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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.
- ItemCzochralski growth of mixed cubic sesquioxide crystals in the ternary system Lu2O3-Sc2O3-Y2O3(Oxford [u.a.] : Wiley-Blackwell, 2021) Kränkel, Christian; Uvarova, Anastasia; Haurat, Émile; Hülshoff, Lena; Brützam, Mario; Guguschev, Christo; Kalusniaka, Sascha; Klimm, DetlefCubic rare-earth sesquioxide crystals are strongly demanded host materials for high power lasers, but due to their high melting points investigations on their thermodynamics and the growth of large-size crystals of high optical quality remain a challenge. Detailed thermal investigations of the ternary system Lu2O3-Sc2O3-Y2O3 revealing a large range of compositions with melting temperatures below 2200°C and a minimum of 2053°C for the composition (Sc0.45Y0.55)2O3 are presented. These reduced temperatures enable for the first time the growth of high optical quality mixed sesquioxide crystals with disordered structure by the conventional Czochralski method from iridium crucibles. An (Er0.07Sc0.50Y0.43)2O3 crystal is successfully grown and characterized with respect to its crystallographic properties as well as its composition, thermal conductivity and optical absorption in the 1μm range. © 2021 International Union of Crystallography. All rights reserved.
- ItemREScO3 Substrates—Purveyors of Strain Engineering(Weinheim : Wiley-VCH, 2019) Klimm, Detlef; Guguschev, Christo; Ganschow, Steffen; Bickermann, Matthias; Schlom, Darrell G.The thermodynamic and crystallographic background for the development of substrate crystals that are suitable for the epitaxial deposition of biaxially strained functional perovskite layers is reviewed. In such strained layers the elastic energy delivers an additional contribution to the Gibbs free energy, which allows the tuning of physical properties and phase transition temperatures to desired values. For some oxide systems metastable phases can even be accessed. Rare-earth scandates, REScO3, are well suited as substrate crystals because they combine mechanical and chemical stability in the epitaxy process with an adjustable range of pseudo-cubic lattice parameters in the 3.95 to 4.02 Å range. To further tune the lattice parameters, chemical substitution for the RE or Sc is possible. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemCrystal growth and thermodynamic investigation of Bi2M2+O4 (M = Pd, Cu)(London : RSC, 2021) Wolff, Nora; Klimm, Detlef; Habicht, Klaus; Fritsch, KatharinaPhase equilibria that are relevant for the growth of Bi2MO4 have been studied experimentally, and the ternary phase diagrams of Bi2O3–PdO2–Pd and Bi2O3–Cu2O–CuO and its isopleth section Bi2O3–CuO were redetermined. It is shown that every melting and crystallization process is always accompanied by a redox process at the phase boundary and that for both title compounds, the valence of the transition metal is lowered during melting. Vice versa, during crystal growth, O2 must be transported through the melt to the phase boundary. Based on these new insights provided by our thermodynamic studies, Bi2CuO4 single crystals with a length of up to 7 cm and a diameter of 6 mm were grown by the OFZ technique to be used for investigations of magnetic, electronic and thermal transport properties. The grown crystals were characterized by powder X-ray diffraction, Laue, magnetization and specific heat measurements.
- 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.
- ItemExperimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides(Cambridge [u.a.] : Cambridge Univ. Press, 2021) Galazka, Zbigniew; Irmscher, Klaus; Pietsch, Mike; Ganschow, Steffen; Schulz, Detlev; Klimm, Detlef; Hanke, Isabelle M.; Schroeder, Thomas; Bickermann, MatthiasWe provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators.
- ItemBulk single crystals and physical properties of β-(AlxGa1-x)2O3(x = 0-0.35) grown by the Czochralski method(Melville, NY : American Inst. of Physics, 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, MatthiasWe 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.
- ItemPhase diagram studies for the growth of (Mg,Zr):SrGa12O19 crystals(Dordrecht [u.a.] : Springer Science + Business Media B.V., 2021) Klimm, Detlef; Szczefanowicz, Bartosz; Wolff, Nora; Bickermann, MatthiasBy differential thermal analysis, a concentration field suitable for the growth of Zr, Mg co-doped strontium hexagallate crystals was observed that corresponds well with known experimental results. It was shown that the melting point of doped crystal is ca. 60 K higher than that of undoped crystals. This higher melting points indicate hexagallate phase stabilization by Zr, Mg co-doping and increase the growth window of (Mg,Zr):SrGa12O19, compared to undoped SrGa12O19 that grows from SrO–Ga2O3 melts.
- ItemSuitability of binary oxides for molecular-beam epitaxy source materials: A comprehensive thermodynamic analysis(Melville, NY : AIP Publ., 2020) Adkison, Kate M.; Shang, Shun-Li; Bocklund, Brandon J.; Klimm, Detlef; Schlom, Darrell G.; Liu, Zi-KuiWe have conducted a comprehensive thermodynamic analysis of the volatility of 128 binary oxides to evaluate their suitability as source materials for oxide molecular-beam epitaxy (MBE). 16 solid or liquid oxides are identified that evaporate nearly congruently from stable oxide sources to gas species: As2O3, B2O3, BaO, MoO3, OsO4, P2O5, PbO, PuO2, Rb2O, Re2O7, Sb2O3, SeO2, SnO, ThO2, Tl2O, and WO3. An additional 24 oxides could provide molecular beams with dominant gas species of CeO, Cs2O, DyO, ErO, Ga2O, GdO, GeO, HfO, HoO, In2O, LaO, LuO, NdO, PmO, PrO, PuO, ScO, SiO, SmO, TbO, Te2O2, U2O6, VO2, and YO2. The present findings are in close accord with available experimental results in the literature. For example, As2O3, B2O3, BaO, MoO3, PbO, Sb2O3, and WO3 are the only oxides in the ideal category that have been used in MBE. The remaining oxides deemed ideal for MBE awaiting experimental verification. We also consider two-phase mixtures as a route to achieve the desired congruent evaporation characteristic of an ideal MBE source. These include (Ga2O3 + Ga) to produce a molecular beam of Ga2O(g), (GeO2 + Ge) to produce GeO(g), (SiO2 + Si) to produce SiO(g), (SnO2 + Sn) to produce SnO(g), etc.; these suboxide sources enable suboxide MBE. Our analysis provides the vapor pressures of the gas species over the condensed phases of 128 binary oxides, which may be either solid or liquid depending on the melting temperature. © 2020 Author(s).
- ItemAlF3-assisted flux growth of mullite whiskers and their application in fabrication of porous mullite-alumina monoliths(Amsterdam : Elsevier, 2021) Abdullayev, Amanmyrat; Klimm, Detlef; Kamutzki, Franz; Gurlo, Aleksander; Bekheet, Maged F.Mullite is a promising material with its competitive thermochemical and mechanical properties. Although mullite could be obtained by several synthesis methods, the flux method emerges with its advantages over other methods. However, obtaining mullite whiskers with a high aspect ratio and length for ceramic reinforcements is still challenging. In this work, mullite whiskers were grown from AlF3-assisted flux. The addition of AlF3 to flux salt not only decreases the formation temperature of mullite to as low as 700 °C and suppresses the formation of corundum side phase, but also increases the length and aspect ratio of the whiskers. The obtained mullite whiskers were used as reinforcement for porous alumina monoliths prepared by the freeze casting route and subsequent sintering at 1500 °C. The fabricated mullite-alumina monoliths show competitive compressive strength of 25.7 MPa while having as high as 70.6% porosity, which makes them a potential candidate for membrane applications.