Evaluation of the sp 3 /sp 2 ratio of DLC films by RF-PECVD and its quantitative relationship with optical band gap
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Carbon Letters https://doi.org/10.1007/s42823-020-00199-x
ORIGINAL ARTICLE
Evaluation of the sp3/sp2 ratio of DLC films by RF‑PECVD and its quantitative relationship with optical band gap Hongxun Sheng1 · Wenwen Xiong1 · Songsheng Zheng1 · Chao Chen1 · Song He1 · Qijin Cheng1 Received: 20 September 2020 / Revised: 15 October 2020 / Accepted: 17 October 2020 © Korean Carbon Society 2020
Abstract DLC has been attractive as semiconductor materials for solar cell due to its biological friendliness, flexible microstructures, and especially its tunable band gap. In order to fabricate high-efficiency multiband gap solar cell, it is important to control the sp3/ sp2 bonds ratio of DLC film corresponding to optical band gap (Eg). There are many references reporting the relations among the fabrication conditions, Eg, sp3/sp2, and I D/IG. However, a more comprehensive database is needed for controllable fabrication. Especially, the quantitative relationship of sp3/sp2 ratio to Eg of DLC film by PECVD is unclear. In this paper, 36 sets of DLC films were fabricated by RF-PECVD. Characterization methods of XPS, Raman spectroscopy, and IR absorption have been used to determine the sp3/sp2 ratio of DLC films. UV/visible light absorption method has applied to evaluate Eg. The Eg obtained is in the range 1.45–3.0 eV. Our results agree well with the references. The XPS spectra gives a linear relationship as Eg = − 0.161 (± 0.136) + 26.095 (± 1.704) · {sp3(XPS)/sp2}, the Raman spectra shows a linear function that Eg = 1.327 (± 0.046) + 0.428 (± 0.036) · (ID/IG), as well as the FTIR analysis demonstrates that Eg = − 0.492 (± 0.093) + 0.464 (± 0.044) · {sp3(FTIR)/sp2}. Keywords Diamond-like-carbon film · Sp3/sp2 ratio · Optical band gap · RF-PECVD
1 Introduction Diamond-like carbon (DLC) is a mixture of sp2, sp3, and sometimes sp1 hybridized carbon in a disordered network [1–3]. DLC films are known to have superior properties including high hardness, wide range of optical transparency, high thermal conductivity and good chemical inertness. Thus DLC coatings have been widely applied on magnetic storage disks [4], cutting tools [5], mechanical parts [6], man-made biomedical parts [7], electrochemical electrode material [8], cold-cathode material [9] and optical devices [10, 11]. Various methods have also been reported on synthesizing DLC films, such as ion-beam deposition [12], ionized evaporation [13], pulsed laser deposition [14], sputtering [15], hot filament chemical vapor deposition (HFCVD) [16], * Songsheng Zheng [email protected] * Chao Chen [email protected] 1
College of Energy, Xiamen University, Xiamen 361005, People’s Republic of China
microwave plasma-enhanced CVD (MW-PECVD) [17], DCPECVD [18, 19] and RF-PECVD [20–22]. Among these methods, RF-PECVD is widely used, because it allows low temperature and uniform coating on large area substrates with different shapes and sizes [22]. Due to the lack of long-range order and a variety of possible structures, amorphous carbon is difficult to be characterized by
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