 C to remove the remaining solvent. Electrical contacts were made using EPO-TEKH20E 0 conductive epoxy, cured at 120 C, and good ohmic contacts to the polymer were thus achieved. The implantation was performed on an Accelerators Incorporated 300 MP model ion implanter for doses ranging between 101' and
Mat. Res. soc. Symp. Proc. Vol. 27 (1984) QEleevier Science Publishing Co., Inc.
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1017/cm2, and with an ion beam energy of 200 keV. The samples were 79 75 implanted with Br, As, and 14N ions at room temperature. The depth di
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