Growth conditions and mechanisms for IrOx nano-platelet formation by reactive sputtering

Abstract

Iridium oxide (IrOx) forms a wide array of tailorable nanostructures, including plates, rods, and cones. Despite a strong body of literature on nanostructured IrOx, prior work did not explore the specific growth conditions and mechanisms for platelet formation. Here we report on how IrOx nanoplatelets take their form during DC reactive sputtering deposition, highlight the conditions and growth mechanisms that lead to unique nanostructures, and demonstrate changes in morphology and crystal orientation via x-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The XRD spectra indicate that IrOx undergoes a substantial transition with increasing oxygen flow rate during deposition, with the initial face-centered cubic (111), (200) and (220) peaks vanishing, while the rutile (101) and (110) peaks emerge, with several intermediate peaks indicating transition species. Ultimately (101) emerges as the preferred orientation of the IrOx nanostructures, as evidenced by the time-series XRD spectra. Corresponding stress analysis indicates that in the absence of oxygen flow, the IrOx films are highly compressive, and that the film stress becomes significantly less compressive with increasing oxygen flow. This unlocks the use of IrOx as a highly tailorable nanostructured material, ranging from dense film to high aspect ratio platelets, by simply adjusting oxygen flow rate and/or sputtering time.