Physical vapour transport growth of aluminium nitride (AlN) single crystals on silicon carbide (SiC) substrates has been optimised and crack-free, large-area, free-standing (0001) AlN wafers were prepared from the grown template crystals. 28 mm diameter single crystals without any polycrystalline surroundings were obtained. Different off-oriented substrates give rise to different growth modes. Sharp and symmetric line shapes of X-ray diffraction (XRD) rocking curves and high intense Raman phonon mode peaks prove high structural quality and homogeneity of the grown crystals. Full width at half maximum of the rocking curves is 72 arcsec for symmetric 00.2 reflection and 200 arcsec for asymmetric 10.3 reflection, representing a low screw as well as edge type threading dislocations. Wet chemical etching results also confirm the above XRD results and the estimated etch pit density is as low as 2–5 × 10⁵ cm⁻². The growth surfaces of all the crystals show only Al-polarity as inferred by the etching analysis. The concentration of silicon and carbon impurities incorporated from the SiC substrate decreases with the growth length of the AlN crystals. These impurities might play a decisive role in determining the optical properties of the crystal and be responsible for the absence of near-bandgap excitonic luminescence. Confocal Raman spectra show only the phonon modes allowed by the selection rules for the measured symmetry. The observed E₂(high) phonon mode frequency very closely matches the reported stress-free phonon frequency of AlN. This work demonstrates that AlN templates prepared on SiC as a foreign substrate can be used as native seeds for the growth of further homo-epitaxial layers and crystalline boules.

Aluminium nitride (AlN) crystalline substrate has emerged as a striking material and received tremendous attention for applications in high power electronics (HPE), deep-ultraviolet (DUV) light sources due to its exceptional properties. Single crystal growth of AlN by physical vapour transport (PVT) technique, and the necessity of large diameter AlN native substrates for the fabrication of HPE and DUV devices are described here. Two competing growth approaches in PVT are utilised to produce initial AlN single crystalline seeds namely, starting with self-nucleation followed by iterative homo-epitaxial growth for enlarging the crystal diameter in steps, and directly seeding on a closely lattice-matched foreign substrate of desired diameter by hetero-epitaxial growth. Both of these approaches are intended to grow bulk single crystals from which wafers might be prepared for further fabrication of devices. The hetero-epitaxial growth approach is specifically and comprehensively reviewed in this present work. A specific attention is given in using 6H- and 4H- polytype silicon carbide (SiC) substrates. The issues in hetero-epitaxially grown crystals such as presence of misfit dislocations, control of low-angle grain boundaries, incorporation of unintentional impurities, are highlighted together with the recent progress made in the achievement of about 2.5-inch dia. free-standing AlN wafer by this approach.

The LEGEND Collaboration is searching for neutrinoless double-beta ( 0 ν β β ) decay by operating high-purity germanium detectors enriched in Ge 76 in a low-background liquid argon environment. Building on key technological innovations from the GERmanium Detector Array (GERDA) experiment and the MAJORANA DEMONSTRATOR experiment, LEGEND-200 has performed a first 0 ν β β decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of 0.5 − 0.2 + 0.3 cts / ( keV ton yr ) in the 0 ν β β decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of 2.8 × 10 26 yr on the half-life of 0 ν β β decay, reveals no evidence for a signal and sets a new observed lower limit at T 1 / 2 0 ν > 1.9 × 10 26 yr (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range m β β < 75 – 200 meV , depending on the adopted nuclear matrix element.