ABSTRACT
Most of the facts known up to date about the electro optic and the microstructural degradation during the lasing process concern GaAs based lasers. (A review can be found in Fukuda, 1991, chapter 5). Many key papers have been written about the microstructural degradation of GaAs based lasers (e.g. Petroff
and Hartman 1973, Henry et a!. 1979). There are, however, very significant differences in the reliability performance of GaAs based and InP based lasers. This can be best demonstrated by comparing the evolution of the lifetime of bulk active emitting 0.85 mm GaAs lasers and InP lasers emitting at 1.3 11m. (see Figure 2). The short lifetimes of0.85 mm lasers were limited by process stresses, non-radiative recombination enhanced defect reactions (REDR) causing dark spot and dark line defects (DSDs and DLDs), catastrophic facet degradation and contact interdiffusion. The lifetimes were progressively increased by using Indium bonds, stripe geometry (e.g., see Figure 3), facet coating and diffusion barriers in the contact metals. By contrast early 1.3 11m lasers using unoptimized substrates and no facet coating gave predicted lifetimes well in excess of that of the best GaAs lasers and the lifetime was subsequently increased by using stripe geometry and improved metallization systems. The reasons for this are outlined later. The remaining causes for degradation in the improved GalnAsP/InP lasers, presented in this paper, are to a great deal due to growth and process induced defects, the nucleation points for which are indicated in the sketch of a buried heterostructure (BH) laser with n-channel current blocking layers in Figure 3. Also shown in this Figure are the positions and the stages in the fabrication process of the BH laser, which give rise to interfacial defects. BH lasers have been grown by different growth methods (LPE, MOCVD) and
(e. g. Si02 ) ~--p-type GainAs
n nr carrier density refractive index
Eop optical field
Transmission Electron Microscopy of Aged Ga/nAsP/InP Buried Heterostructure Lasers 197
various designs have been developed. These are not presented here. However, sketches of the individual design are included on the micrographs of degraded lasers in later paragraphs.
