More stable RW process due to analysis of antiguiding effect in GaN-based laser diodes
Fig. 1: Simplified gain model and refractive index model, as used in simulation.
Fig. 2: Simulated threshold current density (circles, squares) as a function of the ridge waveguide etch depth. Measurements (stars) are in very good agreement with simulation.
Blue and violet emitting laser diodes based on GaN have been available on the market for a couple of years now. Nevertheless, several aspects of the device physics are still not sufficiently understood. For example, several research groups including the FBH have observed that the lasing threshold strongly depends on the depth of the ridge waveguide, an effect that cannot be explained in a conclusive manner. Recently, we have proposed an unusually strong index-antiguiding in the devices, i.e. a high carrier density in the quantum wells (QWs) below the ridge which locally reduces the refractive index. If the carrier-induced change of the effective index is of the same order of magnitude as the built-in effective index step Δneff generated by the ridge etch, the lateral mode confinement is reduced significantly so that the threshold current density increases. Since a shallower ridge implies a lower Δneff value, shallow-ridge devices are expected to be far more sensitive to index-antiguiding than deep-ridge ones.
Using the simplified gain model shown in Fig. 1, we have been able to simulate our laser diodes with a 2D-mode solver. A reasonably good agreement between simulation results and experimental data was thereby obtained. The mode behavior was investigated by varying the model parameters, in particular gain and absorption in the QWs and width of the current spreading. The influence of each parameter was studied in detail. Finally, a realistic set of parameters was found which allows to reproduce the measured laser diode threshold values. The large value for the antiguiding parameter R assumed in the simulation was confirmed by measurements performed in cooperation with the Fraunhofer Institute for Applied Solid State Physics (IAF). Using our model, the dependency of the threshold current density jth on the ridge etch depth was calculated, as shown in Fig. 2. The results reveal that, because of the large antiguiding, deep ridge etching well into the p-waveguiding layer is needed in order to obtain low thresholds in GaN-based laser diodes. This work will enable us to make our RW process more stable in terms of a reproducibly low lasing threshold.
Publications:
L. Redaelli, H. Wenzel, T. Weig, G. Lükens, S. Einfeldt, U.T. Schwarz, M. Kneissl, G. Tränkle, "Effect of index-antiguiding on the threshold of GaN-based narrow ridge-waveguide laser diodes", Conference on Lasers and Electro Optics (CLEO), Technical Digest p. CF1F.3 (2013).
L. Redaelli, H. Wenzel, M. Martens, J. Piprek, S. Einfeldt, M. Kneissl, G. Tränkle, "Index antiguiding in narrow ridge-waveguide (In,Al)GaN based laser diodes", zur Veröffentlichung angenommen im Journal of Applied Physics (2013).
FBH research: 20.08.2013