Cavity optimization for AlGaN heterostructure deep-ultraviolet lasers

The challenges for AlGaN based UV emitters revolve around the wide-bandgap semiconductor materials limitations such as low carrier mobilities, high dopant activation energies and asymmetries between electron and hole transport [1]. For the purpose of a laser diode, these properties translate into the challenge of achieving population inversion, or optical gain, by electrical injection. Since lasing occurs when the optical gain of the laser diode equals the optical losses, the minimization of the optical losses is of paramount importance. The optical losses consist of intrinsic material losses, as well as cavity losses. Here, we quantify the effects of mirror imperfections including slant and roughness on the cavity loss and show that it is a superlinear function of the slant angle and RMS roughness, and scales as the inverse wavelength squared of the principal lasing mode [2]. This highlights the importance of device processing optimization as Fabry-Pérot cavities couple to shorter wavelengths. With the development of a complementary dry and wet etch recipe for both etched and cleaved facets [3], we demonstrate an optically pumped AlGaN double heterostructure laser grown by molecular beam epitaxy (MBE) on bulk AlN, exhibiting peak gain at 284 nm [2]. However, the reflectivity for a smooth and vertical facet is still limited by a single semiconductor-air interface. To reduce the cavity loss further, we develop sidewall deposited oxide Distributed Bragg Reflectors (DBRs) which are compatible with deep-UV coupled Fabry-Pérot cavities.