Strained-layer superlattices for reduction of dislocation density in GaAs1−xPx on GaAs by organometallic vapor-phase epitaxy

Abstract

This paper presents results of organometallic vapor-phase-epitaxial growth of low dislocation density GaAs1−xPx on GaAs, utilizing a thin compositionally graded layer (2 μm) and a strained-layer superlattice (SLS) to reduce the dislocation density. The grown structure consists of three sections: (1) a 2-μm GaAs1−xPx graded layer with final phosphorus composition x1 on a GaAs substrate; (2) a GaAs1−yPy /GaAs1−y′Py′ SLS; and (3) a 1-μm GaAs1−xPx top layer with x=0.4. Three types of grading layers were investigated: sublinear, hyperlinear, and linear. The linear grading is found to give the lowest dislocation density. However, the 2-μm-thick linearly graded region is definitely too thin to release all the misfit strain. The residual strain produces dislocations after the SLS has been grown, resulting in dislocation generation in the constant composition layer. A novel method has been developed to eliminate this problem. The phosphorus composition at the end of the grading, x1, is made intentionally larger than both the average composition of the SLS and the top layer to prevent residual strain relief during the growth of the top layer. Using this ‘‘overshoot grading’’ method, the etch pit density in the top layer is reduced from 3×107/cm2 without the special structure to 6.5×105/cm2. The reduction of dislocation density by the SLS has been systematically studied with variation of the SLS parameters, including the thickness of each layer, the number of periods, and the composition change in the SLS. The results show that the SLS itself can reduce the dislocation density by about one order of magnitude. The combination of the SLS with overshoot grading yields the most effective elimination of dislocations. Using the optimum conditions, GaAs0.6P0.4 with good surface morphology, strong visible photoluminescence intensity, and a dislocation density of 6.5×105/cm2 has been obtained.