2023, Galazka, Zbigniew, Fiedler, Andreas, Popp, Andreas, Ganschow, Steffen, Kwasniewski, Albert, Seyidov, Palvan, Pietsch, Mike, Dittmar, Andrea, Anooz, Saud Bin, Irmscher, Klaus, Suendermann, Manuela, Klimm, Detlef, Chou, Ta-Shun, Rehm, Jana, Schroeder, Thomas, Bickermann, Matthias

We have systematically studied the growth, by the Czochralski method, and basic physical properties of a 2 cm and 2 in. diameter bulk β-(AlxGa1-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.

2022, Tetzner, Kornelius, Schewski, Robert, Popp, Andreas, Anooz, Saud Bin, Chou, Ta-Shun, Ostermay, Ina, Kirmse, Holm, Würfl, Joachim

The present work investigates the use of the refractory metal alloy TiW as a possible candidate for the realization of ohmic contacts to the ultrawide bandgap semiconductor β-Ga2O3. Ohmic contact properties were analyzed by transfer length measurements of TiW contacts annealed at temperatures between 400 and 900 °C. Optimum contact properties with a contact resistance down to 1.5 × 10-5 ω cm2 were achieved after annealing at 700 °C in nitrogen on highly doped β-Ga2O3. However, a significant contact resistance increase was observed at annealing temperatures above 700 °C. Cross-sectional analyses of the contacts using scanning transmission electron microscopy revealed the formation of a TiOx interfacial layer of 3-5 nm between TiW and β-Ga2O3. This interlayer features an amorphous structure and most probably possesses a high amount of vacancies and/or Ga impurities supporting charge carrier injection. Upon annealing at temperatures of 900 °C, the interlayer increases in thickness up to 15 nm, featuring crystalline-like properties, suggesting the formation of rutile TiO2. Although severe morphological changes at higher annealing temperatures were also verified by atomic force microscopy, the root cause for the contact resistance increase is attributed to the structural changes in thickness and crystallinity of the interfacial layer.