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1. Silicon-doped β -Ga ₂ O ₃ films grown at 1 µ m/h by suboxide molecular-beam epitaxy

   https://doi.org/10.1063/5.0139622  

   Azizie, Kathy; Hensling, Felix V.; Gorsak, Cameron A.; Kim, Yunjo; Pieczulewski, Naomi A.; Dryden, Daniel M.; Senevirathna, M. K.; Coye, Selena; Shang, Shun-Li; Steele, Jacob; et al (April 2023, APL Materials)

   We report the use of suboxide molecular-beam epitaxy ( S-MBE) to grow β-Ga 2 O 3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 10 16 to 10 19  cm −3 . In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga 2 O, i.e., gallium suboxide, is supplied. Directly supplying Ga 2 O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga 2 O 3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature ( T sub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO 2 ) producing an SiO suboxide molecular beam are used to dope the β-Ga 2 O 3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 10 17  cm −3 reveal a room-temperature mobility of 124 cm 2  V −1  s −1 that increases to 627 cm 2  V −1  s −1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga 2 O 3 films grown by S-MBE at growth rates of ∼1 µm/h. 

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2. Epitaxial growth of α-(Al x Ga1− x )2O3 by suboxide molecular-beam epitaxy at 1 µm/h

   https://doi.org/10.1063/5.0170095  

   Steele, Jacob; Azizie, Kathy; Pieczulewski, Naomi; Kim, Yunjo; Mou, Shin; Asel, Thaddeus J; Neal, Adam T; Jena, Debdeep; Xing, Huili G; Muller, David A; et al (April 2024, APL Materials)

   We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films. 

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3. Si doping of β -Ga2O3 by disilane via hybrid plasma-assisted molecular beam epitaxy

   https://doi.org/10.1063/5.0142107  

   Wen, Zhuoqun; Khan, Kamruzzaman; Zhai, Xin; Ahmadi, Elaheh (February 2023, Applied Physics Letters)

   Obtaining uniform silicon concentration, especially with low concentrations (ranging from 1 × 1016 to 1 × 1018 cm−3) by molecular beam epitaxy, has been challenging due to oxidation of a silicon solid source in the oxide environment. In this work, Si doping of β-Ga2O3 (010) films by diluted disilane as the Si source is investigated using hybrid plasma-assisted molecular beam epitaxy. The impact of growth temperature, disilane source concentration, and disilane flow rate on Si incorporation was studied by secondary ion mass spectrometry. Uniform Si concentrations ranging from 3 × 1016 to 2 × 1019 cm−3 are demonstrated. Si-doped β-Ga2O3 films with different silicon concentrations were grown on Fe-doped β-Ga2O3 (010) substrates. The electron concentration and mobility were determined using van de Pauw Hall measurements. A high mobility of 135 cm2/V s was measured for an electron concentration of 3.4 × 1017 cm−3 at room temperature. 

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4. Growth of conductive Si-doped α-Ga2O3 by suboxide molecular-beam epitaxy

   https://doi.org/10.1063/5.0299750  

   Steele, Jacob; Chen, Julianne; Burrell, Tamá; Pieczulewski, Naomi A; Bhattacharya, Debaditya; Smith, Kathleen; Gann, Katie; Thompson, Michael O; Xing, Huili G; Jena, Debdeep; et al (October 2025, APL Materials)

   We report a two-step film-growth process using suboxide molecular-beam epitaxy (S-MBE) that produces Si-doped α-Ga2O3 with record transport properties. The method involves growing a relaxed α-(AlxGa1−x)2O3 buffer layer on m-plane sapphire at a relatively high substrate temperature (Tsub), ∼750 °C, followed by an Si-doped α-Ga2O3 overlayer grown at lower Tsub, ∼500 °C. The high Tsub allows the ∼3.6% lattice-mismatched α-(AlxGa1−x)2O3 buffer with x = 0.08 ± 0.02 to remain epitaxial and phase pure during relaxation to form a pseudosubstrate for the overgrowth of α-Ga2O3. The optimal conditions for the subsequent growth of Si-doped α-Ga2O3 by S-MBE are 425 °C ≤ Tsub ≤ 500 °C and P80% O3 = 5 × 10−6 Torr. Si-doped α-Ga2O3 films grown with this method at Tsub > 550 °C are always insulating. Secondary-ion mass spectrometry confirms that both the insulating and conductive films have uniform silicon incorporation. In conductive films with 1019 ≤ NSi ≤ 1020 cm−3, the incorporated silicon is ∼100% electrically active. At NSi ≤ 1019 cm−3, the carrier concentration (n) plummets. A maximum Hall mobility (μ) = 90 cm2V·s at room-temperature is measured in a film with n = 2.9 × 1019 cm−3 and a maximum conductivity (σ) = 650 S/cm at room-temperature in a film with n = 4.8 × 1019 cm−3. A threading dislocation density of (5.6 ± 0.6) × 1010 cm−2 is revealed by scanning transmission electron microscopy, showing that there is still enormous room to improve the electrical properties of doped α-Ga2O3 thin films. 

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5. Quasi-vertical β-Ga2O3 Schottky diodes on sapphire using all-LPCVD growth and plasma-free Ga-assisted etching

   https://doi.org/10.1063/5.0280191  

   Khan, Saleh Ahmed; Ibreljic, Ahmed; Bhuiyan, A_F_M_Anhar Uddin (September 2025, APL Electronic Devices)

   This work demonstrates quasi-vertical β-Ga2O3 Schottky barrier diodes (SBDs) fabricated on c-plane sapphire substrates using an all-low-pressure chemical vapor deposition (LPCVD)-based, plasma-free process flow that integrates both epitaxial growth of a high-quality β-Ga2O3 heteroepitaxial film with in situ Ga-assisted β-Ga2O3 etching. A 6.3 μm thick (2̄01) oriented β-Ga2O3 epitaxial layer structure was grown on c-plane sapphire with 6° miscut, comprising a moderately Si-doped (2.1 × 1017 cm−3) 3.15 μm thick drift layer and a heavily doped (1 × 1019 cm−3) contact layer on an unintentionally doped buffer layer. Mesa isolation was achieved via Ga-assisted plasma-free LPCVD etching, producing ∼60° inclined mesa sidewalls with an etch depth of 3.6 μm. The fabricated SBDs exhibited excellent forward current–voltage characteristics, including a turn-on voltage of 1.22 V, an ideality factor of 1.29, and a Schottky barrier height of 0.83 eV. The minimum differential specific on-resistance was measured to be 8.6 mΩ cm2, and the devices demonstrated high current density capability (252 A/cm2 at 5 V). Capacitance–voltage analysis revealed a net carrier concentration of 2.1 × 1017 cm−3, uniformly distributed across the β-Ga2O3 drift layer. Temperature-dependent J–V–T measurements, conducted from 25 to 250 °C, revealed thermionic emission-dominated transport with strong thermal stability. The Schottky barrier height increased from 0.80 to 1.16 eV, and the ideality factor rose modestly from 1.31 to 1.42 over this temperature range. Reverse leakage current remained low, increasing from ∼5 × 10−6 A/cm2 at 25 °C to ∼1 × 10−4 A/cm2 at 250 °C, with the Ion/Ioff ratio decreasing from ∼1 × 107 to 5 × 105. The devices achieved breakdown voltages ranging from 73 to 100 V, corresponding to parallel-plate electric field strengths of 1.66–1.94 MV/cm. These results highlight the potential of LPCVD-grown and etched β-Ga2O3 devices for high-performance, thermally resilient power electronics applications. 

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