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

Titanium oxynitride (TiO_xN_y) 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, TiO_xN_y, 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/TiO_xN_y/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.