Two inch diameter, highly conducting (Si-doped) bulk β-Ga₂O₃ single crystals with the cylinder length up to one inch were grown by the Czochralski method. The obtained crystals revealed high structural quality characterized by narrow x-ray rocking curves (FWHM ≤ 25 arc sec) and high surface smoothness (RMS < 200 pm) of the epi-ready wafers. The free electron concentration and Hall mobility at room temperature were in the range of 1.6–9 × 10¹⁸ cm⁻³ and 118 – 52 cm² V⁻¹ s⁻¹, respectively, which are not affected by a heat treatment at temperatures up to 1000 °C in an oxidizing atmosphere. Temperature-dependent electrical properties of the crystals revealed a degenerated semiconducting state. Both high structural quality and electrical properties make the crystals well suited as substrates for homoepitaxy and electronic device fabrication in the vertical configuration.

A high growth rate process above 1 µm/h was achieved for Si-doped (100) β-Ga₂O₃ homoepitaxial films grown via metalorganic vapor phase epitaxy (MOVPE) while maintaining high crystalline perfection up to a film thickness of 3 µm. The main growth parameters were investigated to increase the growth rate and maintain the step-flow growth mode, wherein the enhanced diffusion channel due to the formation of a Ga adlayer was proposed to be the possible growth mechanism. Si doping allowed precise control of the n-type conductivity of the films with electron concentrations ranging from 1.5 × 10¹⁷ to 1.5 × 10¹⁹ cm⁻³ and corresponding mobilities from 144 to 21 cm² V⁻¹ s⁻¹, as revealed by Hall effect measurements at room temperature. Secondary ion mass spectrometry confirmed homogeneous Si doping through the film and a one-to-one correlation between the Si concentration and the electron concentration. Low defect density in the films was determined by x-ray diffraction measurements. The demonstration of a high growth rate process of β-Ga₂O₃ films with μm level thickness and smooth surface morphology via MOVPE is critical for high power electronics with vertical device architecture.

In this work, we report on the realization of SnO/β-Ga₂O₃ heterojunction vertical diodes and lateral field-effect transistors for power electronic applications. The p-type semiconductor SnO is grown by plasma-assisted molecular beam epitaxy on n-type (100) β-Ga₂O₃ with donor concentrations of 3 × 10¹⁷ cm⁻³ for the diode devices and 8.1 × 10¹⁷ cm⁻³ for the field-effect transistors. The deposited films show a predominant SnO (001) phase featuring a hole concentration and a mobility of 7.2 × 10¹⁸ cm⁻³ and 1.5 cm²/V s, respectively. The subsequent electrical characterization of the heterojunction diodes and field-effect transistors show stable switching properties with on/off current ratios >10⁶ and specific on-resistances below 4 mΩ cm². Furthermore, breakdown measurements in air of the non-field-plated heterojunction transistor with a gate-to-drain distance of 4 μm yield a breakdown voltage of 750 V, which equals an average breakdown strength of nearly 1.9 MV/cm. The resulting power figure of merit is calculated to 178 MW/cm² demonstrating state-of-the-art properties. This emphasizes the high potential of this heterojunction approach.

We have systematically studied the growth, by the Czochralski method, and basic physical properties of a 2 cm and 2 in. diameter bulk β-(Al_xGa_1−x)₂O₃ single crystal with [Al] = 0–35 mol. % in the melt in 5 mol. % steps. The segregation coefficient of Al in the Ga₂O₃ melt of 1.1–1.2 results in a higher Al content in the crystals than in the melt. The crystals were also co-doped with Si or Mg. [Al] = 30 mol. % in the melt (33–36 mol. % in the crystals) seems to be a limit for obtaining bulk single crystals of high structural quality suitable for homoepitaxy. The crystals were either semiconducting (no intentional co-dopants with [Al] = 0–30 mol. % and Si-doped with [Al] = 15–20 mol. %), degenerately semiconducting (Si-doped with [Al] ≤ 15 mol. %), or semi-insulating ([Al] ≥ 25 mol. % and/or Mg-doped). The full width at half maximum of the rocking curve was 30–50 arcsec. The crystals showed a linear but anisotropic decrease in all lattice constants and a linear increase in the optical bandgap (5.6 eV for [Al] = 30 mol. %). The room temperature electron mobility at similar free electron concentrations gradually decreases with [Al], presumably due to enhanced scattering at phonons as the result of a larger lattice distortion. In Si co-doped crystals, the scattering is enhanced by ionized impurities. Measured electron mobilities and bandgaps enabled to estimate the Baliga figure of merit for electronic devices.

In this work, we report on the realization of vertical (100) β-Ga₂O₃ FinFET devices for the use in power electronics applications. The experiments are carried out on structures consisting of highly conducting (100) β-Ga₂O₃ substrates with a doping concentration N _D of 3 × 10¹⁸ cm⁻³, and epitaxially grown layers with N _D of 5 × 10¹⁶ cm⁻³ for the drift and channel region. The fabricated FinFET devices feature enhancement-mode properties with a threshold voltage of +4.2 V and on/off-current ratio of 10⁵. Moreover, breakdown measurements of these devices reveal an average breakdown strength of 2.7 MV cm⁻¹. Additional device simulation indicates the presence of electric field peaks near the gate edge outside the active device as high as 7 and 5 MV cm⁻¹ in the Al₂O₃ gate oxide and β-Ga₂O₃ semiconductor, respectively.