Electronic Structure of Ultrananocrystalline Diamond Grain Boundaries

Scientific Achievement

Ultrananocrystalline diamond (UNCD) films grown from hydrogen-poor plasmas have grain sizes of 3-10 nm resulting in a large number of grain boundaries. We performed density-functional based tight-binding molecular dynamics calculations of high-energy high-angle twist (100) grain boundaries in diamond as a model for the UNCD grain boundaries. We found that about one-half of the carbons in the grain boundary are threefold coordinated and are responsible for states introduced into the band gap. This will help interpretation of a variety of experimental results. Simulations were also performed for N, Si and H impurities in (100) twist grain boundaries where substitution energies, optimized geometries and electronic structures were calculated. Substitution energies were found to be substantially lower for the grain boundaries compared to the bulk diamond crystal. Therefore, segregation of impurities in the grain boundaries can be expected. Nitrogen increases the number of three-coordinated carbons while hydrogen saturates dangling bonds. Also, a shift in the Fermi energy toward the conduction band of about 0.4 eV at larger nitrogen concentrations was calculated. Nitrogen-doped UNCD was observed experimentally to have conductivities approaching that of graphite and yet still be essentially phase-pure diamond. We propose that GB conduction involving carbon p-states in the GB is responsible for the high electrical conductivities in these films. These findings provide understanding of the unique electronic properties of ultrananocrystalline diamond films.

Significance

This is the first theoretical study of impurities in the high-energy high-angle grain boundaries. It is very difficult to experimentally determine the distribution of the impurities in this material and these simulations have established that nitrogen impurities will be in the grain boundaries. Experiments are underway to confirm this. The increase in density of carbon p states and upward shift of the Fermi level provide an explanation for the increase in conductivity found in nitrogen doped material. This mechanism is quite different from conventional doping by shallow donors and theoretical guidance is invaluable in experimental effort to increase doping efficiency. Further simulations such as these will be crucial to developing an understanding of the conductivity mechanism in nanocrystalline diamond and tailoring the electronic properties for specific applications in microelectronic device technology.

Performers

L.A. Curtiss, D. M. Gruen, J.A. Carlisle, M. Sternberg (grad. student), P. Zapol (postdoc), Th. Frauenheim. (University of Paderborn)

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