  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 {\sigma}* 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 {\pi}*
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.
