Formation of β-C₃N₄ crystals at low temperature

Carbon nitride films with β-C₃N₄ crystals of 200 nm grain size were grown on Si (100) substrates using magnetron sputtering. Reactive deposition was achieved using a graphite target in an argon/nitrogen plasma at room temperature. These films were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. Micro level β-C₃N₄ crystal grains were observed with TEM. According to calculations from electron diffraction pattern, these crystalline structures were in a good agreement with hypothetical b-C₃N₄ structure. AFM measurement also indicated the grain size was around 200 nm and the carbon nitride films had low surface roughness. From XPS data, maximum N/C ratio of 0.5 was achieved in the films. XPS spectra of the films typically showed three peaks in the C 1s core level spectrum (centered at 284.6, 285.9, and 287.2 eV) and two peaks in the N 1s core level spectrum (centered at 398.7 and 400.2 eV). This indicates that there are two types of C-N bonds; N is bonded to sp²- or sp³-coordinated C atoms in the as-deposited films. FTIR spectra showed three absorption bands in the range of 1000-3000 cm^-1. The absorption band around 2367 cm^-1 can be attributed to CN nitrile bond. The absorption bands around 1559 and 1201 cm^-1 demonstrate the existence of CN (sp²) and C-N (sp³) bonds. Analysis of Raman spectrum further demonstrated the coexistence of sp, sp², and sp³ bonds. All of XPS, FTIR, and Raman measurements showed the presence of fourfold coordinated β-C₃N₄ crystals in the films, which is in good agreement with TEM and electron diffraction results.