Heterojunction diodes are constructed by growing n-type (nitrogen-doped) ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films onto p-type Si substrates in nitrogen and hydrogen mixed-gases by coaxial arc plasma deposition. The fabricated heterojunctions are analyzed regarding their film morphology and electrical properties. The complex structure of UNCD/a-C:H films, which consists of nano-sized-sp3 diamond grains and sp2-grain boundaries (GBs), makes it difficult to separate their contribution to electrical conductivity of the films using conventional characterization methods. In this paper we characterize the n-type UNCD/p-type Si heterojunction diode by employing impedance spectroscopy method, which can isolate the differing components that contribute to the overall conductivity of the film. The impedance spectroscopy is measured in the frequency range of 100 Hz to 2 MHz, with AC small signal superimpose on DC bias voltage in the range of 0–1 V. The influence of the bias on UNCD grains and GBs contribution to resistance and capacitance of UNCD/a-C:H film and n-UNCD/p-Si heterojunction is investigated by equivalent circuit model using fitting of the impedance data. The results revealed that the electrical conductivity is mainly controlled by the GBs rather than the UNCD grains. Furthermore, the extracted value of dielectric constant of N-doped UNCD/a-C:H film is found to be comparable with that of microcrystalline diamond, which indicates the capacitance contribution in the device is mainly originated from the UNCD grains. This study demonstrates capability of impedance spectroscopy to provide an obvious separated contribution of sp3 and sp2 bonded carbons to the electrical conductivity in their coexistence materials.

n-Type (phosphorus-doped) diamond is a promising material for diamond-based electronic devices. However, realizing good ohmic contacts for phosphorus-doped diamonds limits their applications. Thus, the search for non-conventional ohmic contacts has become a hot topic for many researchers. In this work, nanocarbon ohmic electrodes with enhanced carrier collection efficiency were deposited by coaxial arc plasma deposition. The fabricated nanocarbon ohmic electrodes were extensively examined in terms of specific contact resistance and corrosion resistance. The circular transmission line model theory was used to estimate the charge collection efficiency of the nanocarbon ohmic electrodes in terms of specific contact resistance at a specific voltage range (5–10 V); they exhibited a specific contact resistance of 1 × 10−3 Ωcm2. The result revealed one order reduction in the specific contact resistance and, consequently, a potential drop at the diamond/electrode interface compared to the conventional Ti electrodes. Moreover, the fabricated nanocarbon electrodes exhibited high mechanical adhesion and chemical inertness over repeated acid treatments. In device applications, the nanocarbon electrodes were evaluated for Ni/n-type diamond Schottky diodes, and they exhibited nearly one order enhancement in the rectification ratio and a fast charge collection at lower biasing voltages.

Ultrananocrystalline diamond/hydrogenated amorphous carbon films were synthesized by coaxial arc plasma deposition. The morphological and structural evolutions of the films driven by laser irradiation (ArF, 193 nm, 20 ns) are examined at different laser energies (0.4–1 J cm−2). Up on laser irradiation, the films revealed hydrogen effusions accompanied by the transformation of sp3 into sp2-hybridization carbon. However, the film irradiated at 0.8 J cm−2 exhibited higher sp3 fractions, which can be attributed to that: a specific value of laser energy density (0.8 J cm−2) can provide a suitable amount of heating that is enough to quench the film and increase the sp3 contents.

A laser-induced doping method was employed to incorporate phosphorus into an insulating monocrystalline diamond at ambient temperature and pressure conditions. Pulsed laser beams with nanosecond duration (20 ns) were irradiated on the diamond substrate immersed in a phosphoric acid liquid, in turns, and a thin conductive layer was formed on its surface. Phosphorus incorporation in the depth range of 40–50 nm below the irradiated surface was confirmed by secondary ion mass spectroscopy (SIMS). Electrically, the irradiated areas exhibited ohmic contacts even with tungsten prober heads at room temperature, where the electrical resistivity of irradiated areas was greatly decreased compared to the original surface. The temperature dependence of the electrical conductivity implies that the surface layer is semiconducting with activation energies ranging between 0.2 eV and 54 meV depending on irradiation conditions. Since after laser treatment no carbon or graphitic phases other than diamond is found (the D and G Raman peaks are barely observed), the incorporation of phosphorus is the main origin of the enhanced conductivity. It was demonstrated that the proposed technique is applicable to diamond as a new ex situ doping method for introducing impurities into a solid in a precise and well-controlled manner, especially with electronic technology targeting of smaller devices and shallower junctions.