Mist-CVD Grown NiO/Ga₂O₃ pn Heterojunctions for High Power Electronics

β-Ga₂O₃ is a promising ultrawide bandgap semiconductor for high-voltage and power devices. However, to date, acceptor doping of β-Ga₂O₃ has proven difficult due to the flat valence band. To address this limitation and improve upon homoepitaxial unipolar device performance, heterojunction p-n devices with p-type NiO are being explored. Heteroepitaxial growth of p-type NiO has currently been demonstrated by several thin-film growth techniques, including MOCVD, sputtering, PLD, and Mist-CVD. Mist-CVD is particularly promising as it enables single-crystal heteroepitaxial films, and is also a cost-effective, plasma-free technique that is scalable to large-area growth. With the availability of a broad range of low-vapor pressure precursors, phase pure single crystals, complex heterostructures, and 2D materials can be readily formed.

Epitaxial Li-doped NiO films were grown on c-plane sapphire and (100) MgO substrates from nickel (II) acetylacetonate and lithium acetylacetonate precursors. Films exhibited excellent crystallinity with FWHM of 15 and 61 arcseconds on sapphire and MgO. Li doping was achieved by varying the Li precursor concentration between 0 and 2 mmol/L while keeping the Ni precursor concentration constant at 20 mmol/L. By van der Pauw, UID NiO films showed an electrical resistivity of 10⁶ Ω-cm, which was controllably reduced to 10¹–10⁵ Ω-cm by Li doping. A high selectivity wet etch (1:1 HCl: DI at 50°C), with an etch rate of 2.6 nm/min, was developed to facilitate lateral and vertical device geometries. Si-doped β-Ga₂O₃ was similarly grown successfully as homoepitaxial films on (010) Fe-doped β-Ga₂O₃ substrates, with a growth rate of ~12 nm/min at 600°C. With increasing temperature, the growth rate decreased and significant γ-Ga₂O₃ inclusions were observed above 800°C. Si doping to 3x10¹⁹ cm^-3 with a mobility of 63 cm²/(V-s) was demonstrated. Phase-pure β-Ga₂O₃ was observed only on β-Ga₂O₃ substrates. In contrast, on sapphire substrates α-phase and γ-phase defects were observed in addition to the dominant β-phase. Two different heterojunctions of NiO/β-Ga₂O₃ and β-Ga₂O₃/NiO were grown on sapphire, MgO, and β-Ga₂O₃ substrates. Both vertical and lateral heterojunction devices were fabricated in these hole-conducting NiO and electron-conducting Ga₂O₃ films. Vertical NiO/Ga₂O₃ heterojunction exhibited near unity ideality with >2V V_on and >1 kV catastrophic breakdown while lateral diodes exhibited forward currents exceeding 300 mA/mm. Film quality, carrier densities, and pn junction IV curves demonstrate this successful approach for fabricating junction diodes with a full set of necessary processes for low-cost Ga₂O₃ devices.