β-Ga₂O₃-Based Heterostructures and Heterojunctions for Power Electronics: A Review of the Recent Advances

Abstract

During the past decade, Gallium Oxide (Ga₂O₃) has attracted intensive research interest as an ultra-wide-bandgap (UWBG) semiconductor due to its unique characteristics, such as a large bandgap of 4.5–4.9 eV, a high critical electric field of ~8 MV/cm, and a high Baliga’s figure of merit (BFOM). Unipolar β-Ga₂O₃ devices such as Schottky barrier diodes (SBDs) and field-effect transistors (FETs) have been demonstrated. Recently, there has been growing attention toward developing β-Ga₂O₃-based heterostructures and heterojunctions, which is mainly driven by the lack of p-type doping and the exploration of multidimensional device architectures to enhance power electronics’ performance. This paper will review the most recent advances in β-Ga₂O₃ heterostructures and heterojunctions for power electronics, including NiO_x/β-Ga₂O₃, β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃, and β-Ga₂O₃ heterojunctions/heterostructures with other wide- and ultra-wide-bandgap materials and the integration of two-dimensional (2D) materials with β-Ga₂O₃. Discussions of the deposition, fabrication, and operating principles of these heterostructures and heterojunctions and the associated device performance will be provided. This comprehensive review will serve as a critical reference for researchers engaged in materials science, wide- and ultra-wide-bandgap semiconductors, and power electronics and benefits the future study and development of β-Ga₂O₃-based heterostructures and heterojunctions and associated power electronics.

1. Introduction

Although Ga₂O₃ was synthesized over six decades ago, it has remained largely ignored as an ultra-wide-bandgap (UWBG) semiconductor material [1]. It was not until 2011 that Higashiwaki et al. [2] demonstrated its potential by featuring a field-effect transistor (FET) constructed using epitaxially grown β-Ga₂O₃ on bulk Ga₂O₃ single crystals. Shortly thereafter, the potential of Ga₂O₃ was recognized, especially for power electronics, leading to extensive research efforts [3]. These developments are particularly promising for future power electronics. For example, the power electronics industry is projected to play a significant role in providing 80% of the nation’s electricity flow by 2030 in the U.S. [4]. This industry will cater to various growing needs, including the expansion of electric vehicles, data centers, renewable energy, consumer electronics, and the space telecommunication sector, with the potential to drastically reduce energy losses in the years to come. The driving force behind this rapid development of β-Ga₂O₃ power devices primarily stemmed from β-Ga₂O₃′s large bandgap of 4.5–4.9 eV, high critical electric field of ~8 MV/cm, impressive Baliga’s figure of merit (BFOM) (i.e., ~3000 times larger than that of Si), presence of large native substrates, and controllable n-type doping capabilities across a wide range of ~10¹⁵–10²⁰ cm⁻³ [1,2,3,5,6,7,8]. Ga₂O₃ exists in five polymorphs, with β-Ga₂O₃ being the most thermodynamically stable phase [8]. β-Ga₂O₃ substrates are readily available in sizes up to 4 inches in diameter, and they can be mass-produced using the Edge-Defined Film-Fed Growth (EFG) method [9]. Additionally, high-quality epitaxial layers can be grown on these substrates using various techniques, including halide vapor-phase epitaxy (HVPE) [10], molecular beam epitaxy (MBE) [11], and metal–organic chemical vapor-phase deposition (MOCVD) [12]. Moreover, Ga₂O₃ and its composites can be synthesized using solution-based techniques such as sol-gel [13], hydrothermal [14], and chemical bath methods [15] and electrochemical deposition [16]. These methods enable large-area deposition and a high yield with simple processing at low temperatures in ambient conditions, thereby reducing the complexity and energy consumption during manufacturing. Additionally, these wet chemical solution processes are effective for preparing various types of Ga₂O₃-based nanomaterials [17]. Ga₂O₃ was also investigated for memristors and neuromorphic computing, with resistive switching mechanisms reliant on the formation and dissolution of conductive filaments in a metal–semiconductor–metal (MSM) structure, mainly due to the oxygen vacancies in Ga₂O₃ [18,19]. For n-type dopants, the Group IV elements Si, Ge, and Sn form shallow donor states in Ga₂O₃ [20]. The theoretical mobility of β-Ga₂O₃ is ~200 cm²/V·s and is primarily limited by interactions with longitudinal optical phonons [21,22,23]. At low temperatures, a remarkably high carrier mobility exceeding 23,000 cm²/V·s was demonstrated [12]. The saturation velocity of β-Ga₂O₃ is estimated to be 1 × 10⁷ cm/s [24]. β-Ga₂O₃ exhibits a considerably lower thermal conductivity in comparison to other WBG materials such as GaN and SiC [25]. Therefore, effective thermal management stands as an important challenge for the practical use of Ga₂O₃ in power electronic device applications. β-Ga₂O₃ possesses a monoclinic crystal structure with anisotropic physical and electrical properties that vary across different crystallographic orientations [8]. Consequently, attributes such as the dielectric constant [26], thermal conductivity [25], and electron mobility [27] exhibit variations depending on the crystallographic direction. This anisotropy presents both challenges in device fabrication and opportunities for innovative device architectures and operations.

One prominent challenge in the field is the absence of p-type β-Ga₂O₃, primarily attributed to significant difficulties in the introduction of acceptor impurities and the generation of holes within the material. Theoretical predictions indicate that the activation energy for acceptors (such as Mg, Zn, Be, and N) in β-Ga₂O₃ is exceptionally high (>1 eV), posing a fundamental hurdle in achieving hole conductivity [28]. Additionally, its flat valence band results in a large effective hole mass, low hole mobility, and a low diffusion constant [29]. To make things worse, holes tend to be localized in β-Ga₂O₃ as small polarons due to lattice distortion rather than behaving as “free” holes [30]. These factors limit the development of electronic devices based on β-Ga₂O₃ and hinder the realization of its full potential as a UWBG semiconductor. Consequently, the majority of the demonstrated β-Ga₂O₃ devices are unipolar, such as FETs [7] and Schottky barrier diodes (SBDs) [31]. Nevertheless, ongoing research endeavors are focused on achieving p-type conductivity in β-Ga₂O₃ through methods such as ion implantation [32], co-doping [33], and the formation of heterojunctions with other materials [34]. Heterojunction formation, in particular, has garnered significant attention due to its demonstrated excellent electrical characteristics, including a low on-resistance (R_on), a high ON/OFF ratio, and a large reverse blocking voltage [35]. These advances enable further exploration of β-Ga₂O₃’s full potential as a UWBG material, challenging its counterparts like GaN and SiC, where p-type doping is available [36,37].

This review primarily focuses on the utilization of β-Ga₂O₃ heterostructures in power electronics applications, which is organized into the following sections. The first section discusses p-NiO_x/β-Ga₂O₃ heterojunctions, which emerge as a viable solution for the lack of p-type doping in β-Ga₂O₃ [35]. The second part of the review mainly focuses on β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures and their applications in both vertical and lateral devices. The formation of two-dimensional electron gas (2DEG) at the β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructure through Si modulation doping enables the development of modulation-doped FETs (MODFETs). Furthermore, the review explores β-Ga₂O₃ heterostructures and heterojunctions with various other materials for innovative device architectures, including GaN, SiC, SnO, Cu₂O, CuI, and UWBG semiconductors like diamond, aluminum nitride (AlN), and boron nitride (BN). Finally, the discussion extends to β-Ga₂O₃ heterostructures and heterojunctions with two-dimensional (2D) materials like graphene, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), and black phosphorus (BP). These novel 2D materials possess unique characteristics suitable for diverse device configurations that can achieve efficient carrier transport. This timely review will serve as a valuable reference for the advancement of β-Ga₂O₃-based heterostructures in power electronics applications.

2. p-NiO_x/β-Ga₂O₃ Heterojunction

Recently, NiO_x has attracted significant attention as a promising p-type material for forming heterojunctions with n-type β-Ga₂O₃ [35]. While the incorporation of NiO_x with β-Ga₂O₃ is a relatively recent development, NiO_x has a well-established history of serving as hole contact in solar cell applications [38]. In particular, NiO_x is highly regarded for its efficient extraction of generated holes, notably in perovskite solar cells [39]. Various NiO_x deposition techniques are available, encompassing diverse methods such as sol-gel spin coating [40], RF sputtering [41], plasma-assisted laser deposition [42], electron beam (e-beam) evaporation [43], and the thermal oxidation of Ni [44]. Each of these methods presents its unique advantages and drawbacks. Among these techniques, RF sputtering is the most extensively researched method due to its capability to produce uniform polycrystalline films and control NiO_x doping by adjusting the O₂/Ar gas ratios and deposition pressure. More information about the RF sputtering deposition details can be found elsewhere [41,45,46]. It is important to note, however, that RF sputtering also causes damage to the β-Ga₂O₃ surface, as the high-energy plasma involved can induce interface states that can detrimentally impact the device performance [47]. Nevertheless, it has been demonstrated that sputtering can yield abrupt NiO_x/β-Ga₂O₃ interfaces.

Li et al. [41] and Xia et al. [45] have conducted investigations into the band alignment, bandgap, and resistivity and the influence of the post-annealing temperature on NiO_x films produced via RF magnetron sputtering. Their studies revealed that the bandgap of NiO_x varied from 3.9 to 3.7 eV due to the annealing process, as illustrated in Figure 1a. In Figure 1b, the X-ray photoelectron spectroscopy (XPS) spectrum shows distinct Ni and O peaks. During the deposition process, Ni²⁺ and Ni³⁺ ions can interact with O₂ to form NiO or Ni₂O₃, and the extent of this reaction strongly depends on the O₂/Ar gas flow, as demonstrated in Figure 1d. Figure 1c,f illustrate that annealing led to a reduction in Ni₂O₃ and an increase in NiO, as further confirmed by X-ray diffraction (XRD) measurements. Additionally, it has been observed that the bandgap and resistivity of the films were closely associated with the O₂/Ar gas flow (Figure 1e). Moreover, as described earlier, annealing resulted in a decrease in the bandgap and an increase in the resistivity of the films. Stoichiometric NiO is a wide-bandgap antiferromagnetic insulator [48]. The p-type properties of NiO_x primarily arise from the presence of Ni vacancies and O interstitials (native accepter defects) [49]. NiO_x can also be p-type-doped with alkali metals such as Li and K [50]. The elevated O₂ flow during deposition and post-deposition annealing can enhance the stoichiometry of the NiO_x films, resulting in films with a high resistivity and an improved hole mobility. In other words, native acceptor defects are thermally unstable and can be readily eliminated through an annealing process [49]. Furthermore, it has been observed that the polycrystalline nature improved with thermal annealing due to the crystalline grain growth [51].

Kokubun et al. [40] pioneered the development of the first NiO_x/β-Ga₂O₃ heterostructure with Li-doped p-NiO_x using a sol-gel spin coating process. While these devices exhibited favorable rectifying behavior, issues such as a high R_on and a low breakdown voltage (BV) limited their performance. It was deduced that the prevailing current transport mechanism was attributed to carrier recombination rather than carrier diffusion. Recent reports have further affirmed that, owing to the type-II band alignment of NiO_x/β-Ga₂O₃ devices, carrier diffusion does not play a significant role under low-bias conditions. Instead, interface recombination (Figure 2a) and multi-step tunneling (Figure 2b) have emerged as the predominant mechanisms governing current transport in these heterojunctions [49,51,52]. Consequently, the quality of the interface holds the utmost importance for such devices. In Figure 2c, a transmission electron microscope (TEM) image of the fabricated NiO_x/β-Ga₂O₃ diodes is presented, showing Au, Ni, NiO_x, and β-Ga₂O₃ layers. It should be noted that Ni/Au contact acts as good Ohmic contact for NiO_x with the aid of thermal annealing [51]. Figure 2d emphasizes the abrupt nature of the heterojunction using a high-resolution TEM (HRTEM) image; however, it also reveals regions that have incurred damage due to the high-energy plasma involved during sputtering (~150 W) [47]. To mitigate such issues, lower-energy plasma (~70 W) can be employed for NiO_x deposition, resulting in an abrupt and smooth NiO_x/β-Ga₂O₃ interface, as depicted in Figure 2e [53]. The damage that occurs during sputtering can introduce interface states that impact the device performance, potentially leading to increased leakage current and degradation of the device functionality. An alternative solution involves the use of e-beam-deposited NiO_x, where the interface quality is improved, as illustrated in Figure 2f [43].

The anisotropic nature of NiO_x/β-Ga₂O₃ in different crystal orientations has also been investigated. Deng et al. [54] investigated the dependence of the band alignment of NiO_x/β-Ga₂O₃ heterostructures on various crystal orientations and revealed differences in the conduction and valence band offsets. Specifically, (–201) exhibited the lowest conduction and highest valence band offsets, while (010) showed the opposite trend. Subsequent investigations involved the use of e-beam-evaporated NiO_x in various β-Ga₂O₃ crystal orientations, confirming clear interfaces across different orientations through HRTEM studies [43]. However, a significant disparity in the electronic properties was observed between (–201) [or (001)] and (010) orientations, manifesting as variations in the turn-on voltage, ON/OFF ratio, interface defect density, and temperature-dependent forward current transport. These differences are primarily attributed to distinct atomic configurations, the density of the dangling bonds, and variations in the interface trap-state densities across different crystal planes [43]. NiO_x/β-Ga₂O₃ heterostructures commonly exhibit hysteresis, attributed to the interface traps at the NiO_x/β-Ga₂O₃ heterojunction. Hao et al. [55] demonstrated a reduction in I–V hysteresis by subjecting the devices to 350 °C thermal annealing for 3 min post-fabrication. This annealing process resulted in a decrease in the interface trap states from 1.04 × 10¹² to 1.33 × 10¹¹ eV⁻¹cm⁻² and a reduction in the ideality factor from 3.02 to 1.27 after annealing. Following optimization, the devices achieved a BV of 1.63 kV with a high power figure of merit (PFOM) of 0.65 GW/cm². Gong et al. [52] and Yan et al. [56] investigated interface trap states using capacitance–frequency (C–F) measurements and fitted the data with equations that assumed the distribution of the interface states into two energy states. The use of two states proved more effective, representing the interface states closer to the conduction band and others with deeper energy levels [43]. Gong et al. [52] and Yan et al. [56] both observed interface states in the order of ~10¹⁰ eV⁻¹cm⁻², which are among the lowest reported values. In addition, Li et al. [57] demonstrated the thermal annealing stability of NiO_x/β-Ga₂O₃ p-n diodes from 300 to 500 °C in comparison with the as-deposited NiO_x/β-Ga₂O₃ p-n diodes. Annealing at 300 °C resulted in a reduced low forward bias current, while the specific on-resistance (R_on,sp) remained relatively stable. However, annealing at higher temperatures from 400 to 500 °C degraded the R_on,sp due to the increased resistivity of the NiO_x and degraded Ni/Au contact. Between 300 and 500 °C, the reverse leakage current decreased compared to in the as-deposited samples, with the 300 °C annealed sample exhibiting the highest BV. This indicates that temperature optimization is crucial to enhancing the heterointerface in these devices.

Lu et al. [58] demonstrated a NiO_x/β-Ga₂O₃ heterojunction p-n diode with a low R_on,sp of 3.5 mΩ·cm² and a high BV of 1.06 kV. A low reserve leakage of <1 µA/cm² was observed until its breakdown. Zhang et al. [59] showcased a p-n diode featuring an impressive BV of 8.32 kV and an R_on,sp of 5.24 mΩ·cm², resulting in a PFOM of 13.2 GW/cm². In Figure 3a, the device configuration is depicted, featuring two NiO_x regions and a field plate (FP) structure designed to withstand high BVs. Utilizing a double NiO_x layer provided a low R_on,sp for the device while maintaining the BV [49]. The devices had drift regions of 7.5 and 13 µm with an unintentional doping (UID) concentration of 5–7 × 10¹⁵ cm⁻³. Figure 3b illustrates the forward I–V characteristics and R_on,sp curves for both device types. Additionally, these devices exhibited remarkable thermal stability up to 150 °C, as indicated in Figure 3c. Figure 3d presents the BV curves for these devices, marking a significant milestone in the NiO_x/β-Ga₂O₃ p-n diode research. Li et al. [47] presented a noteworthy achievement with a 4.7 kV NiO_x/β-Ga₂O₃ p-n diode through meticulous optimization of the thickness of the lower NiO_x layer. Their device fabrication process aligned with the TCAD simulations, which indicated that extending the NiO_x layer beyond the contact area could effectively alleviate electric field crowding, ultimately leading to a high BV. In Figure 3e, the device structure is depicted, while Figure 3f illustrates the forward I–V curves for the fabricated devices with varying thicknesses of the lower NiO_x layer in comparison to a conventional β-Ga₂O₃ SBD. Figure 3g showcases the corresponding BVs, and Figure 3h presents the forward I–V curves in log scale, along with a comparison of the R_on,_sp of the devices. These devices showed a notable PFOM of 2 GW/cm². The same authors conducted a comprehensive study to optimize the NiO_x doping, thickness, and junction termination extension (JTE) for NiO_x/β-Ga₂O₃ devices with kV-class operation using Silvaco TCAD simulations [60]. Additionally, they successfully demonstrated high-BV devices with a maximum of 6.5 kV, achieved through thermal annealing optimization [57]. They also showcased p-n diodes with a 3.6 kV BV and a 4.8 A current capability within a 1 mm² device area [61].

Xiao et al. [62] demonstrated a ~3.5 kV NiO_x/β-Ga₂O₃ p-n diode with multiple JTEs of NiO_x layers with different lengths (Figure 4a). This mechanism allows a gradual decrease in the effective charge carriers away from the device’s active region to smoothen the electric field crowding at the anode edge, as in Figure 4b. The bottom two JTEs consisted of lightly doped NiO_x, while the top JTE was highly doped. This structure reduces the crowded electric field at the anode by reducing the surface electric field. Figure 4c shows the BV of devices with JTEs fabricated on two sets of wafers and without JTEs. Zhou et al. [63] presented a p-n diode featuring a bevel structure, as shown in Figure 4d, which exhibited remarkable characteristics, including a high BV of 2.04 kV and an impressive R_on,sp of 2.26 mΩ·cm². These devices demonstrated a significant BFOM of 1.84 GW/cm² (2.87 GW/cm²) with DC (pulsed) measurements. In Figure 4e, the BV curves of these diodes compared to a conventional β-Ga₂O₃ SBD with and without bevel edge termination are depicted. Figure 4f illustrates an effective reduction in electric field crowding using various bevel angles. Furthermore, this device exhibited a rapid reverse recovery time of 16.4 ns, outperforming Si fast-recovery diodes (Si FRDs), showing a significant advancement in current technology (Figure 4g). Hao et al. [64] demonstrated a 2.6 kV p-n diode by employing a JTE and annealing the device at 300 °C for 10 min. This diode exhibited stability at high temperatures and a BV of 1.77 kV at 250 °C. Furthermore, a low R_on,sp of 2.5 mΩ.cm² resulted in a PFOM of 2.83 GW/cm². Wang et al. [65] achieved a 2.41 kV p-n diode with a JTE and a small-angle beveled FP, boasting a remarkable R_on,sp of 1.12 mΩ·cm² and a record BFOM of 5.18 GW/cm². Gong et al. [66] demonstrated a 1.86 kV NiO_x/β-Ga₂O₃ p-n diode that showed stability at 400 K with the use of double NiO_x layers. Most recently, a NiO_x/β-Ga₂O₃ p-n diode with a 7.5 kV BV, a PFOM of 6.2 GW/cm², and an ON/OFF ratio of >10¹³ was demonstrated [67]. Collectively, these works highlight the immense potential of p-NiO_x/β-Ga₂O₃ heterojunctions for advanced power device applications.

In addition to p-n junctions, NiO_x can serve as a material for edge termination in β-Ga₂O₃ devices. This facilitates charge balance at the heterojunction, reducing electric field crowding and enhancing the device’s high-voltage-handling capabilities. NiO_x-based JTEs [56,62,68], guard rings [69], and reduced surface field structures (RESURFs) [70,71] have been reported as edge terminations for β-Ga₂O₃ devices. Yan et al. [56] demonstrated a β-Ga₂O₃ SBD with NiO_x heterojunction edge termination and a SiO₂ FP structure (Figure 5a). These devices showed an average BV of 1.86 kV and an R_on,sp of 3.12 mΩ·cm², yielding a BFOM of 1.11 GW/cm². Furthermore, these devices showed a low interface trap density of 4.13 × 10¹⁰ eV⁻¹cm⁻². Figure 5b–d show the forward I–V, R_on,sp, and BV of SBDs with different anode diameters. Gong et al. [69] showcased a high-performance SBD with p-NiO_x field-limiting rings (FLRs). A BFOM of 0.46 GW/cm² was obtained with a 1.89 kV BV and an R_on,sp of 7.7 mΩ·cm² for an SBD with an FLR width/spacing of 2 µm. It should be noted that an 8 µm JTE is also included in the design before the FLRs, as in Figure 5e. In this work, the authors fabricated p-n diodes, junction barrier Schottky diodes (JBS), conventional SBDs, and SBDs with 2 and 3 µm FLRs. Figure 5f–h show the forward I–V, R_on,sp, and BV of all the fabricated devices. Wang et al. [68] presented an SBD with three JTEs stacked on top of each other. These JTEs served to inhibit the electric field crowding in proximity to the Schottky contact, thus facilitating efficient charge extraction during device switching. Ma et al. [46] demonstrated the control of p-type doping in NiO_x in the range of <10¹⁸ to 2 × 10¹⁸ cm⁻³ by changing the O₂ partial pressure during RF magnetron sputtering. Precise control of p-type doping allows for charge balance in the n-type Ga₂O₃ region, leading to SBDs with ~8 kV BV. Recently, Qing et al. [71] presented an SBD featuring a NiO_x RESURF structure that achieves a BV exceeding 10 kV, both at room temperature and 200 °C. Achieving a 10 kV performance at both room temperature and 200 °C marks a significant milestone in power electronics, highlighting the substantial potential of NiO_x/β-Ga₂O₃ devices.

The JBS diodes combine the advantages of both SBDs and p-n diodes, offering the low turn-on voltage of SBDs and the low reverse leakage current of p-n diodes. During forward bias conditions, the Schottky regions are activated, allowing for efficient high-current transport, while at reverse bias, the Schottky regions were shielded from a high electric field due to the potential barrier from the p-n junction regions. Therefore, JBS devices have reduced resistive losses during forward bias (low turn-on) and the capability to handle high blocking voltages during reverse bias. The availability of p-type NiO_x enables the fabrication of NiO_x/β-Ga₂O₃ JBS diodes. Multiple JBS diodes have been successfully demonstrated to have the capability for high forward currents and high reverse blocking voltages, some reaching into the kV-class [72,73,74]. These achievements were made possible through the utilization of sputtered and thermally oxidized p-NiO_x. Lv et al. [72] showed a JBS diode featuring an R_on,sp of 3.45 mΩ·cm², along with a BV of 1.75 kV for a small-area device (100 µm²). This small-area device exhibited an impressive BFOM of 0.85 GW/cm². For a larger-area device (1 mm²), it delivered 5 A of forward current and could withstand voltages up to ~0.7 kV. The NiO_x was obtained through thermal oxidation, involving the exposure of a 5 nm Ni layer to a temperature of 600 °C for a duration of 10 min in an O₂ atmosphere. In the resulting devices, SBD exhibited a turn-on voltage of 0.7 V, while the JBS diode had a slightly higher turn-on voltage of 1.0 V. This slight difference in voltage is due to the lateral depletion of the NiO_x/β-Ga₂O₃ heterojunctions in the JBS diode. Yan et al. [73] reported a JBS diode with an R_on,sp of 1.94 mΩ·cm² and a BV of 1.34 kV, resulting in a PFOM of 0.93 GW/cm². The reduction in the β-Ga₂O₃ fin width was identified as a contributing factor to minimizing the reverse leakage current due to the enhanced sidewall depletion in the p-NiO_x. The devices were manufactured using sputtered NiO_x and varying β-Ga₂O₃ fin widths, specifically 1.5, 3, and 5 µm. Wu et al. [74] presented a JBS diode with and without a beveled FP. The incorporation of a beveled FP effectively reduced the electric field crowding from 7.8 to 5.1 MV/cm. These JBS diodes exhibited a di/dt of ~400 A/µs, a reverse recovery time of 26.8 ns, and a reverse recovery charge of 46.2 nC. Additionally, these devices were assembled and used to construct a voltage multiplier, demonstrating the practical applicability of NiO_x/β-Ga₂O₃ JBS diodes. These results highlight the promising future of NiO_x/β-Ga₂O₃ JBS diodes for high-current, high-voltage applications, paving the way for energy-efficient power electronics.

Over the years, β-Ga₂O₃-based lateral FETs have shown significant advancements in their electrical properties [7]. However, to approach the material’s limits with β-Ga₂O₃, achieving a lower R_on and a higher BV is imperative. Introducing NiO_x beneath the gate in β-Ga₂O₃-based lateral FETs can enhance the R_on and improve the breakdown characteristics. This innovative configuration is referred to as a NiO_x/β-Ga₂O₃-based junction field-effect transistor (JFET). In lateral FETs, higher channel doping can lead to a lower R_on, but it may also decrease the device’s BV. However, by incorporating NiO_x under the gate, vertical depletion within β-Ga₂O₃ can allow for higher doping to optimize the R_on without compromising the device’s BV. Moreover, lateral depletion can reduce the electric field crowding on the drain side of the gate, further enhancing the device’s BV. Wang et al. [75] introduced the first NiO_x/β-Ga₂O₃ JFET. The output and transfer characteristics of the devices feature an R_on of 38 mΩ·mm and an ON/OFF ratio of 10¹⁰. The device exhibited a subthreshold slope (SS) of 76 mV/Dec and a BV of 1.12 kV. Zhou et al. [76] presented a normally-off NiO_x/β-Ga₂O₃ JFET with a recessed gate and a NiO_x layer. This device exhibited a threshold voltage (V_th) of 0.9 V, an R_on of 151.5 Ω·mm, a BV of 0.98 kV, and a SS of 73 mV/Dec. These results indicate promising early developments for NiO_x/β-Ga₂O₃ JFETs.

Recent advancements in NiO_x/β-Ga₂O₃ heterojunction devices showcase their remarkable potential in high-voltage and high-power applications. These innovations encompass kV-class p-n diodes, NiO_x edge-terminated Ga₂O₃ SBDs, JBS diodes, and JFETs. These devices offer enhanced electrical characteristics and exhibit impressive voltage handling capabilities ranging from ~1 to 10 kV. It is worth noting that even though p-type doping of β-Ga₂O₃ might be realized in the future, heterojunction p-n diodes still present some advantages over homojunction p-n diodes. Heterojunction diodes allow for tuning of the electronic properties based on the conduction and valence band offsets, which offers unique device characteristics, such as fast switching. Moreover, heterojunctions offer a cost-effective approach to device processing and enable advanced device designs that homojunction devices struggle to achieve. These remarkable advancements highlight the substantial progress made in the application of NiO_x/β-Ga₂O₃ heterojunction devices, setting the stage for more energy-efficient and high-performance power electronics that are well suited to meet the demands of the next decade.

3. β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ Heterostructures

Alloying Ga₂O₃ with Al₂O₃ results in (Al_xGa_1−x)₂O₃ [77], which possesses an increased bandgap (e.g., 4.8–6.2 eV for x = 0 to 0.71) [78]. β-(Al_xGa_1–x)₂O₃ represents a monoclinic ternary alloy expected to offer a higher BFOM than β-Ga₂O₃, rendering it more suitable for power electronics applications [77]. A significant challenge lies in the production of high-quality β-(Al_xGa_1−x)₂O₃ layers with a substantial Al content, such as exceeding 20% in (010) β-Ga₂O₃ substrates [79]. This challenge arises from the fact that β-Ga₂O₃ has a monoclinic crystalline structure, while Al₂O₃ tends to adopt a corundum structure, leading to phase separation with a high Al content [80]. Additionally, the lattice mismatch with a high Al composition makes the epitaxial growth of β-(Al_xGa_1−x)₂O₃ on β-Ga₂O₃ a particularly demanding task. The primary approaches to growing β-(Al_xGa_1−x)₂O₃ are MBE [1] and MOCVD [78]. It is worth noting that other methods, such as pulsed laser deposition (PLD) [81] and sputtering [82], can also be employed for depositing β-(Al_xGa_1−x)₂O₃. This section primarily focuses on β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures grown using MBE and MOCVD for power electronics.

Oshima et al. [83] presented the early stages of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ growth using plasma-assisted MBE (PAMBE) on (100) β-Ga₂O₃ single crystals grown via the floating zone (FZ) method. During PAMBE growth, Ga and Al were evaporated from the effusion cells within RF-generated O plasma, resulting in the formation of β-(Al_xGa_1−x)₂O₃ thin films. More detailed growth parameters are provided elsewhere [76,77]. The Al content in the grown films was estimated by analyzing the shift in the high-resolution X-ray diffraction (HRXRD) spectra between the β-Ga₂O₃ and β-(Al_xGa_1−x)₂O₃ (020) peaks [84]. Using the equation x = 0.4727 × Δθ₀₂₀, the Al composition in the (010) crystal orientations could be accurately determined, where Δθ₀₂₀ represents the shift between the β-Ga₂O₃ and β-(Al_xGa_1−x)₂O₃ (020) peaks. They successfully incorporated Al up to x = 0.61 while maintaining the integrity of the β-phase crystal structure. However, at x = 0.68, the sample became polycrystalline, and the surface roughness of the samples was high. Stephen et al. [85] demonstrated the growth of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ in (010) crystal orientations with a varying Al content in the range of x = 0.1 to 0.18 by optimizing the PAMBE growth conditions, such as substrate temperature and Al flux. These samples showed a good surface morphology with a maximum incorporation of 18% Al.

Zhang et al. [86] demonstrated a two-dimensional electron gas (2DEG) with high electron mobility through modulation doping and the use of Fe-doped semi-insulating β-Ga₂O₃ substrates. In the band diagram of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃, the Fermi level is above the conduction band (Figure 6c), leading to carrier accumulation at the interface. The β-(Al_xGa_1−x)₂O₃ layer can be modulation-doped with Si (δ-doping) (Figure 6a), and these free carriers migrate to the interface, resulting in the formation of a 2DEG with a sheet electron density of approximately 10¹² cm⁻². Figure 6b displays the HRXRD spectra of the grown β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures. Two samples were grown with 130 and 360 nm buffer layers. The thicker buffer layer exhibited a higher 2DEG density due to the reduction in the backside depletion. Hall measurements were done using the van der Pauw geometry of the regrown highly doped β-Ga₂O₃ contact, and the charge density showed a weak temperature dependence (Figure 6d), indicating the presence of a formed 2DEG channel with a high low-temperature mobility of 2790 cm²/V·s at 50 K. The room temperature mobility was measured at 162 cm²/V·s for sample A and 180 cm²/V·s for sample B. The observed mobility was fitted using models for polar optical phonon scattering, remote impurity scattering, background impurity scattering, interface roughness scattering, and acoustic deformation potential scattering. A good agreement between the experimental values and fitting was observed, as demonstrated in Figure 6e,f. These findings demonstrate the formation of a 2DEG channel using β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ with Si modulation doping.

Miller et al. [87] reported the MOCVD growth of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ using the metal–organic precursors trimethylaluminum (TMAl) and triethylaluminium (TEAl) as the Al source and triethylgallium (TEGa) as the Ga source and O₂ for oxidation. They successfully achieved β-(Al_xGa_1−x)₂O₃ alloys with an Al content x up to 43% on sapphire substrates and demonstrated β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ superlattices. Compared to MBE, MOCVD is the industrial standard tool for the mass production of epitaxial compound semiconductor films. However, MBE operates under an ultra-high vacuum and requires the substrate temperature to be kept at <700 °C to reduce the decomposition of Ga₂O₃ into volatile suboxides. This limits the maximum Al₂O₃ incorporation into β-Ga₂O₃ to around 25% using MBE [88]. In contrast, the low to medium pressure in MOCVD allows the growth of β-(Al_xGa_1−x)₂O₃ at substrate temperatures >800 °C, with high solubility of the Al₂O₃ in β-Ga₂O₃. This prevents the formation of volatile suboxides and enables a high Al content in the β-(Al_xGa_1−x)₂O₃ barriers. The MOCVD growth has a large temperature window and precise control of the precursor flow, which makes it suitable for growing high-Al-fraction β-(Al_xGa_1−x)₂O₃ thin films. Such a high Al content is critical to realizing the formation of a 2DEG in the β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterojunctions and achieving a sufficiently large conduction band offset to confine the electrons [85]. Recent efforts involving the incorporation of Al through MOCVD have achieved 99% incorporation in the (100) orientation. However, the (010) and (−201) orientations exhibited lower Al incorporation, which was limited to 29% and 16%, respectively [89].

Ranga et al. [78] demonstrated n-type degenerate doping in β-(Al_0.26Ga_0.74)₂O₃ thin films grown through MOCVD and employed modulation doping in β-(Al_0.26Ga_0.74)₂O₃/β-Ga₂O₃ heterostructures. The Hall measurements unveiled a high carrier concentration ranging from 0.6 to 7.3 × 10¹⁹ cm⁻³, accompanied by a corresponding electron mobility of 53–27 cm²/V·s within the uniformly doped β-(Al_0.26Ga_0.74)₂O₃ layers. To assess the quality of the Si-doped β-(Al_xGa_1−x)₂O₃ epilayers, HRXRD measurements were taken to determine the Al composition of these epilayers. The results indicate Al mole fractions of approximately 0.26, closely matching the precursor molar flow ratio (25%) (Figure 7a), indicating that there were no pre-reactions with a complete incorporation of Al. Following the deposition of Ti/Au, Ohmic contact behavior was observed at room temperature. The temperature-dependent carrier concentration and mobility of the epilayers were assessed using Hall measurements (Figure 7b,c). Among the three samples grown using different silane (SiH₄) flows, it was observed that sample A did not form Ohmic contact below 160 K. All three samples showed degenerate doping, as no carrier freeze-out was evident. To further confirm these observations, Figure 7d shows calculations of the separation from the Fermi level and conduction band considering different Al fraction values (α), where α = 0.22 (non-parabolic bands) and α = 0 (parabolic bands) for the three samples. This analysis confirmed that sample A exhibited weakly degenerate characteristics, while samples B and C were strongly degenerate. To achieve a total electron sheet charge of 2.3 × 10¹² cm⁻², modulation doping in a β-(Al_0.26Ga_0.74)₂O₃/β-Ga₂O₃ heterostructure was utilized, employing a uniformly doped β-(Al_0.26Ga_0.74)₂O₃ barrier layer alongside a thin spacer layer, as illustrated in the inset in Figure 7e. In Figure 7e, the HRXRD spectrum of the β-(Al_0.26Ga_0.74)₂O₃/β-Ga₂O₃ heterostructures on a Sn-doped substrate is presented. C–V measurements were conducted to measure the charge density and depth relationship. A peak corresponding to the 2DEG was observed near the estimated barrier thickness obtained from HRXRD, indicating modulation doping at this interface. The sudden drop in the C–V profile with depth (Figure 7f) confirms the carrier confinement at the heterointerface. Figure 7g presents the simulated band diagram using a Poisson–Schrödinger solver, which is closely aligned with the measured 2DEG value from the C–V profile.

Recently, Wang et al. [90] conducted an extensive study using TCAD simulations to gain insights into β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures. The impact of the δ-doping concentration, the influence of the δ-doping position, and the effect of the δ-doping width were systematically investigated [90]. Multiple MODFETs have been developed after the successful growth of high-quality β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures. Ahmadi et al. [91] reported β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ MODFETs grown via MBE with Ge as the n-type dopant. Their work resulted in a maximum current density of 20 mA/mm and a pinch-off voltage of −6 V, with a 2DEG sheet charge density of 1.2 × 10¹³ cm⁻². Zhang et al. [86] employed the same structure grown using MBE to fabricate a β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ MODFET (Figure 6a). The heterostructure exhibited a high channel mobility at low temperatures, enabling the measurement of the Shubnikov–de Haas (SdH) oscillations in the transverse magnetoresistance under varying magnetic fields perpendicular to the sample surface. The calculated 2DEG densities from the SdH oscillations were 1.15 × 10¹² and 1.96 × 10¹² cm⁻² for samples A and B, respectively, aligning with low-field Hall measurements (Figure 6d). The transconductance (g_m) decreased at a higher gate bias due to the reduced modulation efficiency resulting from the charge transfer into the barrier layer. The device exhibited ~10⁹ rectification and an SS of ~91mV/Dec. The extracted V_th was 0.5 V, indicating a normally-off operation. High-frequency small-signal measurements conducted on this device showed a cutoff frequency of 3.1 GHz and a maximum oscillation frequency of 13.1 GHz at a V_DS of 10 V and a V_GS of 1.5 V. Kalarickal et al. [92] achieved a higher 2DEG sheet charge density, reaching up to 4.7 × 10¹² cm⁻², along with an effective mobility of 150 cm²/V·s using an ultra-thin spacer layer of just 1 nm. Increasing the 2DEG density is important as it aids in reducing the R_on and boosting the drain current in MODFETs. Moreover, a heightened charge density contributes to better screening of the strong polar optical phonon scattering, ultimately enhancing mobility. With a fixed conduction band offset, reducing the spacer thickness can be used to increase the 2DEG density. Furthermore, given that polar optical phonon scattering already limits mobility (~200 cm²/V·s), the potential impact of increased remote ionized impurity scattering on the total mobility is expected to be minimal with a 1 nm spacer. To prevent the diffusion of the Si δ-doping through the thin spacer, a lower growth temperature of 610 °C was used. Additionally, Kalarickal et al. [93] presented an effective field management approach to β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ MODFETs by employing an extremely high-k dielectric, BaTiO₃ (k > 100). This innovative strategy facilitated a more uniform electric field distribution within the gate–drain region of the FET. Using this method, average breakdown fields of 1.5 and 4 MV/cm were achieved at gate-to-drain spacings (L_GD) of 6 and 0.5 μm, respectively, with a high channel sheet charge density of 1.6 × 10¹³ cm⁻². This resulted in a record PFOM of 376 MW/cm² at an L_GD of 3 μm.

Zhang et al. [94] demonstrated a double heterostructure MODFET (DHFET), as in Figure 8a. In this configuration, the β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃/β-(Al_xGa_1−x)₂O₃ heterostructure acts as a quantum well and provides a 2DEG charge density higher than that of a single heterostructure. The Hall measurements indicated a mobility of 1775 cm²/V·s at 40 K and 123 cm²/V·s at room temperature. Figure 8b,c show the output and transfer curves of the demonstrated device with the physical dimensions and the biasing conditions. Figure 8d shows a simulation of the energy band diagram of the double heterostructure with and without parallel conducting channels. The important thing to note is that parallel conducting channels in the β-(Al_xGa_1−x)₂O₃ barrier layers can show up at high-impurity doping levels, as in the upper diagram of Figure 8d. These parasitic channels can lower mobility and affect the device performance. Therefore, precise control of Si doping is necessary to minimize the effect of parasitic conducting channels. The channel mobility at room temperature with the depth profile was extracted from a demonstrated transistor, as in Figure 8e. Later, Kalarickal et al. [95] reported a β-(Al_0.18Ga_0.82)₂O₃/Ga₂O₃ DHFET with an average field of 5.5 MV/cm, where a field management strategy with high-k BaTiO₃ and low-k SiO₂ and Al₂O₃ was also employed. The measured output characteristics of the β-(Al_0.18Ga_0.82)₂O₃/β-Ga₂O₃ DHFET showed an R_on of 28.6 Ω·mm. The corresponding transfer characteristics revealed a rectification ratio of ~10⁷. The device breakdown voltage increased from 0.66 V to 1.11 kV as the L_GD was increased from 1.15 to 5.15 μm. Vaidya et al. [96] reported high-performance β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ MODFETs with an elevated g_m and a peak current density reaching 74 mA/mm. The small signal frequency measurements revealed impressive cutoff frequencies for the current gain and power gain, measuring 30 and 37 GHz, respectively, at a V_GS of 1.2 V and a V_DS of 10 V. Joishi et al. [97] demonstrated the breakdown characteristics of a field-plated β-(Al_0.22Ga_0.78)₂O₃/β-Ga₂O₃ MODFETs with Si₃N₄ passivation. A high BV of 1.37 kV was shown for a L_GD = 16 µm device with an R_on,sp of 120 Ω·cm². A comparably high average breakdown field of 3.9 MV/cm was obtained for an L_GD = 320 nm device. This work shows effective passivation and field management strategies such as gate-connected FPs can significantly enhance the device performance. Recently, Wang et al. [98] explored a novel current aperture vertical transistor (CAVET) using TCAD simulation by incorporating a β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructure. These results indicate the potential of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ MODFETs.

As the fabrication techniques for β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures continue to advance, there is a promising future for employing these heterostructures in vertical devices. Nevertheless, the growth of the β-(Al_xGa_1−x)₂O₃ layer is constrained by the relatively slow growth rate, and as the layer thickness increases with a higher Al content, the film quality tends to deteriorate. Consequently, kV-class vertical devices based on β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ have not yet been realized. However, the use of UID of β-(Al_xGa_1−x)₂O₃ as a cap layer to enhance the BV of β-Ga₂O₃ SBDs has been explored [99]. Ahmadi et al. [100] investigated β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures for the fabrication of SBDs with varying Al contents (Figure 9a). The diodes were produced by growing a 400 nm thick β-Ga₂O₃ layer on Sn-doped β-Ga₂O₃ (010) substrates, followed by a 100 nm thick β-(Al_xGa_1−x)₂O₃ layer with four different Al compositions. The HRXRD measurements (Figure 9b) confirmed the different Al compositions to be 0, 0.038, 0.084, and 0.164. The I–V characteristics revealed that similar Schottky barrier heights were observed in the SBDs with varying Al contents, which is attributed to the lateral fluctuations in the alloy composition leading to no variations in the barrier height. Figure 9c shows the temperature-dependent I–V data for an x = 0.084 composition. Furthermore, an increase in temperature resulted in a decrease in the ideality factor and an increase in the Schottky barrier height of the devices (Figure 9d), indicating an inhomogeneous Ni-Schottky contact. Sundaram et al. [99] introduced a novel structure to enhance the BV by growing a 30 nm β-(Al_xGa_1−x)₂O₃ capping layer to increase the Schottky barrier height, as shown in Figure 9e. They fabricated and compared β-(Al_xGa_1−x)₂O₃-capped (x = 0.22) SBDs with conventional β-Ga₂O₃ SBDs. Electrical measurements were conducted on both devices to determine the Schottky barrier height, and the resulting I–V curves are presented in Figure 9f. The extracted Schottky barrier heights for the β-Ga₂O₃-only and β-(Al_xGa_1−x)₂O₃-capped devices were 1.32 and 1.58 eV, respectively. This indicates an increase of 0.26 eV for the β-(Al_xGa_1−x)₂O₃-capped sample compared to the β-Ga₂O₃-only device. In Figure 9g, the reverse I–V characteristics of the two SBDs are shown, where the β-Ga₂O₃-only device and β-(Al_xGa_1−x)₂O₃-capped device exhibit BVs of 246 V and 387 V, respectively. Figure 9h depicts the simulated electric field around the Schottky contact’s edge at reverse breakdown. For the β-Ga₂O₃-only device, the maximum electric field under the anode edge was 2.5 MV/cm at breakdown (Figure 9h). In the β-(Al_xGa_1−x)₂O₃-capped device, the maximum field was 3.6 MV/cm. The additional Schottky barrier height created by the β-(Al_xGa_1−x)₂O₃ cap at the Schottky interface reduced the thermionic emission (TE) and thermionic field emission (TFE), thereby increasing the device’s BV.

To gain a deeper understanding of the leakage mechanisms of β-(Al_xGa_1−x)₂O₃ SBDs, Mudiyanselage et al. [101] conducted a study on the temperature-dependent reverse I–V characteristics of these devices. β-(Al_xGa_1−x)₂O₃ (x = 0.21) thin films were grown via MBE on a (010) Sn-doped β-Ga₂O₃ substrate, as depicted in Figure 9i. An anode made of Pt/Ti/Au was deposited to form the Schottky contact. Figure 9j presents the temperature-dependent forward I–V characteristics of the devices, showing an increasing current density with increasing temperature. Figure 9k displays the Schottky barrier heights and ideality factors of the vertical β-(Al_xGa_1−x)₂O₃ SBD calculated using the TE theory. As the temperature increased, the barrier height varied from 0.91 to 1.18 eV, while the ideality factor changed from 1.8 to 1.1. These changes are attributed to the non-uniform nature of the metal/semiconductor interface (i.e., inhomogeneous Schottky contact), resulting in the clear temperature-dependent behavior of both parameters. In Figure 9l, the leakage mechanisms for the SBDs are depicted. Different temperature ranges activate distinct processes within the semiconductor, as revealed by the reverse I–V measurements. Because of the high Schottky barrier height exceeding 1 eV in the vertical β-(Al_xGa_1−x)₂O₃ SBD, the most likely mechanisms include Poole–Frenkel emission (PFE), trap-assisted tunneling (TAT), Fowler–Nordheim tunneling (FNT), field emission (FE), and variable range hopping (VRH). To identify the dominant process, researchers often fit the reverse leakage data using the mathematical expressions of each model. Our analysis identified PFE and TAT as the two dominant mechanisms, with the other mechanisms playing a minor role.

Wan et al. [102] presented a lateral rectifier based on a NiO_x/β-(Al_0.21Ga_0.79)₂O₃/β-Ga₂O₃ heterojunction. The structure of these rectifiers is depicted in Figure 10a, with the β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructure grown using a MOCVD system. NiO_x was deposited using RF magnetron sputtering, and a bilayer structure was employed to optimize the sheet resistance and BV. Figure 10b shows the temperature-dependent forward I–V characteristics and the R_on,sp of the devices on a logarithmic scale, while Figure 10c shows the forward I-V characteristics on a linear scale with the extracted turn-on voltage at each temperature. The device current density increased with temperature, while the device turn-on voltage decreased as the temperature increased. Figure 10d shows the reverse I–V characteristics of the devices, where the negative temperature dependence of the BVs was observed, and thus the breakdown mechanism did not involve avalanche breakdown. They further studied devices with different diameters (50, 75, and 100 µm) [103]. The forward I–V curves of the devices with different diameters showed that the inclusion of the alloy did not significantly degrade the turn-on voltage. The highest BV value exceeding 7 kV was achieved in the 75 µm diameter lateral rectifier. A reverse I–V measurement at a low bias (<−100) indicated that the reverse leakage current in this bias range was primarily governed by TFE.

In short, the above research and the development of β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures have shown considerable promise for power electronics. These heterostructures have been investigated in various device configurations, such as SBDs and MODFETs. The outcomes of these studies provide important insights into their electrical characteristics and potential applications. Further exploration and optimization of these heterostructures may lead to significant advancements in the field.

4. β-Ga₂O₃ Heterojunctions with Other p-Type Materials

In addition to NiO_x, other p-type semiconducting materials with wide bandgaps, such as SiC and GaN, and metal oxides like Cu₂O, CuI, and SnO have shown promise in forming p-n heterojunctions with β-Ga₂O₃. However, the exploration and utilization of these heterojunctions, in contrast to NiO_x, are still in the early stages of development. Montes et al. [104] demonstrated a p-n heterojunction created using mechanically exfoliated β-Ga₂O₃ thermally bonded with p-GaN, as depicted in Figure 11a. The electrical characteristics of the devices are presented in Figure 11b on both the linear and logarithmic scales, while Figure 11c provides a comparison between the p-n diode and a simultaneously fabricated β-Ga₂O₃ SBD. These devices exhibited good temperature behavior, as shown in Figure 11d. Due to a significant number of interface states, the ideality factor for these devices was relatively large and decreased with an increasing temperature. The turn-on voltage of the devices was approximately 3.6 V at room temperature and decreased to around 2.2 V at 200 °C (Figure 11e). A band diagram of the devices was simulated using Silvaco TCAD in Figure 11f. The ideal turn-on voltage from the simulation was 3.2 V, while most of the fabricated devices exhibited a higher turn-on voltage, mainly due to the mechanical bonding of the p-n heterojunction (Figure 11g). In Figure 11h, an Arrhenius plot showed the conductance as a function of temperature at a forward bias of 3.5 V, demonstrating improved conductivity with increasing temperature. Moreover, p-GaN/β-Ga₂O₃ diodes were created with varying β-Ga₂O₃ thickness. Figure 11i shows the I–V characteristics of the p-n heterojunction with β-Ga₂O₃ thicknesses of 100 nm and 5 and 20 μm, indicating similar electrical characteristics across the different thicknesses. Mudiyanselage et al. [105] conducted theoretical investigations into edge termination structures for this p-n heterojunction, utilizing a 5 μm thick β-Ga₂O₃ drift layer, which has enabled the development of kV-class power diodes. Nandi et al. [106] have conducted research on JBS diodes utilizing this heterostructure, examined the switching characteristics of the devices, and obtained information on the reverse recovery behavior. Furthermore, Mudiyanselage et al. [107] studied JBS diodes by optimizing the p-GaN regions and varying their shapes to minimize the electric field crowding within the devices. Under forward bias, the Schottky regions became active, while under reverse bias, the p-n junctions effectively reduced the leakage currents. Deep UV photodiodes based on β-Ga₂O₃/SiC heterojunctions were demonstrated [108]. Nakagomi et al. [109] presented a β-Ga₂O₃/SiC heterojunction diode according to the thermal evaporation of Ga in O₂ plasma onto a p-type 6H-SiC substrate. Song et al. [110] investigated a β-Ga₂O₃/4H–SiC MOSFET using fusion bonding.

Alternative materials such as Cu₂O, CuI, and SnO have also been employed to create p-n heterojunctions with β-Ga₂O₃. Watahiki et al. [111] fabricated p-Cu₂O/β-Ga₂O₃ diodes, as depicted in Figure 12a. The heterojunction demonstrated a significant forward current, pointing to a low conduction band offset at the p-Cu₂O/β-Ga₂O₃ interface (Figure 12b). This device exhibited an R_on of 8.2 mΩ·cm² and a BV of 1.49 kV (Figure 12c,d). An ideality factor of 1.31 suggested the diffusion current as the dominant mechanism. p-CuI/β-Ga₂O₃ was also demonstrated [112]. Furthermore, Budde et al. [113] reported vertical p-SnO/β-Ga₂O₃ heterojunction diodes via PAMBE (Figure 12e). The simulated band diagram is depicted in Figure 12f. These diodes exhibited excellent rectification with an ON/OFF ratio of 2 × 10⁸ at ±1 V, an ideality factor of 1.16, and an R_on,sp of 3.9 mΩ·cm². The temperature-dependent I–V measurements indicated that TE dominated the transport in the forward bias (Figure 12g). The built-in voltage, calculated from the C–V measurements, was 0.96 V, as in Figure 12h. Moreover, Tetzner et al. [114] demonstrated an advanced lateral heterojunction FET (HJFET) utilizing p-type SnO in combination with (100)-oriented n-type β-Ga₂O₃ (Figure 12i). These transistors displayed stable switching properties with ON/OFF current ratios exceeding 10⁶ and an R_on,sp below 50 Ω·mm, as demonstrated in Figure 12j,k. In the breakdown measurements, a heterojunction transistor with an L_GD of 4 µm achieved a breakdown voltage of 0.75 kV (Figure 12l) and a calculated BFOM of 178 MW/cm². In summary, p-SnO/β-Ga₂O₃ and p-Cu₂O/β-Ga₂O₃ diode structures exhibit relatively lower ideality factors, indicating their potential for high-frequency applications and offer promising performance in terms of R_on, BV, and BFOM. However, the calculated critical breakdown field still falls short of the theoretical value for β-Ga₂O₃, possibly due to surface defects during film growth and the absence of electric field management design. Future studies should systematically explore these characteristics as well.

These findings highlight the potential to enhance novel materials when combined with β-Ga₂O₃. A key challenge in developing these devices is effectively managing the interface states. The presence of interface states, resulting from factors like the growth or deposition method and lattice mismatch, can limit the device performance. The introduction of a transition layer, such as h-BN, between these materials could help reduce interface states and lead to an improved device performance.

5. β-Ga₂O₃ Heterostructures with Other UWBG Semiconductors

Researchers are also investigating the use of other UWBG materials such as diamond, AlN, and BN to form heterostructures with β-Ga₂O₃. Diamond is a promising material for next-generation power electronic devices due to its high carrier mobilities (4500 cm²/V·s for electrons and 3800 cm²/V·s for holes), higher critical electric field (>10 MV/cm), higher thermal conductivity (2000 W/m·K), and ultra-wide bandgap (5.5 eV) [115,116]. It also has p-type doping with boron (B) and has an acceptor concentration ranging from 10¹⁵ to 10²¹ cm⁻³, which make it an attractive candidate material for exploiting the full potential of β-Ga₂O₃ [117]. Matsumae et al. [118] have demonstrated the direct bonding of β-Ga₂O₃ and diamond substrates. OH-terminated β-Ga₂O₃ and diamond surfaces were directly bonded using a thermal dehydration reaction at low temperatures (250 °C). Recently, MOCVD growth of β-Ga₂O₃ on (001) diamond substrates was demonstrated [119]. Furthermore, heteroepitaxial growth of β-Ga₂O₃ thin films on single crystalline diamond (111) wafers using RF magnetron sputtering was also demonstrated [120]. Mishra et al. [121] conducted a theoretical investigation into the electrical and thermal characteristics of diamond/β-Ga₂O₃ super-junction Schottky barrier diodes (SJSBDs) with a 20 nm thick Al₂O₃ interlayer. This interlayer will play a crucial role in practical device realization because it protects the Ga₂O₃ from microwave plasma etching during the chemical vapor deposition (CVD) growth of diamond. Furthermore, the oxide interlayer aids in securing the nano-diamond seeds necessary for the growth of polycrystalline diamond (PCD). Additionally, this design permits the attainment of high electric fields in both the p-type and n-type regions, exploiting the UWBG characteristics of these materials. Achieving a charge balance between the p and n regions is essential for the device to operate effectively at the highest BV. A charge imbalance would diminish the device performance by inducing an asymmetric electric field within the device, leading to premature breakdown. Notably, the incorporation of a p-type diamond into this structure also enhanced the device’s thermal performance, addressing one of the main challenges associated with β-Ga₂O₃, i.e., low-thermal conductivity [3], as pointed out earlier. Sittimart et al. [117] employed a low-temperature direct bonding method to fabricate diamond/β-Ga₂O₃ p-n heterojunction diodes (inset Figure 13a). These diodes exhibited an impressive rectifying ratio exceeding 10⁸ at ± 10 V. Additionally, the leakage current was exceptionally low, measuring less than 10⁻¹² A. The forward I–V curve is presented in Figure 13a. The ideality factor and barrier height for these diodes were estimated to be 2.7 and 1.5 eV, respectively. The band alignment of this heterojunction is illustrated in Figure 13b. In Figure 13c, a comparison is presented between the β-Ga₂O₃ SBD and the diamond/β-Ga₂O₃ p-n heterojunction, highlighting their differences and characteristics. Figure 13d shows the C–V characteristics of one of the demonstrated p-n diodes. Malakoutian et al. [122] demonstrated polycrystalline diamond epitaxial growth on β-Ga₂O₃ with a CVD method for device-level thermal management. A thermal conductivity of 110 W/m·K for the diamond and a thermal boundary resistance of 30.2 m²K/GW were measured. Recently, Rahaman et al. [123] demonstrated the mechanical integration of diamond/β-Ga₂O₃ as an effortless fabrication method to demonstrate a p-n heterojunction, as in Figure 13e. The devices show a remarkable ON/OFF ratio of 10⁸ and a low ideality factor of 1.28, while the device operation spans from 25 °C up to 125 °C (Figure 13f,g). This work provides a novel pathway for integrating UWBG material with different thermal expansion coefficients.

AlN exhibits a wide bandgap of 6.2 eV, making it highly suitable for power applications [124]. Chen et al. [125] conducted a study on the band alignment of AlN/β-Ga₂O₃ heterostructures and determined valence band offsets of 0.09 eV (type II alignment) for AlN grown via plasma-enhanced atomic layer deposition (PEALD) on β-Ga₂O₃ and 0.72 eV (type I alignment) for AlN prepared using thermal atomic layer deposition (T-ALD) on β-Ga₂O₃. Song et al. [126] explored an AlN/β-Ga₂O₃ heterostructure, employing polarization-induced doping to create high-performance enhancement-mode transistors with a 2DEG concentration reaching up to 8.1 × 10¹⁹ cm⁻³ (Figure 14a). A p-GaN gate was utilized to achieve a normally-off operation with a positive V_th. In Figure 14b, the distribution of the polarization charge in the different material regions is depicted, and Figure 14c shows the band diagram and 2DEG confinement, with Figure 14d displaying the device’s output characteristics. Nonetheless, growing high-quality AlN or Al-rich AlGaN thin films on β-Ga₂O₃ with good interfaces remains a challenge. Furthermore, since GaN has a narrower bandgap compared to β-Ga₂O₃, p-GaN itself has a lower critical field, which could potentially impact the device breakdown performance. Alternatively, using wider-bandgap p-type materials like p-AlGaN layers may offer advantages for gate materials. Furthermore, researchers are actively exploring the use of UWBG materials such as AlN and h-BN thin interlayers for β-Ga₂O₃ SBDs to achieve higher Schottky barrier heights and BVs. Recently, Baskaran et al. [127] investigated an AlN/β-Ga₂O₃ high-electron-mobility transistor (HEMT) on a SiC substrate for power converters and RF applications. Similar to AlN, BN is emerging as a UWBG material with a bandgap of ~6.0 eV and a critical field of ~10 MV/cm [128,129,130]. However, BN predominantly functions as a dielectric material rather than a semiconductor. A recent study by Xu et al. [130] showcased a kV-class β-Ga₂O₃ metal–insulator–semiconductor (MIS) diode incorporating a thin BN nitride layer aimed at enhancing device breakdown. The introduction of a 2.8 nm thin BN layer (Figure 14e) significantly elevated the device breakdown from 0.73 to 1.03 kV, highlighting the potential of this approach (Figure 14h). It is worth noting that this enhancement may result in an increased turn-on voltage and R_on,sp in the devices. Nevertheless, this technique serves as an effective means of tuning the device breakdown voltage. Figure 14f,g depict the forward I–V curves of both diodes, with and without the BN interlayer, on the linear and log scales. These results indicate that the combination of UWBG materials with Ga₂O₃ opens up new device architectures to explore.

6. β-Ga₂O₃ Heterostructures with 2D Materials

Recent years have witnessed significant advancements in 2D materials such as MoS₂, WS₂, WSe₂, BP, and graphene [131,132]. Researchers have also explored the integration of 2D materials with β-Ga₂O₃. This section discusses some of the noteworthy research efforts and the latest developments. Yan et al. [133] demonstrated graphene/β-Ga₂O₃ vertical barristor heterostructures, and the device structure is depicted in Figure 15a. The fabrication process began with the mechanical exfoliation and transfer of graphene onto a SiO₂/Si wafer, with the SiO₂ layer measuring 285 nm in thickness. Next, β-Ga₂O₃ was exfoliated and transferred onto the wafer, where it overlapped with the graphene flake. β-Ga₂O₃ was intentionally kept undoped to provide a reasonable estimate of the breakdown field. After that, the contact regions were defined using e-beam lithography, followed by Ti/Au (5/45 nm) metal evaporation. Figure 15b presents a cross-sectional energy band diagram of the heterostructure, illustrating the charge transport mechanisms under different biasing conditions. The graphene layer plays a crucial role in enabling electron injection while enabling a high electric field due to the back-gate (Si substrate) bias. Cross-sectional HRTEM images of the fabricated graphene/β-Ga₂O₃ heterostructure are showcased in Figure 15c. Electrical measurements revealed a remarkable breakdown field of 5.2 MV/cm perpendicular to the (100) plane.

Kim et al. [134] demonstrated a depletion-mode β-Ga₂O₃ HJFET with an n-channel configuration, achieved via van der Waals bonding to an exfoliated p-WSe₂ flake, as shown in Figure 15d. The use of van der Waals interaction allows for layered materials separated from their bulk crystals to be assembled into a strain-free heterostructure. Figure 15e shows a band diagram of the device with its conduction and valence band offsets. The p-WSe₂/β-Ga₂O₃ heterostructure diode exhibited typical diode characteristics with a rectifying ratio of ~10⁵. The layered HJFETs displayed exceptional output (Figure 15f) and transfer characteristics. These devices boasted a high ON/OFF ratio of ~10⁸ and a low SS of ~133 mV/Dec. Saturated output currents were observed with a V_th of −5.1 V, and a three-terminal breakdown voltage of 144 V was obtained. Leblanc et al. [135] demonstrated p-n heterojunctions using p-WSe₂, WS₂, and BP on β-Ga₂O₃, and the fabricated device structure is depicted in Figure 15g. Their study focused on optimizing 2D/3D vertical diodes on β-Ga₂O₃ by considering three key factors, including the β-Ga₂O₃ crystal orientation, the choice of 2D material, and the type of metal contact. The I–V characteristics of the p-n diodes in (–201) β-Ga₂O₃ orientations are presented in Figure 15h. Figure 15i shows the variation in the ideality factor with the choice of 2D material and β-Ga₂O₃ crystal orientations. These results suggest that 2D/3D β-Ga₂O₃ vertical heterojunctions can be optimized by employing substrate orientation (–201), combining it with 2D WS₂ exfoliated layers and Ti contact. These optimized devices exhibited record rectification ratios (>10⁶) while maintaining a high ON-current density (>10³ Acm⁻²), making them promising for applications in power rectifiers. The above results showcase the potential of 2D material/β-Ga₂O₃ for transistors and diodes.

7. Conclusions and Outlook

β-Ga₂O₃ heterostructures and heterojunctions hold significant promise for future power electronics technology advancements. A wide range of heterojunctions/heterostructures and configurations have been explored to harness the unique properties of β-Ga₂O₃ for high-power electronics. NiO_x/β-Ga₂O₃ heterojunctions open the door for advanced power devices, high-frequency applications, and enhanced functionality. Further optimizations and innovations in incorporating NiO_x/β-Ga₂O₃ heterostructures into power devices can be expected, and the stability and reliability of such devices warrant special attention. Precise control over the p-type doping of NiO_x is essential to achieve the necessary charge balance between the p- and n-type regions, as any imbalance can hinder the device BV and overall performance. While researchers have demonstrated some degree of tunability in p-type doping, further optimization of NiO_x/β-Ga₂O₃ heterojunction diodes is warranted. Exploring various edge termination structures and multidimensional device architectures is crucial to surpass unipolar BFOM limitations. Novel device architectures hold promise for achieving higher performance levels. β-(Al_xGa_1−x)₂O₃/β-Ga₂O₃ heterostructures have unique properties, such as high breakdown voltages and excellent 2DEG control, making them the workhorse for high-frequency devices. In these heterostructures, precise control over the Si δ-doping channel width and position is paramount. At elevated doping levels, parasitic conduction channels may arise, emphasizing the importance of accurate Si doping for an optimal device performance. Implementing multiple-channel devices with multiple quantum wells could enhance the device performance compared to single- or double-channel devices. The inclusion of UWBG materials like diamond, AlN, and BN brings new opportunities for β-Ga₂O₃ electronics as both device active regions and passivation structures. The growth of wide- and ultra-wide-bandgap materials on Ga₂O₃ necessitates reducing the defect density and improving the material quality due to significant lattice mismatches. Interface engineering and optimized growth techniques hold the potential to enhance the material quality and consequently the device performance. Furthermore, the versatile nature of β-Ga₂O₃ allows it to be combined with different materials, including 2D materials like graphene, MoS₂, WS₂, WSe₂, and BP for novel device applications, and new and exciting material combinations are expected. Incorporating 2D materials presents challenges, as the surface roughness plays a crucial role in promoting the adhesion between Ga₂O₃ and 2D materials. Future developments may involve exploring new material combinations and device configurations that leverage the anisotropic nature of β-Ga₂O₃. Additionally, improving the Ohmic and Schottky contacts in 2D/3D β-Ga₂O₃ devices is essential to enhancing their current transport capabilities. While there are challenges, such as interface engineering and material compatibility, active research is ongoing to address these issues. Furthermore, issues arising from the low thermal conductivity of Ga₂O₃ can be mitigated with the use of heterostructures involving integration with high-thermal-conductivity diamond and SiC. The research on β-Ga₂O₃ heterostructures and heterojunctions has the potential to significantly expand the capability of Ga₂O₃ power electronics and facilitate their maturity. This review can be a valuable reference for recent developments in Ga₂O₃-based heterojunctions and serve as an important gateway to further development of Ga₂O₃-based electronics and their heterostructures to address future energy demands.