Point Defect-Induced UV-C Absorption in Aluminum Nitride Epitaxial Layers Grown on Sapphire Substrates by Metal-Organic Chemical Vapor Deposition

Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV-C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm(-1). The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary-ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV-C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV-C devices by MOCVD on sapphire substrates are outlined.