Li-Doped NiO/Un-Doped NiO/β-Ga₂O₃-Based p-i-n Diode for Power Device Application

Recently, beta gallium oxide (β-Ga₂O₃, 4.7–4.9eV), which has a wide band gap than SiC and GaN, has been attracting attentions in the field of power electronic applications [1]. The Baliga’s figure of merit(B-FOM) of Ga₂O₃ is about 3000, which is 4 times that of GaN and 10 times that of SiC, and is expected to achieving high breakdown voltage at low on-resistance in the power device [2]. However, despite these high numbers, the actual reported performance of the power unit is much lower than expected. This is because it is difficult to implement p-type Ga₂O₃, which can be used as PN junction termination to improve breakdown voltage value [3]. For further improvement of devices which require lower on-resistance(R_on) and higher breakdown voltage, it is important to form a junction termination structure such as a guard ring and a merged structure using a p-type material even to reduce the maximum electric field of a wide bandgap materials. However, the development of p-type β-Ga₂O₃ remains insufficient, only the theoretical studies and few experimental results reported. Because a very low mobility of self-trap holes and a deep acceptor level are expected, p-type β-Ga₂O₃ may intrinsically not be practical for power device applications. As a strategy to compensate for this is to construct p–n heterojunctions by integrating n-type Ga₂O₃ with other p-type semiconductors if the interface quality is controlled in an appropriate manner [4].
In this study, a diode to form a p-n heterojunction with optimized β-Ga₂O₃ was fabricated using NiO, a material with p-type conductivity [5]. Among p-type oxide families, the wide-bandgap NiO material has promising potentials in the applications of various optoelectronic and power devices due to its high visible spectral transparency and p-type conductivity stemming from nickel vacancies or monovalent impurities [6].
It was confirmed that the p-type NiO was inserted between the β-Ga₂O₃ and the Ni Schottky junction to ensure the p-n characteristics and thus the depletion layer expanded. In addition, the conductivity control of nickel oxide was attempted by lithium doping and oxygen concentration regulation. As a result, lithium-doped nickel oxide exhibited improved ohmic contact properties with Ni due to the thin film's low specific resistance characteristics compared to undoped nickel oxide, which induced the diode's low on-resistance. Therefore, using these current characteristics, NiO was stacked in two layers to design a heterojunction diode with a Li-NiO/NiO/β-Ga₂O₃ structure and a device that achieves a high breakdown voltage of -1678 V while maintaining a low on-resistance of 7.1 mΩcm²

Acknowledgments This work was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (P0012451, The Competency Development Program for Industry Specialist), the Technology Innovation Program - (20016102, Development of 1.2kV Gallium oxide power semiconductor devices technology and RS-2022-00144027, Development of 1.2kV-class low-loss gallium oxide transistor) funded by MOTIE, and Hyundai Motor Group.

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