Beta-gallium oxide (β-Ga₂O₃), an ultrawide bandgap semiconductor, has become a prominent candidate for high-voltage and high-power applications due to its outstanding intrinsic properties in comparison to its SiC and GaN counterparts, including an ultrawideband of ~4.8 eV and a theoretical critical electric field up to 8 MV/cm^[1]. Advances in synthesis of single-crystal β-Ga₂O₃ substrates, especially through the cost-competitive melt growth method, also benefit the development of Ga₂O₃-based vertical high-voltage power devices^{[2, 3]}. A conventional vertical-structured Schottky barrier diode (SBD) based on β-Ga₂O₃ was fabricated with the anode and cathode metals forming Schottky and Ohmic contacts, respectively. Abundant strategies, including field plate, trench metal−oxide−semiconductor (MOS), mesa termination and implanted high-resistance edge termination techniques^[4−8], have been proposed to approach the theoretical limit of β-Ga₂O₃ SBD. However, under high electric field, a significant increase in leakage current is observed at the metal/Schottky interface, which may be attributed to the combined effects of barrier lowering and enhanced tunneling phenomena^{[9, 10]}. Junction engineering is expected to effectively lower the reverse leakage current and achieve high-performance β-Ga₂O₃ rectifiers. Limited by the deficiency of p-type epitaxy and p-type ion implantation technique in β-Ga₂O₃, attention has been focused on p-type oxides^[11]. For example, Kokubun et al.^[12] reported the first all-oxide β-Ga₂O₃-based p−n heterojunction with Li-doped NiO in 2016. Since then, extensive research has been conducted on NiO/β-Ga₂O₃ heterojunction diodes (HJDs). Through adopting a sputtered continuous NiO film as edge termination to alleviate the electric field crowding, Hao et al.^[13] reported a NiO/β-Ga₂O₃ HJD achieving a record high breakdown voltage (BV) of 2.66 kV and specific ON-resistance (R_on,sp) low to 2.5 mΩ·cm². Watahiki et al.^[14] fabricated a Cu₂O/β-Ga₂O₃ HJD in 2017, whose reverse leakage current (I_R) was far below that of the reference SBD. Zheng et al.^[15] reported a high-performance IrO_x/β-Ga₂O₃ p−n heterojunction with a remarkable BV of 1005 V as well as a relatively low R_on,sp of 4 mΩ·cm². SnO is another candidate for p-type oxides, which shows hole mobility much higher than that of magnetron sputtered^[16] and pulsed laser deposition (PLD)-grown NiO^[17] and outstanding thermal stability up to 300 °C^{[18, 19]}. Budde et al.^[20] fabricated the vertical SnO/β-Ga₂O₃ HJD by growing SnO film on β-Ga₂O₃ (2¯01) substrate through plasma-assisted molecular beam epitaxy (MBE). However, the improvement of the BV is urgently needed.

Herein, we demonstrate the fabrication of vertical SnO/β-Ga₂O₃ HJD via reactive magnetron sputtering. The electrical characteristics of the heterojunction as well as its band alignment are investigated. Compared to its SBD counterpart, the HJD maintains a relatively low R_on,sp (2.8 mΩ·cm²) and presents a higher BV (1675 V) as well as a lower I_R (<15 μA/cm²), giving a power figure of merit (PFOM) of 1.0 GW/cm² and implying that the HJD has potential applications in high-performance power devices.

(001)-oriented β-Ga₂O₃ substrates (640 µm, 6 × 10¹⁸ cm⁻³) with an epitaxial layer 10 µm thick (2 × 10¹⁶ cm⁻³) provided by Novel Crystal Technology, Inc. were used for the device manufacturing. The substrates were rinsed sequentially with acetone, isopropanol, and deionized water. The epitaxial layer was rinsed in H₂O₂ (30%) for 4 min and exposed to O₂ plasma for 30 s to improve the reverse breakdown voltage^[21], while the rear surface was treated in Ar plasma for 30 s to enhance the n-type doping and reduce contact resistance (R_c)^[22]. Subsequently, Ti/Au stack (20 nm/80 nm) was deposited on the rear surface using electron beam evaporation and performed rapid thermal annealing (RTA) at 470 °C for 1 min under N₂ atmosphere to form an Ohmic contact ^[23]. 120-nm-thick SnO film with a diameter of 80 µm was defined on the substrate through conventional photolithography and deposited via reactive radio frequency (RF) magnetron sputtering from a high-purity Sn target (RF power 30 W, chamber pressure 0.4 Pa) for 15 min, using the mixture of Ar₂/O₂ (49 sccm/1 sccm) as the sputtering gas. The film was further annealed at 200 °C in N₂ for 10 min for a better crystallinity^[24]. A Ni/Au stack (30/100 nm) with a diameter of 50 µm was deposited through the photolithography and lift-off processes to function as the anode. A β-Ga₂O₃ SBD with the same Ni/Au anode (diameter of 50 µm) was also fabricated for comparison.

Chemical bonding states of the SnO/β-Ga₂O₃ heterojunction were studied by XPS spectra conducted on a Thermo ESCALAB250 X-ray photoelectron spectroscope, employing an Al Kα monochromatic source with an energy of 1486.6 eV. Current−voltage (I−V) curves of the devices were characterized on a semiconductor parameter system for power devices (Keysight B1505A and B1500A), while capacitance−voltage (C−V) measurements were performed on a semiconductor characterization system (FS-Pro/PX600).

In Fig. 1(a), one can see the cross-sectional schematic of a typical SnO/β-Ga₂O₃ HJD, and Fig. 1(b) provides its top-view optical microphotograph. Sn 3d core level spectrum (as depicted in Fig. 1(c)) exhibits a spin−orbit split doublet with peaks located at approximately 485.98 (Sn 3d_5/2) and 494.38 eV (Sn 3d_3/2), respectively. The values are consistent with the previously reported binding energies of Sn^2+[24]. The peak of O 1s core level spectrum (shown in Fig. 1(d)) at 529.83 eV can also be assigned to O-Sn²⁺. Further, the energy loss structure on the high binding energy side of O 1s core level spectrum indicates an estimated E_g of approximately 2.9 eV for the obtained SnO film^[25]. The Hall mobility (μ_h) and Hall hole concentration (p_h) of the film are defined to be 7 cm²/(V·s) and 5 × 10¹⁷ cm⁻³ through van-der-Pauw Hall measurement. Notably, the hole mobility is about one order of magnitude higher than that of NiO film with similar hole concentrations (for example, 0.94^[16] and 0.87 cm²/(V·s)^[26]).

To study the energy band offset of the SnO/β-Ga₂O₃ heterojunction interface, a 300 nm SnO film was deposited on β-Ga₂O₃ substrate and the SnO film was etched through repeated Ar⁺ ion sputtering till both Sn 3d and Ga 3d photoelectron peaks appeared in the XPS spectrum. The core level spectra of Ga 3d and Sn 3d for bulk β-Ga₂O₃ and SnO film, along with their valence band maxima (VBM), are presented in Figs. 2(a) and 2(b). The value of VBM of β-Ga₂O₃ (EVBMGa2O3) is measured to be approximately 4.4 eV, which means that the VBM lies 4.4 eV beneath the Fermi level. The VBM of SnO (EVBMSnO) is determined to be about 1 eV, revealing the p-type conductivity of the SnO film. Considering the bandgap of 2.9 and 4.8 eV for SnO and β-Ga₂O₃, the valence band offset (∆E_v) was determined to be 2.65 eV, while the conduction band offset (∆E_c) was calculated to be 0.75 eV, using the following formula^[27]:

1ΔEv=(ESn3d3/2SnO−EVBMSnO)−(EGa3dGa2O3−EVBMGa2O3)−(ESn3d3/2SnO/Ga2O3−EGa3dSnO/Ga2O3),

2ΔEc=EgGa2O3−EgSnO−ΔEv,

where (ESn3d3/2SnO/Ga2O3−EGa3dSnO/Ga2O3) represents the energy difference between Sn 3d_3/2 and Ga 3d core level spectra at the SnO/β-Ga₂O₃ heterojunction interface (Fig. 2(c)). Fig. 2(d) presents the band alignment of the SnO/β-Ga₂O₃ interfaces, revealing a type-Ⅱ band alignment with a theoretical built-in potential (qV_bi) of 0.75 eV derived from the difference in the Fermi level (E_F) of SnO and β-Ga₂O₃^[28]. Therefore, the electrons need to overcome a potential barrier (V_a = V_bi + ΔE_C/q) of about 1.5 eV to inject from the β-Ga₂O₃ side into the SnO side. This is smaller than that of NiO/β-Ga₂O₃ heterojunction (~3.6 V), suggesting the possibility of a lower turn-on voltage (V_on) ^[29].

Figs. 3(a) and 3(b) present the forward current density−voltage (I−V) curves of SnO/β-Ga₂O₃ HJD and β-Ga₂O₃ SBD plotted on linear and semi-logarithmic plot, respectively. Both exhibit a rectification ratio of ~10¹¹ at ±5 V, which is similar to the reported one of the well-researched NiO/β-Ga₂O₃ HJDs^{[15, 30]}, revealing an excellent heterojunction interface. V_on (defined at I_F = 0.1 A/cm²) of the HJD is determined to be 1.3 V, showing a positive shift of about 0.3 V when compared to that of the SBD. Meanwhile, V_on of HJD is significantly smaller than the theoretical electron potential barrier (V_a) determined from energy band diagram. This phenomenon has been attributed to defect-assisted tunneling and interface recombination^{[29, 31]}. What is more, the V_on of our SnO/β-Ga₂O₃ HJD is also less than that of the well-researched NiO/β-Ga₂O₃ HJD (1.4−2.72 V) devices reported to date, suggesting potential application in the low-power and high-efficiency power devices^{[12, 32]}. R_on,sp is determined to be 2.8 and 1.9 mΩ·cm² for the HJD and SBD, respectively. The higher V_on and R_on,sp of the HJD should be attributed to the higher barrier height (qφ_b) of the heterojunction compared to that of the Schottky barrier^[33]. As presented in Fig. 3(b), qφ_b of the HJD and SBD are obtained to be 1.12 and 0.72 eV, respectively, according to the formula J=A*T2e−qφb/kT(eqV/nkT−1)^{[31, 34]}, where A* is the Richardson’s constant (41 A/cm²/K² for β-Ga₂O₃)^[2]. Further, the ideal factor (n) of the HJD and SBD are extracted to be 1.57 and 1.20, respectively (inset in Fig. 3(b)). The deviation of n from the unity may be ascribed to the defect-related tunneling current in the heterojunction interface^[35].

Fig. 3(c) illustrates the capacitance−voltage (C−V) and 1/C²−V behavior of SnO/β-Ga₂O₃ HJD at 10 kHz. The qV_bi is derived to be 1.34 eV from the intercept of 1/C²−V curve on the x-axis^[35]. Notably, the qV_bi extracted from C−V characteristics exceeds the theoretical one derived from the energy band diagram. This may be attributed to the presence of a significant series contact resistance in the device, which substantially affects the C−V measurements and may lead to deviations in the extracted qV_bi value^[28].

For the reverse I−V characterization, the cathode of the device was securely clamped to a metal chuck, and the anode was connected using a four-point probe configuration to ensure measurement accuracy. Considering the close proximity of the BV to the air breakdown voltage, the device surface was coated with electronic fluorinated fluid (3M Fluorinert FC-40) to mitigate air breakdown effects. The reverse I−V curves of the SnO/β-Ga₂O₃ HJD and β-Ga₂O₃ SBD are shown in Fig. 3(d). The catastrophic breakdown is confirmed through optical microscope inspection, as can be observed from the inset of Fig. 3(d). Compared to the SBD, the HJD achieves a significantly higher BV of 1675 V, without employing any refined electric field management techniques. The two devices exhibit I_R of the same level (~15 μA/cm²) at lower reverse bias (<200 V), which is comparable to the detection limit of our semiconductor device analyzer (~300 pA at 3000 V). However, I_R of the HJD is far below that of the SBD at higher reverse bias above (>200 V), suggesting that the p−n junction can reduce the surface electric field and in turn alleviate the leakage current^[36].

Two-dimensional distribution of the electrical field for the two devices at a fixed reverse voltage of 500 V were obtained through simulation in Silvaco TCAD and illustrated in Figs. 4(a) and 4(b). Obviously, a serious electric field crowding can be observed at the edge of anode for the SBD (Fig. 4(a)), showing a maximum electric field value of 3.41 MV/cm (Figs. 4(c) and 4(d)). Upon the incorporation of p-type SnO film, the electric field is spread out and the maximum electric field value is decreased to 2.20 MV/cm (Figs. 4(c) and 4(d)). This is primarily attributed to the fact that SnO extending outside of the anode is acting as a junction termination extension^[37]. Meanwhile, the electric field beneath the anode decreases from 1.89 MV/cm of the Schottky interface to 1.72 MV/cm of the heterojunction interface, which accounts for the suppressed I_R of the HJD.

Fig. 5 benchmarks the BV and R_on,sp of the cutting-edge β-Ga₂O₃ HJDs^{[13−15, 27, 38−41]}. As marked, our SnO/β-Ga₂O₃ HJD shows a R_on,sp of 2.8 mΩ·cm² and BV of 1675 V with the PFOM (BV²/R_on,sp) of 1.0 GW/cm². Although the performance of our device is to be improved compared to the NiO/β-Ga₂O₃ HJD with the similar structure^[13], it surpasses most reported p-type oxide/β-Ga₂O₃ HJDs such as Cu₂O/β-Ga₂O₃^[14], p-IrO_x/β-Ga₂O₃^[15] and TiO₂/β-Ga₂O₃^[27].

Furthermore, SnO/β-Ga₂O₃ HJDs with different diameters were fabricated. As plotted in Figs. 6(a) and 6(b), the V_on shows a slight increase from 1.3 to 1.5 V with the increasing of diameter from 50 to 100 μm. Concurrently, there is an increase in the R_on,sp from 2.8 to 4.1 mΩ·cm². However, the BV drops from 1675 and 1090 V. The unexpected decrease in BV may be attributed to the fact that more defects may be incorporated at the interface or a non-uniform interface quality may be obtained for the larger-area diodes, leading to premature breakdown of the device^[41].

In summary, by depositing a p-type SnO layer onto a β-Ga₂O₃ substrate with a lightly doped epitaxial layer, we fabricate a vertical SnO/β-Ga₂O₃ HJD. A type-Ⅱ band alignment with a theoretical built-in potential (qV_bi) of 0.75 eV is revealed through XPS spectra. The HJD presents R_on,sp and BV of 2.8 mΩ·cm² and 1675 V, respectively, giving a PFOM of 1.0 GW/cm². Compared with its SBD counterpart, the HJD maintains a relatively low R_on,sp and the reverse blocking characteristics are greatly enhanced, giving a higher BV and a lower I_R. The results demonstrate the superior quality of the SnO/β-Ga₂O₃ heterojunction interface and reveal its prospective use in high-performance power devices.