Electrical Conductivity as a Function of Temperature of Diamond films Grown by Downstream Microwave Plasma Chemical Vapo
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ELECTRICAL CONDUCTIVITY AS A FUNCTION OF TEMPERATURE OF DIAMOND FILMS GROWN BY DOWNSTREAM MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION Brian R. Stoner, Jesko A. von Windheim, Jeffrey T. Glass, North Carolina State University, Department of Materials Science, Raleigh, NC 27695-7919; Danny Zoltan and Jan W. Vandersande, Jet Propulsion Laboratory, Pasadena, CA 91109.
Abstract Electrical conductivity measurements were used to study the effects that sample distance from the plasma during growth has on the carrier transport properties of undoped CVD diamond. The films were grown by downstream microwave plasma chemical vapor deposition at distances from 0.5 to 2.0 cm from the edge of plasma glow. Electrical conductivity measurements were performed between room temperature and 1000 °C to gain a better understanding of the CVD growth process and the resulting electrical properties of the diamond film's. Room temperature electrical conductivity was found to vary by over 5 orders of magnitude with increasing growth distance from the plasma, and this is attributed to decreasing hydrogen incorporation efficiencies at further distances from the plasma.
I.
Introduction
There are many applications for highly electrically insulating materials that will remain stable at extreme temperatures, such as the insulator in high temperature thermoelectric power conversion devices. Diamond appears to be a good candidate for such high temperature applications due to its intrinsically high electrical conductivity and very high thermal conductivity. Chemical vapor deposited (CVD) diamond thin-films are thus an attractive alternative to alumina, which is the material currently used in such devices. The quality of CVD diamond films is greatly improving yet their electrical conductivities are still several orders of magnitude higher than that of natural II-A diamond 1 . However this report and others 2' 3,4 have shown that the conductivity of the as grown diamond films may be greatly altered, without appreciable change in diamond quality as determined by Raman
Mat. Res. Soc. Symp. Proc. Vol. 270. 01992 Materials Research Society
414
4 spectroscopy, by various post deposition treatments. Landstrass and Ravi and Albin and Watkins 3 showed that the conductivity of both bulk and CVD diamond could be decreased by exposing the samples to a hydrogen plasma. Nakahata and coworkers 2 showed that this effect could be reversed and the resistivities increased to near that of natural diamond by either a post deposition anneal in oxygen or an oxygen containing plasma. Adams et al. 1 in a study of the electrical conductivity as a function of temperature, found that the as-grown films initially showed a high conductivity, but after heating to 1000 °C and then cooling to room temperature they became very resistive.
Some of the above researchers have speculated and it is the belief of the current authors that the low resistivities associated with the as-grown, or hydrogenated diamond films is due to hydrogen passivation of electrical traps at the surface or
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