High quality UV AlGaN/AlGaN distributed Bragg reflectors and microcavities

We demonstrate high-reflectivity crack-free Al 0.18 Ga 0.82 N/Al 0.8 Ga 0.2 N distributed Bragg reflectors (DBR) and monolithic microcavities grown by molecular beam epitaxy on thick c-axis GaN templates. The elastic strain energy in the epilayer is minimized by compensating the compressive and tensile stress in every period of the DBR structure. A 25 period DBR mirror provides a 26nm-wide stop band centered at 347 nm with the maximum reflectivity higher than 99%. The high-reflectivity DBRs can be used to form high Q -factor monolithic AlGaN/AlGaN microcavities. INTRODUCTION High-reflectivity distributed Bragg reflect ors (DBR) in the ultraviolet region ar e essential for the development of GaN-based optical devices. In particular, the region around 350 nm is important for devices containing pure GaN as an active medium. The growth of DBRs remains challenging in the nitrid e system. The difficulty lies in maintaining the structural integrity of a relatively thick structure that contains materials with a large lattice constant mismatch and different thermal expansion coefficients. Only a few groups have reported high-reflectivity Al