Cathodoluminescence of compositionally graded layers of GaAs1−xPx

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Abstract
Compositionally graded layers of GaAs1−xPx grown on (100) planes of GaAs substrates were investigated as a function of the distance t from the substrate, using a 77 °K cathodoluminescence technique. It was found that luminescence intensity L of these layers was strongly dependent on the profile of the compositional gradient, dx/dt. A number of the nonradiative recombination centers are three−dimensionally distributed in the shape of a cross−grid. This is related to misfit dislocations associated with impurities in the graded layers. The gradient dependence of L demonstrates that the density of these nonradiative recombination centers, N, is proportional to (dx/dt)2. This quadratic relation shows that only grading profiles which satisfy the condition d2x/dt2N continuously with increasing t. Discontinuous variations of x and/or dx/dt cause a significant degradation in the subsequent layers. From the spatial variation in the ratio of the intensity of the 1.35−eV band to the near−gap band, it was found that an anomalous accumulation of Cu contaminant occurred at the substrate−epitaxial−layer interface region, where a relatively high density (∼4×108 cm−2) of these nonradiative recombination centers was present. This is because of a very steep compositional gradient (∼20% P/μ). Here, the concentration of Cu was optically estimated through cathodoluminescence data from Cu−diffused GaAs samples.