Based on the test results, Hydrogen diffusion is probably responsible for the quick degradation process. During the first hundred hours of the test, hydrogen slowly diffuses towards the p-metal/semiconductor interface, passivating the Mg acceptors in the GaN contact layer, and thus leading to an increase in the device contact resistance. A high Mg-doping level (above 2×1019 cm-3) is likely to lead to the formation of compensating defects (e.g., inversion domains and/or pyramidal defects). The poor p-type quality is responsible for high series resistances and non-uniform current injection into the active region, resulting in lower optical output powers under CW operation. Additionally, complete hydrogen removal could not be achieved with the thermal activation process in such highly-doped layers.

In contrast to these findings, the devices with optimized p-type layers showed no considerable degradation of the electrical characteristics after 2000 hours. The output power drop after this time was limited to 6%. The samples with optimized Mg-doping levels achieved a higher hydrogen removal with the post-growth thermal annealing than highly-doped samples. Under the specific test conditions, the estimated lifetime for those devices was higher than 5000 hours. Furthermore, maximum output powers as high as 350 mW (for an injection current of 500 mA) have been achieved in CW operation at room temperature.