Improved Burn In With In Situ Etch
buried-heterostructure lasers with claimed 50 per cent lower rates of burn-in
degradation is to be introduced by Bookham Technology at the Optical Fiber
Communication Conference and Exposition (OFC) this month (March 2003). The
burn-in improvement promises increasing long-term reliability of devices,
which also exhibit 20 percent lower threshold currents. The new process is also
compatible with newer aluminium gallium indium arsenide (AlGaInAs)
materials.
Buried heterostructures are used to mitigate the effects of thermally
induced chirp and power rollover that can adversely impact laser
performance. Such power efficient uncooled directly modulated lasers are an
important class of communications laser.
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 lasers long-term reliability.
This effect is particularly severe for AlGaInAs. This materials system has
better performance characteristics and the 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 etch, making surfaces highly vulnerable to
damage and contamination.
Bookhams in-situ process eliminates the problem of post-etch surface damage
or contamination by performing the etch within the metallorganic chemical
vapour deposition (MOCVD) reactor. Overgrowth is performed immediately on
the clean freshly etched surface, thereby preventing the surface
contamination and oxidation that can occur in the standard process of using
an external etcher.
The paper details how InP 2.5Gbit/s 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 6nm quantum wells in a separate confinement
heterostructure, with a 1541nm target wavelength gain-coupled grating etched
above the quantum-well stack.