Boron-Doping Effects on Local Structures of Semiconducting Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Thin Films Fabricated via Coaxial Arc Plasma: an X-ray Absorption Spectroscopic Study
Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma possess a marked structural feature of diamond grains embedded in an amorphous carbon and a hydrogenated amorphous carbon matrix which are the largest constituents of the films. Since the amorphous nature yields much larger light absorption coefficients as well as a generation source of photo-induced carriers with UV rays, these films can be potential candidates for deep-UV photodetector applications. From some previous studies p-type conduction of the films has been realized by doping boron in experimental conditions. In addition, their optical and electrical characteristics were investigated previously. However, the bonding structures which largely affect the physical properties of the devices have not been investigated. In this work, near-edge X-ray absorption fine structure spectroscopy characterizations are carried out. The result reveals that a bonding state σ* C-B of diamond surfaces is formed preferentially and structural distortion is caused at an early stage of boron-doping. Further doping into the films lessens the amount of unsaturated bonds such as π* C=-C, which may be a cause of the device performance degradations. Our work suggests a fundamental case model of boron-doping effects on a local structure of the film.