Influence of post-annealing on structural, optical and electrical properties of tin nitride thin films prepared by atomic layer deposition

Post-thermal annealing treatment is an effective process employing the thin film structure modifications and compositional properties. In this article, we present a detailed investigation and understanding of the changes in the phase, crystal structure, microstructure, and optoelectrical properties of tin nitride (SnNx) thin films. These were deposited by atomic layer deposition (ALD) followed by annealing at temperatures ranging from 300 to 550 degrees C. The results suggest that post-annealing significantly influences the properties of as-deposited ALD SnNx thin films at 150 degrees C. For instance, X-ray diffractometry (XRD) and transmission electron microscopy (TEM) results demonstrate that the as-deposited film predominantly forms an amorphous structure. After annealing up to 350 degrees C, the film retains its amorphous structure with a minor enhancement in crystallinity. The bonding state to reveal its phase was confirmed by X-ray photoelectron spectroscopy (XPS). The as-deposited film predominantly forms SnN bonding from Sn2+ states, and it is changed after annealing at 350 degrees C, where the fraction of Sn4+ from the Sn3N4 phase considerably increases. However, the XRD and TEM results do not distinguish the differences between as-deposited or 300 degrees C-annealed and 350 degrees C-annealed samples. After the annealing temperature is increased to 400 and 450 degrees C, both processes of crystallization into the mixed phase of hexagonal SnN and cubic Sn3N4, and their decomposition into metal Sn with simultaneous nitrogen release occur. At a further elevation of the annealing temperature (500 degrees C and beyond), a considerably distorted morphology and agglomeration of the as-deposited film structure was observed. This was due to the formation of island-like structures or droplets of metallic Sn by significantly releasing (or almost all) the nitrogen within the films. As the film properties vary upon annealing, spectroscopic ellipsometry (SE) was used to investigate the optical and electrical parameters of the as-deposited and annealed films. The optical band gap of the as-deposited film is 1.5 eV and remains unchanged up to 400 degrees C; it then increases to 1.9 eV at higher annealing temperatures. The electrical resistivity of the films decreases monotonically as the annealing temperature increases, which is attributed to the change in carrier concentration. The change in the optoelectronic properties can be associated with the change in crystallinity and escape of the nitrogen content connected with the change in stoichiometry.