Comparison of the aging behavior of diffused and epitaxial GaAlAs heterojunction electroluminescent diodes

Two Types of GaAlAs heterojunction electroluminescent diodes are compared. In the planar-type diffused diode the p-n homojunction is obtained by zinc diffusion in areas limited by a silicon nitride mask; the p-p heterojunction confines electrons in the bulk and avoids surface recombinations. The epitaxial diode is a conventional double heterojunction; the active area is limited by proton bombardment. The aging behaviors of these two types appear quite different. Diffused diodes degrade relatively fast; under continuous operation at 550 A/cm²(100 mA) the device lifetime (emitted light power falling down to 50 percent of the initial value) is 10 000 to 50 000 h. Epitaxial diodes degrade much more slowly; their device lifetime is estimated at several 10⁵h. The degradation is followed during aging, by using electrical and optical measurements, together with SEM and TEM analyses. It is confirmed that fast degradation occurs with simultaneous growth of dark line defects (DLD) and that these DLD´s result from dislocation dipoles of interstitial nature, emitted either from scratches or, more frequently, from previously existing dislocations. It is shown that epitaxial diodes are practically free of dislocations; on the contrary, in the case of planar-type p-n junctions, a large number of dislocation loops are developed at the junction periphery, resulting from the mechanical stress at the edge of the junction during processing. This is the main source of DLD´s and fast degradation of planar-type diffused diodes.