Sixfold ring clustering in $s{p}^2$-dominated carbon and carbon nitride thin films: A Raman spectroscopy study

The atomic arrangement in $s{p}^2$-dominated carbon (C) and carbon nitride $\left({\mathrm{CN}}_x\right)$ thin films has been studied by Raman spectroscopy as a function of substrate temperature and, in the case of ${\mathrm{CN}}_x$, different N incorporation routes (growth methods). In this way, materials composing graphitelike, fullerenelike (FL), and paracyanogenlike structures have been compared. The results show that each type of arrangement results in a characteristic set of the Raman spectra parameters, which describe the degree of aromatic clustering, bond length, and angle distortion and order in sixfold structures. In the case of C films, the atomic structure evolves with substrate temperature from a disordered network to nanocrystalline planar graphitic configurations, with a progressive promotion in size and ordering of sixfold ring clusters. Nitrogen incorporation favors the promotion of sixfold rings in highly disordered networks produced at low temperatures, but precludes the formation of extended graphiticlike clusters at elevated substrate temperatures $(>700\mathrm{K})$. In the latter case, N introduces a high degree of disorder in sixfold ring clusters and enhances the formation of a FL microstructure. The formation and growth of aromatic clusters are discussed in terms of substrate temperature, N incorporation, growth rate, film-forming sources, and concurrent bombardment by hyperthermal particles during growth.

Figure 1

Normalized Raman spectra of films grown by IBS, DIBD (a,b), MS (c), carbon evaporation, and IBAE (d). IBS ${\mathrm{CN}}_x$ films were grown at RT (a) and $450°\mathrm{C}$ (b) for different ${\mathrm{N}}_2$ mass flow ratios. DIBD ${\mathrm{CN}}_x$ film was grown at $450°\mathrm{C}$ with pure ${\mathrm{N}}_2$ sputtering and assisting beams (b). MS ${\mathrm{CN}}_x$ films were grown at different temperatures and ${\mathrm{N}}_2$ ratios in the sputtering gas (c). Evaporated $a$-C film was grown at RT, while IBAE ${\mathrm{CN}}_x$ films were grown with assisting ${\mathrm{N}}_2^{+}+{\mathrm{N}}^{+}$ ion current density of $71\mathrm{\mu }\mathrm{A}{\mathrm{cm}}^{-2}$ at RT, $300$, $400$, and $450°\mathrm{C}$ (d).

Figure 2

Position (a), width ${\Gamma }_G$ (b), and coupling parameter ${\Gamma }_G∕q$ (c) of the $G$ peak, width ${\Gamma }_D$ of the $D$ peak (d), and cluster diameter $L_a$ (e) vs N concentration for the C and ${\mathrm{CN}}_x$ films grown by IBS at different temperatures. In (a) and (b), lines represent linear fits, while in (c)–(e), lines are guiding the eye only. The error bars represent the 95% confidence limits of the fittings.

Figure 3

${\Gamma }_G$ vs $G$ peak position (a) and ${\Gamma }_D$ vs ${\Gamma }_G∕q$ (b). Dashed lines represent linear fits.

Figure 4

$G$ peak position (a) and coupling parameter ${\Gamma }_G∕q$ (b) vs $I_D∕I_G$. Lines are guiding the eye only.