Gallium oxide (Ga₂O₃) is considered to be competitive for high-power applications in the next generation because of its outstanding characteristics. For instance, its bandgap is ultra-wide (~4.8 eV), furthermore the theoretical breakdown field is as high as 8 MV/cm^[1]. To evaluate the suitability of semiconductor materials for high-power applications, the Baliga’s figure-of-merit is commonly used, which is extremely high for Ga₂O₃, almost 4 times and 10 times larger than GaN and SiC^[1].

After years of development, there has been remarkable progress in Ga₂O₃-based metal–oxide–semiconductor field effect transistors (MOSFETs), Schottky barrier diodes (SBDs)^[2-10] and photodetectors^{[11, 12]}. Nevertheless, the absence of p-type Ga₂O₃ is still a main obstacle for the bipolar devices based on Ga₂O₃. To conquer the difficulty, a natively p-typed oxide of NiO is brought in to realize p-NiO/n-Ga₂O₃ HJDs^[13]. Based on this technology, devices with R_on,sp and V_br of 10.6 mΩ·cm²/1860 V and 3.5 mΩ·cm²/1059 V have been reported^{[14, 15]}. However, the V_br and PFOM (PFOM = V_br²/R_on,sp) of reported NiO/β-Ga₂O₃ heterojunction p–n diodes were still far from the theoretic limit of the material.

In this work, we demonstrated p-NiO/n-Ga₂O₃ vertical HJDs with a double-layer junction termination extension (DL-JTE) consisting of two p-typed NiO layers with varied lengths. The double-layer JTE structure effectively avoids electric field concentration and also allows more holes into the Ga₂O₃ drift layer to reduce drift resistance. The fabricated device with DL-JTE demonstrates high breakdown voltage of 2830 V and a small R_on,sp of 1.34 mΩ∙cm², generating a high PFOM of 5.98 GW/cm². The result indicates that the DL-JTE structure has great potential for fabricating high power devices.

Fig. 1 schematically illustrates the cross-sectional structure of the devices with JTE and DL-JTE fabricated on the same wafer. By using halide vapor phase epitaxy (HVPE), a drift layer of 7-μm lightly Si-doped n-type β-Ga₂O₃ was grown on a heavily Sn-doped (001) β-Ga₂O₃ substrate. The electron concentration of the β-Ga₂O₃ substrate and the drift layer is 1 × 10¹⁹ and 1.5 × 10¹⁶ cm⁻³, respectively. Electron-beam evaporation was employed for Ohmic metal deposition of 30/300 nm Ti/Au on the back of the wafer. Then a rapid thermal annealing was performed in a N₂ atmosphere at 480 °C for 1 min to improve Ohmic contact. After that, RF magnetron sputtering was used to coat a full area NiO layer with 60-nm thickness on the upper surface of the sample at room temperature. Subsequently, another 60-nm p-NiO layer with 10 μm larger than the square anode electrode was sputtered with a lift-off process to form a double-layer junction termination extension (DL-JTE) structure. The power of the RF magnetron sputtering and the deposition rate of NiO are 80 W and 0.85 nm/min. The ratio of Ar/O₂ is chosen to be 15/10 sccm. The target material is NiO ceramics with 99.99% purity. The hole concentration of the grown p-NiO layer is found to be around 1 × 10¹⁸ cm⁻³ after Hall measurements. Finally, the Ni/Au (50/300 nm) metal stack was formed on the p-NiO layer by electron-beam evaporation and lift-off process to make an Ohmic contact. The area of anode electrode is 100 × 100 μm². Besides, the p-NiO/ n-Ga₂O₃ heterojunction diode with a single-layer JTE structure was also fabricated on the same wafer using the same device processing.

The C–V and 1/C²–V characteristics of the β-Ga₂O₃ SBD without termination under 1 MHz are shown in Fig. 2. The concentration of N⁻ drift layer is confirmed to be around 1.5 × 10¹⁶ cm⁻³ according to

1Nd=2qϵsϵ0A2×1dC−2dVAnode,

where q, ε_s, ε₀, A is the electron charge, relative dielectric constant, vacuum dielectric constant and the anode area, respectively.

The forward current–voltage (I–V) characteristics in the linear-scale and R_on,sp of the fabricated NiO/β-Ga₂O₃ vertical HJDs with JTE and DL-JTE are shown in Fig. 3. The device with DL-JTE exhibited a lower R_on,sp (1.34 mΩ∙cm²) than that with JTE (1.93 mΩ∙cm²). The electron concentration and thickness of the NiO layer are only 1 × 10¹⁸ cm^–3 and 60 nm, respectively, which cannot inject enough holes into the Ga₂O₃ drift layer. The second NiO layer under the anode is introduced in the DL-JTE, which will inject more holes into the Ga₂O₃ drift layer to reduce drift resistance. Therefore, a lower R_on,sp is realized by introducing DL-JTE. The turn-on voltage for both devices was about 2 V, which is higher than most Ga₂O₃-based SBDs, suggesting a higher barrier height.

The reverse breakdown characteristics of the fabricated vertical HJDs with JTE and DL-JTE are shown in Fig. 4. Compared with the heterojunction diode with JTE exhibiting a V_br of 2020 V, the one with DL-JTE yielded a much higher V_br of 2830 V. With V_anode rising reversely, an almost constant leakage current was observed till the breakdown points were reached for both devices.

To further evaluate the effect of the JTE and DL-JTE structure on the device electric field engineering, the electric field distributions of both devices were simulated using commercial TCAD software. As shown in Fig. 5, when V_Anode reaches –2020 V, for the heterojunction diode with JTE, a peak electric field of 8 MV/cm is observed at the anode edge resulting from electric field concentration effect, while for that with the DL-JTE structure, the electric field spreads and the crowding is effectively relieved, resulting in a decreased peak field of 4.7 MV/cm at the anode edge, which increases to 8.1 MV/cm when reverse bias grows to 2830 V, while the other peak field around the second NiO layer remains at 3.9 MV/cm.

Fig. 6 shows the V_br vs R_on,sp plot of the most advanced HJDs^[14-21], hetero-junction barrier Schottky (HJBS)^[22-24] diodes, and SBDs^[2-6] based on β-Ga₂O₃. Our HJD with DL-JTE is marked using a red star. As shown, the fabricated device with DL-JTE structures achieves a high V_br of 2830 V. Together with the low R_on,sp value of 1.34 mΩ·cm², a PFOM of 5.98 GW/cm² is gotten, which is 2.8 times larger than that of a conventional single-layer JTE structure.

By introducing a double-layer junction termination extension structure, NiO/β-Ga₂O₃ vertical HJDs with significant improvement were realized. Compared with the one with JTE fabricated on the same sample, the device with DL-JTE has a lower R_on,sp of 1.34 mΩ·cm² and a higher breakdown voltage of 2830 V, corresponding to a high power figure-of-merit of 5.98 GW/cm², suggesting that NiO/β-Ga₂O₃ HJDs with DL-JTE have great potential in high power applications.