【Member Papers】Xidian University---Interface modification-induced high-performance vertical NiOₓ/β-Ga₂O₃ heterojunction diodes via O₂ plasma treatment

      Researchers from the Xidian University have published a dissertation titled "Interface modification-induced high-performance vertical NiO_x/β-Ga₂O₃ heterojunction diodes via O₂ plasma treatment" in Applied Surface Science.

Project Support

      This work was supported by the National Key R&D Program of China (No. 2022YFB3605402); the National Natural Science Foundation of China (under Grants 62274132, 62004151, 62274126).

Background

      β-Gallium oxide (β-Ga₂O₃), as a potential material for the next generation of electronic devices due to its excellent material properties, elicits an incredible amount of interest. Moreover, large-sized, low-cost single-crystal substrates can be achieved through the melting or casting methods, which will facilitate the large-scale industrialization of Ga₂O₃ devices. Due to a lack of p-type Ga₂O₃, heterojunctions are the optimal solution to the technological bottleneck in the development of Ga₂O₃ devices. Recently, several groups have demonstrated various p-n heterojunction power diode (HJD) structures aimed at enhancing Ga₂O₃ device performance, including double-layered NiO, beveled-mesa, junction termination extension (JTE), field plate assisted deep mesa (FPDM). Concurrently, we have developed a composite terminal structure for NiO/β-Ga₂O₃ HJD with a power figure-of-merit of 13.2 GW/cm2 , that exceeds the 1-D unipolar limit of GaN and SiC. Notably, Jian-Sian Li et al. achieved breakdown voltages up to 13.5 kV and low on-resistance by optimizing the structure and process of NiO/β-Ga₂O₃ vertical heterojunction diodes, demonstrating their great potential for high power applications. In addition to developing advanced terminal structure designs, strengthening the research on the characteristics of heterojunction interfaces and transport mechanisms is crucial for improving the performance and accelerating industrialization of β-Ga₂O₃ heterojunction devices. Lu et al. achieved a performance-enhanced NiO/β-Ga₂O₃ HJD by obtaining high-quality heterojunction interfaces via TMAH treatment after ICP etching. Deng et al. investigated the interface energy band structure of NiO/β-Ga₂O₃ heterojunctions with different Ga₂O₃ orientations and its effect on the electrical properties. Wang et al. determined that bipolar charge transport in NiO/β-Ga₂O₃ HJD is dominated by the modified Shockley-Read-Hall recombination and minority carrier diffusion. Despite these advancements, research on the metal–semiconductor interface and optimization of ohmic contacts in NiO_x/β-Ga₂O₃ HJDs remains largely unexplored.

Abstract

      The advancement of NiO_x/β-Ga₂O₃ heterojunctions is a crucial approach to overcoming the technical limitations of bipolar Ga₂O₃ power devices. However, the development of interface engineering and the investigation of ohmic characteristics at the metal-NiO_x interface remain largely unexplored. In this study, we demonstrated that O₂ plasma treatment at a specific time can achieve low ohmic contact resistance (5.8 × 10^-4 Ω·cm²) and high breakdown voltage (1340 V) for Ni/NiO_x/β-Ga₂O₃ heterojunction diodes, which outperforms most of state-of-the-art Ga₂O₃ heterojunction diodes and Schottky barrier diodes without any terminal structures. The comparative analysis revealed that the variations of NiOOH content at the interface play a pivotal role in affecting the interface properties and the device’s forward characteristics. Following O₂ plasma treatment at a specific duration, the increased presence of the strong dipole NiOOH altered the Ni/NiO_x interface state, leading to a significant increase in hole concentration on the NiO_x surface, enhanced transport ability of interface carriers, and a substantial reduction in ohmic contact resistance. This study bridges the knowledge gap between the characteristics of metal–semiconductor interfaces and the ohmic transport mechanism in β-Ga₂O₃ heterojunctions, offering significant insights for the design and optimization of β-Ga₂O₃ heterojunction devices.

Highlights

      • O₂ plasma treatment achieves low ohmic resistance and high breakdown voltage in NiO_x/β-Ga₂O₃

      • Variations in NiOOH content at the interface affect the interface properties and device’s forward characteristics.

      • Specific O₂ plasma treatment duration modifies Ni/NiO_x interface, enhancing ohmic contact properties.

Conclusions

      In summary, we have developed a controllable interface treatment technique for high-performance NiO_x/β-Ga₂O₃ heterojunction diodes (HJDs) using O₂ plasma treatment. This method significantly enhances device performance by modulating the energy band structure and reducing ohmic contact resistance at the Ni/NiO_x interface. The 1-minute O₂ plasma treatment results in a smoother surface morphology, as evidenced by AFM images, which reduces interface defects and improves metal–semiconductor contact quality. XPS analysis shows an increase in NiOOH and Ni₂O₃ content, leading to higher surface hole concentrations and improved carrier transport. The increased NiOOH content plays a crucial role in enhancing device performance. The strong dipole of NiOOH causes a displacement in the surface state energy levels, resulting in an increased surface state density. This, in turn, enhances the transport capability of carriers at the interface by reducing the contact barrier and increasing the probability of carriers crossing the potential barrier. These chemical and structural changes collectively modulate the energy band structure, resulting in more favorable band alignment and reduced specific on-resistance. Consequently, the treated HJDs exhibit a significantly higher Baliga’s figure of merit (BFOM) of 0.675 GW/cm², a 209.63 % improvement over untreated devices. This study provides a novel strategy for optimizing metal–semiconductor interfaces, paving the way for the commercialization and scalable manufacturing of high-performance β-Ga₂O₃ devices.