High-reflectivity ultraviolet AlGaN∕AlGaN distributed Bragg reflectors

We demonstrate high-reflectivity crack-free Al0.18Ga0.82N∕Al0.8Ga0.2N distributed Bragg reflectors (DBR) for the spectral region around 350nm grown by molecular-beam epitaxy on thick GaN templates. The structural quality of the DBR layers is maintained by compensating the compressive and tensile stress in each λ∕4 pair. This approach results in the lowest elastic strain energy and allows the growth of thick coherently strained DBRs. A 25 period mirror provides a 26nm wide stop band centered at 347nm with the maximum reflectivity higher than 99%.