Structural and Optical Properties of Luminescent Silicon Carbonitride Thin Films

A series of silicon carbonitride (SiCN) thin films, a relatively unstudied material, were fabricated by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR PECVD) located at McMaster University. For comparison, a set each of silicon nitride and silicon carbide samples were also fabricated. To study the effect of introducing rare earth ions into SiCN host matrix, we synthesised a series of SiCN co-doped with Ce and Tb. This work investigates the influence of film composition and annealing conditions on the luminescence and structure of rare earth doped and undoped SiCN thin films through photoluminescence (PL), X-ray emission (XES), X-ray absorption near edge structure (XANES), and X-ray diffraction (XRD) experiments. The XES measurements were measured on Bl. 8.0.1 at the Advanced Light Source in Berkeley, USA. XANES has been carried out on the high resolution spherical grating monochromator (SGM) 11ID-1, soft X-ray microcharacterization beamline (SXRMB) 06B1-1, and variable line spacing plane grating monochromator (VLS PGM) beamline at the Canadian Light Source (CLS) synchrotron facility. PL emission was measured at the room temperature using a 17 mW 325 nm HeCd laser as the excitation source. For both doped and un-doped thin films, the XANES measurements were recorded at the N K-edge, C K-edge, and Si L_3,2-edge while total fluorescence yield (TFY), total electron yield (TEY), and partial fluorescence yield (PFY) were simultaneously measured. Both the XES and XANES spectra were calibrated using the same reference materials. For doped sample the XANES spectra of Ce N_5,4-edge and Ce M_5,4-edge are also measured. The XES measurements included the Ce N_5,4, Si L_3,2, C K, and N K spectra. XRD also was used to determine the local bonding coordination of rare earth ions when incorporated in amorphous as well as crystalline structures. Film composition was determined using Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analysis carried out at the Tandetron Laboratory at Western University. The thickness of film varying from 170 to 200 nm allowed the conduction of ion beam and X-ray absorption spectroscopy measurements without interference from the substrate. The variable angle spectroscopic (VASE) ellipsometer was employed to investigate the thickness and optical constants of the films.

The PL spectra of SiCN samples annealed at the lower temperatures consist of a broad peak ranging from 400 to 700 nm. Bright Ce³⁺ and Tb³⁺ related emission lines are observed for the doped films annealed in nitrogen at 1200°C for one hour. TEM analysis reveals the formation of randomly distributed nanocrystals in doped SiCN films following 1200°C annealing, while the undoped SiCN samples did not appear to show any crystalline structure at any temperature. Comparisons of the XANES and XES results indicated that in a the doped SiCN films, structural re-ordering occurs at the higher annealing temperatures leading to the formation of Si clustering along with a decrease in Si-N bonding. Conversely, little to no difference was observed in the electronic structure of undoped  SiCN films following annealing at different temperatures. This suggests that the electronic structure of the un-doped SiCN films may not change with respect to the Si and N atoms.