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- 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.
- ItemEffect of post-metallization anneal on (100) Ga2O3/Ti–Au ohmic contact performance and interfacial degradation(Melville, NY : AIP Publ., 2022) Lee, Ming-Hsun; Chou, Ta-Shun; Bin Anooz, Saud; Galazka, Zbigniew; Popp, Andreas; Peterson, Rebecca L.Here, we investigate the effect of post-metallization anneal temperature on Ti/Au ohmic contact performance for (100)-oriented Ga2O3. A low contact resistance of ∼2.49 × 10−5 Ω·cm2 is achieved at an optimal anneal temperature of ∼420 °C for (100) Ga2O3. This is lower than the widely-used temperature of 470 °C for (010)-oriented Ga2O3. However, drastic degradation of the (100)-oriented contact resistance to ∼1.36 × 10−3 Ω·cm2 is observed when the anneal temperature was increased to 520 °C. Microscopy at the degraded ohmic contact revealed that the reacted Ti–TiOx interfacial layer has greatly expanded to 25–30 nm thickness and GaAu2 inclusions have formed between (310)-Ga2O3 planes and the Ti–TiOx layer. This degraded interface, which corresponds to the deterioration of ohmic contact properties, likely results from excess in-diffusion of Au and out-diffusion of Ga, concurrent with the expansion of the Ti–TiOx layer. These results demonstrate the critical influence of Ga2O3 anisotropy on the optimal post-metallization anneal temperature. Moreover, the observed Ti/Au contact degradation occurs for relatively moderate anneal conditions (520 °C for 1 min in N2), pointing to the urgent necessity of developing alternative metallization schemes for gallium oxide, including the use of Au-free electrodes
- 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.
- ItemTransport Properties and Finite Size Effects in β-Ga2O3 Thin Films([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Ahrling, Robin; Boy, Johannes; Handwerg, Martin; Chiatti, Olivio; Mitdank, Rüdiger; Wagner, Günter; Galazka, Zbigniew; Fischer, Saskia F.Thin films of the wide band gap semiconductor β-Ga2O3 have a high potential for applications in transparent electronics and high power devices. However, the role of interfaces remains to be explored. Here, we report on fundamental limits of transport properties in thin films. The conductivities, Hall densities and mobilities in thin homoepitaxially MOVPE grown (100)-orientated β-Ga2O3 films were measured as a function of temperature and film thickness. At room temperature, the electron mobilities ((115 ± 10) cm2/Vs) in thicker films (>150 nm) are comparable to the best of bulk. However, the mobility is strongly reduced by more than two orders of magnitude with decreasing film thickness ((5.5 ± 0.5) cm2/Vs for a 28 nm thin film). We find that the commonly applied classical Fuchs-Sondheimer model does not explain sufficiently the contribution of electron scattering at the film surfaces. Instead, by applying an electron wave model by Bergmann, a contribution to the mobility suppression due to the large de Broglie wavelength in β-Ga2O3 is proposed as a limiting quantum mechanical size effect.