Low voltage drop AlGaN UV-A laser structures with transparent tunnel junctions and optimized quantum wells

This paper presents the design, material growth and fabrication of AlGaN laser structures grown by plasma-assisted molecular beam epitaxy. Considering hole transport to be the major challenge, our ultraviolet-A diode laser structures have a compositionally graded transparent tunnel junction, resulting in superior hole injection and a low contact resistance. By optimizing active region thickness, a five-fold improvement in photoluminescence intensity is obtained compared to that of our own non-optimized test structures. The electrical and optical characteristics of processed devices demonstrate only spontaneous emission with a peak wavelength at 354 nm. The devices operate up to a continuous-wave current density of 11.1 kA cm-2 at room temperature, which is the highest reported for laser structures grown on AlGaN templates. Additionally, they exhibit a record-low voltage drop of 8.5 V to achieve this current density.