Diamond films grown by plasma chemical vapor deposition techniques display a fairly low resistivity (∼106 Ω cm). Heat treating the films causes an increase in the resistivity by up to six orders of magnitude. The low resistivity of the as-grown films is postulated to be due to hydrogen passivation of traps in the films. Annealing causes dehydrogenation resulting in the electrical activation of deep traps with an attendant increase in the resistivity. This mechanism has been confirmed by an observed reduction of the resistivity of the heat-treated films when they are subjected to a plasma hydrogen treatment.
Subjecting natural diamond single crystals to the action of atomic hydrogen in a hydrogen plasma is shown to result in the passivation of interband states in the crystal resulting in a marked reduction in the resistivity to about 105 Ω cm from the expected high resistivity of∼1016 Ω cm. When the hydrogenated crystals are heat treated in a neutral ambient, the hydrogen can be expelled from the crystals, restoring the high resistivity. The behavior of natural diamond crystals, with respect to the effects of hydrogen, is shown to be similar to the behavior of diamond thin films synthesized by plasma-enhanced chemical vapor deposition techniques.
Morphological instabilities attending the high growth rate of diamond films are examined. Pertinent literature on morphological instabilities and microstructure evolution in vapor deposited films is reviewed and theoretical treatments related to the case of diamond growth are discussed. Diamond films of various thicknesses have been synthesized utilizing the combustion flame synthesis technique involving diamond growth rates of ∼1 μm/min. Films of thicknesses under 20 μm are found to be dense and the surface smoothness of such films is governed by facets on the individual crystallites that make up the film. Increasing film thicknesses, at high growth rates, results in extremely rough surfaces, the trapping of voids and discontinuities, and the incorporation of non-diamond phases in the growing film. These characteristics are typical of morphological instabilities when surface diffusion and re-evaporation processes are absent and instability is promoted by the high rate arrival of the appropriate species from the flame ambient to the surface. Factors contributing to morphological instabilities include competitive shadowing and nutrient starvation and growth anisotropy of the different crystallographic faces on individual diamond crystals. It is shown that surface temperature and the presence of oxidizing species in the flame ambient contribute to anisotropic growth of diamond crystals and hence to morphological instabilities in diamond films. An approach to avoiding these instabilities is briefly discussed.
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