Synthesis of tin-incorporated nanocomposite diamond like carbon films by plasma enhanced chemical vapor deposition and their characterizationNitrogen-incorporated carbon nanowalls are prepared by microwave plasma-enhanced chemical vapor deposition using acetylene and methane. n-type conduction in the nanowalls is confirmed by Hall-and Seebeck-effect measurements. We show that increasing the amount of C 2 radicals by adding Ar enables catalyst-free growth of nanowalls at a high rate up to about 1 m / min and reduces the deposition temperature ͑T D ͒ down to around 650°C. A substrate pretreatment using diamond powder results in a composite of nanowalls and nanocrystalline diamond films, suggesting that the nanowall growth is limited by gas-phase conditions rather than surface conditions. The low conductivity nanowalls for low T D exhibit thermal activation in the Arrhenius plot, indicative of semiconducting conduction, while the high conductivity nanowalls for high T D are almost temperature independent, indicative of quasimetallic conduction. The high conductivity is attributed to a global increase in the sp 2 cluster size and crystallinity, which is responsible for increasing delocalization of defect states associated with bonding and, hence, quasimetallic character.

Structural and electrical conduction properties of nitrogen-doped nanocrystalline diamond films are studied as a function of deposition temperature (TD) in a microwave Ar-rich/CH4 plasma with 30%N2 addition. Hall- and Seebeck-effect measurements confirm n-type conduction for TD above 1100 K. For TD from 1100 and 1220 K, the electron concentration increases up to 1020 cm−3 and the electron mobility is in the range of 4–8 cm2 V−1 s−1. For TD above 1250 K, the mobility decreases to ∼1 cm2 V−1 s−1. Low conductivity films deposited at low TD exhibit semiconductorlike thermal activation in the Arrhenius plots, while high conductivity films deposited at high TD are almost temperature independent, indicative of quasimetallic conduction. The nitrogen concentration in the films is about 0.3 at. %, independent of TD. As TD is increased, the sp2 content and order increase. This is responsible for the appearance of midgap states, their delocalization, and the larger distance between diamond grains. The high conductivity at high TD is due to the amount and crystallinity of sp2 carbon, rather than the nitrogen concentration.

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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.

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 ∼20nm 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.