High-reflectivity ultraviolet AlN/AlGaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition

High-reflectivity ultraviolet distributed Bragg reflectors (DBRs), based on AlN/AlGaN quarter-wave layers, have been designed and grown on 2 in (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition. The growth of 20-pair AlN/Al0.23Ga0.77N DBR shows no observable cracks in the structure and achieves peak reflectivity of 90% at 367 nm together with a stop-band width of 24 nm. Furthermore, the growth of 34-pair AlN/Al0.23Ga0.77N DBR shows partial cracks from optical microscopy images. According to the room-temperature photoluminescence measurement, the emission spectrum of 34-pair AlN/Al0.23Ga0.77N DBR is broader than that of 20-pair AlN/Al0.23Ga0.77N DBR, which could be due to strain inhomogeneity generated by cracks. Despite the crystal quality problem, the peak reflectivity of 34-pair AlN/Al0.23Ga0.77N DBR could still achieve 97% at 358 nm and the stop-band width is 16 nm.