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

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 InxGa12xN/GaN (nominal x50.25) 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

Raman scattering by the E2h and A1(LO) phonons of InxGa1−xN epilayers (0.25 < x < 0.75) grown by molecular beam epitaxy

“…However, the RSMs did not allow us to discern between composition and strain gradients, as performed for instance in Ref. 29. Nevertheless, the broadening of the x-ray peaks in Fig.…”

“…13,19 The presence of in-depth strain and composition gradients in the InGaN alloy seems to depend on the particular growth method and growth conditions. For instance, Pereira et al 29 showed that the relaxation of strain in InGaN epilayers grown by metal organic chemical vapor deposition (MOCVD) is accompanied by increased In contents. Similarly, Wang et al 41 reported increased In contents along the growth direction in Ga-rich InGaN layers grown by MOCVD.…”

Composition measurement of epitaxial ScxGa1−xN films

“…This effect became more pronounced as the Sc flux was increased to 4 nA, where the Sc contents measured by XPS are 0.31, 0.27 and 0.21 respectively. This is likely due to the differences in surface polarity expected to occur between III-nitride films grown by MBE (normally N-polar) and MOVPE (normally Ga-polar) [49,50], as well as differences in the film strain which are well known to affect the relative incorporation of alloy constituents having different ionic radii [51,52] and which may also affect the relative surface accumulation.…”

Basic Research Needs for Solid-State Lighting. Report of the Basic Energy Sciences Workshop on Solid-State Lighting, May 22-24, 2006

“…Laboratory systems have improved significantly in recent years, becoming easier to operate and equipped with more functional optics and detectors. For example, the availability of high-flux, high-resolution monochromators on laboratory four-circle diffractometers has made specular x-ray scattering and reciprocal space mapping workhorses for determining composition and strain in compound semiconductor systems [Pereira et al 2002]. New advances in x-ray optics [Kang et al 2006] hold forth the promise of focusing hard x-rays down to the nanometer level, opening up a range of scattering and fluorescence methods for application at high spatial resolutions.…”