Evidence of hydrogen termination at grain boundaries in ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma

Abstract

Ultranonocrystalline diamond / hydrogenated amorphous carbon composite thin films consist of three different components; ultrananocrystalline diamond crystallites, hydrogenated amorphous carbon, and grain boundaries between them. Since grain boundaries contain a lot of dangling bonds and unsaturated bonds, they would be a cause of carrier trap center degrading device performance in possible applications such as UV photo-detectors. We experimentally demonstrate hydrogen atoms preferentially incorporate at grain boundaries and terminate dangling bonds by means of several spectroscopic techniques. XPS measurements cannot detect quantitative transitions of sp 2 -and sp 3 -hybridized carbons in the films, resulting in 55-59 % of sp 3 contents. On the other hand, FT-IR and NEXAFS exhibit some variations of the amounts of certain carbon hybridization for sure. The former confirms the transformation from sp 2 to sp 3 hydrocarbons by ~10 % by additional hydrogenation, and the latter represents chemical configuration changes from π* C ≡ C and π* C = C to σ* C − H . These results can be an evidence of localized hydrogen at grain boundaries, which plays a part in terminating dangling bonds and unsaturated bonds, and hydrogenation can be an effective tool of an enhancement of photovoltaic performance in the above sensing applications.

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

“…Afterwards, arctangent function steps are applied as an approximation of the largest absorption located at ∼291 eV that denotes the ionization potential peculiar to diamonds. As it has been already studied on our recent paper, the NEX-AFS C K-edge can be sorted out on the basis of the derivations; specifically, the areas larger than ∼291 eV come from UNCD grains, an a-C and a a-C:H matrices [31]. It should be noted here that the threshold of this absorption differs depending on the morphologies of any carbon-related materials (see, for instance, [45].…”

“…The correlation with oxygen impurities is unclear at present and should be investigated by conducting measurements on NEXAFS O K-edge. The amount of π * C=C in the films increases approximately accompanied [31]. These spectra were obtained by total electron yield technique, different from the referred work, and processed in a proper manner; therefore, note that both figures cannot be compared simply with each other.…”

“…One of the most noticeable appearance changes are illustrated by the areas of Y and Z in figure 1(b). To study the details of these areas, the processed spectra were decomposed to gaussian functions that represent x-ray absorption in each carbon hybridization by use of analytical fitting program Athena [46] based on our recent work [31] and they are shown in figure 2. Specifically, backgrounds of the obtained raw data were subtracted at the first setout, and the pre-and post-edges of processed data were normalized.…”

“…The decomposed spectrum deriving from carbon-oxygen bonds is split into two gaussian functions positioned at 289.3 and 290.3 eV which denote π * C=O and π * COOH, respectively, from that in reference to our latest work [31]. The area around here is highly convoluted with several absorptions such as an ionization potential of UNCD crystallites and a core electron excitation to σ * C=C states as well as that denotes a transition of π * C=O and π * COOH.…”

“…In this work, we report on the boron-doping effects by use of near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, which is functional in quantitative analysis of local structures, studying the boron-and carbon-related bonds since C K-edge obtained by this technique is able to decern any bonding states of carbon. Recently, we have investigated the hydrogenation effect similarly by use of NEXAFS spectroscopy as well as x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy, and demonstrated an effectiveness of hydrogenation induced with a technological development of the film fabrication process (see [30] for the detail) with its detail mechanism by studying comprehensively the chemical bonding state changes accompanied by the amounts of hydrogen content in the films [31]. Results obtained in this work reveal that the films would own double-step transitions of doping toward the bonding states dependent on their amounts and a certain bonding state related to boron surely serves to create electrical levels between the optical gaps closer to the top of valence bands, which leads to a substantial improvement of the electrical conductivities, by correlating with the electrical characteristics investigated previously.…”