Layer intermixing and related long-term instability in heavily carbon-doped AlGaAs/GaAs superlattices

Superlattices (SLs) of Al_o.3Ga_o.7As/GaAs grown by metalorganic chemical vapor deposition and heavily doped with carbon using CClp₄ were annealed for 24 h at 825° C under a variety of ambient and surface encapsulation conditions. Photoluminescence at T = 1.7 K has been employed to determine approximate Al-Ga interdiffusion coefficients (D _Ai-Ga) for different annealing conditions. For all encapsulants studied D _Al-Ga increases with increasing As₄ pressure in the annealing ampoule. This result disagrees with trends reported for Mg-doped crystals, and with predictions of the charged point-defect (Fermilevel) model. The Si₃N₄ cap provides the most effecitve surface sealing against ambientstimulated layer interdiffusion (D _Al-Ga ≈ 1.5-3.9 x 10^-19 cm²/sec). The most extensive layer intermixing has occurred for an uncapped SL annealed under As-rich ambient (D _Al-Ga ≈ 3.3 x 10^-18 cm²/sec). These values are up to ~40 times greater than those previously reported for nominally undoped AIGaAs/GaAs SLs, implying that the C _As doping slightly enhances layer intermixing, but significantly less than predicted by the Fermi-level effect. The discrepancies between the experimental data and the model are discussed. Pronounced changes in the optical properties of the annealed SLs with storage time at room temperature are also reported. These changes may indicate a degraded thermal stability of the annealed crystals due to high-temperature-induced lattice defects. A possibly related effect of the systematic failure to fabricate buried heterostructure quantum well lasers via impurity-induced layer disordering in similarly doped AIGaAs/GaAs crystals is discussed.