Strain and composition distributions in wurtzite InGaN/GaN layers extracted from x-ray reciprocal space mapping

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Abstract
Strain and composition distributions within wurtzite InGaN/GaN layers are investigated by high-resolution reciprocal space mapping (RSM). We illustrate the potential of RSM to detect composition and strain gradients independently. This information is extracted from the elongation of broadened reciprocal lattice points (RLP) in asymmetric x-ray reflections. Three InxGa1−xN/GaN (nominal x=0.25) samples with layer thickness of 60, 120, and 240 nm, were grown in a commercial metal-organic chemical vapor deposition reactor. The RSMs around the (105) reflection show that the strain profile is nonuniform over depth in InGaN. The directions of “pure” strain relaxation in the reciprocal space, for a given In content (isocomposition lines), are calculated based on elastic theory. Comparison between these directions and measured distributions of the RLP shows that the relaxation process does not follow a specific isocomposition line. The In mole fraction (x) increases as the films relax. At the start of growth all the films have x∼0.2 and are coherent to GaN. As they relax, x progressively increases towards the nominal value (0.25). Compositional gradients along the growth direction extracted from the RSM analysis are confirmed by complementary Rutherford backscattering measurements.