Nickel oxide thin films grown by chemical deposition techniques: potential and challenges in next-generation rigid and flexible device applications

Nickel oxide (NiOx), a p-type oxide semiconductor, has gained significant attention due to its versatile and tunable properties. It has become one of the critical materials in wide range of electronics applications, including resistive switching random access memory devices and highly sensitive and selective sensor applications. In addition, the wide band gap and high work function, coupled with the low electron affinity, have made NiOx widely used in emerging optoelectronics and p-n heterojunctions. The properties of NiOx thin films depend strongly on the deposition method and conditions. Efficient implementation of NiOx in next generation devices will require controllable growth and processing methods that can tailor the morphological and electronic properties of the material, but which are also compatible with flexible substrates. In this Review, we link together the fundamental properties of NiOx with the chemical processing methods that have been developed to grow the material as thin films, and with its application in electronic devices. We focus solely on thin films, rather than NiOx incorporated with 1D or 2D materials. This Review starts by discussing how the p-type nature of NiOx arises and how its stoichiometry affects its electronic and magnetic properties. We discuss the chemical deposition techniques for growing NiOx thin films, including chemical vapor deposition, atomic layer deposition, and a selection of solution processing approaches, and present examples of recent progress made in the implementation of NiOx thin films in devices, both on rigid and flexible substrates. Furthermore, we discuss the remaining challenges and limitations in the deposition of device-quality NiOx thin films with chemical growth methods