IWGO 2026 Session IWGO-MoP: IWGO Poster Session I

This study demonstrates advanced surface treatment and microfabrication techniques for β-Ga₂O₃ using TMAH etching. (i) Mild etching of (001) substrates with 2.38 wt% TMAH (a standard photolithographic developer) at 25–40 °C transforms relatively rough, CMP-finished surfaces into atomically flat step-and-terrace morphologies [1]. On (010) substrates, aggressive etching with 25 wt% TMAH at 90 °C reveals significant crystallographic anisotropy. (ii) Specifically, pronounced lateral etching along the [001] direction facilitates the undercutting of dry-etched mesa structures to form β-Ga2O3/air-gap structures, such as air-bridges and cantilevers [2] . (iii) Furthermore, the high etch resistance perpendicular to the (−201) plane allows for the refinement of dry-etched trenches, transforming rough, slanted sidewalls into smooth, vertical (−201) facets [3]. These results provide straightforward pathways for the surface preparation, the development of high-performance β-Ga₂O₃ MEMS and fin/trench-based power devices.

This work was supported by ARIM program (JPMXP1225NM5079)andJSPS KAKENHI (JP24K01368).

[1] T. Oshima, Jpn. J. Appl. Phys. 64, 088001 (2025).[2] T. Oshima, Appl. Phys. Express 18, 116501 (2025).

[3] T. Oshima, AIP Advances16, 025145 (2026).

One-dimensional wide-bandgap heterostructures have attracted increasing interest in recent years because of their significant potential applications in future optoelectronic devices. We developed a novel technique to investigate the electrical properties of individual nanorods [1], nanostripes [2], or axial/core–shell p–n junctions [3,4] using a nanomanipulator in the chamber of a scanning electron microscope. In contrast to conventional approaches, this method eliminates the use of extensive processing chemistry and allows the observation of morphological changes in situ during electrical characterisation.

1. S. Tiagulskyi, O. Černohorský, N. Bašinová, R. Yatskiv, J. Grym, Materials Research Bulletin, 164 (2023) 112286.
2. P. Wojnar, S. Chusnutdinow, A. Kaleta, M. Aleszkiewicz, S. Kret, J.Z. Domagala, P. Ciepielewski, R. Yatskiv, S. Tiagulskyi, J. Suffczyński, A. Suchocki, T. Wojtowicz, Nanoscale, 16 (2024) 19477-19484.
3. S. Tiagulskyi, R. Yatskiv, H. Faitová, Š. Kučerová, J. Vaniš, J. Grym, Materials Science in Semiconductor Processing, 107 (2020) 104808.
4. S. Tiagulskyi, R. Yatskiv, M. Sobanska, K. Olszewski, Z.R. Zytkiewicz, J. Grym, Nanoscale, 17 (2025) 8111-8117.

⁺Author for correspondence: yatskiv@ufe.cz

β-Ga₂O₃ has attracted great attention in power electronics because of its ultra-wide bandgap, which allows high breakdown voltage and excellent power device performance. However, it is still difficult to fabricate p–n junction devices based on β-Ga₂O₃ because reliable p-type doping in β-Ga₂O₃ is not easily achieved. One possible solution is to form heterojunctions using p-type oxide semiconductors such as NiO. [1-2]

In this study, we show that changing the widths of the p-NiO and p+NiO layers can significantly improve the breakdown voltage of β-Ga₂O₃/NiO heterojunction diodes. The device was fabricated using an n-type β-Ga₂O₃ wafer with a cathode electrode on the backside. On the top surface, p-NiO and p+NiO layers were stacked sequentially and covered with an anode electrode to form a double-layer heterojunction diode.

To investigate the effect of the device structure, the widths of the p-NiO and p+NiO layers were systematically varied. When both layers had the same width, the device showed a breakdown voltage of −326 V. However, when the width decreased toward the top, making the p+NiO layer narrower than the p-NiO layer, the breakdown voltage increased significantly to −670 V.

This improvement is related to the reduction of electric field crowding near the junction edge due to electric field dispersion in the width-modulated structure. These results indicate that controlling the lateral structure of p-type layers is an effective method for improving the reverse voltage performance of β-Ga₂O₃ heterojunction diodes. Further optimization of the device structure is expected to enhance the device performance.

β-Ga₂O₃ has a large bandgap of approximately 4.8 eV and bulk substrates can be fabricated by melt-growth methods, making it a promising candidate for power device applications. Whereas β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures have been the primary focus for β-Ga₂O₃-based HEMT structures [1], β-(Al_xGa_1-x)₂O₃/β-(In_yGa_1-y)₂O₃ heterostructures have recently attracted attention due tothe expectation of a larger band offset. Nevertheless, reports on the high-quality growth of β-(In_xGa_1-x)₂O₃ remain scarce, and understanding its physical properties is essential for HEMT applications. In particular, since β-(In_xGa_1-x)₂O₃ serves as the channel layer, understanding its electrical properties is important. In this study, Si doping in β-(In_xGa_1-x)₂O₃ was investigated using the mist CVD method, which has been successfully used for crystal growth in our group [2].

Figure 1 shows the XRD 2θ–ω measurement results for samples with In concentrations of 1.8 and 3 vol.% in the solution. As shown in Fig. 1, a diffraction peak accompanied by clear fringes was observed for the sample with an In concentration of 1.8 vol.%, indicating the growth of a thin film with high crystallinity. In contrast, when the In concentration was 3 vol.%, the fringes disappeared, and degradation of crystallinity due to lattice relaxation was observed. Figure 2 shows the electron mobility as a function of In concentration with [Si]/[Ga]=0.1 at.%. As shown in Fig. 2, the mobility decreases with increasing In concentration, indicating that the In composition notably influences the carrier transport properties, likely due to alloy scattering.

[1] Zhang et al., Appl. Phys. Lett. 112, 173502 (2018).

[2] H. Nishinaka, et al., Sci. Technol. Adv. Mater. 25 (2024).

β-Ga₂O₃ has an ultrawide bandgap and has been attracting attention as a next-generation power-semiconductor device material [1]. To ensure reliability in of β-Ga₂O₃ devices, it is important to investigate the impact of defects. We investigated evaluation of β-Ga₂O₃ epitaxial wafer using photoluminescence (PL), which is non-destructive, non-contact technique.

(001) oriented β-Ga₂O₃wafers with epitaxial layer thickness of 5, 10 or 15 µm were used as samples. For evaluation, PL spectra and time-resolved PL (TR-PL) measurements were carried out using a 266 nm laser under the conditions of a pulse width of 1 ns and an excitation light intensity of 0.6 mW. These measurements were conducted both of epitaxial layer (Epi side) and substrate (Sub side) sides.

Figure 1 shows PL spectra the samples.For Sub side, a weaker emission was observed compared to those from Epi side for all the samples.On Epi side, the thicker the epitaxial layer, the stronger the light emission. The sample with an epitaxial layer thickness of 15 µm exhibited emission at wavelengths of approximately 500 nm and 560 nm, but none was observed in the samples with epitaxial layers of 5 and 10 µm.Figure 2 shows TR-PL decay curves. The TR-PL decays were faster on Sub side than on Epi side. The decays can be broadly divided into two components: fast decay with a small time constant, and the proportion of the slow decay component in Epi side depends on the epitaxial layer thickness. These results demonstrate that the time constant of the slow component would reflect crystalline quality of the epitaxial layers.

This paper is based on results obtained from a project, JPNP22007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

[1] M. Higashiwaki: Phys. Status. Solidi. Rapid. Res. Lett. Vol15, Issue11, 5 August (2021)

It is widely known that β-Ga₂O₃ substrates are mechanically fragile and cleaves easily along the (100) and (001) planes. Dicing by wafer sawing β-Ga₂O₃ substrates into coupons, dies, or chips has been reported to be a challenging task as edges of β-Ga₂O₃ are susceptible to cracking. The damaged edges from wafer sawing cracks and in some cases distort the substrate so severely that a significant area adjacent to the edge becomes unusable for either epitaxy or wafer bonding. In the case of (001) and (100) substrates, the surface also delaminates at uncontrolled depths along the thickness of the substrate upon cracking at the edges. We demonstrate successful crack-free dicing for various orientations of β-Ga₂O₃ substrates by employing pulsed UV laser dicing. The edges of wafer-saw coupons are structurally characterized and directly compared to edges of laser diced coupons, primarily the impact of each edge type on direct wafer bonding yield and exfoliation via light-atom ion implantation. Laser diced coupons produce sharp, clean edges that do not distort the substrate that would reduce wafer bonding yield. In fact, entire ~cm scale, laser-diced β-Ga₂O₃ coupons yield ~100% fully bonded areas when bonded to 4H-SiC substrates including areas up to the edge of the sample. On the other hand, cracks and delamination from wire sawed edges hinder wafer bonding altogether. Interestingly, wafer saw edges and cracks greatly impact the nucleation mechanics of light-atom ion implantation for the exfoliation and transfer of thin films. The cracks and damage induced by the wafer saw process serve as nucleation sites for the implanted light atoms to nucleate and grow to induce surface blistering or exfoliation (which would have induced layer transfer if bonded prior to annealing). However, these surface blisters appear much earlier than expected compared to the blisters in the rest of the uncracked bulk material. Control over the timing and uniformity of exfoliation is important for producing consistent thin-film β-Ga₂O₃ composite substrates. Mitigating damage and lattice distortion at substrate edges are important for either subsequent coupon processing or for dicing dies or chips from processed wafers.

The authors acknowledge the support from Air Force SBIR Phase II program. TPOC Dr. Thaddeus Asel.

β-Ga₂O₃ is a promising material for next-generation low-power switching devices due to its high breakdown electric field. Understanding the temperature dependence of carrier transport and Schottky barrier properties is crucial for practical device operation and reliability. We investigated the electrical characteristics of HVPE-grown β-Ga₂O₃ vertical Schottky diodes (SBDs) using different Schottky electrodes from 95 K to 408 K.

Si-doped n-type (001) β-Ga₂O₃ homoepitaxial layers, with a net donor density of 4×10¹⁶ cm^-3, were grown via halide vapor phase epitaxy on an n⁺-type substrate. Ni and Cu Schottky electrodes were formed on the epitaxial surface. C-V and I-V characteristics of the SBDs were evaluated. The Schottky barrier heights (φ_B0) was determined from C-V characteristics, while the ideality factor (n) and the apparent Schottky barrier height (φ_B) were extracted from I-V characteristics using thermionic emission (TE) models.

The temperature dependence of φ_B0 for Ni (φ_B0,Ni) and Cu (φ_B0,Cu) are evaluated. At each temperature, φ_B0,Ni was ~0.3 eV higher than φ_B0,Cu, consistent with the difference in their work functions. Extracted temperature coefficient (α) for φ_B0,Ni and φ_B0,Cu were -5.9×10^-4 and -7.0×10^-4 eV/K, respectively. The obtained coefficients are consistent with the typical values reported for other metals (such as Pt and Ni), confirming that they reflect the intrinsic temperature dependence of the bandgap.

As for the carrier transport, n approached unity above approximately 200 K, confirming TE dominance. Below 200 K, n increases while φ_B decreases with decreasing temperature. Specifically, at around 100 K, n reached 2.58 for Ni and 1.57 for Cu. The onset temperatures for the increase in n and decrease in φ_B were lower for Cu than for Ni, reflecting the difference in their work function. However, deviations at lower temperatures seem to suggest that considering tunneling and/or patch effects is necessary.

This work reports the superior electrical stability of Cr₂O₃/β-Ga₂O₃ heterojunction diodes (HJDs) compared with co-fabricated NiO_x/β-Ga₂O₃ HJDs and explores the orientation-dependent breakdown electric field (E_Br,||) of Cr₂O₃/β-Ga₂O₃ HJDs fabricated on five commercially available β-Ga₂O₃ orientations. The HJDs are fabricated on highly doped n-type bulk substrates and exhibit breakdown electric field anisotropy across various orientations with the highest E_Br,|| value obtained at 12.9 MV/cm on (110) β-Ga₂O₃. Type-II band alignment between Cr₂O₃ and β-Ga₂O₃ is also reported by first-principles calculations.

The Cr₂O₃/(001) HVPE β-Ga₂O₃ HJD fabrication begins with a backside Ti/Au (50/150 nm) Ohmic metallization on n⁺ β-Ga₂O₃ bulk substrate using e-beam evaporation followed by a 60-seconds rapid thermal annealing (RTA) at 470 °C in N₂. A p^- and p⁺⁺ Cr₂O₃ stack is reactively sputtered on HVPE β-Ga₂O₃ drift region with a pre-patterned photoresist liftoff mask by optical lithography. Following the Cr₂O₃ deposition, a Ni/Au/Ni (50/50/150 nm) anode cap/metal hard mask stack is deposited via e-beam evaporation. The fabricated HJDs are later dry-etched ~2 μm into the β-Ga₂O₃ drift region below the heterojunction interface using inductively coupled plasma (ICP) for edge termination. Similar fabrication process is also applied to NiO_x/(001) HVPE β-Ga₂O₃ HJDs for stability comparison and Cr₂O₃/n⁺ β-Ga₂O₃ HJDs for breakdown electric field analysis.

The as-fabricated Cr₂O₃/β-Ga₂O₃ HJDs exhibit similar electrical characteristics compared to NiO_x-based HJDs in terms of forward current density (~125 A/cm² at 5 V), differential specific on-resistance (R_on,sp~12 mΩ•cm²), and breakdown voltages (~2 kV) on 7.48-μm thick drift region with 9.45×10¹⁵ cm^-3 apparent charge density. The NiO_x/β-Ga₂O₃ HJDs show significant forward current density degradation (<1 A/cm² at 5 V) after 10-days of ambient exposure while that of Cr₂O₃/β-Ga₂O₃ HJDs remains relatively constant. It is qualitatively determined that the sheet resistance (R_sh) degradation of sputtered NiO_x causes forward current density reduction in the ambient conditions, and water (H₂O) vapor in the ambient air is the main agent that increases the sheet resistance of NiO_x thin film from reactive sputtering. Cr₂O₃/β-Ga₂O₃ HJDs fabricated on n⁺ β-Ga₂O₃ bulk substrates reveal E_Br,|| anisotropy across five orientations of β-Ga₂O₃ with (110) orientation exhibiting the highest breakdown electric field value at 12.9 MV/cm. Relative permittivity values used for E_Br,|| values extraction are found using first-principles calculations.

Progress in next-generation advanced electronic and functional devices, based on complex heterostructures and advanced materials integration, is increasingly constrained by the rigidity of conventional thin-film processing and patterning workflows. While these approaches deliver high material quality and uniformity, they offer limited flexibility for spatially localized, multi-material fabrication, three-dimensional thickness engineering, and rapid experimentation at the nanoscale within a single process flow.

ATLANT 3D introduces Direct Atomic Layer Processing (DALP®), a nanofabrication technology enabling digitally controlled, spatially localized deposition of multiple materials with atomic-scale precision. DALP allows different materials to be deposited sequentially and locally in a unified workflow, enabling manufacturing of complex material stacks, heterostructures, interfaces, and thickness gradients without intermediate lithographic patterning steps.

This presentation describes the DALP process architecture and its role in both combinatorial materials discovery and targeted device manufacturing for next-generation devices. By enabling programmable material placement, controlled thickness variation, and repeatable execution within a single platform, DALP supports accelerated materials exploration while also enabling the direct production of device-ready structures as part of broader manufacturing flows. Representative examples include multi-material nanoscale structures for advanced semiconductor and functional material applications, where precise interface control, spatial selectivity, repeatability, and manufacturability are critical. DALP expands the accessible design space of nanoscale fabrication and provides a a direct pathway from materials discovery to device-ready, manufacturable structures.

Vertical Ga₂O₃ heterojunction diodes (HJDs) were fabricated and systematically studied to evaluate the impact of lateral contact geometry on forward and reverse device performance. The devices were formed on 32 µm thick, lightly doped (8.6 × 10¹⁵ cm⁻³) β-Ga₂O₃ drift layers grown by halide vapor phase epitaxy on heavily doped n⁺ substrates. A NiO/Ni/Au heterojunction anode was employed together with dielectric edge termination using an 80 nm SiO₂ / 45 nm SiN field-plate structure. Three device designs with different field plate dimensions (D = 10, 20, and 30 µm) between the first- and second-layer contact metals were investigated. Capacitance–voltage measurements confirmed uniform drift-layer doping, while forward I–V characteristics showed comparable saturation currents and turn-on voltages for all designs. Increasing D resulted in a reduction in on-resistance and a significant suppression of reverse leakage current at low to moderate reverse bias. The optimized device with D = 30 µm exhibited the highest breakdown voltage of approximately 8.1 kV, compared with 5.5 kV and 4.5 kV for D = 20 µm and D = 10 µm, respectively. These results demonstrate that lateral contact spacing is a critical design parameter for electric-field management and breakdown enhancement in Ga₂O₃ heterojunction diodes.

Ultrawide bandgap (UWBG) semiconductors are promising for high-power electronics; however, reliable p-type doping in β-Ga₂O₃ is fundamentally hindered by its flat O 2p–derived valence band [1], which results in large hole effective mass, low mobility, and strong self-trapping [2]. As a result, high-performance β-Ga₂O₃ p–n junctions require external p-type UWBG materials, though most oxide candidates suffer from low mobility [3]or unfavorable band alignment [4]. Here, we report Li_yNi_1-x-yMg_xO as a novel p-type UWBG semiconductor grown by co-sputtering with controlled Mg incorporation for β-Ga₂O₃-based power devices. XRD and XPS confirm successful Mg substitution and improved crystallinity. The bandgap widens from 4.27 to 5.44 eV with increasing Mg power. While hole concentration decreases (1.72×10¹⁸ to ~10¹⁶ cm^-3), hole mobility dramatically improves from 0.798 to 33.39 cm²V^-1s^-1. DFT calculations using VASP attribute this enhancement to reduced localization of valence band edge states. Heterojunction devices achieve breakdown voltages up to -1450 V and reduced on-resistance (8.83 mΩ.cm²). Despite increased band offset, the reduced turn-on voltage at higher Mg content is explained by Mg-induced trap-assisted tunneling. These results establish Li_yNi_1-x-yMg_xO as a promising p-type contact for high-performance β-Ga₂O₃ UWBG power electronics.

To achieve high breakdown voltage (V_BR) of Ga₂O₃ power devices, edge termination (ET) is essential because of electric field (E-field) crowding at the main junction edge, which results in premature breakdown [1]. Among various ET technologies, junction termination extension (JTE) is widely used in commercial power devices due to its simple formation by single-step p-type ion implantation or diffusion. However, selective p-type doping in Ga₂O₃ remains significantly challenging [2]. To this end, Ga₂O₃ power devices commonly rely on a sputtered p-type nickel oxide (NiO) as an alternative material for JTE formation. Notably, NiO has a wide bandgap of ~3.8 eV and tunable conductivity [3]. Nevertheless, NiO single-zone JTE (SZ-JTE) exhibits large variations since its effective JTE dose depends on both doping concentration (N_JTE) and thickness (t_JTE). As a result, small deviations in both NJTE and tJTE can degrade JTE’s blocking capability, resulting in a narrow process window and premature breakdown [4]. Here, we proposed a ring-assisted JTE (RA-JTE) for vertical NiO/Ga₂O₃ p-n heterojunction diodes (HJDs). The RA-JTE consists of a sputtered NiO JTE layer augmented with multiple NiO floating field rings. In addition, the multiple p⁺-NiO rings are designed with their spacing gradually increasing away from the main junction. Owing to this design, RA-JTE can suppress peak E-field at the main junction edge and promote a more uniform E-field distribution, resulting in a V_BR exceeding 3 kV. TCAD simulations further indicate that RA-JTE preserves a more uniform E-field profile under JTE dose deviations, offering improved robustness compared with SZ-JTE. These results suggest that RA-JTE is a promising ET method for Ga₂O₃ power devices with enhanced process tolerance.

[1] H. Gong et al., IEEE Electron Device Lett. 45, 1421(2024).

[2]S. J. Pearton et al., Appl.Phys. Rev. 12, 031336 (2025).

[3] Y. Ma et al., Adv. Elect. Mater. 11, 230062 (2025).

[4] W. Sung et al., IEEE Electron Device Lett. 37, 1609(2016).

⁺Author for correspondence: youseung@sejong.ac.kr

The industrial deployment of metastable ε-Ga₂O₃ on sapphire is currently impeded by intrinsic crystallographic incompatibility and the difficulty of achieving macroscopic homogeneity via solution-based growth techniques. When grown on a strictly hexagonal substrate like sapphire, the inherent symmetry mismatch inevitably induces the formation of three equivalent 120°-rotational domains, which act as carrier scattering centers and severely broaden the X-ray rocking curve (XRC) full-width at half-maximum (FWHM). Furthermore, the complex interplay of precursor mist flow and evaporation dynamics in Mist-CVD often leads to non-uniform thickness and quality gradients. To address these limitations, we implement a synergistic uni-element engineering strategy via Tin (Sn) alloying [1]. Beyond its conventional role as a dopant, we demonstrate that heavy Sn alloying effectively modulates the cation sublattice, promoting a structural evolution from the ordered orthorhombic phase toward a pseudo-hexagonal symmetry. This symmetry regulation significantly mitigates the lattice misfit with the substrate, culminating in an ultranarrow (004) XRC FWHM of 0.04°.TEM investigations reveal a rapid lattice recovery mechanism, where initial interfacial disorder is annihilated within a few nanometers. Furthermore, we successfully surmounted the uniformity barriers typical of Mist-CVD, realizing superior wafer-level consistency with a thickness deviation of merely ~2 nm across a 2-inch wafer. This work identifies Sn-alloyed ε-Ga₂O₃ as a versatile and high-fidelity template, offering a simplified single-source route for the scalable manufacturing of next-generation electronics.

The ever-increasing demand in energy necessitates efficient power converters, with high power handling capability. β-Ga₂O₃ based diodes are promising devices for the next generation high voltage power-switching applications, owing to large bandgap of Ga₂O₃. To attain large blocking voltage, the β-Ga₂O₃ drift layer of vertical diodes should be thick(>10µm) with around 10¹⁶cm^-3 free carriers. There are several reports of thick unintentionally doped or low Si doped β-Ga₂O₃ layer with nearly 10¹⁶cm^-3 carrier concentration for PiN or SBD diode applications. Majority of those films were grown using HVPE growth technique . Interestingly, up to our knowledge there has been no report of thick (>10µm) β-Ga₂O₃ epilayers doped with Sn(tin)having less than 10¹⁶cm^-3free carrier concentrations, grown using MOCVD. The challenges, in having low Sn doped β-Ga₂O₃ and eventually low carriers, stems from “wrong” site occupancy of Sn in crystal lattice or compensating defect formation, resulting in resistive films . In this work, we report 12µm thick homoepitaxial β-Ga₂O₃ film with 6×10¹⁶cm^-3 Sn dopants (SIMS) on Sn-doped β-Ga₂O₃ (001) substrate grown in a RF-heated horizontal MOCVD reactor (MR Semicon). Growth temperature was T=875⁰C with TMGa and TESn as Gallium and Sn precursor, respectively. The rocking curves comparison for (002) reflection of our epilayers, with (002) reflection of commercially available Sn-doped β-Ga₂O₃ substrate from NCT, Japan is performed. The full-widths at half maximum (FWHMs) of our epilayer (0.020⁰) is comparable to the commercially available substrate (0.026⁰), suggesting it’s high structural quality. Capacitance-Voltage measurement is performed on the grown sample, using Hg-probe. The extracted charge concentration (N_d-N_a) from the CV plot, in the frequency range of 0.1MHz to 1MHz, is calculated to be around 6×10¹⁵cm^-3. Thus, up to our knowledge, we demonstrate for the first time, 12µm thick MOCVD grown Sn:β-Ga₂O₃ epilayers, with very low charge concentration n<10¹⁶cm^-3, necessary for high power PiN diode fabrication.

Rutile-structured germanium dioxide (r-GeO₂) has attracted considerable attention as a high-power switching device material owing to its high breakdown field (7.0 MV/cm), capability for ambipolar doping, and feasibility of bulk crystal growth using flux-based techniques [1]. However, these substrates are not commercially available yet; therefore, heterogeneous substrates, such as TiO₂ and sapphire, have been utilized to grow r-GeO₂. Lattice mismatches between r-GeO₂ and these heterogeneous substrates can induce phase separation with amorphous and α-quartz phases [2]. This study investigated the insertion of buffer layers to reduce lattice mismatch. Rutile structured Ge_xSn_1-xO₂, which has compositions that can be lattice-matched to both r-GeO₂ and TiO₂, was employed as a buffer layer. The Ge_xSn_1-xO₂ buffer layer consisted of six layers, each with a Ge composition from 70% to 95% in increments of 5%, gradually reducing the lattice mismatch.

GeO₂ and graded Ge_xSn_1-xO₂ buffer layers were grown by mist chemical vapor deposition (CVD). Figure 1 shows scanning electron microscopy (SEM) images and the inverse pole figure (IPF) map obtained from electron backscatter diffraction (EBSD) analysis of GeO₂ thin films on a (001) TiO₂ substrate with and without a graded buffer layer. In addition to rutile phase, α-quartz phase was detected on bare TiO₂ substrates, as shown in Figure 1 (a)-(c). The IPF map of the rutile phase is predominantly red (Figure 1 (b)), corresponding to the (001) plane, which is consistent with the substrate orientation and suggests epitaxial growth. In contrast, the IPF map in Figure 1 (c) exhibits color gradation, suggesting polycrystalline growth of α-quartz phase. As shown in Figure 1 (e), only signals attributed to (001) oriented rutile phase were observed, indicating that single-phase r-GeO₂ growth was achieved by a graded Ge_xSn_1-xO₂ buffer layer. These results revealed that reducing the lattice mismatch is effective for the growth of single-phase r-GeO₂ thin films on TiO₂ substrates.

[1] K. Bushick, et al., Appl. Phys. Lett. 117, 182104 (2020)

[2] I. Rahaman, et al., ACS Appl. Electron. Mater. 7, 2848–2854(2025)

β-(AlₓGa₁₋ₓ)₂O₃ has emerged as a highly promising ultra–wide–bandgap semiconductor for multi–kV power electronics. Key challenges—including particle formation, parasitic gas–phase reactions, and controllable Al incorporation—are typically addressed by tuning conventional MOCVD parameters such as VI/III ratio, reactor pressure, and growth temperature. We identify the precursor reaction zone path length, before impingement on the substrate, as an independent and selective process variable. This geometric parameter modulates the residence time without significantly affecting surface–directed mass transport or surface reaction kinetics, because the concentration boundary layer thickness remains unchanged.

We tested this hypothesis in an AIXTRON CCS 3 × 2″ MOCVD reactor. Two gas–phase Al fractions—22% TMAl/(TMAl+TMGa) for samples A and B, and 52% for samples C and D were combined with two showerhead–substrate gaps: 6 mm (A, C) and 15 mm (B, D). This matrix was designed to isolate whether precursor reaction zone path length meaningfully affects Al incorporation and particle formation.

XRD measurements show the dominant β–Ga₂O₃ (002) substrate peak near 31.7°, accompanied by a lower–angle feature near 30.4° consistent with the rotated–domain (–401) reflection previously reported for MOCVD growth on (001) β–Ga₂O₃ substrates. Notably, the β–(AlₓGa₁₋ₓ)₂O₃ layers exhibit a strong rotated–domain contribution as well, while a distinct alloy (002) peak at ~32.3–32.4° is much weaker to be clearly resolved. This absence is attributed to the requirement for coherently strained films and the potential complications of alloy inhomogeneity or Al segregation. Therefore, Al incorporation can be more reliably estimated from the separation Δ(2θ)_(4̅01). Using this approach, we estimate solid–phase Al fractions of ~22% (A), ~12% (B), ~42% (C), and ~35% (D). These results show that, at fixed TMGa, TMAl flows, reducing the gap consistently increases incorporated Al while simultaneously suppressing particle formation, as confirmed by optical microscopy.

Quantitative transport modelling incorporating the actual reactor geometry shows that the mean residence time in the parasitic reaction zone (T > 250 °C) increases from 5.1 ms at 6 mm gap to 12.8 ms at 15 mm gap—providing 2.5× more time for deleterious TMAl + O₂ oxidation reactions, which preferentially deplete TMAl relative to TMGa. Importantly, concentration boundary layer calculations show that δc remains constant—approximately 1.19 mm for TMAl and 0.94 mm for TMGa—across both gap settings. This confirms that surface–directed mass transport and reaction kinetics are not affected by the gap variation.

Deep-ultraviolet (DUV) photodetectors have attracted considerable attention for applications such as flame detection, space exploration, and sterilization monitoring [1]. Among various materials, β-Ga₂O₃ is a promising candidate for solar-blind photodetectors owing to its ultrawide bandgap (~4.8 eV), which enables selective absorption in the solar-blind region [2].

In this work, β-Ga₂O₃-based DUV photodetectors were fabricated using epitaxial thin films grown by mist chemical vapor deposition (Mist-CVD) [3]. The Mist-CVD technique offers several advantages, including a simple non-vacuum process, cost-effectiveness, and the ability to grow high-quality epitaxial films.

The effects of atomic layer deposition (ALD)-deposited Al₂O₃ passivation on the electrical and photoresponse characteristics of the devices were systematically investigated. Prior to passivation, the devices exhibited shifted and asymmetric I–V characteristics due to surface defects and trap states. After applying the Al₂O₃ passivation layer, the I–V curves shifted toward the origin and became more symmetric, indicating effective suppression of surface trap states.

Furthermore, the I–T characteristics also showed a noticeable improvement. Before passivation, the devices showed a triangular-shaped response with a pronounced persistent photoconductivity (PPC) effect [4]. In contrast, after Al₂O₃ passivation, the I–T curves exhibited a square-shaped response with fast rise and decay times, indicating effective suppression of the PPC effect.

The device performance was significantly enhanced following passivation, with responsivity increasing from 2.67 × 10^-2 A/W to 2.20 A/W – an improvement of approximately two orders of magnitude. Simultaneously, the on/off ratio rose from 3.63 × 10² to 6.84 × 10⁴ due to the simultaneous reduction in dark current and increase in photocurrent.

These results demonstrate that Al₂O₃ passivation effectively suppresses surface trap states and significantly enhances the electrical stability and photoresponse performance of β-Ga₂O₃-based DUV photodetectors.

[1] C. Avila-Vendano, J. A. Carvajal-Fresno, and M. A. Quevedo-Lopez, IEEE Sensors J. 21, 14815 (2021)

[2] Z. Galazka, Semicond. Sci. Technol. 33, 113001 (2018)

[3] K. Uno, M. Ohta, and I. Tanaka, Appl. Phys. Lett. 117, 052106 (2020)

[4] S. Hullavarad, N. Hullavarad, D. C. Look, and B. Claflin, Nanoscale Res. Lett. 4, 1421–1427 (2009)

+Author for correspondence: youseung@sejong.ac.kr

In the growth of bulk β-Ga₂O₃ crystals by the edge-defined film-fed growth (EFG) method, twin defects frequently arise during the shouldering stage. This phenomenon is initiated when the degree of supercooling at the growth interface reaches the energy threshold required for twin nucleation as the expanding crystal diameter perturbs thermal stability. The resulting twin defects disrupt the structural integrity of the crystal and significantly degrade the efficiency of the devices [1].

In this study, we propose an optimized growth process using a wide-width seed that corresponds to the die width, thereby fundamentally suppressing twin formation by bypassing the diameter expansion stage. Initially, a (-101)-oriented β-Ga₂O₃ ingot was grown using a conventional seed (Figure 1(a)); although it appeared to be a single crystal to the naked eye, polarized microscopy (Figure 1(b)) and HRXRD θ-2θ scans (Figure 1(c)) revealed distinct twin boundaries and crystallographic non-uniformity within the same ingot.

To resolve this issue, a wide-width seed fabricated by the vertical Bridgman (VB) method was introduced (Figure 1(d)). Since the seed width matches the die, it facilitates immediate vertical growth, enhancing interface stability and effectively eliminating the conditions for twin nucleation. Consequently, a twin-free single-crystalline β-Ga₂O₃ ingot was successfully grown (Figure 1(e)), and microscopy after polishing (Figure 1(f)) confirmed that twin defects were completely suppressed, resulting in a high-quality crystalline state.

Application of p-type conductive oxides, such as nickel oxide (NiO) allows to produce high quality, high voltage, and low leakage current gallium oxide (β-Ga₂O₃) power diodes. NiO active layers are often deposited using reactive magnetron sputtering method, and process parameters can affect both material and resulting power devices properties. However, there is a lack of direct comparison of different magnetron sputtering configurations for deposition of p-type NiO layers for β-Ga₂O₃ devices. Here, in this work, properties of vertical NiO/β-Ga₂O₃ heterojunction diodes (HJDs) obtained by confocal and planar sputtering magnetron system were compared. The devices were fabricated on n⁺-β-Ga₂O₃ epi layers (N_D~6.5x10¹⁵cm^-3) grown by HVPE on bulk (001) β-Ga₂O₃ substrates. The NiO/Ni/Au anodes were fabricated using lift-off photolithography and deposition of 100 nm NiO layers by means of RF magnetron sputtering using ceramic NiO target. Nickel oxide layers were deposited with different oxygen contents in the Ar/O₂ gas mixture equal to 0, 10, 20, 30% in both planar down pO₂ (oxygen partial pressure) and confocal down fO₂ (oxygen partial flow) sputtering magnetron systems.Diodes with a NiO layer deposited in pure Ar atmosphere were characterized by Schottky-like or double-barrier I-V characteristics. Devices with a NiO layer from a confocal magnetron sputtering possess a lower ideality factor (n = 1.02), higher forward current over 200 A/cm² and breakdown voltage over 1600V as compared to diodes produced with a NiO layer obtained from a planar system. However, it shows “double-barrier” like forward characteristics. Higher breakdown voltage can be associated with higher energy bandgap (E_g=3.71 eV) as well as higher resistivity of layers deposited using confocal sputtering system. Devices with NiO layer deposited in a reactive atmosphere (pO₂ = fO₂ = 10-30%) were characterized by a typical p-n heterojunction characteristics with higher turn-on voltages. Higher forward current and breakdown voltage were observed for HJDs fabricated means of confocal magnetron sputtering system as compared to planar one. Ideality factors as low as n~1,15 were obtained for all diodes, regardless of oxygen partial flow. This work has been partially supported by the Wide Bandgap (WBG) Pilot line, which is funded jointly by the Chips Joint Undertaking, through the European Union’s Digital Europe programme and Horizon Europe programme, as well as by the participating states Italy, Sweden, Poland, Finland, Austria, France and Germany, under Grant Agreement n. 101183211

Rutile germanium dioxide (r-GeO₂) has drawn a lot of attention due to its large bandgap, structural stability, and high thermal conductivity. Enhanced stability offered through alloying with SnO₂ makes these rutile structured oxides excellent candidates for next generation power electronic devices. Due to rutile oxides only recently being recognized as ultra-wide bandgap semiconducting materials, potential dopant and contact materials remain speculative.

In this work, we confirm Sb as an effective dopant in single crystal r-Sn_0.4Ge_0.6O₂ thin films grown on (10-10)-oriented sapphire substrates. Through variation of doping concentrations from 0.03-3.3%, we identify lower and upper doping limits for thin films. Additionally, we can achieve carrier densities up to 1.7 x 10²⁰ cm^-3 and mobilities up to 33 cm2/Vs at room temperature. Challengingly, it is observed that under oxygen rich annealing conditions, germanium vacancies can act as electron sinks, drastically reducing carrier mobilities and rendering films insulating. We confirm that Sb can still behave as an effective donor so long as annealing occurs under reduced oxygen environments. Our work establishes optimized conditions for synthesizing Sb-doped thin films using pulsed laser deposition and the interplay between electron transport and deposition conditions.

β-Ga₂O₃ has an ultrawide bandgap of 4.7 eV and superior material properties that make it a promising candidate for next-generation power devices. However, realization of effective p-type doping in β-Ga₂O₃ remains a significant challenge, limiting the performance of devices. To address this issue, current-blocking layers (CBLs) with deep levels introduced by nitrogen (N) ion implantation have been proposed as an alternative to p-type wells. In this study, CBL test element groups (CBL-TEGs) with varying N implantation doses were fabricated to evaluate CBL performance and reduce leakage current. Light emission analyses were performed to identify the leakage locations and elucidate the leakage mechanism.

CBL-TEGs have an epitaxial layer grown on a β-Ga₂O₃ (001) substrate, with a carrier concentration of 1.0 × 10¹⁶ cm⁻³ and a thickness of 10 μm. N ion implantation was performed to create a box profile with a depth of 0.8 μm. Total N doses were 4.0 × 10¹³, 4.0 × 10¹⁴, and 4.0 × 10¹⁵ cm⁻². An n⁺ layer with a depth of 0.2 μm was formed by silicon (Si) ion implantation. Following N and Si implantations, annealing was performed at 900 °C for 30 min. and 1 min., respectively. The top and bottom electrodes were formed by depositing Ti/Au with thicknesses of 20/230 nm. Finally, the top electrode was patterned, and the samples were annealed at 470 °C for 1 min.

The reverse-bias characteristics of the CBL-TEGs with various N doses were measured. Increasing the N dose effectively reduces the leakage current. Specifically, for the sample with a dose of 4.0 × 10¹⁵ cm⁻², the leakage current remained below 2 × 10⁻⁵ A/cm², a substantial improvement over previously reported values. After that, plan-view images of light emission and the emission spectrum under leakage condition were obtained. Clear emission is observed from the n⁺ region surrounding the top electrode for the 4.0 × 10¹³ cm⁻² sample, indicating that leakage current was distributed throughout the CBL under the entire n⁺ region. An emission peak near 700 nm (1.77 eV) suggests electron transitions involving the deep level. For the 4.0 × 10¹⁴ cm⁻² sample, emission from the n⁺ region around the electrode was observed, as with the case for the 4.0 × 10¹³ cm⁻² sample. In contrast, for the 4.0 × 10¹⁵ cm⁻² sample, emission was confined to the edge of the n⁺ region. These observations indicate that a N dose of 4.0 × 10¹⁵ cm⁻² effectively suppresses leakage through the CBL under the n⁺ region.

Acknowledgement: This study is based on results obtained from a project, JPNP22007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

β-Ga₂O₃ has gained strong interest for power electronics because of its advantages over Si, SiC, and GaN. Homoepitaxy on β-Ga₂O₃ substrates is especially important, as it enables films with the highest crystalline quality. Among available techniques, Molecular Beam Epitaxy (MBE) offers atomic–level precision and excellent interface control, but its growth rate is limited by volatile Ga₂O formation. Achieving (001) homoepitaxy at low temperatures is difficult for any method. Overcoming the growth–rate limitation requires a more efficient oxygen plasma source capable of further oxidizing Ga₂O into solid β–Ga₂O₃. A high–density plasma is therefore essential.

To address this, we developed a High–Density Oxygen Radical Source (HD–ORS) that uses a mixture of ozone and oxygen to generate atomic oxygen for both MBE and Physical Vapor Deposition (PVD). Ozone was chosen instead of O₂ because its peak dissociation energy is an order of magnitude lower, and its dissociation produces singlet oxygen (¹D), which is significantly more reactive than triplet oxygen (³P).

This study demonstrates homoepitaxial β–Ga₂O₃ growth on Sn–doped Ga₂O₃ substrates by MBE using the HD–ORS. Optimal growth was achieved at a substrate temperature of 300 °C. We further show homoepitaxial β–Ga₂O₃ growth by PVD using both a commercial Low–Impedance Antenna Inductively Coupled Plasma (LIA–ICP) source and the HD–ORS. Notably, the HD–ORS enabled stable and reproducible (001)–oriented homoepitaxy on (001) substrates, achieving a growth rate of 1 µm/h-an order of magnitude higher than typical Ga₂O₃ growth rates obtained by conventional MBE.

This work studies the leakage current characteristics of Ga₂O₃ Schottky barrier diodes (SBDs) employing metallic magnesium (Mg) as an ex-situ diffusion source for Schottky barrier engineering. The epitaxial structure consists of a 5 µm unintentionally doped (UID) β-Ga₂O₃ layer (n ≈ 2.6×10¹⁶ cm^-3) grown by hydride vapor-phase epitaxy (HVPE) on a (001)-oriented n-type Sn-doped β-Ga₂O₃ substrate (NCT). A 10 nm thick Mg layer was deposited by sputtering on patterned regions, followed by liftoff. The Mg diffusion process was performed by thermal annealing at various temperatures in O₂ or N₂ ambient for different durations. The wafers were then cleaned by acid to remove Mg residue and other amorphous compound formed during annealing. Ni/Au Schottky contact was deposited on the Mg-treated mesa regions by e-beam evaporation, and an Al backside contact was subsequently deposited by sputtering. No edge termination or passivation was applied in these SBDs. A reference sample without Mg annealing process was also fabricated.

The forward and reverse J-V measurement results for 200µm diameter SBD are shown in Fig. 1. Under forward bias, almost all SBDs have the same behavior as the reference device except that annealing at 800°C shows an increased turn-on voltage. Compared to the annealing in N₂ ambient, the annealing process of Ga₂O₃ in O₂ ambient better suppresses the reverse leakage current with stable device performance. It was found that with thin Mg deposition thickness of 10nm, the mild annealing temperature of 600-650°C shows lower leakage current compared to annealing at higher temperature of 800°C. Furthermore, extending the annealing time from 5 minutes to 10 minutes at 650°C increases the reverse leakage, indicating that Mg diffusion needs to be controlled such that there exists a window for moderate Mg diffusion concentration for the optimized reverse leakage characteristics of Mg-annealed β-Ga₂O₃ SBD.

The MOVPE growth of gallium oxide remains challenging, as precise control of layer thickness, surface morphology, and doping levels is required to produce device-grade epitaxial wafers for lateral and vertical transistors and to enable scaling to larger wafer sizes. In-situ optical metrology methods such as pyrometry and reflectometry therefore play an important role in understanding and controlling the growth process [1]. For fundamental studies, gallium oxide layers can be grown heteroepitaxially on sapphire substrates. Due to the refractive index contrast between the film and the substrate, strong Fabry–Pérot oscillations appear in the reflectance signal, enabling straightforward determination of growth rate and layer thickness. However, transistor applications require growth on native gallium oxide substrates, resulting in homoepitaxy where the refractive index contrast is minimal and Fabry–Pérot oscillations are largely suppressed.

In this work, we demonstrate that in-situ reflectometry can nevertheless provide critical growth information during homoepitaxial growth. Small variations in refractive index enable extraction of growth rate and layer thickness using an autocorrelation-based analysis of the reflectance signal. In addition, short-wavelength reflectometry at 280 nm provides a sensitive method for real-time monitoring of surface roughness during growth, independent of the substrate material. Furthermore, we present results on the determination of doping levels in Si-doped gallium oxide using in-situ reflectometry. Significant refractive index changes with doping level are observed and attributed to plasmonic effects described by the Drude approximation. This enables correlation of doping levels with the in-situ reflectance signal, demonstrating the potential of optical in-situ metrology for real-time monitoring of doping during MOVPE growth.

With such in-situ monitoring, independent of the substrate and even the MOVPE tool, the growth can be precisely monitored.

[1] Journal of Crystal Growth 603 (2023) 127003

Author for correspondence: kolja.haberland@laytec.de

One major drawback of Ga₂O₃ is its low electron mobility. To overcome this limitation in homoepitaxial β-Ga₂O₃, a modulation-doped β-(Al_xGa_1-x)₂O₃/Ga₂O₃ heterostructure has been proposed to induce a high-mobility two-dimensional electron gas (2DEG) at the interface, which can be used in high electron mobility transistors (HEMT). However, a comprehensive understanding of the growth mechanisms, as welle as the surface and reaction behavior of aluminum-alloyed gallium oxide layers is still lacking.

We report on the growth of fully strained epitaxial thin layers (15-60 nm thickness) of β–(AlₓGa_1–x)₂O₃ on β–(100) Ga₂O₃ with an offcut angle of 4° by metal-organic vapor phase epitaxy (MOVPE) with an Al content around 40%. With increasing the Ga flow, the growth rate increased linearly up to approximately 30 nm/min at constant Al flow (Fig. 1a). With a higher Ga flow, a smoother surface with a step-bunching morphology with less particles is observed (Fig. 1b). The Al content in the layers stayed roughly constant with increasing Ga flow, might suggesting that a Ga wetting-layer is formed. The Ga wetting layer influences adatom diffusion, allowing Al adatoms to migrate over the Ga-rich surface and incorporate into the crystal lattice, effectively “sinking” from the adlayer to form a thin layer. Thus, Ga acts as a surfactant for the growth of β-(Al_xGa_1-x)₂O₃, possibly attributed to a metal-exchange catalysis mechanism similar to the reported In-catalyzed Ga₂O₃ growth via MBE.[1]

This work is an important step toward understanding the growth of Al-alloyed heterostructures for achieving high-efficiencyHEMT devices.

[1] P. Vogt, O. Brandt, H. Riechert, J. Lähnemann, O. Bierwagen, PRL 119, 196001 (2017).

β-Ga₂O₃ has emerged as a promising ultra-wide bandgap semiconductor for high-voltage power electronics due to its large bandgap (~4.8 eV) and high critical electric field. However, the absence of reliable p-type doping remains a key limitation for the realization of bipolar devices. One approach to overcome this challenge consists of using p-type oxides to form heterojunction PiN structures with n-type Ga₂O₃.

In this work, vertical Ni₁₋ₓO/β-Ga₂O₃ PiN diodes were fabricated on Si-doped β-Ga₂O₃ substrates with a 10-µm-thick n⁻ Ga₂O₃ drift layer (N_D ≈ 10¹⁶ cm⁻³). The Ni₁₋ₓO layer used to form the heterojunction was optimized in a companion study focusing on the electrical properties of p-type Ni₁₋ₓO thin films. A 3 µm-deep mesa structure was implemented to mitigate electric field crowding at the device periphery and improve the overall electrical performance.

Electrical characterization was performed on devices with anode diameters ranging from 80 µm to 450 µm. Forward measurements show rectifying behavior with a turn-on voltage of approximately 1.9 V. The forward current density increases with the device diameter, while smaller diodes exhibit reduced current density. This size dependence suggests that forward transport is not purely limited by bulk series resistance but is influenced by perimeter-related effects and non-uniform carrier injection at the Ni₁₋ₓO/β-Ga₂O₃ heterojunction.

Reverse measurements reveal low leakage current densities in the 10⁻⁷–10⁻⁸ A/cm² range and a diameter-dependent breakdown behavior, where larger devices exhibit earlier breakdown, likely due to electric-field crowding at the device periphery, combined with increased sensitivity to local defects in larger-area devices.

Temperature-dependent measurements further reveal a transition from injection-limited transport in small devices to drift-limited conduction in larger diodes. While larger diodes exhibit a decrease of forward current with increasing temperature, consistent with mobility degradation in the Ga₂O₃ drift layer, smaller devices show a thermally activated current increase, suggesting injection-limited transport at the heterojunction.

A benchmark of leakage current density versus breakdown voltage is presented and compared with reported Ni₁₋ₓO/β-Ga₂O₃ heterojunction diodes. These results highlight the competitive among the lowest reported leakage performance of the proposed devices and provide insights into the transport mechanisms governing the scaling of vertical Ga₂O₃ heterojunction power diodes.

β-Ga₂O₃ is known as a novel material for power electronics. [1,2] We have been focusing on Mg_xNi_1-xO alloy as a material for pn heterojunction with β-Ga₂O₃. Electric property, bandgap, and band alignment have been reported for Mg_xNi_1-xO with x≤32%. [2,3] In this study, Mg_xNi_1-xO films were deposited by RF magnetron sputtering for x≤0.58, and their electronic and electrical properties were evaluated.

Mg_xNi_1-xO films were deposited by RF magnetron sputtering in an O₂ atmosphere at an ambient temperature. A NiO target and c-plane sapphire or quartz glass substrate were set with a distance of 3 cm. To alloy with MgO, the NiO target was co-sputtered with 0 to 16 pieces of 10×10 mm² square-shaped MgO substrates. RF power and sputtering pressure were fixed at 150 W and 0.75 Pa, respectively. Since sputtering rate decreased with increasing the number of MgO substrate, sputtering time was varied in a range 10-30 minutes to adjust film thickness in a range 106-140 nm. Resistivity was measured by the four-point probe method with van der Pauw configuration. Mg composition was quantified by energy dispersive X-ray spectrometry. Optical transmittance spectra were measured by UV-Vis spectrophotometer. Valence band offset (ΔE_v) at the MgNiO/sapphire interface was determined by X-ray photoelectron spectroscopy.

As shown in Fig. 1, room temperature resistivity increased monotonically with increasing x. All the films were confirmed to show a p-type conductivity by Seebeck effect measurement. Absorption edge E_edge, which was determined by the Tauc plot, increased with x in a range 3.69-4.28 eV. The Tauc plot showed additional onset in a range 3.74-5.49 eV. The former is attributed to O 2p to Ni 3d charge transfer transition, and the latter is originated from valence to conduction band transition giving bandgap E_g. [5] ΔE_v decreased with x in a range 2.31-1.57 eV.The results may imply that breakdown voltage is expected to be higher for large x with maintaining low on-voltage.

We report on β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ (AlGaO/GaO) heterostructures grown on c-plane sapphire by liquid-injection MOCVD. AlGaO buffers were compositionally-graded with varying starting Al content (5, 15, 30 at. %) via novel fully-autonomous liquid precursor dosing to achieve linear gradient for mitigation of the lattice mismatch. As opposed to our previous results, Si-doped GaO films grown on AlGaO buffers exhibited improved crystallinity: GaO X-ray rocking curve showed best FWHM of ~1.6°, while only ~0.9° for AlGaO buffer with 5% Al content. However, signs of phase separation were observed for AlGaO films with 15% and 30% Al content. Significantly improved was RT electron mobility μ_e ~8 cm²/Vs at n_e ~1.3×10¹⁹ cm^-3 (ρ = 0.078 Ω∙cm) compared to the films prepared directly on sapphire which showed μ_e ~0.6-1.7 cm²/Vs and strong electron localization. Depletion-mode MOSFETs showed output current of ~9 mA/mm but did not fully pinch-off (before gate dielectric breakdown) without gate recess. Further, strong suppression of parasitic (310)-oriented crystallites, previously achieved via the use of off-cut substates, was observed for 5% Al content AlGaO buffers, while only minor content was observed in subsequently-grown GaO films. Implementing the AlGaO buffer led to several times larger GaO grains than previously; RMS surface roughness of AlGaO and GaO films were 3.8 and 11 nm, respectively. We further evaluate electronic structure, depth-resolved chemical composition, stoichiometry, and electric transport properties via photoluminescence, X-ray photoelectron spectroscopy, elastic recoil detection analysis, and low-T I-V measurements, respectively.

β-Ga₂O₃, one of the ultra-wide-bandgap semiconductors with a bandgap of 4.6-4.9 eV, has attracted attention because of its potential applications for power electronics. Thus far, the control of carrier concentration in the range of 5 × 10¹⁷ cm⁻³ to 2 × 10²⁰ cm⁻³ has been achieved for thin films obtained by mist-CVD [1]. However, compared to other growth processes, the carrier concentration remains relatively high. The carrier concentration below 10¹⁷ cm⁻³ is required for achieving a breakdown voltage above 1 kV. This is partly because unintentional impurities such as Si are incorporated into thin films during the growth process. In this work, an attempt was made to reduce the concentration of unintentional impurities, especially, Si by inner-wall coating of silica tube used for the thin film growth and cleaning of substrate surface prior to the preparation of β-Ga₂O₃ thin films on β-Ga₂O₃ substrates.

Figure 1 shows SIMS profiles of Si in thin films and substrates before and after the cleaning of substrate surface with hydrofluoric acid and inner-wall coating of silica glass tube. It is clear that the concentration of Si was reduced by both treatment with hydrofluoric acid and inner-wall coating. Figure 2 depicts relationship between electron mobility and carrier concentration obtained by the Hall effect measurements. The carrier concentration was reduced by both hydrofluoric acid treatment and inner-wall coating. For the thin film grown in the silica glass tube the inner-wall of which was coated with both Ga₂O₃ and Al₂O₃, the carrier concentration is decreased to 6.3 × 10¹⁶ cm⁻³, and the electron mobility reaches 129 cm²V⁻¹s⁻¹.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under project JPNP21005.

[1] T. Ogawa et al., Jpn. J. Appl. Phys. 62, SF1016 (2023).

Mist CVD has evolved as a safe and cost-effective method for growing oxide thin films such as those used for transparent conductors, passivation layers, and coating films. However, its application to the growth of device-quality semiconductor layers has hardly been investigated, as it is considered inferior to MOCVD or MBE in terms of source purity and the potential for impurity incorporation. At Kyoto University, based on the experience that β-Ga₂O₃ films grown on β-Ga₂O₃ substrates exhibit good surface morphology and crystallinity, owing to homoepitaxy, efforts have been made to reduce residual impurities in β-Ga₂O₃ films. Recently, the donor concentration became controllable in the order of 10¹⁷cm^-3, where the breakdown voltage could be >800 V.

To demonstrate the potential applications of mist CVD-grown β-Ga₂O₃ films in devices, we presented successful operation of MESFETs with the gate length (L_G) of 2 μm [1], followed by those with a L_G of 0.45 μm and an effective gate width (W_G) of 150 μm showing a current gain cut-off frequency ( f_T) and a maximum oscillation frequency ( f_max) of8.3 and 18.4 GHz, respectively [2]. In this presentation, we demonstrate advanced RF characteristics of MESFETs with larger W_Gshowing higher power and gain.

We used a (010)-oriented 2-inch β-Ga₂O₃ substrate grown by the vertical Bridgeman method. The substrate was divided into four pieces on which the growth was performed by mist CVD. The β-Ga₂O₃ epitaxial layer was Ge-doped and had a thickness of 61 nm. The electron concentration and mobility were estimated at 1.5x10¹⁸ cm^-3 and 85 cm²/Vs, respectively, based on the Hall effect measurements. The MESFETs were fabricated at the C-TEFs of Nagoya Universityusing stepper lithography. The L_G and W_G were 0.4 and 600 μm, respectively. An example of the DC characteristics revealed the maximum drain current and transconductance of 93 mA/mm and 33 mS/mm, respectively, at the drain voltage of 10 V. An amplifier using the MESFET showed a maximum output power of 19 dBm (80 mW), a linear gain of 10dB, and a maximum added efficiency of 15 % at 2.48 GHz by placing the matching circuit near the device. The output power was more than six times higher than that of our previous device [2] owing to the larger gate width. However, the drain current is not high enough, and overcoming this is one of our future objectives. Nevertheless, these results encourage the use of mist CVD may as a cost-effective device fabrication technology.

We acknowledge Novel Crystal Technology, Inc. for supplying a 2-inch β-Ga₂O₃ substrate.

[1] H. Takane et al., Appl. Phys. Express 16, 081004 (2023); [2] H. Ikeda et al., Jpn. J. Appl. Phys. 64, 108002 (2025).

Bandgap engineering of β-Ga₂O₃ has been widely investigated for enhancing Baliga’s figure of merit of the materials and for developing heterojunction-based devices. These studies employed the heteroepitaxy of a ternary (Al_xGa_1−x)₂O₃ (AGO) alloy; however, a phase-pure and high-quality AGO epitaxial layer is practically limited to an Al content of approximately 20% and a certain layer thickness due to the lattice mismatch. In contrast, a quaternary (AlxScyGa1−x−y)2O3 (ASGO) is expected to exhibit a wider tunability of the bandgap energy (E_g) and layer thickness owing to its lattice matching to β-Ga₂O₃. Here, we have investigated the crystal structures and E_g of ASGO powders and thin films for future heterojunction-based power-device applications.

The four lattice parameters of the ASGO powder samples are identical to those of β-Ga2O3 when x/y is between 1.5 and 3.2. We thus prepared several ASGO films with the lattice-matched x/y range on β-Ga₂O₃ (100) by using pulsed-laser deposition. The lattice-matched ASGO (100) epilayer exhibited superior crystallinity as confirmed by a sharp reflection from the thin film in x-ray diffraction reciprocal space maps. The estimated E_g of the ASGO (100) films determined by electron energy loss spectroscopy varied from 4.5 to 5.8 eV while maintaining the lattice matching to β-Ga₂O₃.

β-Ga₂O₃ has potential for use in high-power electronics due to its unique properties and facile crystalline growth. Commercial realization of β-Ga₂O₃ power electronics will require intimate knowledge of performance-limiting structural and extended defects. Here, we use photoemission electron microscopy (PEEM) and spectroscopy to measure the surface electronic properties and observe spatial inhomogeneities of β-Ga₂O₃ substrates and homoepitaxy films.

The electronic properties of β-Ga₂O₃ substrates with (010), (001), and (-201) orientations are measured and show nanoscale electronic variations on all three facets that correspond to local regions of different doping and gap state density (Fig 1 a-c). The work function and ionization potential of the (010) surface are substantially lower than the other two orientations (Fig 1 d), which is consistent with reported differences in Schottky barrier heights.¹

In (010) β-Ga₂O₃ homoepitaxially grown via hydride vapor phase epitaxy (HVPE), extended structural defects are observed. We observe two types of linear, surface defects aligned along the [001] crystal axis. One defect consists of a micrometer-sized particle and a tail of protruding material, while the other is a groove in the surface. The large particle is a Ga-rich phase that is likely present early in the HVPE growth that disrupts the surface, while the groove defect appears purely structural in nature.²

1. Fu et al. IEEE Trans. Electron Devices 65, 3507–3513 (2018).
2. Kim et al., Appl. Phys. Lett. 126, 231605 (2025).

Ga₂O₃ and (Al_xGa_1−x)₂O₃ alloys have attracted attention as materials for next-generation high-power devices. In this study, the composition dependence of the formation enthalpy, ΔH, of (Al_xGa_1-x)₂O₃ alloys in the α and β phases was investigated by first-principles calculations. For simplicity, the calculated results were approximated using a regular solution model. Based on these results, thermodynamic analysis was performed for plasma-assisted molecular beam epitaxy (PA-MBE) growth of (Al_xGa_1−x)₂O₃.

For β-(Al_xGa_1-x)₂O₃, three Al occupation models were considered: octahedral-site occupation (β-oct), random occupation of octahedral and tetrahedral Ga sites (β-rand), and tetrahedral-site occupation (β-tet). The results indicate that Al preferentially occupies the octahedral site. The composition-dependent formation enthalpies for α- and β-(Al_xGa_1-x)₂O₃ were approximated using the regular solution model, ΔH=Ωx(1−x). The fitted Ω values were 511 cal/mol-cation for β-oct, 3021 cal/mol-cation for β-tet, and 2108 cal/mol-cation for α-(Al_xGa_1-x)₂O₃, corresponding to critical temperatures (T_c) of −145, 487, and 257 °C, respectively. The low T_c for β-oct suggests that compositional separation is unlikely within the β phase.

The relationship between the gas-phase composition ratio, R_Al(=P°_Al/P°_III), and the solid-phase Al composition x was also investigated under various oxygen input partial pressures. Under O-rich growth conditions, R_Al was nearly equal to x. In contrast, under metal-rich growth conditions, x shits toward Al₂O₃-rich compositions with increasing R_Al because the input Ga becomes Ga₂O(g) and remains in the gas phase. The calculated results agree well with the experimental data. These results indicate that, under the present PA-MBE conditions, the growth of (Al_xGa_1-x)₂O₃ is thermodynamically limited.

This work was supported in part by the Collaborative Research Program of Research Institute for Applied Mechanics, Kyushu University.

[1] T. Ohtsuki, J. Vac. Sci. Technol. A 41, 042712 (2023).

Gate charge Q_G is a useful metric for understanding and predicting the switching speed and loss of high-voltage power switches, but is difficult or impossible to directly measure on-wafer for ultra-wide band gap devices due to their extremely low gate charges and fast switching speeds. New methods for extracting Q_G are therefore necessary in order to effectively benchmark Ga₂O₃ devices. We have explored three means of estimating gate charge: split C-V, s-parameter extraction, and geometric approximation. The geometric method provides a conservative order-of-magnitude estimate of Q_G. Q_G from split C-V measurements are in good agreement with datasheet values for COTS parts and agree within an order of magnitude with the Q_G,geometric for lateral Ga₂O₃ devices. S-parameter extraction agrees well with split C-V data and has a lower noise floor, improving accuracy for small-periphery devices. Work is ongoing to correlate these values to transient switching measurements and to determine the best practices for accurate Q_G extraction. Overall, these methods provide a useful suite of techniques for researchers looking to benchmark switching performance for small, UWBG devices with low Q_G.

This work focuses on the characterization of defects in an axial slice from seed to dome of b-Ga2O3 commercially grown by SYNOPTICS using the Czochralski (CZ) method. High resolution x-ray topography (XRT) was used to observe threading dislocations that propagate throughout the slice. To obtain clear XRT images, the sample surfaces were subjected to chemical–mechanical polishing (CMP) to remove damage introduced during mechanical processing prior to XRT observation. Figure 1 shows two XRT images taken with g = 912 and g = 605. Note that the horizontal lines that appear in Fig. 1(a) do not appear in Fig. 1(b). Using contrast analysis, the invisibility criterion, the Burgers vectors of these horizontal lines must contain a component. As the growth direction for CZ grown b-Ga2O3 is these striations can be characterized as threading screw dislocations as their Burgers vector and dislocation direction are parallel. Using the data, it was calculated that the density of the TSDs on the seed side was ~1000/cm2 which dropped to ~400/cm2 on the dome side. It was observed that the TSDs either annihilated, converted to basal plane dislocations (BPDs), or combined into a single TSD. The while circular contrasts in Fig 1(a) are basal plane dislocations and their density in the slice also decreases from seed side to dome. The average density dropped from ~400/cm2 to ~100/cm2. Finally, the long dislocations in these images are threading mixed dislocations TMDs as their line direction changes as they migrate through the slice and they appear in both Fig 1(a) and 1(b) The density of the TMDs once again drops from seed side to dome side, with the averages of ~150/cm2 and ~50/cm2 respectively. Further details of the defects and their evolution will be presented.

Among ultra-wide bandgap (UWBG) semiconductors Ga₂O₃ is a promising candidate for application in high-power electronic devices due to its high breakdown field [1]. To further widen the bandgap, alloying with isoelectric Al is a viable strategy. Because of the limited solubility of Al in β–Ga₂O₃, α-Ga₂O₃ is the preferred polymorph for such investigations. Being isostructural to Al₂O₃, α-Ga₂O₃ allows for Al-alloying over the entire composition range [2].

In this contribution we investigate the growth of α-(Al_xGa_1-x)₂O₃ layers by plasma-assisted suboxide molecular beam epitaxy (S-MBE) in combination with the presence of the growth-enhancing additive Indium on the surface. Using these approaches, a comparative study of the Al-alloying of α-Ga₂O₃ on m-plane Al₂O₃ is presented. The findings pave the way to investigate the incorporation of donors into the α-(Al_xGa_1-x)₂O₃ layers. Furthermore, we take the concept of Vogt et al. [3] into account, to use a low temperature α-Ga₂O₃ buffer layer to suppress the defect formation in the desired conductive layer on top. In this framework we study doping with Sn, Ge, and Si for layers at different Al-concentrations. The incorporation of the dopants and their electrical activity are examined in relation to structural and morphological characteristics, analyzed by using high-resolution X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM).