GRADED-COMPOSITION BUFFER LAYERS USING DIGITAL ALGAASSB ALLOYS

We describe step-graded digital-alloy buffers using alternate layers o f Al0.5Ga0.5As and Al0.5Ga0.5As0.65Sb0.35 grown on GaAs substrates by molecular beam epitaxy. The buffers consist of three sets of superlatt ices with AlGaAs/AlGaAsSb layer thicknesses of 7.7/2.3 nm, 5.4/4.6 nm, and 3.1/6.9 nm, respectively, terminating in a lattice constant equal to that of bulk In0.32Ga0.68As. Transmission electron micrographs sho w that most of the misfit-generated dislocations lie near the steps in pseudoalloy composition, and atomic force micrographs indicate a rms surface roughness of 3.6 nm. A 20.5-period lattice-matched InGaAs/InAl As reflector stack grown on such a buffer has a peak reflectivity of 9 8% near 1.3 mu m. These buffers provide potentially useful substrates for optoelectronic device applications near 1.3 mu m using strained In GaAs active regions. (C) 1995 American Institute of Physics.