Shinya Ohmagari

Near-Edge X-ray Absorption Fine-Structure, X-ray Photoemission, and Fourier Transform Infrared Spectroscopies of Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films

The chemical bonding structure of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films prepared by pulsed laser deposition was examined by near-edge X-ray absorption fine-structure (NEXAFS), X-ray photoemission, and Fourier transform infrared (FTIR) spectroscopies. An intense sp3-CH peak was observed in the FTIR spectrum. This implies that the sp3-CH peak originates from the grain boundaries between UNCD crystallites, wherein dangling bonds are terminated with hydrogen atoms. The presence of an intense σ*C–C peak in the NEXAFS spectrum and a narrow sp3 peak in the photoemission spectrum was specific to UNCD/a-C:H films; these confirm the existence of UNCD crystallites.

Near-edge X-ray absorption fine structure of ultrananocrystalline diamond/hydrogenated amorphous carbon films prepared by pulsed laser deposition

The atomic bonding configuration of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films prepared by pulsed laser ablation of graphite in a hydrogen atmosphere was examined by near-edge X-ray absorption fine structure spectroscopy. The measured spectra were decomposed with simple component spectra, and they were analyzed in detail. As compared to the a-C:H films deposited at room substrate-temperature, the UNCD/a-C:H and nonhydrogenated amorphous carbon (a-C) films deposited at a substrate-temperature of 550°C exhibited enhanced π* and σ*C≡C peaks. At the elevated substrate-temperature, the π* and σ*C≡C bonds formation is enhanced while the σ*C-H and σ*C-C bonds formation is suppressed. The UNCD/a-C:H film showed a larger σ*C-C peak than the a-C film deposited at the same elevated substratete-temperature in vacuum. We believe that the intense σ*C-C peak is evidently responsible for UNCD crystallites existence in the film.

Heterojunction Diodes Comprising p-Type Ultrananocrystalline Diamond Films Prepared by Coaxial Arc Plasma Deposition and n-Type Silicon Substrates

Heterojunction diodes, which comprise boron-doped p-type ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films prepared by coaxial arc plasma deposition and n-type Si substrates, were electrically studied. The current–voltage characteristics showed a typical rectification action. An ideality factor of 3.7 in the forward-current implies that carrier transport is accompanied by some processes such as tunneling in addition to the generation–recombination process. From the capacitance–voltage measurements, the built-in potential was estimated to be approximately 0.6 eV, which is in agreement with that in a band diagram prepared on the assumption that carriers are transported in an a-C:H matrix in UNCD/a-C:H. Photodetection for 254 nm monochromatic light, which is predominantly attributable to photocurrents generated in UNCD grains, was evidently confirmed in heterojunctions. Since dangling bonds are detectable by electron spin resonance spectroscopy, their control might be an important key for improving the rectifying action and photodetection performance.

Carrier transport and photodetection in heterojunction photodiodes comprising n-type silicon and p-type ultrananocrystalline diamond/hydrogenated amorphous carbon composite films

Carrier transport and photodetection in heterojunction photodiodes comprising n-type Si substrates and p-type B-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were investigated. Their transport model was discussed mainly on the basis of electrical measurements. It was revealed that an a-C:H matrix in UNCD/a-C:H would predominantly be responsible for carrier transportation in the photodiodes. The photodiodes exhibited high external quantum efficiencies of 72 and 23% under 254 and 365 nm UV illuminations, respectively. These superior responses might be attributable to the photocarrier generation in UNCD grains accompanied by an efficient carrier transport to the a-C:H matrix.

Boron-Induced Dramatically Enhanced Growth of Diamond Grains in Nanocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films Deposited by Coaxial Arc Plasma Deposition

Boron-doped nanocrystalline diamond/hydrogenated amorphous carbon composite films were prepared by coaxial arc plasma deposition. The X-ray diffraction measurement exhibited that the diamond grain size is remarkably increased from 2 nm (undoped films) to 82 nm and the lattices of the grains are dilated accompanied by the incorporation of boron atoms into the lattices. The near-edge X-ray absorption fine-structure showed a weak exciton peak of diamond due to the enlarged grains. The enhanced growth mechanism is discussed on the basis of a defect-induced diamond growth model.

Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films for Metal-Semiconductor-Metal Photodetector

Photoconduction of p-type ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were investigated in metal-semiconductor-metal (MSM) structure. The Cu contacts exhibited the highest contact resistance among examined metals and are suitable for photodetection in a MSM structure because of the high contact resistance being able to suppress dark current. Photoconduction spectra showed clear response in the ultraviolet (UV) and visible wavelength ranges, which might be attributable to UNCD grains and grain boundaries, respectively. At wavelength of 200 nm, the photocurrent and external quantum efficiency reached 80 mA/W and 44%, respectively. We firstly demonstrated the photoresponse in single-layered p-type UNCD/a-C:H films.