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We demonstrate the epitaxial growth of single-phase (100) BaTiO3 films on (100) β-Ga2O3 substrates at substrate temperatures ranging from 600 to 700 °C using molecular-beam epitaxy. Characterization of a 47 nm thick BaTiO3 film by atomic force microscopy reveals a step-and-terrace morphology with unit-cell-high BaTiO3 steps and an rms surface roughness of 0.26 nm. Scanning transmission electron microscopy (STEM) images show that in some regions the β-Ga2O3 substrate terminates with a (100)A plane as it transitions to BaTiO3 and in other regions with a (100)B plane. The (100) BaTiO3 films are fully relaxed and consist of a mixture of two types of a-axis domains: a1 and a2. The orientation relationship determined by X-ray diffraction and confirmed by STEM is (100) BaTiO3 || (100) β-Ga2O3 and [011] BaTiO3||[010] β-Ga2O3. Despite the average linear lattice mismatch of 3.8%, BaTiO3 films with rocking curve full width at half maximum widths as narrow as 28 arc sec are achieved. From capacitance–voltage measurements on a metal–oxide–semiconductor capacitor structure with a-axis BaTiO3 as the oxide layer and Si-doped β-Ga2O3 as the semiconducting layer, we extract a dielectric constant of K11 = 670 for the BaTiO3 epitaxially integrated with (100) β-Ga2O3. We anticipate that this high-K epitaxial dielectric will be useful for electric-field management in β-Ga2O3-based device structures. 

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. 

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. 

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of ∼1 μm/h with control of the silicon doping concentration from 5 × 1016 to 1019 cm−3 . In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting frst step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of ∼1 μm/h is readily achieved at a relatively low growth temperature (Tsub ≈ 525 ○C), resulting in flms with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 μm thick flms). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 μm thick flm with a mobile carrier concentration of 2.7 × 1017 cm−3 reveal a room-temperature mobility of 124 cm2 V−1 s −1 that increases to 627 cm2 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 feld-effect transistors made from these silicon-doped β-Ga2O3 flms grown by S-MBE at growth rates of ∼1 μm/h.