2020, Sun, Xiaoxiao, Wang, Ping, Wang, Tao, Chen, Ling, Chen, Zhaoying, Gao, Kang, Aoki, Tomoyuki, Li, Mo, Zhang, Jian, Schulz, Tobias, Albrecht, Martin, Ge, Weikun, Arakawa, Yasuhiko, Shen, Bo, Holmes, Mark, Wang, Xinqiang

We identify and characterize a novel type of quantum emitter formed from InGaN monolayer islands grown using molecular beam epitaxy and further isolated via the fabrication of an array of nanopillar structures. Detailed optical analysis of the characteristic emission spectrum from the monolayer islands is performed, and the main transmission is shown to act as a bright, stable, and fast single-photon emitter with a wavelength of ~400 nm. © 2020, The Author(s).

2020, Hassa, Anna, Wouters, Charlotte, Kneiß, Max, Splith, Daniel, Sturm, Chris, von Wenckstern, Holger, Albrecht, Martin, Lorenz, Michael, Grundmann, Marius

In this paper, the growth of orthorhombic and monoclinic (Al x Ga1 - x )2O3 thin films on (00.1) Al2O3 by tin-assisted pulsed laser deposition is investigated as a function of oxygen pressure p(O2) and substrate temperature Tg. For certain growth conditions, defined by Tg = 580°C and p(O2) = 0.016 mbar, the orthorhombic ?-polymorph is stabilized. For Tg = 540°C and p(O2) = 0.016 mbar, the ?-, and the ß-, as well as the spinel ?-polymorph coexist, as illustrated by XRD 2?-?-scans. Further employed growth parameters result in thin films with a monoclinic ß-gallia structure. For all polymorphs, p(O2) and Tg affect the formation and desorption of volatile suboxides, and thereby the growth rate and the cation composition. For example, low oxygen pressures lead to low growth rates and enhanced Al incorporation. This facilitates the structural engineering of polymorphic, ternary (Al,Ga)2O3 via selection of the relevant process parameters. Transmission electron microscopy (TEM) studies of a ? - (Al0.13Ga0.87)2O3 thin film reveal a more complex picture compared to that derived from x-ray diffraction measurements. Furthermore, this study presents the possibility of controlling the phase formation, as well as the Al-content, of thin films based on the choice of their growth conditions. © 2020 The Author(s). Published by IOP Publishing Ltd.

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, Matthias

Large 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.

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, Andreas

This 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.

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, Andreas

In 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

2016, Mohn, Stefan, Stolyarchuk, Natalia, Markurt, Toni, Kirste, Ronny, Hoffmann, Marc P., Collazo, Ramón, Courville, Aimeric, Di Felice, Rosa, Sitar, Zlatko, Vennéguès, Philippe, Albrecht, Martin

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.