Ultraviolet GaN photodetectors on Si via oxide buffer heterostructures with integrated short period oxide-based distributed Bragg reflectors and leakage suppressing metal-oxide-semiconductor contactsWe report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/a-Al 2 O 3 (0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al 0.4 Ga 0.6 N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.