Low-temperature, high-throughput spatial atomic layer deposition of NiOx nanocrystalline thin films from [Ni(ipki)2]

Abstract

Spatial atomic layer deposition (SALD) is a recent ALD variant enabling much faster deposition rates, even at atmospheric pressure, making it ideal for scalable, low-cost devices. NiO_x, a transparent p-type oxide, is widely used in emerging technologies like perovskite solar cells. However, no suitable SALD process for NiO thin films has been reported so far. In this work, we present the deposition of nanocrystalline NiO_x thin films via SALD using a recent Ni precursor not yet explored for the ALD of NiO, namely, bis(4-(isopropylamino)pent-3en-2-onato)nickel(II) or [Ni(ⁱpki)₂]. O₂ has been used as coreactant, with higher deposition rates being achieved if H₂O is added to the O₂ flow. A narrow ALD window has been obtained between 230 °C and 250 °C, where a GPC of 0.023 nm is observed. This corresponds to a deposition rate of 1.4 nm/min, which is 2 to 10 times faster than the rates reported for conventional ALD of NiO thin films. Remarkably, the growth onset of NiO_x starts around only 170 °C. The transmittance of the films reaches nearly 97 % in the visible for 55 nm thick films. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed a high homogeneity of the films. X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) studies confirm the presence of a cubic NiO_x crystalline phase. Finally, NiO_x films have been deposited on Ag nanowire networks, demonstrating the possibility of depositing homogeneous and conformal coatings with this new process.