The utilization of power electronic devices will be crucial in future applications involving high temperatures, high voltages, and high radiation levels, where conventional silicon-based devices encounter limitations^[1]. Power devices based on wide bandgap (WBG) materials, particularly silicon carbide (SiC) and gallium nitride (GaN), have undergone a revolution in recent years, supplementing the lack of growth of silicon-based power electronics in the medium- and high-voltage markets^[2−4]. The ultra-wide band gap semiconductor gallium oxide (Ga₂O₃) exhibits a larger band gap (~4.9 eV), higher breakdown field strength (8 MV/cm), and superior Baliga's figure of merit (>3000) compared to SiC and GaN, thereby highlighting its immense potential in the realm of ultra-high voltage applications^[5−7]. The current availability of a well-established, cost-effective, high-quality, and large-scale substrate and epitaxial material product for β-Ga₂O₃ greatly facilitates the advancement of Ga₂O₃ power electronic devices^[8−11].

In recent years, the rapid development of vertical β-Ga₂O₃ Schottky barrier diodes (SBDs) can be attributed to the availability of commercial Ga₂O₃ materials and the ability to achieve highly controllable electrical conductivity^{[12, 13]}. The conventional configuration of Ga₂O₃ SBDs typically consists of a vertical structure, wherein Ga₂O₃ forms a Schottky contact with the anode metal and an ohmic contact with the cathode metal^[14]. Furthermore, the Schottky barrier plays a crucial role in the electron transport process, thus it is imperative to carefully select an appropriate anode metal to achieve optimal performance of the SBD device. The barrier heights for contacts of various work function barrier metals, such as Au, Ni, Gu, Mo, W, Pd, etc., in contact with Ga₂O₃ have been extensively studied^[15−17]. It is worth noting that the observed differences in results are minimal and may contribute to the Fermi pinning effect at the surface/interface of Ga₂O₃^[17−19]. However, the electrical properties of Ga₂O₃ SBDs require further investigation. In addition, the current focus of research lies in the exploration of Ga₂O₃ pn heterojunction diode devices (HJDs) with alternative p-type material substitutions, owing to the more difficult p-type doping. The current focus of research lies in NiO, which is widely regarded as the most suitable choice due to its controlled p-type doping with a bandgap of 3.6−4.0 eV^[20−23]. The pn junction diodes exhibit outstanding performance in terms of their ability to withstand reverse voltage and minimize leakage current. Thus, the enhancement of the reverse electrical characteristics of NiO/Ga₂O₃ HJDs warrants dedicated investigations.

In this work, the vertical β-Ga₂O₃ SBDs device was prepared without a termination structure, and the anode electrodes were chosen from high-work function barrier metals such as Au, Ni, and Pd. In comparison with Ga₂O₃ SBDs, the barrier metals of Ni/Au showed better forward current output electrical performance with an on resistance (R_on) of 0.11 Ω·cm², built-in voltage (V_bi) of 1.47 V, and ultra-low leakage current. In addition, the advantages of NiO/Ga₂O₃ HJDs in terms of enhanced breakdown voltage and reduced leakage are further confirmed through TCAD simulations and experiments. This work provides some guidance for the electrode selection and optimization of future Ga₂O₃ SBDs.

The homogeneous n⁻ Si-doped β-Ga₂O₃ epitaxial films with a thickness of 1.45 μm were fabricated using an MOCVD on n⁺ (100) Sn-doped substrate with a carrier concentration (N_d) of 5 × 10¹⁸ cm⁻³. Ga₂O₃ single crystal substrates and epitaxial wafers used in this research project are from Beijing GAO Semiconductor Co. Ltd. The Schottky barrier diode (SBD) was fabricated by depositing anode electrodes of various metals (Pd/Au, Ni/Au, Au) via sputtering, with each electrode measuring 600 μm × 600 μm. Ti/Au (50/100 nm) was used as the cathode back electrode.

The crystal structure and rocking curve peak of the sample were evaluated via X-ray diffraction (XRD) patterns. The optical absorption was characterized using a ultraviolet−visible (UV−Vis) spectrophotometer. The surface morphologies and roughness of all samples were investigated by scanning microscope (SEM) and atomic force microscopy (AFM). The thickness of the film was determined by a step profiler (KLA D-300). The capacitance−voltage (C−V) and current−voltage (I−V) characteristics of the device were tested by using a semiconductor analyzer (Keithley, 4200-SCS).

The XRD patterns of the Ga₂O₃ film and substrate exhibit typical β-Ga₂O₃ structure with single orientations of (400), (600), and (800), as depicted in Fig. 1(a). The X-ray rocking curve peak of Ga₂O₃ film from the (400) plane exhibited an FWHM of 111.6 arcsec, as shown in Fig. 1(b), indicating excellent crystalline properties of the β-Ga₂O₃ film^{[24, 25]}. The UV−Vis absorption spectra observed are also presented in Fig. 1(c). Based on the analysis, the bandgap of the β-Ga₂O₃ films was determined to be 4.9 eV, which aligns well with the findings reported in previous research^[26−28]. The SEM and AFM surface morphology of the β-Ga₂O₃ film is depicted in Figs. 1(d)−1(f). The results demonstrate that the surface of the β-Ga₂O₃ film exhibits a clean, uniform, and atomically flat structure.

Fig. 2(a) illustrates the schematic cross-sectional view of the fabricated vertical β-Ga₂O₃ SBD without a terminal structure. The anode electrodes for vertical β-Ga₂O₃ SBD were deposited using magnetron sputtering with Pd/Au, Ni/Au, and Au as the Schottky contact, respectively. All of these electrodes have work functions exceeding 5 eV. The epitaxial film is grown using the MOCVD technique, and a film thickness of 1.45 μm was investigated by a step profiler. The carrier concentration (N_d) of the film is approximately 2.6 × 10¹⁶ cm⁻³, as obtained from Fig. 2(b). The N_d can be estimated by extracting the slope from the 1/C²−V curve, which can be described by following Eqs. (1) and (2)^[29].

11C2=2qNDϵSϵOA2(Vbi−V−KTq),

2ND=2qϵSϵOA2⋅1Slope.

The Sn-doped substrate with a (100) orientation has a thickness of approximately 500 μm and a N_d of 5 × 10¹⁸ cm⁻³. Ti/Au electrodes serve as the ohmic contacts on the backside.

The electrical performance of all devices was assessed by conducting forward and reverse current density−voltage (J−V) measurements on vertical Ga₂O₃ SBDs, as illustrated in Fig. 3. All devices exhibit slightly different forward electrical properties. The β-Ga₂O₃ SBDs with Pd, Ni, and Au electrodes exhibit respective built-in voltages (V_bi) of 1.52, 1.47, and 1.59 V, which were determined by extracting the intercept from the linear fitting of the J−V curve shown in Fig. 3(a). The R_on of β-Ga₂O₃ SBDs with Pd, Ni, and Au electrodes is determined as 0.09, 0.11, and 0.12 Ω·cm², respectively, based on the slope of the linear region in the J−V characteristics. Additionally, all devices exhibit comparable saturation currents, as illustrated in Fig. 3(b). According to the theory of thermal electron emission (TE) mode^{[30, 31]}, the J−V characteristic of the Schottky diode can be described by the following equations.

3J=JSexp(eVnKT),

4JS=A*T2exp(−ΦJ−VKT),

where e is the electron charge, n is the ideality factor, K is the Boltzmann constant, J_S is the saturation current, A is the contact area, A^* is the Richardson constant, and Φ_J−V is the Schottky barrier height. Therefore, the ideality factor (n) and Schottky barrier height (Φ_J−V) can be estimated by extracting from Eqs. (3) and (4), which can be described as:

5n=eKT⋅1Slope,

6ΦJ−V=KTeLn(A*T2JS).

The comparison of n and Φ_J-V for β-Ga₂O₃ SBDs is summarized in Table 1. Although the n of an ideal Schottky diode is equal to 1, in practice, the values of all samples, as indicated in Table 1, surpassing the threshold of 3, signify a more pronounced deviation from the TE model in terms of the current conduction mechanism. The first possibility is that there might exist additional conduction mechanisms, such as field-emitting (FE) or thermal field-emitting (TFE), which contribute to the current transport and result in J−V characteristics of the device that deviate from those predicted by the single thermionic emission (TE) model^[32]. On the contrary, an alternative explanation arises from the presence of dislocations or defects in the material, leading to lateral inhomogeneity of the Schottky barrier height and consequently resulting in this relatively high ideal factor^{[19, 33]}. Similarly, the Φ_J−V of the β-Ga₂O₃ Schottky contact is affected by different conduction mechanisms and material defects. Therefore, the work function of the metal itself can not directly determine the n and Φ_J−V of the contact between the barrier metal and β-Ga₂O₃.

Due to the relatively simple device structure, vertical β-Ga₂O₃ SBDs exhibit comparatively lower withstand voltages, with breakdown voltages of 33 V for Ni electrode devices, 30.8 V for Pd electrode devices, and 31.5 V for Au electrode devices, as depicted in Fig. 3(c). As observed from the local enlargement in Fig. 3(d), the β-Ga₂O₃ SBDs with Ni electrode exhibit a reduced reverse leakage current. The utilization of Ni Schottky electrodes is highly recommended for enhancing the electrical characteristics of β-Ga₂O₃ SBDs. Overall, a comparison of the parameters from the table reveals that there is no significant correlation between the electrical performance of β-Ga₂O₃ SBDs and the calculated values of n and Φ_J−V in ET mode. Furthermore, it suggests that multiple factors influence the electrical performance of β-Ga₂O₃ SBDs.

The device performance is further enhanced through the utilization of NiO/β-Ga₂O₃ heterojunction diodes (HJDs), the device schematic is shown in Fig. 4(a). Based on the AFM images, a 200 nm thick NiO film was obtained, as illustrated in Fig. 4(b). Compared with the β-Ga₂O₃ SBDs, the positive electrical characterization properties of NiO/β-Ga₂O₃ HJDs decline due to hole injection from the NiO, as shown in Fig. 4(c). However, the breakdown voltage of the NiO/β-Ga₂O₃ HJDs has been enhanced from 33 to 44.5 V, and the reverse leakage current is significantly reduced from 3.37 × 10⁻⁶ to 4.82 × 10⁻⁸ A/cm², as illustrated in Fig. 4(d). Additionally, the TCAD simulation has confirmed that both the β-Ga₂O₃ SBDs and NiO/β-Ga₂O₃ HJDs exhibit an electric field peak precisely at the edge of the Schottky electrode. Furthermore, the incorporation of heterogeneous film NiO effectively alleviates the concentration of electric field at the edges, as depicted in Fig. 5. Therefore, the breakdown voltage of NiO/β-Ga₂O₃ HJDs increases greatly.

In this study, the fabrication of vertical β-Ga₂O₃ with different barrier metals was carried out on homogeneous epitaxial films by using MOCVD technology, excluding the use of edge terminals. The results show that the vertical Ga₂O₃ SBDs with Ni electrodes showed better forward current output electrical performance and maintained a reduced reverse leakage current. Additionally, we further enhance the breakdown voltage of the device by fabricating NiO/β-Ga₂O₃ HJDs and significantly reduce the reverse leakage current of the device from 3.37 × 10⁻⁶ to 4.82 × 10⁻⁸ A/cm². Our results show that Ga₂O₃ power detailed investigations into Schottky barrier metal and NiO/Ga₂O₃ heterojunction of Ga₂O₃ homogeneous epitaxial films are of great research potential in high-efficiency, high-power, and high-reliability applications.