We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by CH 4 /H 2 plasma chemical vapor deposition.Diamond thin films with a thickness below 350 nm and with boron concentration ranging from 10 17 cm -3 to 10 21 cm -3 have been investigated. UV Raman spectroscopy and AFM have been used to assess the quality and morphology of the diamond films. Hall effect measurements confirmed the expected p-type conductivity. At room temperature, the conductivity varies from 1.5x10 -8 Ω -1 cm -1 for a non-intentionally doped film up to 76 Ω -1 cm -1 for a heavily B-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 cm 2 V -1 s -1 down to 0.2 cm 2 V -1 s -1 . For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature-dependent for films with the highest boron doping, and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2K. The critical boron concentration for the metal-insulator transition is in the range from 2x10 20 cm -3 up to 3x10 20 cm -3 . We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence band transport dominates at low boron concentration and high temperatures, 2 whereas hopping between boron acceptors is the dominant transport mechanism for boron doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 -1.0 eV, where both Boron impurities and the sp 2 carbon phase in the grain boundaries govern the optical absorption.

We studied the transport properties of highly boron-doped nanocrystalline diamond thin films at temperatures down to 50 mK. The system undergoes a doping-induced metal-insulator transition with an interplay between intergranular conductance g and intragranular conductance g 0 , as expected for a granular system. The conduction mechanism in the case of the low-conductivity films close to the metal-insulator transition has a temperature dependence similar to Efros-Shklovskii type of hopping. On the metallic side of the transition, in the normal state, a logarithmic temperature dependence of the conductivity is observed, as expected for a metallic granular system. Metallic samples far away from the transition show similarities to heavily borondoped single-crystal diamond. Close to the transition, the behavior is richer. Global phase coherence leads in both cases to superconductivity ͑also checked by ac susceptibility͒, but a peak in the low-temperature magnetoresistance measurements occurs for samples close to the transition. Corrections to the conductance according to superconducting fluctuations account for this negative magnetoresistance.

Because of its large band gap and variety of stable surface terminations, diamond is a suitable material to study the optical and electronic properties of organic films. Optical absorption and photocurrent experiments with pentacene on hydrogen-and oxygen-terminated diamond surfaces reveal a strong, polarization-dependent photoresponse of pentacene films. The diamond surface reconstruction as well as the molecule-surface interactions influence the morphology and the molecular structure of the films, causing the associated polarization dependence. On oxygen-terminated diamond, the pentacene thin-film phase typical for electronically inert substrates such as SiO 2 is formed. On hydrogen-terminated diamond, on the other hand, a three-dimensional growth mode of a filamentlike pentacene morphology is observed by atomic force microscopy, with pentacene molecules arranged with their long molecular axis oriented along the hydrogen-terminated diamond surface, as confirmed by x-ray diffraction. Furthermore, on hydrogen-terminated single crystalline diamond, the b axis of the pentacene unit cell is found to orient preferentially perpendicular to the surface, in agreement with photocurrent and optical-absorption experiments.

The low temperature electronic transport of highly boron-doped nanocrystalline diamond films is studied down to 300 mK. The films show superconducting properties with critical temperatures T c up to 2.1 K. The metal-insulator transition and superconductivity is driven by the dopant concentration and greatly influenced by the granularity in this system, as compared to highly boron-doped single crystal diamond. The critical boron concentration for the metal-insulator transition lies in the range from 2.3 × 10 20 cm −3 up to 2.9×10 20 cm −3 , as determined from transport measurements at low temperatures. Insulating nanocrystalline samples follow an Efros-Shklovskii type of temperature dependence for the conductivity up to room temperature, in contrast to Mott variable range hopping in the case of insulating single crystal diamond close to the metalinsulator transition.The electronic transport in the metallic samples not only depends on the properties of the grains (highly borondoped single crystal diamond) alone, but also on the intergranular coupling between the grains. The Josephson coupling between the grains plays an important role for the superconductivity in this system, leading to a superconducting transition with global phase coherence at sufficiently low temperatures. Metallic nanocrystalline samples show similarities to highly boron-doped single crystal diamond. However, metallic samples close to the metal-insulator transition show a more rich behaviour. A peak in the low-temperature magnetoresistance measurements for samples close to the transition is explained due to corrections to the conductance according to superconducting fluctuations.