Effects of Ta3N5 Morphology and Composition on the Performance of Si-Ta3N5 Photoanodes

The use of thin Ta₃N₅ films in tandem Si-Ta₃N₅ photoelectrochemical (PEC) devices motivates understanding of the surface Ta₃N­₅ properties, as they may have a strong effect on the device performance. The bulk and surface properties can change as a function of nitridation temperature; thus its effect is studied, ranging from 700 to 1000 °C, on the PEC performance, morphology, and composition of thin (10 nm) Ta₃N₅ films deposited on planar and nanostructured Si substrates. Scanning electron microscopy (SEM), scanning Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) are employed to gain fundamental understanding in the differences of the Ta­₃N₅ films. By controlling Ta₃N­₅ morphology and composition with nitridation temperature, it is determined that Ta₃N₅ with high crystallinity and surface N/Ta ratio, synthesized at 800 °C, yields the highest PEC performance with the earliest photocurrent onset and highest photocurrent. Samples nitrided at 700 °C have lower crystallinity and that likely leads to lower performance. For samples nitrided at temperatures above 800 °C, the N/Ta ratio decreases forming chemically reduced tantalum nitride phases, as well as N-deficient and correspondingly O-rich morphological domains that can adversely affect the PEC performance as hole-blocking layers or O trap-mediated recombination centers at the surface of Ta₃N₅.