Structural and optical properties of 100 mm AlN bulk single crystals

AlN is a critical ultrawide bandgap (UWBG) semiconductor used in high-power electronics and deep-ultraviolet optoelectronic devices. While 2-in. AlN wafers are commercially available, scaling up to larger diameters is crucial for the widespread adoption of UWBG technology. In this study, we investigated the structural and optical properties of a 100 mm diameter bulk AlN crystal. The transparent regions of the wafer exhibited an x-ray diffraction rocking curve linewidth as low as 27 arcsec and a strong band edge transition at 5.97 eV. However, the wafer showed significant spatial variations in optical transparency. Our detailed characterization results revealed a dominant absorption peak near 2.7 eV, attributed to the presence of aluminum vacancies (VAl); a series of emission peaks involving VAl, VAl− ON complex, and CN impurities; and a correlation between less transparent regions and higher concentrations of these specific defects. Understanding the effects of specific impurities/defects on the structural and optical properties is crucial for improving bulk crystal growth and processing methods to produce more uniform, large-diameter AlN substrates, which is vital for the scalable integration of AlN-based devices.