Design of InGaAs linear graded buffer structures

Abstract

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear-graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation-free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x-ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain-free cap layer with high crystalline quality is discussed.