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- ItemCharge carrier density, mobility, and Seebeck coefficient of melt-grown bulk ZnGa2O4 single crystals(New York, NY : American Inst. of Physics, 2020) Boy, Johannes; Handwerg, Martin; Mitdank, Rüdiger; Galazka, Zbigniew; Fischer, Saskia F.The temperature dependence of the charge carrier density, mobility, and Seebeck coefficient of melt-grown, bulk ZnGa2O4 single crystals was measured between 10 K and 310 K. The electrical conductivity at room temperature is about σ = 286 S/cm due to a high electron concentration of n = 3.26 × 1019 cm−3 caused by unintentional doping. The mobility at room temperature is μ = 55 cm2/V s, whereas the scattering on ionized impurities limits the mobility to μ = 62 cm2/Vs for temperatures lower than 180 K. The Seebeck coefficient relative to aluminum at room temperature is SZnGa2O4−Al = (−125 ± 2) μV/K and shows a temperature dependence as expected for degenerate semiconductors. At low temperatures, around 60 K, we observed the maximum Seebeck coefficient due to the phonon drag effect. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
- 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.
- 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.
- ItemHigh-mobility 4 μm MOVPE-grown (100) β-Ga2O3 film by parasitic particles suppression(Bristol : IOP Publ., 2023) Chou, Ta-Shun; Seyidov, Palvan; Bin Anooz, Saud; Grüneberg, Raimund; Rehm, Jana; Tran, Thi Thuy Vi; Fiedler, Andreas; Tetzner, Kornelius; Galazka, Zbigniew; Albrecht, Martin; Popp, AndreasIn this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016cm−3
- ItemEnhancement-mode Ga2O3 wrap-gate fin field-effect transistors on native (100) β-Ga2O3 substrate with high breakdown voltage(Melville, NY : American Inst. of Physics, 2016) Chabak, Kelson D.; Moser, Neil; Green, Andrew J.; Walker, Dennis E.; Tetlak, Stephen E.; Heller, Eric; Crespo, Antonio; Fitch, Robert; McCandless, Jonathan P.; Leedy, Kevin; Baldini, Michele; Wagner, Gunter; Galazka, Zbigniew; Li, Xiuling; Jessen, GreggSn-doped gallium oxide (Ga2O3) wrap-gate fin-array field-effect transistors (finFETs) were formed by top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate. The fin channels have a triangular cross-section and are approximately 300 nm wide and 200 nm tall. FinFETs, with 20 nm Al2O3 gate dielectric and ∼2 μm wrap-gate, demonstrate normally-off operation with a threshold voltage between 0 and +1 V during high-voltage operation. The ION/IOFF ratio is greater than 105 and is mainly limited by high on-resistance that can be significantly improved. At VG = 0, a finFET with 21 μm gate-drain spacing achieved a three-terminal breakdown voltage exceeding 600 V without a field-plate.
- ItemPerspectives on MOVPE-grown (100) β-Ga2O3thin films and its Al-alloy for power electronics application([Melville, NY] : American Institute of Physics, 2022) Rehm, Jana; Chou, Ta-Shun; Bin Anooz, Saud; Seyidov, Palvan; Fiedler, Andreas; Galazka, Zbigniew; Popp, AndreasBeta gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor with attractive physical properties for next-generation high-power devices, radio frequency electronics, and solar-blind ultraviolet radiation detectors. Here, we present an overview and perspective on the development of MOVPE-grown (100) β-Ga2O3 thin films and its role in supplementing high-power electronics. We review the development path of the growth process on (100) β-Ga2O3 thin films with a discussion regarding the solved and remaining challenges. The structural defect formation mechanism, substrate treatment strategies, and different growth windows are analyzed to optimize the grown film to fulfill the requirements for device fabrication. Toward industrial applications, MOVPE-grown β-Ga2O3 thin films are evaluated in two aspects: thick layers with smooth surface roughness and the electrical properties in terms of high carrier mobility and low doping concentration. Based on the reviewed results, we propose strategies in substrate preparation treatments and supportive tools such as the machine learning approaches for future growth process optimization and envision the rising interest of the β-Ga2O3-related alloy, β-(AlxGa1-x)2O3.
- ItemSuppression of particle formation by gas-phase pre-reactions in (100) MOVPE-grown β -Ga2O3films for vertical device application(Melville, NY : American Inst. of Physics, 2023) Chou, Ta-Shun; Seyidov, Palvan; Bin Anooz, Saud; Grüneberg, Raimund; Pietsch, Mike; Rehm, Jana; Tran, Thi Thuy Vi; Tetzner, Kornelius; Galazka, Zbigniew; Albrecht, Martin; Irmscher, Klaus; Fiedler, Andreas; Popp, AndreasThis work investigated the metalorganic vapor-phase epitaxy (MOVPE) of (100) β-Ga2O3 films with the aim of meeting the requirements to act as drift layers for high-power electronic devices. A height-adjustable showerhead achieving a close distance to the susceptor (1.5 cm) was demonstrated to be a critical factor in increasing the stability of the Ga wetting layer (or Ga adlayer) on the surface and reducing parasitic particles. A film thickness of up to 3 μm has been achieved while keeping the root mean square below 0.7 nm. Record carrier mobilities of 155 cm2 V-1 s-1 (2.2 μm) and 163 cm2 V-1 s-1 (3 μm) at room temperature were measured for (100) β-Ga2O3 films with carrier concentrations of 5.7 × 1016 and 7.1 × 1016 cm-3, respectively. Analysis of temperature-dependent Hall mobility and carrier concentration data revealed a low background compensating acceptor concentration of 4 × 1015 cm-3.
- ItemAnisotropic optical properties of highly doped rutile SnO2: Valence band contributions to the Burstein-Moss shift(New York : American Institute of Physics, 2018) Feneberg, Martin; Lidig, Christian; White, Mark E.; Tsai, Min Y.; Speck, James S.; Bierwagen, Oliver; Galazka, Zbigniew; Goldhahn, RüdigerThe interband absorption of the transparent conducting semiconductor rutile stannic oxide (SnO2) is investigated as a function of increasing free electron concentration. The anisotropic dielectric functions of SnO2:Sb are determined by spectroscopic ellipsometry. The onsets of strong interband absorption found at different positions shift to higher photon energies with increasing free carrier concentration. For the electric field vector parallel to the optic axis, a low energy shoulder increases in prominence with increasing free electron concentration. We analyze the influence of different many-body effects and can model the behavior by taking into account bandgap renormalization and the Burstein-Moss effect. The latter consists of contributions from the conduction and the valence bands which can be distinguished because the nonparabolic conduction band dispersion of SnO2 is known already with high accuracy. The possible originsof the shoulder are discussed. The most likely mechanism is identified to be interband transitions at jkj > 0 from a dipole forbidden valence band.
- 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.
- ItemOptical properties of In2O3 from experiment and first-principles theory: influence of lattice screening([Bad Honnef] : Dt. Physikalische Ges., 2018) Schleife, André; Neumann, Maciej D.; Esser, Norbert; Galazka, Zbigniew; Gottwald, Alexander; Nixdorf, Jakob; Goldhahn, Rüdiger; Feneberg, MartinThe framework of many-body perturbation theory led to deep insight into electronic structure and optical properties of diverse systems and, in particular, many semiconductors. It relies on an accurate approximation of the screened Coulomb electron–electron interaction W, that in current implementations is usually achieved by describing electronic interband transitions. However, our results for several oxide semiconductors indicate that for polar materials it is necessary to also account for lattice contributions to dielectric screening. To clarify this question in this work, we combine highly accurate experimentation and cutting-edge theoretical spectroscopy to elucidate the interplay of quasiparticle and excitonic effects for cubic bixbyite In2O3 across an unprecedentedly large photon energy range. We then show that the agreement between experiment and theory is excellent and, thus, validate that the physics of quasiparticle and excitonic effects is described accurately by these first-principles techniques, except for the immediate vicinity of the absorption onset. Finally, our combination of experimental and computational data clearly establishes the need for including a lattice contribution to dielectric screening in the screened electron–electron interaction, in order to improve the description of excitonic effects near the absorption edge.