Characterization of Aggregated Carbon Compounds at SiO2/SiC Interface after Plasma Oxidation at Near Room Temperature

Abstract

Si sublimation from a silicon carbide (SiC) surface at elevated temperatures is a convenient and simple method of forming graphene [1]. The increase in carbon concentration at the monolayer scale on SiC prior to graphene growth is the key to obtaining graphene with reduced pit density. There have been some attempts to achieve this, one example of which is the use of vacuum evaporation from a solid carbon source [2, 3]. We have proposed a novel chemical route to enrich the carbon concentration on SiC. Namely, we have found that aggregated carbon compounds with a few monolayers are formed when an initial hexagonal SiC surface is treated by plasma oxidation at near room temperature and then the oxide layer (SiO2) with a thickness of ~10 nm is stripped off by HF etching [4, 5]. No additional carbon clusters were formed in the conventional thermal oxidation of SiC at 1000°C. Neither the composition of the aggregated compounds nor the mechanism of the generation of such carbon species by the plasma-assisted procedure is clear, however. In this study, we reveal that, after the plasma oxidation followed by wet etching, the SiC surface is more hydrophobic than the initial SiC surface terminated by hydroxyls. Taking the electronegativities of Si, C and O atoms into account, this result indicates the formation of C-O and C-C, which is confirmed by the chemical-shift analyses of O1s and C1s spectra taken by X-ray photoelectron spectroscopy. In thermal oxidation, it is widely accepted that the reaction (2SiC+3O2 -> 2SiO2+2CO) forms a SiO2 layer whereas CO molecules diffuse into the oxide and are finally emitted in the gas phase. However, it has also been reported that CO molecules can diffuse a high-quality SiO2 film at temperatures higher than 900°C [6]. It is reasonable to suppose that, in the case of plasma oxidation at near room temperature, CO molecules reside at the SiO2/SiC interface and form carbon compounds. In addition, a consideration based on thermodynamics implies the direct formation of solid carbon clusters during oxidation at low temperatures by the reaction SiC+O2 -> SiO2+C(s) [7]. These growth kinetics of SiO2 on SiC well explain the obtained results described in the previous paragraph. Such a carbon residue at the SiO2/SiC interface can degrade the performance of metal-oxide-semiconductor (SiC) devices. But it may serve as an additional carbon source for epitaxial graphene growth on SiC. [1] C. Berger et al., J. Phys. Chem. B, 108, 19912 (2004). [2] A. Al-Temimy et al., Appl. Phys. Lett., 95, 231907 (2009). [3] J. Park et al., Adv. Mater., 22, 4140 (2010). [4] N. Saito, K. Arima et al., Carbon, 80, 440-445 (2014). [5] N. Saito, K. Arima et al., ECS Transactions, 64, 17, 23 (2014). [6] O.H. Krafcsik et al., Jpn. J. Appl. Phys. 40, 2197 (2001). [7] K. Kita et al., ECS Transactions, 64, 8, 23 (2014).