Bookham Technology plc has developed what it claims is a novel in-situ etch and regrowth process for uncooled Indium Phosphide (InP) buried-heterostructure lasers that results in 50% lower rates of burn-in degradation than can be obtained by current standard processes. The times-2 burn-in improvement is extremely promising for increasing the long-term reliability of the devices, which also exhibit 20% lower threshold currents, says the company. The new process is also compatible with the newer Aluminium Gallium Indium Arsenide (AlGaInAs) materials system and promises significant reliability gains for this developing technology.
The company will unveil these findings at the Optical Fiber Communication Conference and Exposition later this month in Atlanta, GA, in a paper entitled, "In-situ Etched Buried Heterostructures for Uncooled and Extended Reach Modulated Lasers," by K. Hinzer; G. Knight; D. G. Goodchild; T. Grevatt; G. Letal; R. J. Finlay; and J. K. White.
Power-efficient uncooled directly modulated lasers with extended reach are an important class of communications laser for which buried heterostructures are essential, as these structures mitigate the effects of thermally induced chirp and power rollover that adversely impact laser performance. However, the process needed to fabricate buried heterostructures is crucial to device reliability. The etching and regrowth required to make buried heterostructures can leave defects or introduce surface contamination on the mesa sidewalls. When the buried heterostructure is subsequently regrown on the material surface, any etch damage or impurities that remain at the surface can introduce a leakage current that increases the laser threshold current during device operation and degrades the laser's long-term reliability.
This effect is particularly severe for AlGaInAs, a new laser materials system of growing industry interest because of its performance characteristics and potential to operate at higher temperatures than standard InP. In principle, this would allow more compact devices to be fabricated as less passive cooling is needed. Unfortunately, AlGaInAs oxidises rapidly after etching, making the etched surfaces highly vulnerable to damage and contamination.
According to Bookham, its in-situ etching process eliminates the problem of post-etching surface damage or contamination by performing the etching step within the MOCVD reactor itself. Overgrowth is performed immediately on the clean freshly etched surface, thereby preventing the surface contaminations and oxidation that can occur in the standard process of using an external etcher to which the material has to be transferred at the risk of damage and contamination.
"The in-situ process will improve even further the outstanding reliability of our current InP laser design, since all material systems are subject to some extent to the surface defects/contaminations of the conventional process for buried heterostructures," explains Karin Hinzer, advisor, New Materials and joint author of the OFC paper describing the process. "But reliability is a very big issue for AlGaInAs right now because the reliability of existing uncooled InP devices from vendors like Bookham is just so good--and getting better. So the results of the new process suggest that AlGaInAs could have similar reliability to InP if AlGaInAs technology goes mainstream in the future," she adds.
The paper details how InP 2.5-Gbit/sec directly modulated lasers have been grown and fabricated by the process and also how two formulations of AlGaInAs buried heterostructures have been grown. The InP buried-heterostructure lasers consisted of an active layer with six compressively strained 6-nm quantum wells in a separate confinement heterostructure, with a 1541-nm target wavelength gain-coupled grating etched above the quantum-well stack. The final structures for both InP and AlGaInAs showed smooth planes and excellent surface morphology.