Structural and light emitting properties of silicon-rich silicon nitride films grown by plasma enhanced-chemical vapor deposition

Si-rich Silicon nitride films fabricated by plasma enhanced chemical vapor deposition (PECVD) on p-type silicon substrates were investigated by means of photoluminescence (PL) and X-ray diffraction (XRD) methods. The film stoichiometry was controlled via varying the NH₃/SiH₄ ratio (R) in the range R=0.56-1.0. Thermal annealing at 1100 °C for 30 min in the nitrogen flow was applied to form the Si nanocrystals (NCs) embedded in the films. XRD experiment has revealed also the formation of hexagonal silicon nitride NCs with the sizes of 26-30 nm in the films grown with R=1. When R decreasing, the sizes of silicon nitride NCs decreases down to 2.8-3.0 nm (R=0.63). In the films grown with R=0.56 the amorphous silicon nitride, amorphous Si and crystallized Si phases have been detected. PL study showed the non-monotonic behavior of PL intensity with the R decreasing. The highest PL intensity was detected for the films grown with R=0.63. PL spectra of all films were found to be complex containing a set of PL bands peaked at about 2.8-3.0, 2.5-2.7, 2.1-2.2 and 1.8-2.0 eV. Temperature behavior of PL spectra was investigated in the range 20-300 K that allowed analyzing the nature of each PL component. It was shown that the peak position of all PL bands varies with the SiN_x stoichiometry and Si NC sizes, demonstrating a "red" shift with R decreasing. It was revealed that the former three PL bands did not change their peak positions versus temperature of measurement that permits to assign them to the carrier recombination via radiative defects in the silicon nitride matrix. The PL band at 1.8-2.0 eV has demonstrated a "blue" spectral shift with cooling similar to the Si band gap shrinkage. Based on this behavior, the 1.8-2.0 eV PL band has been attributed to exciton emission inside of Si NCs. The role of silicon nitride NCs in photoluminescence and its excitation is discussed.