A Strained Superlattice Buffer Layer for InGaAs/GaAs Quantum Wells

Good electroabsorption has been demonstrated recently in InGaAs/GaAs quantum wells. 1,2 A structure described previously 1 had only ten quantum wells, and although InGaAs and GaAs are not lattice-matched, the total strain energy in this structure was small enough that the quantum wells grew pseudomorphically, i. e., the InGaAs was strained so that its in-plane lattice constant was equal to the lattice constant of the GaAs substrate. This structure showed good electroabsorption characteristics, but only 6% modulation depth. For a device with 50% modulation depth, about 100 quantum wells are required. However, structures containing only 20 quantum wells appeared to be non-pseudomorphic. Thus 100 quantum wells can not be grown pseudomorphic to the substrate, yet they must be grown with no dislocations to have good electroabsorption characteristics. This can be accomplished by growing the quantum well structure on top of a thick, uniform buffer layer with a lattice constant equal to the weighted average of the lattice constants of the quantum well and buffer layers. Lattice relaxation by means of dislocations occurs only in the buffer layer, and both quantum wells and barriers grow pseudomorphic to the buffer layer rather than the substrate. 3 A similar result can be achieved by eliminating the buffer layer; the lattice relaxation occurs in the first few quantum wells, and the rest are dislocation-free. 2,4,5 We have employed this method to produce a structure with 80 quantum wells which exhibits good electroabsorption characteristics. We have also grown 50 quantum wells on a strained superlattice buffer layer consisting of alternating layers of InGaAs and GaAs, each 20Å thick. The purpose of this is to remove the dislocations from the quantum wells, which are optically active, and place them in the inactive superlattice.