Insight into high-reflectivity AlN/GaN Bragg reflectors with spontaneously formed (Al,Ga)N transient layers at the interfaces

This work gives a detailed insight into how the formation of (Al,Ga)N transient layers (TLs) at the interfaces of AlN/GaN Bragg reflectors modifies their structural and optical properties. While abrupt AlN/GaN interfaces are typically characterized with a network of microcracks, those with TLs are characterized with a network of nanocracks. Transmission electron microscopy reveals a strong correlation between strain and the TLs thickness, identifying thus the strain as the driving force for TLs formation. The AlN/GaN intermixing preserves the targeted stopband position (∼410 nm), whereas the peak reflectivity and the stopband width are both reduced, but still significantly high: >90% and >30 nm, respectively. To model their optical properties, a reduced refractive index contrast approximation is used, a novel method which yields an excellent agreement with the experiment.