We report on the use of nitric oxide (NO) diluted in nitrogen as a nitrogen dopant source for beta-Ga2O3 grown by metal-organic chemical vapor deposition. The effects of the NO/N-2 flow rate, Ga precursor molar flow rate, and substrate temperature on nitrogen incorporation, doping controllability, and film growth rate were systematically investigated. Introducing small NO flows with O-2 increased film growth rates by similar to 1.8-3.2 & times; and enabled controllable nitrogen incorporation in films grown using triethylgallium and trimethylgallium precursors. Secondary ion mass spectrometry revealed a strong dependence of nitrogen incorporation on the NO flow rate, gallium supply, and substrate temperature. Nitrogen concentrations up to 6.5 & times; 10(18) cm(-3) were achieved, while NO/N-2 doping yielded substantially lower hydrogen incorporation, with N/H ratios up to similar to 28. The resulting films exhibited excellent crystallinity, smooth morphology, and thermally activated conduction with activation energies ranging from similar to 27 to 509 meV, depending on nitrogen concentration. These results establish NO as an alternative nitrogen source that enables controlled N doping with minimal hydrogen incorporation-offering a practical route for realizing semi-insulating beta-Ga2O3 layers suitable for high-power electronic applications.

High-current-density (>1 kA/cm2) quasi-vertical AlN Schottky barrier diodes (SBDs) were fabricated on native AlN substrates by metal–organic chemical vapor deposition. The quasi-vertical AlN SBDs exhibited a turn-on voltage of ∼2.5 V, a rectification ratio exceeding 107, an on-state current density of >2.5 kA/cm2, and a low specific on-resistance of 1.5 mΩ cm2. Temperature-dependent current–voltage (I–V) characterizations were performed at both forward and reverse biases to study the key device parameters and leakage at high temperatures. At forward bias, the Schottky barrier height increased, and the ideality factor decreased with increasing temperature, indicating an inhomogeneous metal/AlN interface. The inhomogeneous behavior of the Schottky barrier was comprehensively studied, and the extracted mean Schottky barrier height was 2.55 ± 0.27 eV. Temperature-dependent reverse leakage analysis revealed that Poole–Frenkel emission and trap-assisted tunneling are the dominant leakage mechanisms. This work can be beneficial for advancing the development of ultrawide bandgap AlN-based electronics.

Solid-phase epitaxy (SPE) of β-Ga2O3 thin films by radio frequency (RF) sputtering, followed by crystallization through high-temperature postdeposition annealing, is employed on sapphire substrates, yielding a high-quality pseudosubstrate for subsequent buffer growth via MOCVD and LPCVD. Low roughness (<0.5 nm) and sharp single-crystalline diffraction peaks corresponding to the (-201), (-402), and (-603) reflections of β-Ga2O3 were observed in the SPE β-Ga2O3 film and the subsequent epitaxial buffer layer. N-doped Ga2O3 film grown by LPCVD on SPE Ga2O3 film showed step-assisted growth mode with reasonable electronic behavior, with 45 cm2/V·s mobility at a bulk carrier concentration of 1.3 × 1017 cm-3. These results suggest that SPE Ga2O3 is a promising pathway to advance the development of β-Ga2O3 on foreign substrates.

In this work, we demonstrate device fabrication and characterization of ultrawide bandgap (UWBG) β-(AlxGa1−x)2O3 channel metal–semiconductor field-effect transistors (MESFETs) with Si-implanted source/drain contacts. Films of Si-doped β-(AlxGa1−x)2O3 and unintentionally doped (UID) β-Ga2O3 were grown on an Fe-doped (010) β-Ga2O3 substrate using close-injection showerhead metal–organic chemical vapor deposition (CIS-MOCVD). The Al concentration (x) of the β-(AlxGa1−x)2O3 film was estimated to be ≈21% using x-ray diffraction (XRD), and the Si doping concentration of the n-type β-(Al0.21Ga0.79)2O3 film was measured to be 2 × 1018 cm−3 using secondary ion mass spectroscopy (SIMS). Both gate-recessed and non-recessed MESFET structures were fabricated and had on/off ratios of ≈105. The gate-recessed MESFETs had a lower saturation drain current (15 mA/mm) than the non-recessed structures (26 mA/mm), but they also had a slightly lower off-state leakage current. The gate-recessed structures also enabled better modulation of the channel conductivity, which led to a positive threshold voltage shift of +8V compared with the non-recessed structures. The maximum breakdown voltages of 730 and 858 V were measured for the gate-recessed and non-recessed MESFETs, respectively. Because the thermal conductivity of the disordered alloy β-(Al0.21Ga0.79)2O3 channel is expected to be much lower than β-Ga2O3, thermoreflectance imaging was used to assess the device-level thermal performance. At a DC power density of 0.58 W/mm (VGS = 0 V), an area-averaged gate temperature rise of 54 K was measured. This work demonstrates the potential of leveraging the large-area, low-cost, high-quality β-Ga2O3 substrate platform to develop next-generation power electronics using UWBG β-(AlxGa1−x)2O3 as the active semiconductor.

In this work, we report on the anisotropic etching characteristics of beta-Ga2O3 using triethylgallium (TEGa) performed in situ within an MOCVD chamber. At sufficiently high substrate temperatures, TEGa can act as a strong etchant for beta-Ga2O3 utilizing the suboxide reaction between Ga and Ga2O3 [4 Ga(s) + Ga2O3 (s) -> 3Ga(2)O (g)]. We observe that due to the monoclinic crystal structure of beta-Ga2O3, TEGa etching on both (010) and (001) substrates is highly anisotropic in nature, in terms of both sidewall roughness and lateral etch rate. Smooth sidewalls are only obtained along crystal orientations that minimize sidewall surface energy. Utilizing this technique, we also demonstrate deep sub-micrometer fins with smooth sidewalls and high aspect ratios. Furthermore, we also demonstrate the damage-free nature of TEGa etching by fabricating Schottky diodes on the etched surface, which display no change in the net donor concentration. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Aluminum nitride (AlN) and high aluminum content AlGaN alloys, as ultrawide and direct bandgap semiconductors, are well suited for high-power, high-frequency applications, as well as optoelectronic devices like UV-C LEDs, laser diodes, and photodiodes. However, challenges arise in manufacturing contacts due to the presence of the stable Al₂O₃ native oxide and other contaminants, resulting from minimal exposure to atmosphere and moisture. To address these challenges, this study aims to investigate the impact of ohmic metallization and contact fabrication processes on the electrical properties of Schottky barrier diodes (SBD) and photodetectors. In this work, we manufactured quasi-vertical SBDs with ~75 at% Al AlGaN samples, sample A and B. Both samples have a top 630 nm n - -AlGaN layer and a bottom 1.26 μm n + -AlGaN layer, which were grown on native AlN. Additional samples C and D have an identical structure except for the substrate that is AlN-on-sapphire. The doping of the top active layer in Sample A and C is 5 x 10 16 cm -3 while the doping for the top active layer in Sample B and D is 1.5 x 10 17 cm -3 . The doping of the bottom AlGaN layer for all samples is 5 x 10 18 cm -3 . Mesas were etched down to the bottom n + -AlGaN layer using a Cl 2 /Ar RIE/ICP dry etch. Upon this exposed sub-layer, an ohmic Ti/Al/Ti metallization (20/120/80 nm) was deposited using an e-beam evaporator, based on the low contact resistance process reported by Cao et al [1]. Ni/Au (40/100 nm) circular Schottky contacts of diameters 1000, 500, 250, and 100 μm were deposited on the mesa tops using an e-beam evaporator. To further study the contact processes, we also explored various pre-metal cleaning processes such as 1) wet chemical treatment, 2) ex-situ plasma etching using Cl 2 -based ICP, and 3) in-situ cleaning using argon backsputtering. The results of the contact study will be presented. I-V and C-V characterization was performed to evaluate the electrical properties of the device. The ohmic contact resistivity was characterized using a C-TLM pattern, and the SBD will be evaluated to quantify barrier height, on-resistance, and ideality factor. A particular challenge for AlN and AlGaN structures is the relatively deep levels associated with donor species, which limit doping capability and ohmic contact resistance. To improve transport properties and study doping efficiency, temperature-dependent I-V behavior of the C-TLM and SBD structures will be presented. The photoresponse of the diode using illumination at various sub-bandgap wavelengths will also be evaluated and presented. References [1] 1. H. Cao et al. , Low Contact Resistivity at the 10 −4 Ω cm 2 Level Fabricated Directly on n-Type AlN. Applied Physics Letters . 125 (2024).

Metalorganic chemical vapor deposition (MOCVD) has become a pivotal technique for developing wafer-scale transition metal dichalcogenide (TMD) 2D materials. This study investigates the impact of MOCVD growth conditions on achieving uniform and selective polymorph phase control of MoTe2 over large wafers. We demonstrated the controlled and uniform growth of few-layer MoTe2 in pure 2H, 1T', and mixed phases at various temperatures on up to 4 in. C-plane sapphire wafers with hexagonal boron nitride templates. At 600 °C, high-quality 2H-MoTe2 was obtained within a narrow temperature window, verified with absorption and TEM analysis. In addition, we observed strong exciton-phonon coupling effects in multiwavelength Raman spectroscopy when the excitation wavelength was in resonance with the C-exciton. Our findings indicate that temperature-induced Te vacancies play a crucial role in determining the MoTe2 phase. This study highlights the importance of precise control over the MOCVD growth temperature to engineer the MoTe2 phase of interest for device applications.

Homoepitaxial Si-doped (010) beta-Ga2O3 epitaxial layers with thicknesses ranging from 1.2 to 12 mu m were grown simultaneously via close-injection showerhead metal-organic chemical vapor deposition on both Czochralski and edge-defined, film-fed grown (010) beta-Ga2O3:Fe substrates. Structural characterization was performed via x-ray diffraction [full-width at half maxima of 21-72 arcsec depending on epilayer thickness and substrate, measured from the symmetric (020) reflection], x-ray topography, Raman spectroscopy, cathodoluminescence spectroscopy, and transmission electron microscopy. Surface characterization was performed via atomic force microscope (AFM) and Nomarski imaging, and electrical characterization was performed via electron paramagnetic resonance spectroscopy and Hall effect measurements. In addition, thermal conductivity was measured via steady-state thermoreflectance. AFM imaging revealed a number of extended defects forming in the epilayer, and transmission electron microscopy imaging detailed a V-shaped pit defect formed by misoriented twin defects likely originating from a defect at or near the substrate surface. The Hall effect data (N-S = 2.81-3.1 x 10(17) cm(-3) and mu(H) = 81.5-103.5 cm(2)/V s), surface roughness (1.78-4.25 nm rms), and thermal conductivity measurements (19.3-19.8 W/m K) yielded comparable results among all samples, independent of epilayer thickness. These results suggested that even thicker, high quality epitaxial (010) beta-Ga2O3 could be grown via this technique, provided that the development of the observed extended defects can be mitigated. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).

Beta-phase gallium oxide (β-Ga2O3) has exceptional electronic properties with vast potential in power and radio frequency electronics. Despite the excellent demonstrations of high-performance unipolar devices, the lack of effective p-type dopants in β-Ga2O3 has hindered the further development of Ga2O3-based bipolar devices. In this work, we applied the semiconductor grafting approach and fabricated monocrystalline Si/β-Ga2O3p–n heterojunctions, of which the characteristics were systematically studied. The heterojunctions demonstrated a diode rectification over 1.3 × 107 at ±2 V with a diode ideality factor of 1.13. Furthermore, capacitance–voltage (C–V) measurement showed frequency dispersion-free characteristics from 10 to 900 kHz. The interface defect density (Dit) was calculated as 1–3 × 1012/cm2 eV. Scanning transmission electron microscopy (STEM) and x-ray photoelectron spectroscopy (XPS) revealed that an ultrathin oxygen-rich layer existed on the Ga2O3 surface and later formed an ultrathin interfacial layer after bonding with Si. It is speculated that the excessive oxygen at the Ga2O3 surface enhanced the passivation of the Si dangling bonds and thus reduced Dit. This work improved our understanding of interface properties of the semiconductor grafting approach, providing useful guidance on the future development of Si/Ga2O3 heterojunction devices.

We report on the demonstration of β-Ga2O3 MOSFETs fabricated on 1-inch bulk substrates using metalorganic vapor phase epitaxy (MOVPE) with disilane (Si2H6) as the silicon precursor. Sheet charge uniformity of the as-grown films was measured via Hall and ranged from 5.9 - 6.7 × 1012 cm-2 with a uniform Hall mobility of 125-129 cm2 /V.s across the sample. MOSFET devices with a source-drain width of 5.1 μm were measured across the wafer and had a minimum on-resistance (RON) of 47.87 Ω.mm with a maximum on-current (ION) of 165mA/mm. For these same devices, the on-current (ION) and pinch-off voltage (VP) uniformity across the wafer were 137±12 mA/mm and -27.3±7.3 V respectively. Devices showed low reverse leakage current until catastrophic breakdown occurred, with measured breakdown voltages (VBR) of up to 2.15 kV. This work provides valuable insights into understanding the growth, fabrication, and characterization processes for β-Ga2O3 FETs on full waferscale substrates. It also projects the promise of developing lateral β-Ga2O3 FETs with high current carrying capabilities and breakdown voltages, especially on substrates of 1 inch or larger.

In this work, we report a single crystalline p-Si/(001) β-Ga 2 O 3 heterojunction diode fabricated using semiconductor grafting technology. The diode showed a breakdown voltage (V br ) of ~0.86 kV, which is the highest breakdown voltage reported for Si/(001) Ga 2 O 3 heterojunction diode to date. The average peak electric field (E m ) was calculated to be >2 MV/cm near breakdown, while the specific on resistance (R on,sp ) was measured to be 7.7 mΩ·cm 2 , corresponding to a power figure of merit of ~96 MW/cm 2 . Catastrophic breakdown is confirmed by optical microscope inspection. The turn on voltage (V on ) of the diode was measured to be around 1.12 V, the on-off ratio was calculated to be 9 × 10 9 at -2 and 2 V, and the ideality factor was extracted to be approximately 1.18. The band structure of the diode was analyzed, and C-V measurements were also performed to understand the trapping behavior at the Si/Ga 2 O 3 interface.

The characteristics of NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 heterojunction lateral geometry rectifiers with the epitaxial layers grown by metal organic chemical vapor deposition were measured over a temperature range from 25 °C–225 °C. The forward current increased with temperature, while the on-state resistance decreased from 360 Ω.cm2 at 25 °C to 30 Ω•cm2 at 225 °C. The forward turn-on voltage was reduced from 4 V at 25 °C to 1.9 V at 225 °C. The reverse breakdown voltage at room temperature was ∼4.2 kV, with a temperature coefficient of −16.5 V•K−1. This negative temperature coefficient precludes avalanche being the breakdown mechanism and indicates that defects still dominate the reverse conduction characteristics. The corresponding power figures-of-merit were 0.27–0.49 MW•cm−2. The maximum on/off ratios improved with temperature from 2105 at 25 °C to 3 × 107 at 225 °C when switching from 5 V forward to 0 V. The high temperature performance of the NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 lateral rectifiers is promising if the current rate of optimization continues.

In this work, we report on the anisotropic etching characteristics of e̱ṯa̱-Ga2O3 using triethylgallium (TEGa) performed in-situ within an MOCVD chamber. At sufficiently high substrate temperature, TEGa can act as a strong etchant for e̱ṯa̱-Ga2O3 utilizing the suboxide reaction between Ga and Ga2O3. We observe that due to monoclinic crystal structure of e̱ṯa̱-Ga2O3, TEGa etching on both (010) and (001) substrates is highly anisotropic in nature, both in terms of sidewall roughness and lateral etch rate. Smooth sidewalls are only obtained along crystal orientations that minimize sidewall surface energy. Utilizing this technique we also demonstrate deep sub-micron fins with smooth sidewalls and high aspect ratios. Furthermore, we also demonstrate the damage free nature of TEGa etching by fabricating Schottky diodes on the etched surface which display no change in net donor concentration.

In this work, we demonstrate an in situ etch technique for β-Ga2O3 inside a metalorganic chemical vapor deposition (MOCVD) reactor using triethylgallium (TEGa) as the etching agent. At sufficiently high substrate temperatures (Tsub), TEGa is introduced into the MOCVD reactor which undergoes pyrolysis, resulting in the deposition of Ga on the β-Ga2O3 surface. These Ga adatoms react with Ga2O3 to form gallium suboxide (Ga2O), which desorbs from the β-Ga2O3 surface resulting in the etching of the epilayer. MOCVD chamber parameters such as TEGa molar flow rate, substrate temperature, and chamber pressure were shown to be key in controlling the etch rate and surface morphology. A wide range of etch rates from ∼0.3 to 8.5 μm/h is demonstrated by varying the etch parameters. In addition, smooth surface morphology on (010) and (001) β-Ga2O3 substrates is also demonstrated. This new etch technique could enable damage free fabrication of 3D structures like fins and trenches, which are key components in many β-Ga2O3 device structures.

Beta-phase gallium oxide ($\beta$-Ga$_2$O$_3$) research has gained accelerated pace due to its superiorly large bandgap and commercial availability of large-diameter native substrates. However, the high acceptor activation energy obstructs the development of homojunction bipolar devices employing $\beta$-Ga$_2$O$_3$. The recently demonstrated semiconductor grafting technique provides an alternative and viable approach towards lattice-mismatched $\beta$-Ga$_2$O$_3$-based p-n heterojunctions with high quality interfaces. Understanding and quantitatively characterizing the band alignment of the grafted heterojunctions is crucial for future bipolar device development employing the grafting method. In this work, we present a systematic study of the band alignment in the grafted monocrystalline Si/$\beta$-Ga$_2$O$_3$ heterostructure by employing X-ray photoelectron spectroscopy (XPS). The core level peaks and valence band spectra of the Si, $\beta$-Ga$_2$O$_3$, and the grafted heterojunction were carefully obtained and analyzed. The band diagrams of the Si/$\beta$-Ga$_2$O$_3$ heterostructure were constructed using two individual methods, the core level peak method and the valence band spectrum method, by utilizing the different portions of the measured data. The reconstructed band alignments of the Si/$\beta$-Ga$_2$O$_3$ heterostructure using the two different methods are identical within the error range. The band alignment is also consistent with the prediction from the electron affinity values of Si and $\beta$-Ga$_2$O$_3$. The study suggests that the interface defect density in grafted Si/$\beta$-Ga$_2$O$_3$ heterostructure is at a sufficiently low level such that Fermi level pinning at the interface has been completely avoided and the universal electron affinity rule can be safely employed to construct the band diagrams of grafted monocrystalline Si/$\beta$-Ga$_2$O$_3$ heterostructures.

The addition of CHF3 to Cl-2/Ar inductively coupled plasmas operating at low dc self-biases (<100 V, corresponding to incident ion energies <125 eV) leads to etch selectivity for Ga2O3 over (Al0.18Ga0.82)(2)O-3 of >30, with a maximum value of 55. By sharp contrast, without CHF3, the etching is nonselective over a large range of source and rf chuck powers. We focused on low ion energy conditions that would be required for device fabrication. This result has a direct application to selective removal of Ga2O3 contact layers to expose underlying (Al0.18Ga0.82)(2)O-3 donor layers in high-electron-mobility transistor structures. It is expected that formation of nonvolatile AlF3 species helps produce this selectivity. X-ray photoelectron spectroscopy does detect F residues on the etched surface for the Cl-2/Ar/ CHF3 plasma chemistry.

This paper presents the advantages and disadvantages of using triethylgallium (TEGa) and trimethylgallium (TMGa) for realization of high-purity, carbon-free, epitaxial Ga2O3 films. Critical process conditions and MOCVD reactor geometries on achieving high purity β-Ga2O3 films with high electron mobility and low background carrier concentration, including doping control in this range, will be discussed. This paper will also discuss the MOCVD growth of high Al composition (up to 30%) high quality strained β-(AlGa)2O3/Ga2O3 heterostructures. To obtain semi insulating films, we will discuss Ga2O3 doping with nitrogen, comparing various nitrogen precursors. We will present the limitations of MOCVD technique for achieving heavily doped (>1020 1/cm3), highly conductive β-Ga2O3. We will present a new in-situ etching process for Ga2O3 in MOCVD using Ga and Cl based chemistry.

Beta-phase gallium oxide (β-Ga2O3) research has gained accelerated pace nowadays. However, the high acceptor activation energy obstructs the development of homojunction bipolar devices employing β-Ga2O3. The recently demonstrated semiconductor grafting technique provides an alternative and viable approach towards lattice-mismatched β-Ga2O3-based p-n heterojunctions with high quality interfaces. Understanding and quantitatively characterizing the band alignment of the grafted heterojunctions is crucial for future bipolar device development employing the grafting method. In this work, we present a systematic study of the band alignment in the grafted monocrystalline Si/β-Ga2O3 heterostructure by employing X-ray photoelectron spectroscopy (XPS). The band diagrams were constructed using two individual methods, the core level peak method and the valence band spectrum method, by utilizing the different portions of the measured data. Both methods resulted in an identical band alignment within the error range, which is also consistent with the prediction from the electron affinity values of Si and β-Ga2O3. The study suggests that the interface defect density in grafted Si/β-Ga2O3 heterostructures is at a sufficiently low level such that Fermi level pinning at the interface has been completely avoided and the universal electron affinity rule can be safely employed to construct the band diagrams of grafted monocrystalline Si/β-Ga2O3 heterostructures.

Three different metal stacks, namely Ti/Au, Ni/Au and Sc/Au, were examined as Ohmic metal contacts to Si-doped, n-type (4.1 × 1019 cm−3), 300-nm thick (Al0.18Ga0.82)2O3 layers grown by metal-organic chemical vapor deposition. This is a typical composition used for (AlxGa1-x)2O3 /Ga2O3 heterostructure field effect transistors. The effects of postdepositional annealing (300–475 °C) were examined through circular transfer length method (CTLM) measurements to determine both the transfer resistance and specific contact resistivity. The lowest resistances were achieved with Ti/Au, with specific contact resistivity 1.2 × 10–4 Ω·cm2 and transfer resistance 3.82 Ω·mm for as-deposited contacts. Annealing was found to degrade both of these resistances in all cases from the as-deposited values, even though the AGO sheet resistance decreased slightly, from 1191 Ω/□ to 905 Ω/□ after annealing at 475 °C. The temperature dependence of specific contact resistivity is also investigated.