Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiNx:H alloys prepared by reactive evaporation

Abstract

A wide range of amorphous hydrogenated silicon nitride thin films with an excess of silicon was prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions. A strong visible photoluminescence at room temperature was observed for the as-deposited films as well as for films annealed up to 1100°C. The chemical composition and the structure of the films were investigated using x-ray photoelectron, thermal desorption, and Raman spectroscopies, infrared absorption measurements, grazing incidence x-ray diffraction experiments, and transmission electron microscopy. Two luminescence mechanisms were identified for the films depending on the annealing temperature. For annealing temperatures below 650°C, the films are made of amorphous silicon-rich phases mixed with nitrogen-rich phases. These inhomogeneities in the chemical composition, coupled with the evolution of the photoluminescence energies and intensities with the hydrogen content, suggest that the emission is due to the recombination process of the photogenerated carriers within the band-tail states. For temperatures higher than 800°C, a phase separation occurs and the films could be described as silicon nanoclusters embedded in an insulating amorphous silicon nitride matrix. The clusters are amorphous, and then crystallized when the annealing temperature is high enough. The correlation between the clusters sizes and the photoluminescence results suggests that the emission observed after annealing treatments at temperature higher than 900°C is due to the quantum confinement of the carriers inside the silicon clusters. By carefully choosing the preparation and the annealing conditions, it is possible to tune the photoluminescence energy in the visible range.