High-power lasers with buried implantation for current confinement
Improving efficiency and brightness of laser diodes are key targets for high-power laser systems in order to reduce the cost per usable photon. This holds for direct diode systems as well as for diode pumping of fiber or solid-state lasers.
Current spreading and loss of carriers to regions which do not reach the threshold carrier density is one process limiting device efficiency. Accumulation of carriers at the edges of the active region, which again is related to current spreading, results in reduced beam quality in lateral direction, which limits the coupling efficiency for example into optical fibers. Introduction of buried current apertures can thus increase both efficiency and beam quality.
Broad area lasers emitting near 915 nm have been fabricated using a 2-step epitaxial growth process with an intermediate implantation of ions outside of the active laser stripe. The second growth step removes the crystal damage and activates the implanted dopants so that highly resistive regions are formed. This approach allows for the introduction of buried lateral current confinement layers at moderate cost in terms of process complexity.
Two main variants have been tested: the first (V1) with the implantation in the p-cladding layer, the second (V2) with the implantation in the waveguide layers and in or very near the quantum well (QW). For both variants, two different ions were employed, Si+ and O+, using different implantation doses and energies. The best results in terms of threshold current reduction and increased efficiency were obtained with oxygen implantation in both cases. The first approach, V1, resulted in up to ≈12% reduction of threshold current and ≈15% increase of slope efficiency with respect to standard lasers. It therefore offers significant advantages over standard lasers without such a buried current aperture. With the second approach, V2, an improvement in beam quality has been obtained with a significant reduction of the lateral beam product parameter, from 3.8 mm´mrad at 5 A for standard lasers to 2.2 mm´mrad for implanted and regrown lasers. However, the improved beam quality comes at the cost of reduced efficiency. Further work is dedicated to combine the higher efficiency of V1 with the better beam quality of V2.
P. Della Casa, O. Brox, J. Decker, M. Winterfeldt, P. Crump, H. Wenzel and M. Weyers, "High-power, broad area, buried mesa lasers", Semicond. Sci. Technol. 32 065009 (2017)
D. Martin, P. Della Casa, T. Adam, C. Goerke, A. Thies, K. Häusler, O. Brox, H. Wenzel, P. Crump, M. Weyers and A. Knigge, "Current Spreading Suppression by O- and Si-Implantation in high Power Broad Area Diode Lasers", Photonics West LASE 2019, paper 10900-19 (2019).
P. Della Casa, D. Martin, A. Maaßdorf, T. Adam, A. Thies, M. Beier, K. Häusler, A. Knigge and M. Weyers, "High power broad-area lasers with buried implantation for current confinement", Semicond. Sci. Technol., vol. 34, no. 10, pp. 105005 (2019).