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RESEARCH/RESEARCHERS

Nitrogen-Doped Ultrananocrystalline Diamond Films Exhibit High RoomTemperature n-Type Conductivity Microwave plasma-enhanced chemical vapor deposition has been used by a group of scientists at the Argonne National Laboratory to produce ultrananocrystalline diamond thin films with n-type conductivity as high as 143 Ω-1cm-1, much higher than reported for microcrystalline diamond films. The use of typical microcrystalline diamond films doped with nitrogen in electronic devices is precluded by their low room-temperature conductivity. However, nitrogen doping of UNCD films improves the conductivity, as reported by S. Bhattacharyya, O. Auciello, D.M. Gruen, and their co-workers at Argonne in the September 3 issue of Applied Physics Letters. The researchers propose that grainboundary conduction, which increases with increased nitrogen concentration in the growth plasma, is responsible for the observed increase in overall conductivity. While the nitrogen incorporated into the substrate saturates, the grain boundaries continue to change as the nitrogen concentration in the plasma increases. The ultrananocrystalline diamond (UNCD) films with grain sizes between 2 nm and 5 nm were grown at 800°C on Si(100) and insulating silica using a gas mixture of CH4(1%)/Ar/N2(1 to 20%) with a total pressure of 100 torr and 800 W of microwave power. As the fraction of nitrogen in the plasma increased to 5%, the densities of the C2 and CN radicals increased substantially, as measured by absorption spectroscopy. The density of C2 radicals increased faster than that of the CN radicals at low nitrogen concentrations. Secondaryion mass spectroscopy (SIMS) showed a saturation of nitrogen in the diamond films at 2 × 1020 atoms/cm3 when the nitrogen concentration in the plasma reached 5%. However, the microstructure of the films at these low concentrations did not differ much from the microstructure of undoped films. For films obtained using nitrogen concentrations larger than 10%, the resultant grain size and grain-boundary width increased significantly to 12 nm and 1.5 nm, respectively. High-resolution imaging showed evidence that the grain boundaries were less dense than the grains, which the researchers interpret as evidence of increased sp2 bonding in the grain boundaries. The conductivity at room temperature increased with nitrogen in the plasma, from 0.016 Ω-1cm-1 (for 1% N2) to 143 Ω-1cm-1 (for 20% N2). The highest value reported for nitrogen-doped microcrystalline diamond films in the literature is 10-6 Ω-1cm-1. MRS BULLETIN/SEPTEMBER 2001

Results from Hall measurements for 10% and 20% nitrogen in the plasma show that carrier concentration and carrier mobility increased with nitrogen concentration (reaching 1.5 × 1020/cm3 and 10 cm2/Vs for 20% nitrogen in the plasma), exhibiting significantly higher values than other diamond films with n-type conductivity. Temperature-dependent conductivity measurements did not produce a linear Arrhenius plot, indicating evidence of multiple conduction mechanisms thermally activate