Synthesis of carbon films by electrochemical etching of SiC with hydrofluoric acid in nonaqueous solvents

30.01.2014 19:41

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Synthesis of carbon films by electrochemical etching of SiC with hydrofluoric acid in nonaqueous solvents*

Jaganathan Senthilnathan^a, Chih-Chiang Weng^a, Wen-Ta Tsai^a, Yury Gogotsi^b^, Этот e-mail адрес защищен от спам-ботов, для его просмотра у Вас должен быть включен Javascript , Masahiro Yoshimura^a^,

^a Promotion Centre for Global Materials Research (PCGMR), Department of Material Science and Engineering, National Cheng Kung University, Tainan, Taiwan

^b Department of Materials Science and Engineering, and A. J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA

*In Press, Accepted Manuscript, Available online 24 January 2014, http://dx.doi.org/10.1016/j.carbon.2014.01.028

Abstract

Carbon films on SiC have many applications, ranging from tribology to electrical energy storage. Formation of epitaxial or heteroepitaxial layers of carbon on SiC by “soft solution process,” such as electro- or photochemical ones, are attractive for various fields of application, decreasing the energy consumption and making the process compatible with electronic device fabrication. We have demonstrated formation of a carbon layer on SiC ceramics by electrochemical etching in a nonaqueous electrolyte. The selective etching of Si from SiC in a single step reaction with hydrofluoric acid (HF) in different organic solvents has been carried out and the role of polarity, surface tension, density, and viscosity of the organic solvents in the formation of the carbon layer has been investigated. The solution of 1:4.6 ratio HF and ethanol at low current densities (10 and 20 mA/cm²) allows the best control over selective etching of Si forming amorphous and ordered carbon on the SiC surface. The presence of an intense G band of graphitic carbon in Raman spectra and high resolution transmission electron microscopy analysis indicate formation of ordered carbon on the surface of SiC. X-ray diffraction shows that the etching rate of α-SiC is much higher when compared to β-SiC.


Fig. 1. Raman spectra of unetched and etched SiC. (a) Unetched SiC (b) Etched at 10 mA/cm2, (c) Etched at 20 mA/cm2. Note: G = graphite band; D = disorder induced band; Inset shows the magnified carbon range of Raman spectra of samples etched at 10 and 20 mA/cm2 current densities		Fig. 2. SEM images of SiC etched with HF solutions in different solvents at fixed current densities 10 mA/cm2 (a) acetonitrile (b) acetone (c) water (d) isopropanol

Fig. 3. (a) Voltage vs time diagram for SiC etched with HF solution in (i) water, (ii) alcohol, (iii) acetone, (iv) isopropanol, and (v) acetonitrile. (b) Voltage vs time diagram for SiC etched with 5 molar HF solution in ethanol at (i) 2.5 mA/cm2, (ii) 5.0 mA/cm2, (iii) 10 mA/cm2, (iv) 20 mA/cm2, (v) 40 mA/cm2, (vi) 60 mA/cm2, and (vii) 80 mA/cm2		Fig. 4. SEM images of SiC etched in HF solution in ethanol at different current densities (a) unetched SiC (b) 5 mA/cm2 (c) 10 mA/cm2 (d) 20 mA/cm2 (e) 40 mA/cm2 (f) 60 mA/cm2. Note: Fig. 4 (d) inset shows a backscattered electron image of SiC etched at 20 mA/cm2. SEM micrographs of unetched SiC (g) and etched SiC (h) (at 20 mA/cm2)

Fig. 5. TEM images of SiC etched in HF-ethanol (20 mA/cm2) produced at 200 kV		Fig. 6. XRD patterns of unetched and etched SiC (a) unetched SiC (b) etched at 10 mA/cm2 (c) etched at 20 mA/cm2.

Fig. 7. Proposed reaction mechanism of SiC etching with HF in (5 M) ethanol solution at different current densities: (a) Si reacts with HF2- and follows the single-step mechanism at low current density (b) Si reacts with HF2- and follows single step mechanism at the optimum current density (c) Si reacts with OH- follows the two-step mechanism at high current density.