On the impact of atomic layer deposition processing on the chemical and opto-electrical properties of NiO thin films

Nickel oxide (NiO) is a promising p-type semiconductor extensively investigated for applications in photovoltaics, electrochemical energy storage, and gas sensors. Several deposition techniques, including solution processing, sputtering, chemical vapor deposition, and atomic layer deposition (ALD), are adopted for the synthesis of NiO films. Among these techniques, ALD is recognized for the merit of delivering pinhole-free and conformal films, relevant for processing on textured surfaces. In the present study, we have adopted three ALD NiO processes to deliver films differing in stoichiometry, crystallographic, and optoelectrical properties. To this purpose, the Ni(tBu-MeAMD)2-based thermal ALD process (“NiOBu-MeAMD”), the Ni(MeCp)2-based plasma-assisted ALD (PA-ALD) process (“NiOMeCp”), and the newly developed PA-ALD process, based on the novel AlanisTM precursor (“NiOAlanis”), are compared. The NiOAlanis-based process exhibits a linear growth on c-Si/ native oxide with a growth per cycle (GPC) of 0.27 ± 0.02 Å/cycle at 150 °C, similar to the NiOMeCp process, whereas the NiOBu-MeAMD process is characterized by a higher GPC of 0.43 ± 0.01 Å/cycle. The NiOAlanis films have the lowest Ni-to-O ratio (0.82 ± 0.04) compared to NiOMeCp (0.96 ± 0.05) and NiOBu-MeAMD (0.93 ± 0.05). The excess oxygen in the NiOAlanis films points to a larger relative content of Ni3+ acceptor states. The presence of Ni3+ states contributes to sub-bandgap absorption, leading to a lower transmittance of the NiOAlanis films (∼79%) as compared to the NiOMeCp (∼84%) and NiOBu-MeAMD films (∼90%) in the visible range. In parallel, the resistivity of the (∼9 nm) NiOAlanis films (6 ± 2 Ω cm) is lower than NiOMeCp (66 ± 33 Ω cm) and NiOBu-MeAMD films [(1.7 ± 0.1) × 104 Ω cm], supporting the observation of the presence of a larger Ni3+ content. Transmission electron microscopy images reveal the presence of a relatively higher density of grain boundaries in NiOBu-MeAMD films that can affect carrier mobility. Furthermore, the PA-ALD NiO films exhibit a higher density (6.9 ± 0.8 g/cm3) than the thermal ALD NiOBu-MeAMD films (5.1 ± 0.5 g/cm3) due to the ion flux impinging on the surface during the O2 plasma half-cycle. Our study demonstrates that the ALD process influences the stoichiometry and microstructure of the NiO film, ultimately impacting its electrical and optical properties. Additionally, we reflect on the selection of ALD processes for specific application areas.