Tuning the optoelectronic properties of NiOx thin films by atomic layer deposition with a precise incorporation of aluminum atoms

Nickel oxide (NiO) is a promising p-type semiconductor widely explored for various applications. A key challenge in producing functional NiOx films—particularly with suitable optoelectronic properties and well-defined valence and conduction band positions—lies in achieving precise control over the material’s substoichiometry. We investigate the processes and outcomes involved in incorporating aluminum (Al) atoms during the fabrication of nickel oxide films by atomic layer deposition when using bis(N,N′-di-t-butylacetamidinato)nickel(II) Ni(amd)2, trimethyl aluminum, and water as the growth precursors to address this challenge. By using both a supercycle technique and tuning the pulse sequence, very fine control over the incorporation of Al into the matrix is demonstrated. This incorporation impacts the film properties in myriad ways, changing the conductivity by 5 orders of magnitude, while also decreasing its optical transparency and decreasing its crystallinity. Further characterization by quartz-crystal microbalance and x-ray photoelectron spectroscopy (XPS) reveals that choosing the pulse sequence and thereby modifying the reactive film surface observed by the Al and Ni precursors dramatically impact their relative uptake into the film. High-resolution XPS measurements at the surface and within the layers further show the fine structure details of this uptake. The incorporation of Al correlates with the detection of a low energy Ni contribution in the films, whose proportion increases with the Al fraction and C content of the films, this latter presenting a specific C signature. These observations suggest either Ni—O—Al films composed of an intermixed compound of Ni—Al—O—C or NiOx films with small AlOx domains and incorporated nickel carbide.