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The interactions between hydrogen-rich plasmas and nondiamond phases contained in nanocrystalline diamond films have been examined to explore a possibility of reducing the amount of nondiamond phases. Diamond films with average grain sizes of 15–25 nm were deposited on silicon in microwave Ar-rich/H2/CH4 plasmas. The 1140 cm-1 peak in the visible Raman spectra for the films was used as a signature of trans-polyacetylene (TPA) segments. The amount of TPA relative to amorphous carbon increased with increasing H2/CH4 ratio in the Ar-rich/H2/CH4 plasmas or simply by exposing the as-grown film to hydrogen plasmas after deposition. However, the Raman peak of diamond, which was superimposed by the D peak of amorphous carbon, was little enhanced by exposure to these hydrogen-rich plasmas. It was also found that an increase in the relative amount of TPA was not always accompanied by an increase in hydrogen content in the films. The results were interpreted along the possible mechanisms of TPA formation, which are correlated to the content of hydrogen in the gas phase as well as in the solid phase. [ABSTRACT FROM AUTHOR]

Carrier transport mechanism in n-type nanocrystalline diamond (NCD)/p-type Si heterojunction diodes prepared by microwave plasma-enhanced chemical vapor deposition is studied in a temperature range of room temperature to 473 K. Current-voltage measurements show at most three orders of magnitude of rectification at ±20 V of biasing and room temperature, depending upon the deposition temperature. The current-voltage characteristics are described with the high ideality factor and the low current injection barrier due to the disordered NCD/Si heterojunction interface, mainly associated with grain boundaries in the NCD film. The Arrhenius plots of the currents reveal that the thermal excitation of carriers limits the conduction, and the apparent activation energy decreases drastically upon the bias voltage change from reverse to forward. The current injection mechanism at the interface is explained along the predicted energy-band diagrams, such that the major carriers from the defect states of the NCD are injected into the conduction band of the Si by forward biasing. [ABSTRACT FROM AUTHOR]

Highly conductive, nitrogen-incorporated nanocrystalline diamond films with quasimetallic character emit electrons at low turn-on fields (∼3 V μm-1). These films exhibit stronger delocalization of carriers, indicative of smaller energy separation between the defect bands in the band gap. We show that the emission level derived from the measured emission characteristic and electron affinity shifts upward (up to a few eV) with increasing the film conductivity, thereby decreasing the effective potential barrier height for the emission. This is attributed to higher probabilities of electron injection into upper defect levels during the transport process, originating from internal band bending and increasing band continuity. [ABSTRACT FROM AUTHOR]

The reverse leakage current in n-type nanodiamond film/p-type silicon wafer heterojunction diodes is examined as a function of the film conductivity. We observe that as the delocalization of carriers in the films becomes stronger, the leakage current becomes larger (from 10-3 to 101 A cm-2). The major leakage source is attributed to the minority carriers from the π defect bands introduced by sp2 carbon. Both a current injection barrier and a dielectric barrier at the interface are found to be small (∼0.6 eV). The simultaneous reduction in leakage current and turn-on voltage for low-loss diodes needs optimization of the interfacial valence band connection. [ABSTRACT FROM AUTHOR]

A route to high-purity nanocrystalline diamond films from C2 dimers and related mechanisms have been investigated by enhancing C2 growth chemistry in Ar-rich microwave plasmas. Efficient C2 production by direct dissociation from acetylene causes the micro- to nanocrystal transition with a low threshold Ar concentration of ∼70% and produces films of ∼20 nm grains with a distinct visible-Raman peak of diamond. C2 grows nanodiamond on diamond surfaces but rarely initiates nucleation on foreign surfaces. The phase purity can be improved by increasing the dominance of nanodiamond growth from C2 over nondiamond growth from CHx(x=0–3) and large radicals. [ABSTRACT FROM AUTHOR]