Neodymium-coated PECVD-designed silicon nanostructures: A comprehensive structural, morphological and spectroscopic investigation

Abstract

We report the optical, structural, and spectroscopic properties of thermally evaporated neodymium oxide (Nd2O3) thin films deposietd on a nanostructured silicon (Si-ns) matrix. The Si-ns thin films were grown by depositing silicon nitride (SiN) thin films using plasma enhanced chemical vapour deposition (PECVD). The gas ratio R=NH3/SiH4 was varied between 2.5 and 5 to obtain different stoichiometries and Si amounts in the SiN layers. Two sets of samples were analysed. The first set comprised samples prepared with gas ratios R=2.5, 3, 4, and 5 and annealed at 900°C for 15 min. For the second set, R was 2 and 4 for the samples annealed at 1100°C for 2 h. Both sets were thermally treated under nitrogen (N2) ambient. The nanocrystalline formation and layer thickness were assessed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). UV-Vis absorption spectra revealed that the reflectance was lower for the Nd2O3-coated samples than for to the uncoated ones. This trend remained unchanged after the annealing. Optical band gap (Eg) was found to increase with increasing R (2.5, 3, and 4) with Eg = 1.28, 1.31 and 1.55 eV respectively for annealed samples. Refinement of nanostructures size was observed with higher Eg values. The presence of Nd, Si, oxygen (O) and phosphorus (P) was confirmed by energy dispersive spectroscopy (EDS) and secondary ion mass spectrometry (SIMS) data. Moreover, X-ray diffraction (XRD) and Raman spectroscopy were employed to determine and confirm amorphous silicon (a-Si), crystalline silicon (c-Si), Nd2O3 and SiN phases present in the Nd2O3-SiN bilayers. Crystallite sizes for Nd and Si found to be 40.7 and 6.82 nm respectively. After the annealing process, a Raman shift toward lower wavenumbers was observed for Si peak. XPS data revealed the formation of Nd2O3 thin films with a Nd-O bonding incorporating trivalent Nd ions (Nd3+).