Plasma-Enhanced Atomic Layer Deposition of Titanium Oxynitride (TiOxNy) Thin Films and Their Neuromorphic Applications

Titanium oxynitride (TiOxNy) thin films were successfully coated on silicon and fluorine-doped tin oxide (FTO) glass substrates using an on-surface plasma-enhanced atomic layer deposition technique. Depositions were carried out using titanium tetrachloride (TiCl4) as a precursor and oxygen–nitrogen plasma at various plasma powers. The structural analyses of these films reveal that all deposited films are crystalline in nature, exhibiting the formation of TiO2, TiOxNy, and TiN mixed phases. Further elemental analysis of these films using X-ray photoelectron spectroscopy reveals that with increasing plasma power, the nitrogen content in the films starts increasing until 33 W and reduces again at 48 W. Using the deposited films, artificial synapses were fabricated with gold (Au) as the top electrodes, and the Au/TiOxNy/FTO devices emulate the functions of biological neurons. The neuromorphic properties of the devices were proportional to the nitrogen content of the films. These synaptic devices exhibit the anti-Hebbian behavior of associative learning. Furthermore, these devices were used for pattern recognition by training it with a set of images and by testing it with a fresh set of images. The pattern recognition accuracy is found to enhance significantly in samples deposited by intermediate plasma power, where the nitrogen content was the highest.