β-Ga2O3 Schottky Barrier Diode with Ion Beam Sputter-Deposited Semi-Insulating Layer

Abstract

Vertical Schottky barrier diodes based on an ion beam sputter (IBS)-deposited β-Ga₂O₃ film on a single-crystalline ($\overline{2}$01) unintentionally doped (UID) β-Ga₂O₃ with a Ni contact were developed. To form ohmic Ti/Ni contacts, the IBS-Ga₂O₃/UID β-Ga₂O₃ structures were wet-etched, and an indium tin oxide (ITO) intermediate semiconductor layer (ISL) was deposited on the opposite surface of the UID β-Ga₂O₃. The IBS-deposited Ga₂O₃ layer was polycrystalline and semi-insulating. Low leakage currents, rectification ratios of 3.9 × 10⁸ arb. un. and 3.4 × 10⁶ arb. un., ideality factors of 1.43 and 1.24, Schottky barrier heights of 1.80 eV and 1.67 eV as well as breakdown voltages of 134 V and 180 V were achieved for diodes without and with ITO-ISL, respectively. The surface area of the IBS-Ga₂O₃ film acted as a thin dielectric layer and, together with the preliminary wet etching, provided low leakage currents and relatively high Schottky barrier heights. Diodes with a Schottky barrier based on a Ni/IBS-deposited Ga₂O₃ film contact were demonstrated for the first time.

1. Introduction

In order to improve the efficiency of electrical equipment, inductive motor controllers and power supplies require the development of power electronics components that differ from Si-based devices by having superior characteristics. β-Ga₂O₃ is an interesting base material for such devices.

β-Ga₂O₃ bulk crystals are grown, using a real bulk growth method, from a melt [1,2] and therefore have a better crystalline quality in comparison with the other electronics with technology-relevant wide-bandgap semiconductors: SiC [3], GaN [4] and AlN [5]. Today, the typical dislocation density of bulk β-Ga₂O₃ single crystals grown via the vertical Bridgman method, the edge-defined film-fed growth (EFG) method, the floating zone process and other melt-growth techniques is ~10³–10⁴ cm⁻² [6,7,8,9,10,11] versus 10⁵–10⁷ cm⁻² for vapor- and solution-grown GaN [12,13,14,15,16,17,18,19,20,21] and SiC [22,23,24,25], correspondingly. It is worth noting that heteroepitaxy also allows the deposition of layers of different Ga₂O₃ polymorphs with high structural quality and low dislocation density [26,27].

In addition, the feasibility of using β-Ga₂O₃ is due to its ultra-large bandgap energy E_g = 4.5 eV without alloying like AlGaN, which determines the theoretical breakdown field strength of ε_c = 8 MV/cm, the ability to withstand high temperatures, and the strong dependence of the electrical properties on the presence and concentration of impurities [28]. Estimates have shown that the Baliga figure of merit (BFOM) of β-Ga₂O₃ is approximately 3000 times higher than that of Si. In addition, β-Ga₂O₃ significantly outperforms GaN and SiC in these parameters. The ability to dope β-Ga₂O₃ with shallow donors up to a concentration of 10²¹ cm⁻³ is a significant advantage over the ultra-wide bandgap semiconductors Al_0.7Ga_0.3N and diamond [27,29]. In addition, β-Ga₂O₃ is characterized by high thermal and chemical stability.

The first publications devoted to power devices based on β-Ga₂O₃ appeared in 2010 [28,30,31,32]. To date, Schottky barrier diodes (SBDs) and field effect transistors (FETs) have been developed in a variety of designs, with lateral and vertical transport [28,33,34]. In addition, among the known five different phases of gallium oxide [35], which are widely studied mainly for the development of various detectors [36,37], β-Ga₂O₃ is the most thermodynamically stable polymorph, which significantly expands the methods of material modification [38]. In addition to power electronics, stable β-Ga₂O₃ and metastable α- and κ(ε)-Ga₂O₃ polymorphs are being actively investigated for the development of solar-blind shortwave UV detectors and gas sensors.

SBDs based on β-Ga₂O₃ with a breakdown voltage V_br of 2–4 kV have been successfully developed within the last few years [39,40,41]. The vertical SBDs are of large interest due to the relative simplicity of the fabrication technology, the ability to employ a variety of β-Ga₂O₃ modification methods and designs to optimize the SBDs, as well as the ability to handle higher currents and powers. Despite the promising results achieved worldwide, further research is needed to improve the performance of β-Ga₂O₃-based SBDs and to modify their design. In addition, there are difficulties in making ohmic contacts to β-Ga₂O₃. Promising approaches for modifying β-Ga₂O₃ to improve the parameters of the SBDs are, e.g., the formation of an ISL, preliminary wet-etching of the semiconductor surface, additional annealing procedures and the formation of gate dielectric layers [42,43,44,45,46,47]. The deposited highly conductive indium tin oxide (ITO) thin film, followed by annealing, is an intermediate semiconductor layer that is used to form an ohmic contact with low contact resistance and high temperature stability [48,49]. In this work, such approaches were employed to fabricate SBDs based on β-Ga₂O₃. A β-Ga₂O₃ layer deposited by IBS, followed by high-temperature annealing was used as a gate dielectric layer for the first time. An ITO ISL was deposited followed by a rapid thermal annealing (RTA) to form an ohmic contact. The effect of etching in HCl solution followed by treatment in H₂O₂ on the device performance of the β-Ga₂O₃-based SBDs was investigated in detail.

IBS deposition is characterized by the highest energies of the particles involved in the formation of thin films, as well as a large number of parameters affecting the sputtering process and the ability to achieve a higher vacuum in the operating mode [50,51,52,53]. This allows a more subtle variation in the electrically conductive, structural, optical, mechanical and other properties of the films during the deposition process and allows us to obtain more homogeneous layers in terms of thickness and composition over larger areas of the substrates. Thin films deposited via the IBS method are characterized by high adhesion, a dense structure, high stoichiometry and high purity. Films of the α- and β-Ga₂O₃ phases can be deposited by IBS by varying the parameters of the IBS deposition process [53]. The possibility of depositing metastable κ(ε)-Ga₂O₃ films has also been suggested. In this work, the IBS-deposited β-Ga₂O₃ film was used for the first time to develop an electronic device.

2. Experimental Methods and Materials

Unintentionally doped β-Ga₂O₃ commercial wafers (Tamura Corp., Tokyo, Japan) with a surface orientation of ($\overline{2}$01), a thickness of 650 µm and a donor net concentration N_d of ~10¹⁷ cm⁻³ were used as a substrate in the fabrication of vertical Schottky barrier diodes.

A Ga₂O₃ film of ~500 nm in thickness was homoepitaxially grown via the IBS method on the surface of UID β-Ga₂O₃ ($\overline{2}$01) wafers. This film is denoted as IBS-Ga₂O₃. The IBS synthesis process was carried out using an Aspira-200 (Belarus) system equipped with a ring beam ion source. The sputtered target was a 5″ disc of compressed Ga₂O₃ powder with a purity of 99.995 wt.% (Lanhit LLC, Moscow, Russia). The Ga₂O₃ powder was pressed under a load of 60 tons for 3 h at room temperature (RT). The diameter of the ion beam focused on the target was ~25 mm. UID β-Ga₂O₃ wafers were cleaned using an auxiliary ion source at ~40 W source power and ~150 eV ion energy for 10 min in the chamber before the deposition of IBS on the Ga₂O₃ film. The temperature of the UID β-Ga₂O₃ wafers during the deposition of IBS on the Ga₂O₃ film was 250 °C. Ar and O₂ of high purity (99.995 vol.%) were used as working gases in the system. The Ar/O₂ flow ratio was ½ with a total flow of 30 cm³/min. The gas pressure in the chamber during the IBS-Ga₂O₃ films deposition was 3.750 μTorr. The deposition time of an IBS-Ga₂O₃ film with a thickness of ~500 nm was within 8 h. The IBS Ga₂O₃/UID β-Ga₂O₃ structures were annealed at a temperature of T_ann = 800 °C in air atmosphere for 30 min.

Some of the IBS-Ga₂O₃/UID β-Ga₂O₃ structures were etched in a 10% HCl solution for 5 min at RT, followed by treatment in H₂O₂ for 5 min at T = 85 °C. Wet etching was used to prepare the surface of IBS Ga₂O₃/UID β-Ga₂O₃ structures prior to the deposition of contacts. Continuous Ti layers of 20 nm in thickness and Ni of 100 nm in thickness were then deposited on the surface of UID β-Ga₂O₃ using vacuum thermal sputtering at a pressure of ~0.750 μTorr. The structure was subjected to RTA at T_ann = 400 °C in vacuum for 1 min after deposition of the Ti/Ni layers. A Ni anode with a diameter of 1 mm and a thickness of 100 nm was deposited on the surface of the IBS Ga₂O₃ film by means of vacuum thermal sputtering through a shadow mask.

For the second part of the IBS Ga₂O₃/UID β-Ga₂O₃ structures, an ITO film was deposited on the β-Ga₂O₃ surface via radio frequency magnetron sputtering of an oxide target in Ar/O₂ plasma using an Edwards A-500 (UK) system. Furthermore, the IBS Ga₂O₃/UID β-Ga₂O₃ structures were treated in acetone, isopropyl alcohol, and deionized water before the ITO film deposition. The IBS Ga₂O₃/UID β-Ga₂O₃ structure was not specially heated during the ITO film deposition. The working pressure and power of the system were 5.250 mTorr and 70 W, respectively. The oxygen concentration in the O₂ + Ar mixture was 10 ± 0.5 vol.%. The thickness of the ITO films was within 210–260 nm for 20 min deposition time. A Ti/Ni cathode was deposited on the surface of the ITO film, and a Ni anode was deposited on the surface of the IBS Ga₂O₃ film. The structure was subjected to RTA at T_ann = 400 °C in vacuum for 1 min after deposition of the ITO film and the Ti/Ni layers.

Thus, two types of SBDs with vertical electron transport and different ohmic contacts were fabricated. The series of samples without an ITO layer is denoted as SBD A, and the series of samples with an incorporated ITO layer is denoted as SBD B. Schematic images of the fabricated SBDs are illustrated in Figure 1.

In addition, the electrically conductive characteristics of the structures based on IBS-Ga₂O₃ and ITO films deposited on sapphire substrates in the modes described above were investigated. On the surface of the films, symmetrical Pt contacts were deposited by means of direct current magnetron sputtering.

A focused ion beam (FIB) scanning electron microscope (SEM) and Helios G4 CX nanomanipulators (Thermo Fisher Scientific Inc., Waltham, MA, USA) were used to prepare lamellae of samples for transmission electron microscopy (TEM) analysis. The sample preparation method is described in detail in Refs. [54,55]. The preparation of the samples for TEM by means of the FIB SEM technique involved the following steps:

• Choosing of the samples region to produce the lamella and its orientation;

• Deposition of a Pt layer to protect the samples surface against ion etching by means of a gas injection system at an accelerating voltage of U₁ = 30 kV and a current of I₁ = 24 pA;

• Formation of two trapezoidal depressions on either side of the protective Pt layer by means of an FIB at U₁ = 30 kV and I₁ = 0.23 pA;

• Cutting of lamellae along the perimeter by means of an FIB at U₁ = 30 kV and I₁ = 0.79 pA;

• Bringing and welding the Pt probe of the nanomanipulator to the lamella and then removing it from the sample;

• Moving the lamella with the nanomanipulator onto the grid for TEM and then welding it with Pt using a gas injection system;

• FIB thinning of a lamella welded to a grid at U₁ = 30 kV and I₁ = 80 pA;

• Final polishing of the lamella surface with an ion beam at U₁ = 30 kV and I₁ = 24 pA.

Cross-sectional images of the annealed IBS-Ga₂O₃/UID β-Ga₂O₃ structures were examined using the Jeol JEM 2100 PLUS TEM (JEOL USA, Inc., Peabody, MA, USA) equipment at an accelerating voltage of 200 kV in a bright field (BF) mode. The elemental composition of the films was determined by the BF-TEM mode by means of a Jeol EX-24261M1G5T (JEOL USA, Inc., Peabody, MA, USA) energy dispersive X-ray (EDX) spectroscopy analyzer at a beam current of 1 nA.

Room temperature transmission spectra of the UID β-Ga₂O₃ wafers, IBS Ga₂O₃/UID β-Ga₂O₃ and ITO/UID β-Ga₂O₃ structures were measured using a combined DH-2000 radiation source based on deuterium and tungsten halogen lamps and an Ocean Optics spectrometric system (Ocean Insight, Orlando, FL, USA). The DH-2000 radiation source provides stable radiation in the wavelength range of Δλ = 190–2500 nm. The transmitted radiation was focused by collecting optics and transmitted via optical fiber to the input of the Ocean Optics spectrometers. The Ocean Optics USB 2000+ spectrometer with a wavelength range of 320–517 nm and the Ocean Optics Flame spectrometer with a wavelength range of 200–850 nm were used. Measurements were controlled using Ocean View 1.5.0 software. The wavelength λ was measured with an optical resolution of 1 nm.

The electrically conductive characteristics of the structures under dark conditions were measured in a sealed Nextron MPS-CHH (Nextron Corp., Busan, Republic of Korea) chamber equipped with microprobes. The samples were placed on a ceramic table which allowed for heating of the samples from RT to T = 750 °C with an accuracy of ±0.1 °C. The atmosphere in the chamber was pure dry air. Pure dry air was pumped throughout the chamber containing the samples for 5 min prior to the measurements of the electrically conductive characteristics to eliminate the influence of the atmosphere. A special generator was used as a source of pure dry air. The flow rate of the pure dry air was 1000 cm³/min. The I-V (J-V) and C-V characteristics were measured using a Keithley 2636A (Keithley Instruments LLC, Cleveland, OH, USA) source meter and an E4980A RLC (Agilent Technologies, Santa Clara, CA, USA) meter, respectively, where I is the current, J is the current density, C is the electrical capacitance, and V is the applied voltage. A Keithley 2636A source meter allows us to measure extremely low currents, up to 10⁻¹² A. The measurements of the I-V (J-V) and C-V characteristics were carried out in an automated mode using a program developed on the basis of the Labview platform.

3. Results and Discussion

3.1. Structural Properties of IBS Ga₂O₃/UID β-Ga₂O₃ and ITO/UID β-Ga₂O₃ Structures

A typical TEM image of the IBS Ga₂O₃/UID β-Ga₂O₃ structure, illustrated in Figure 2a, shows an abrupt interface. The thickness of the IBS Ga₂O₃ film is ~521 nm according to the SEM. Figure 2b shows an HRTEM image of an annealed IBS Ga₂O₃/UID β-Ga₂O₃ structure near the interface. The interplanar distances d for the UID β-Ga₂O₃ substrate and the IBS Ga₂O₃ film were determined via the fast Fourier transform (FFT) method based on the phase contrast. The IBS-Ga₂O₃ film, like the UID β-Ga₂O₃ substrate, is a relevant monoclinic β-Ga₂O₃ phase with a structural group of C2/m(12). The d corresponding to the reflections of the ($\overline{2}$01), (−$\overline{1}$10), ($\overline{1}$11) and (31$\overline{1}$) planes of the β-Ga₂O₃ phase (ICSD # 83645) are 0.468 nm, 0.295 nm, 0.267 nm and 0.234 nm, respectively. The IBS-Ga₂O₃ film, which is in contact with the Ni anode, is polycrystalline and, as shown below, is semi-insulating. We believe this is due to the nature of the film deposition process [51,52,53]. It is worth noting that such relatively thick β-Ga₂O₃ films deposited via the IBS method on UID β-Ga₂O₃ substrates and annealed at the conditions described above are being studied for the first time. According to Refs. [51,52,53], it was expected that IBS β-Ga₂O₃ films would be monocrystalline and of high structural quality.

TEM EDX maps of the IBS Ga₂O₃/UID β-Ga₂O₃ interface and EDX spectra of the IBS-Ga₂O₃ and UID β-Ga₂O₃ near the IBS-Ga₂O₃/UID β-Ga₂O₃ interface are depicted in Figure 3a,b, respectively. According to the EDX, the Ga and O contents in the IBS Ga₂O₃ films are ~42 at.% and ~58 at.%. Thus, a reduced content of oxygen atoms in the IBS-deposited layer is observed. No peaks corresponding to other elements are detected in the EDX spectra. The Sn content in the ITO films is determined to be ~5 at.% according to the EDX.

Transmission spectra of the UID β-Ga₂O₃ wafers, IBS Ga₂O₃/UID β-Ga₂O₃ and ITO/UID β-Ga₂O₃ structures are exhibited in Figure 4. The transmission values of the UID β-Ga₂O₃ wafers and IBS Ga₂O₃/UID β-Ga₂O₃ structures in the range of λ = 350–475 nm are above 75%. A sharp drop in the transmittance is observed at short λ due to the band-to-band absorption of photons. An analysis of the absorption edge of the samples showed that the E_g is 4.6 eV and 4.5 eV for the UID β-Ga₂O₃ and IBS Ga₂O₃/UID β-Ga₂O₃ structures, respectively. Interference phenomena are observed at the IBS Ga₂O₃/UID β-Ga₂O₃ interface. The thickness of the IBS Ga₂O₃ film determined from an analysis of the position of the interference maxima is 518 nm and is close to the declared value and result of TEM. The transmission spectra of the ITO/UID β-Ga₂O₃ structures are determined by the ITO film. The transmittance values of these structures are above 75% within the range of λ = 500–650 nm. A decrease in the transmittance of the ITO/UID β-Ga₂O₃ structures is observed in the range of λ = 350–450 nm due to the band-to-band absorption of photons. The E_g of the ITO film is determined to be 3.6 eV.

3.2. Electrically Conductive Characteristics of Diodes Based on IBS Ga₂O₃/UID β-Ga₂O₃ and IBS Ga₂O₃/UID β-Ga₂O₃/ITO Structures with Ni Gate

We used UID β-Ga₂O₃ commercial wafers grown via the EFG technique as substrates. During the growth process, donor type defects are formed in the β-Ga₂O₃ wafer, resulting in a donor concentration of the order of ~10¹⁷ cm⁻³ [56]. The electrical properties and defects of EFG UID β-Ga₂O₃ are summarized in Refs. [57,58,59].

Preliminary measurements showed that the IBS-deposited Ga₂O₃ film is characterized by semi-insulating behavior at RT and that ITO is highly conductive, despite the relatively low annealing temperature. The resistance of the IBS Ga₂O₃ film at RT is above the measurement limit. It is possible to measure the I-V characteristics of the IBS Ga₂O₃ film with Pt contacts at T = 600 °C. The resistivity of the IBS Ga₂O₃ film is determined to be 1.24 × 10⁷ Ohm × cm at this temperature. The resistivity of the ITO film at RT is 0.035 Ohm × cm.

The experimental dependencies of the current density on the applied voltage of the diodes in the range from –2 V to 2 V at RT are depicted in Figure 5. Evidently, both types of diodes demonstrate rectifying characteristics. The rectification ratios at ±2 V are 3.9 × 10⁸ arb. un. and 3.4 × 10⁶ arb. un. for SBD A and SBD B, respectively. An analysis of the J-V characteristics was carried out to estimate the ideality coefficient n, the saturation current density of the diodes J_s and the height of the potential barrier Φ_b. According to the thermionic emission (TE) model, the direct branch of the J-V characteristics can be approximated by the following expression [6,41]:

J = J_s(exp[eV/(nkT)] − 1),(1)

where e is the electron charge; k is the Boltzmann constant; and T is the absolute temperature of the semiconductor. The ideality coefficient was determined from the analysis of the linear portion of the J-V characteristics using the following formula:

ln(J/J_s) = eV/(nkT).(2)

The J_s was obtained by extrapolating ln(J/J_s) to the point of V = 0. According to theories expressed in [6,41],

J_s= A*T² × exp(−eΦ_b/(nkT)),(3)

where A* is the Richardson constant. A* = 33.65 A/(cm⁻² × K⁻²) according to estimates from experimental data at the effective mass of electrons m_n* = 0.28m₀, where m₀ is the mass of an electron in vacuum. Using expression (3), Φ_b can be estimated using the following formula:

Φ_b = ln(A*T²/J_s)×(nkT/e),(4)

The calculated n, J_s and Φ_b, and other experimental parameters of the diodes are summarized in Table 1.

The C-V characteristics of the SBD A and SBD B measured at different amplitudes of the signal u and frequencies f are illustrated in Figure 6. The largest changes in the C-V characteristics are observed at u = 100 mV and f = 10 kHz (shown in Figure 6). N_d was determined via an analysis of the C-V characteristics in the coordinates of S²/C² vs. V by means of the following formula:

N_d = 2/(eεε₀b),(5)

where b is the slope of the reverse branch of the C-V characteristics in the coordinates of S²/C² vs. V (inset of Figure 6). The C-V characteristics obtained are typical of SBD. The N_d for SBD A and SBD B is (1.7–3.1) × 10¹⁷ cm⁻³. Meanwhile, the value of b did not change with f.

The values of leakage current densities obtained are in agreement with the vast majority of experimental works on Schottky diodes based on Ni/β-Ga₂O₃ [1,6,60]. It should be noted that Schottky diodes based on Ni/β-Ga₂O₃ and Pt/β-Ga₂O₃ are currently under extensive investigation. Even lower values of leakage current densities are not yet achievable due to imperfections in the surface structure of β-Ga₂O₃ crystals, imperfections in the methods of preparing the semiconductor surface prior to the deposition of contacts, and the effects of the concentrating electric field. It should be noted that for diodes based on EFG β-Ga₂O₃, no obvious relationship between dislocations and the leakage current has been found [60].

The breakdown voltages (V_br) of the SBD A and SBD B (see Figure 7) are determined to be 134 V and 180 V, respectively. The low V_br values are probably due to the poor quality of the ohmic contact. The SBD A are characterized by higher values of n, Φ_b and the rectifier ratio as well as by low J_s. However, n for the SBD B is close to 1. In addition, these structures are characterized by a higher V_br. For an ideal Schottky contact based on Ni/β-Ga₂O₃, Φ_b = Φ_M − χ_s [6] and should correspond to 1.15 eV, where Φ_M is the work function of the metal and χ_s is the electron affinity of the semiconductor. For Ni, Φ_M = 5.15 eV, and for β-Ga₂O₃, χ_s = 4.00 eV. The experimental and theoretical values of Φ_b are close to or higher than those reported in the literature [6,44]. The structures studied are characterized by significantly high Φ_b compared to the literature data [1,6]. We suppose the increase in Φ_b and the decrease in J_s is facilitated by the presence of an IBS Ga₂O₃ layer between the Ni anode and the UID β-Ga₂O₃. The IBS Ga₂O₃ film has a polycrystalline structure and demonstrates semi-insulating behavior. Th surface area of this layer acts as a thin dielectric layer, the presence of which leads to an increase in Φ_b. A similar increase in Φ_b is typical for the metal–insulator–semiconductor (MIS) diodes [61]. Pre-etching of the structure in an HCl solution followed by treatment in H₂O₂ also results in higher Φ_b values of the SBD A. Such a treatment was employed to increase the Φ_b for a Schottky contact based on β-Ga₂O₃ and various metals [44]. It has been suggested that this treatment reduces the concentration of oxygen vacancies in the near-surface region of the semiconductor and prevents Fermi level pinning.

4. Conclusions

The development of vertical Schottky barrier diodes based on a contact of Ni and β-Ga₂O₃ film deposited by ion beam sputtering on single-crystalline ($\overline{2}$01) unintentionally doped β-Ga₂O₃ was demonstrated, and their properties were studied. The IBS Ga₂O₃/UID β-Ga₂O₃ structures were wet-etched in HCl solution followed by a treatment in H₂O₂ before forming the ohmic Ti/Ni contacts. In addition, two series of diodes were manufactured employing an ITO intermediate semiconductor layer deposited by radio frequency magnetron sputtering. The IBS Ga₂O₃ film is polycrystalline and semi-insulating. For diodes without an ITO intermediate semiconductor layer, a low leakage current, a rectification ratio of 3.9 × 10⁸ arb. un. at ±2 V, an ideality factor of 1.43, a Schottky barrier height of 1.80 eV, and a breakdown voltage of 134 V were achieved. For diodes with an ITO intermediate semiconductor layer, the rectification ratio was 3.4 × 10⁶ arb. un. at ±2 V, the ideality factor was 1.24, the Schottky barrier height—1.67 eV and the breakdown voltage was 180 V. We believe that the low leakage currents and relatively high Schottky barrier heights for both types of diodes are due to the effect of the wet-etching and the deposition of an IBS Ga₂O₃ film, the surface area of which acts as a thin dielectric layer. The IBS Ga₂O₃ films were used to fabricate power diodes with a Schottky barrier for the first time.

Author Contributions

Conceptualization, N.N.Y. and A.V.A.; methodology, N.N.Y., B.O.K. and A.V.A.; formal analysis, N.N.Y., M.V.P., M.G.V., M.M.Z. and A.V.A.; investigation, N.N.Y., M.V.P., M.G.V., M.M.Z. and A.V.A.; resources, M.V.P., B.O.K., M.G.V., M.M.Z. and A.V.A.; data curation, N.N.Y. and A.V.A.; writing—original draft preparation, N.N.Y. and A.V.A.; writing—review and editing, N.N.Y. and A.V.A.; visualization, N.N.Y. and A.V.A.; supervision, A.V.A.; project administration, A.V.A.; funding acquisition, A.V.A. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All data that support the findings of this study are included within the article.

Conflicts of Interest

The authors declare no conflicts of interest. Authors Aleksei V. Almaev and Mikhail M. Zinovev were employed by the company Fokon LLC and Laboratory of Optical Crystals <<LOC LLC>>, respectively. The remaining authors declare that the research was con-ducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Table

Figure 1. Schematic images of the fabricated two types of vertical Schottky barrier diodes—SBD A and SBD B.

Figure 2. (a) TEM image of IBS Ga2O3/UID β-Ga2O3 structure. (b) HRTEM image of IBS Ga2O3/UID β-Ga2O3 interface. The insertions are the FFT.

Figure 3. (a) Elementwise TEM EDX mapping of cross-section of IBS Ga2O3/UID β-Ga2O3 interface; (b) EDX spectra for IBS Ga2O3 and UID β-Ga2O3 near the IBS Ga2O3/UID β-Ga2O3 interface.

Figure 3. (a) Elementwise TEM EDX mapping of cross-section of IBS Ga2O3/UID β-Ga2O3 interface; (b) EDX spectra for IBS Ga2O3 and UID β-Ga2O3 near the IBS Ga2O3/UID β-Ga2O3 interface.

Figure 4. Transmission spectra of UID β-Ga2O3, IBS Ga2O3/UID β-Ga2O3 and ITO/UID β-Ga2O3 structures.

Figure 5. J-V characteristics SBD A and SBD B at RT in double logarithmic coordinates. The insets represent J-V characteristics of SBD A and SBD B at RT in linear coordinates.

Figure 6. C-V characteristics of SBD A and SBD B at RT, f = 10 kHz and u = 0.1 V. Inserts show dependencies of S2/C2 on the applied voltage at RT, various frequencies and u = 0.1 V.

Figure 7. Reverse I-V characteristics of SBD A and SBD B at RT in semi-logarithmic coordinates.

Table 1

Experimental and calculated * parameters of SBD A and SBD B according to the TE model.

SBD Type	A	B
Ideality factor * n	1.43	1.24
Schottky barrier height * Φb (eV)	1.80	1.67
Saturation current density * Js (A/cm2)	6.69 × 10−14	6.24 × 10−11
Leakage current density at –2 V(A/cm2)	7.01 × 10−11	8.38 × 10−9
Rectification ratio at ±2 V (arb. un.)	3.9 × 108	3.4 × 106
Breakdown voltage Vbr (V)	134	180

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