Nickel oxide (NiO) is a significant p-type ultra-wide bandgap semiconductor with a bandgap ranging between 3.7 and 4.0 eV. Its intrinsic properties, including high transparency in the visible spectrum and good chemical stability, make it a promising candidate for transparent electronics and power devices. However, achieving efficient p-type conductivity in NiO remains challenging due to its natural tendency for self-compensation through native defects. Recent advances in doping strategies and heterostructure engineering have opened new pathways for optimizing its performance in practical applications.

The intrinsic p-type behavior of NiO arises from nickel vacancies (V_Ni) and interstitial oxygen (O_i), which act as acceptors. However, these defects alone do not provide sufficient hole concentrations for high conductivity. To enhance p-type conductivity, extrinsic doping with monovalent cations such as lithium (Li) and silver (Ag) has been explored. Lithium, with its small ionic radius, substitutes for nickel sites (Li_Ni), introducing shallow acceptor levels approximately 0.1–0.2 eV above the valence band maximum. This results in improved hole mobility, with reported values reaching up to 10 cm²/V·s in optimally doped NiO:Li films. Silver doping, on the other hand, introduces deeper acceptor levels due to the larger size mismatch between Ag⁺ and Ni²⁺, leading to lower hole mobilities but better stability under thermal stress. The trade-offs between dopant choice, conductivity, and stability must be carefully balanced depending on the intended application.

Hole transport in NiO is governed by a combination of band conduction and polaron hopping mechanisms. At low doping concentrations, holes move primarily through the valence band, but as dopant density increases, localized states near the valence band edge dominate, leading to variable-range hopping behavior. This transition is particularly evident at high doping levels (>5 at.% Li), where structural disorder and defect clustering reduce mobility. Temperature-dependent Hall effect measurements reveal an activation energy of ~0.1–0.3 eV for hole conduction, consistent with the presence of shallow acceptor states. Understanding these transport mechanisms is critical for designing NiO-based devices with optimal performance.

One of the most promising applications of NiO is in heterojunction diodes formed with n-type ultra-wide bandgap oxides such as gallium oxide (Ga₂O₃). The large band offsets between NiO (p-type) and Ga₂O₃ (n-type) create a type-II band alignment, facilitating efficient carrier separation. These heterojunctions exhibit rectification ratios exceeding 10⁶ at room temperature, with turn-on voltages around 1.5–2.0 V. However, interfacial defects and lattice mismatch between the two materials can lead to recombination losses, reducing diode efficiency. Strategies such as interfacial passivation with ultrathin dielectric layers (e.g., Al₂O₃) or graded doping profiles have been employed to mitigate these effects. Additionally, the high breakdown field of Ga₂O₃ (>8 MV/cm) combined with the p-type conductivity of NiO enables high-voltage rectifiers capable of operating at elevated temperatures.

Transparent electronics is another key area where NiO excels. Its wide bandgap ensures high optical transparency (>80% in the visible range), making it suitable for transparent conductive electrodes and thin-film transistors (TFTs). NiO-based TFTs exhibit on/off ratios >10⁵ and field-effect mobilities of 1–5 cm²/V·s, though stability under bias stress remains a challenge due to charge trapping at grain boundaries. Doping with elements like copper (Cu) or magnesium (Mg) has been shown to improve bias stability by reducing oxygen vacancy concentrations. These advances are paving the way for fully transparent circuits integrating NiO with other wide-bandgap oxides.

Despite these advantages, several challenges hinder the widespread adoption of NiO in power and optoelectronic devices. Defect compensation is a major issue, as high concentrations of native defects (e.g., oxygen vacancies) can counteract the effects of intentional doping. Post-deposition annealing in oxygen-rich atmospheres has proven effective in reducing oxygen vacancies, but excessive annealing can lead to dopant diffusion and degradation of electrical properties. Another challenge is the limited understanding of minority carrier lifetimes in NiO, which affects the performance of bipolar devices such as solar cells and light-emitting diodes. Further research into defect passivation techniques and minority carrier dynamics is necessary to unlock the full potential of NiO.

In power electronics, NiO’s high critical electric field (~5 MV/cm) and thermal stability make it attractive for high-voltage rectifiers and transistors. However, the lack of high-quality single-crystal substrates for epitaxial growth results in polycrystalline films with high defect densities. Heteroepitaxial growth on lattice-matched substrates like magnesium oxide (MgO) has shown promise, but scalability remains an issue. Recent efforts in pulsed laser deposition (PLD) and sputtering have improved film quality, with defect densities reduced to the mid-10¹⁶ cm⁻³ range. These advancements are critical for achieving reliable device performance in high-power applications.

Looking ahead, the integration of NiO with emerging ultra-wide bandgap materials such as AlN and diamond could enable next-generation devices with unprecedented performance. The development of low-resistance ohmic contacts to p-type NiO is another area requiring attention, as current contact materials often introduce high Schottky barriers. Alloying NiO with other transition metal oxides (e.g., CuO or CoO) may offer additional avenues for tailoring its electronic properties while maintaining transparency and stability.

In summary, NiO stands out as a rare p-type ultra-wide bandgap oxide with significant potential in transparent electronics, power rectifiers, and heterojunction devices. Advances in doping strategies, interfacial engineering, and defect control are steadily overcoming its inherent limitations. Continued research into material synthesis and device integration will be essential to fully exploit its capabilities in future technologies.