Temperature Dependence of DC Conductivity in Ion-Beam-Irradiated Glassy Carbon
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TEMPERATURE DEPENDENCE OF DC CONDUCTIVITY IN ION-BEAMIRRADIATED GLASSY CARBON Dougal McCulloch and Steven Prawer, Department of Applied Physics and Microelectronics and Materials Research Center and Dipankar Sengupta, Department of Communications & Electrical Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Victoria, 3001, Australia. ABSTRACT The electrical conductivity of ion beam irradiated Glassy Carbon has been investigated in the temperature range 100 to 300 K. Ion species used were C+ and N+ with doses between 1014 and 1018 ions/cm 2. Ion beam irradiation was found to lower the conductivity of Glassy Carbon by up to six orders of magnitude. The temperature dependence of the conductivity in ion beam modified Glassy Carbon has been measured. The functional dependence was found to remain largely unchanged by ion irradiation despite the large overall decrease in the conductivity. The results are interpreted in terms of a model which includes a variable range hopping and strongly scattering metallic components. INTRODUCTION Of the many forms of synthesized carbon, Glassy Carbon (GC) is one of the more interesting. Produced by the decomposition of highly cross-linked polymers, GC is a nongraphitizing carbon with a microstructure best described as a tangle of cross-linked graphite-like ribbons 1 . Interest in ion beam irradiation of GC began when it was discovered that ion implantation significantly improved the mechanical wear resistance of GC2 . Since then a number of authors have studied the effects of ion irradiation on both the structure and wear of GC3 -4 . In an attempt to further understand the structure of ion beam modified GC, its electrical conductivity at room temperature was studied 5 . It was found that ion beam irradiation leads to large increases in the resistivity of GC within the modified surface layer which is of the order of 2000 A thick. In this paper we report an extension of the previous work to include the temperature dependence of conductivity in the ion beam modified material in an attempt to shed more light on the conduction mechanism. EXPERIMENT Substrates of GC were cut from plates supplied from Atomergic Chemetals Corporation (New York). According to the manufacturer, the GC used was heat treated to 2500 OC, has a density of 1.55 g/cm 3 and conductivity of 2.2x10 2 (fl-cm)-'. Prior to implantation the samples were polished with progressively finer diamond paste down to a 1 micron finish. Implants were conducted at room temperature in a vacuum better than 10-5 Torr. The samples were implanted with 50 keV C+ (lx1015 to 1.5x10 18 C+/cm 2 ), 43 keV N+ (5x10 14 to 1x10 15 N+/cm 2) and 50 keV N+ (5x1016 to 1.5x1018 N+/cm 2 ). TRIM 6 calculations suggest that these energies will create a modified layer approximately 2000 A thick. The resistance measurements were performed in the configuration shown in figure 1. The 1 mm diameter contacts were placed on the ion beam modified surface of the GC 2 mm apart and were made using Electrodag paint. The resistance was dete
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