Improved stability of InGaN-based laser diodes by overcoming facet degradation
Fig. 1: Cross section TEM image of the facet of a stressed LD showing the formed SiOx-layer.
Fig. 2: Ageing curve showing the change in operation current due to the growth of an SiOx-layer.
At FBH, we develop GaN-based single-mode ridge waveguide laser diodes in the blue-violet spectral range. Our devices have already achieved lifetimes of more than 10,000 hours at a constant output power of 20 mW. This potentially enables a wide range of applications, such as high-resolution spectroscopy, laser displays, biophotonics, and external-cavity diode laser. However, real-world applications require not only failure-free operation but also stable performance. Especially in complex optical systems, consistent operating current and beam characteristics are essential. We have identified a facet degradation mechanism which affects both aspects. In order to understand and finally mitigate this effect, we have carried out extensive aging studies combined with material analyses.
When running the lasers at constant output power, we observed an oscillating diode current along with a constantly changing far-field pattern within the first few hundred hours. LIV measurements during this period showed that the laser threshold shifts back and forth. TEM analysis combined with EDX measurements showed the formation of a SiOx layer on the front facet coating, with a thickness distribution corresponding to the intensity profile of the optical mode at the facet (see Fig. 1). Literature suggests that under laser irradiation, water molecules together with Si from the atmosphere form this layer on the facet.
To test this hypothesis, we hermetically encapsulated lasers in TO cans filled with dry air or nitrogen. We found that an SiOx layer still forms under these conditions, however, its growth rate was considerably slower. Leakage tests confirmed that the TO cans remained sealed. TEM measurements on unaged laser diodes revealed that a thin layer of SiOx, only a few nanometers thick, was already homogeneously present on the entire facet before aging. This indicates that in a Si-containing atmosphere, an SiOx layer can form even without laser operation.
We observed that this effect depends to a large extent on the material of the top facet coating layer. To counteract it, we tested different materials as a top facet coating layer, namely TiO2, Ta2O5, Al2O3, and SiO2. However, when these laser diodes were operated in the presumably Si-containing laboratory atmosphere, a SiOx layer still formed on the facet. We are currently investigating a suitable coating material in combination with a hermetic encapsulation in the TO housing to effectively prevent this effect.
Furthermore, we have been able to prove the described degradation effect not only on our lasers but also on commercial devices. After removing the TO cap these lasers showed clear SiOx deposits on their facet after 1,500 hours of operation, whereas the same lasers with the cap intact showed no deposits at all even after 4,000 hours.
This work was partly supported by the Federal Ministry for Economic Affairs and Climate Action under contract 50WM2179 (project LARUS).
Publication
Freier, E. et al. "Analyses of degradation mechanisms in single mode InGaN based laser diodes." 2024 IEEE Photonics Conference (IPC). IEEE, 2024.