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- ItemUltra-wide bandgap, conductive, high mobility, and high quality melt-grown bulk ZnGa2O4 single crystals(Melville, NY : AIP Publ., 2019) Galazka, Zbigniew; Ganschow, Steffen; Schewski, Robert; Irmscher, Klaus; Klimm, Detlef; Kwasniewski, Albert; Pietsch, Mike; Fiedler, Andreas; Schulze-Jonack, Isabelle; Albrecht, Martin; Schröder, Thomas; Bickermann, MatthiasTruly bulk ZnGa2O4 single crystals were obtained directly from the melt. High melting point of 1900 ± 20 °C and highly incongruent evaporation of the Zn- and Ga-containing species impose restrictions on growth conditions. The obtained crystals are characterized by a stoichiometric or near-stoichiometric composition with a normal spinel structure at room temperature and by a narrow full width at half maximum of the rocking curve of the 400 peak of (100)-oriented samples of 23 arcsec. ZnGa2O4 is a single crystalline spinel phase with the Ga/Zn atomic ratio up to about 2.17. Melt-grown ZnGa2O4 single crystals are thermally stable up to 1100 and 700 °C when subjected to annealing for 10 h in oxidizing and reducing atmospheres, respectively. The obtained ZnGa2O4 single crystals were either electrical insulators or n-type semiconductors/degenerate semiconductors depending on growth conditions and starting material composition. The as-grown semiconducting crystals had the resistivity, free electron concentration, and maximum Hall mobility of 0.002–0.1 Ωcm, 3 × 1018–9 × 1019 cm−3, and 107 cm2 V−1 s−1, respectively. The semiconducting crystals could be switched into the electrically insulating state by annealing in the presence of oxygen at temperatures ≥700 °C for at least several hours. The optical absorption edge is steep and originates at 275 nm, followed by full transparency in the visible and near infrared spectral regions. The optical bandgap gathered from the absorption coefficient is direct with a value of about 4.6 eV, close to that of β-Ga2O3. Additionally, with a lattice constant of a = 8.3336 Å, ZnGa2O4 may serve as a good lattice-matched substrate for magnetic Fe-based spinel films.
- ItemMelt Growth and Physical Properties of Bulk LaInO3 Single Crystals(Weinheim : Wiley-VCH, 2021) Galazka, Zbigniew; Irmscher, Klaus; Ganschow, Steffen; Zupancic, Martina; Aggoune, Wahib; Draxl, Claudia; Albrecht, Martin; Klimm, Detlef; Kwasniewski, Albert; Schulz, Tobias; Pietsch, Mike; Dittmar, Andrea; Grueneberg, Raimund; Juda, Uta; Schewski, Robert; Bergmann, Sabine; Cho, Hyeongmin; Char, Kookrin; Schroeder, Thomas; Bickermann, MatthiasLarge bulk LaInO3 single crystals are grown from the melt contained within iridium crucibles by the vertical gradient freeze (VGF) method. The obtained crystals are undoped or intentionally doped with Ba or Ce, and enabled wafer fabrication of size 10 × 10 mm2. High melting point of LaInO3 (≈1880 °C) and thermal instability at high temperatures require specific conditions for bulk crystal growth. The crystals do not undergo any phase transition up to 1300 °C, above which a noticeable thermal decomposition takes place. The good structural quality of the crystals makes them suitable for epitaxy. The onset of strong optical absorption shows orientation-dependent behavior due to the orthorhombic symmetry of the LaInO3 crystals. Assuming direct transitions, optical bandgaps of 4.35 and 4.39 eV are obtained for polarizations along the [010] and the [100], [001] crystallographic directions, respectively. There is an additional weak absorption in the range between 2.8 and 4 eV due to oxygen vacancies. Density-functional-theory calculations support the interpretation of the optical absorption data. Cathodoluminescence spectra show a broad, structured emission band peaking at ≈2.2 eV. All bulk crystals are electrically insulating. The relative static dielectric constant is determined at a value of 24.6 along the [001] direction.
- ItemRefractory metal-based ohmic contacts on β-Ga2O3 using TiW(Melville, NY : AIP Publ., 2022) Tetzner, Kornelius; Schewski, Robert; Popp, Andreas; Anooz, Saud Bin; Chou, Ta-Shun; Ostermay, Ina; Kirmse, Holm; Würfl, JoachimThe 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.
- ItemRecent progress in the development of β-Ga2O3 scintillator crystals grown by the Czochralski method(Washington, DC : OSA, 2021) Drozdowski, Winicjusz; Makowski, Michał; Witkowski, Marcin E.; Wojtowicz, Andrzej J.; Irmscher, Klaus; Schewski, Robert; Galazka, ZbigniewA high-quality bulk single crystal of β-Ga2O3 has been grown by the Czochralski method and its basic scintillation characteristics (light yield, energy resolution, proportionality, and scintillation decay times) have been investigated. All the samples cut from the crystal show promising scintillation yields between 8400 and 8920 ph/MeV, which is a noticeable step forward compared to previous studies. The remaining parameters, i.e. the energy resolution slightly above 10% (at 662 keV) and the scintillation mean decay time just under 1 μs, are at the same level as we have formerly recognized for β-Ga2O3. The proportionality of yield seems not to deviate from standards determined by other commercial scintillators.
- ItemHeading for brighter and faster β-Ga2O3 scintillator crystals(Amsterdam : Elsevier, 2022) Drozdowski, Winicjusz; Makowski, Michał; Bachiri, Abdellah; Witkowski, Marcin E.; Wojtowicz, Andrzej J.; Swiderski, Lukasz; Irmscher, Klaus; Schewski, Robert; Galazka, ZbigniewCzochralski-grown β-Ga2O3 and β-Ga2O3:Si crystals with the free electron concentrations between 2.5·1016 and 4.3·1018 cm−3 have been characterized by means of pulse height and scintillation time profile measurements in order to assess their basic scintillation properties. At room temperature, with increasing free electron concentration in the studied range, the scintillation yields decrease from 8920 to 1930 ph/MeV, while the mean scintillation decay times pare down from 989 to 61 ns. However, when the brightest β-Ga2O3 sample is cooled down below 100 K, its scintillation yield exceeds 20000 ph/MeV.
- ItemLow temperature thermoluminescence of β-Ga2O3 scintillator(Amsterdam : Elsevier, 2022) Witkowski, Marcin E.; Drozdowski, Konrad J.; Makowski, Michał; Drozdowski, Winicjusz; Wojtowicz, Andrzej J.; Irmscher, Klaus; Schewski, Robert; Galazka, ZbigniewLow temperature thermoluminescence of β-Ga2O3, β-Ga2O3:Al and β-Ga2O3:Ce has been investigated. Glow curves have been analyzed quantitatively using a rate equations model in order to determine the traps parameters, such as activation energies, capture cross-sections and probabilities of recombination and retrapping.