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

A depth-resolved study of the optical and structural properties of wurtzite InGaN/GaN bilayers grown by metallorganic chemical vapor deposition on sapphire substrates is reported. Depth-resolved cathodoluminescence ͑CL͒ and Rutherford backscattering spectrometry ͑RBS͒ were used to gain an insight into the compositional profile of a 75-nm thick InGaN epilayer in the direction of growth. CL acquired at increasing electron energies reveals a peak shift of about 25 meV to the blue when the electron beam energy is increased from 0.5 to ϳ7 keV, and shows a small shift to lower energies between ϳ7 and 9 keV. For higher accelerating voltages the emission energy peak remains constant. This behavior can be well accounted for by a linear variation of In content over depth. Such an interpretation conforms to the In/Ga profile derived from RBS, where a linear decrease of the In mole fraction from the near surface ͑ϳ0.20͒ down to the near GaN/InGaN interface ͑ϳ0.14͒ region fits the random spectra very well. Furthermore, by measuring the tetragonal distortion at different depths, using RBS/channeling, it is shown that regions of higher In content also appear to be more relaxed. This result suggests that strain hinders the incorporation of In atoms in the InGaN lattice, and is the driving force for the compositional pulling effect in InGaN films.

The effect of strain on the compositional and optical properties of a set of epitaxial single layers of In x Ga 1Ϫx N was studied. Indium content was measured free from the effects of strain by Rutherford backscattering spectrometry. Accurate knowledge of the In mole fraction, combined with x-ray diffraction measurements, allows perpendicular strain (⑀ zz ) to be evaluated. Optical band gaps were determined by absorption spectroscopy and corrected for strain. Following this approach, the strain free dependence of the optical band gap in In x Ga 1Ϫx N alloys was determined for xр0.25. Our results indicate an ''anomalous,'' linear, dependence of the energy gap on the In content, at room temperature: E g (x)ϭ3.39-3.57x eV. Extension of this behavior to higher concentrations is discussed on the basis of reported results.

The presence of two, or more, x-ray diffraction (XRD) peaks from an InGaN epilayer is sometimes regarded as an indicator of phase segregation. Nevertheless, detailed characterization of an InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) shows that splitting of the XRD peak may be completely unrelated to phase decomposition. Wurtzite InGaN/GaN layers were grown in a commercial reactor. An XRD reciprocal space map performed on the (105) plane shows that one component of the partially resolved InGaN double peak is practically aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. The other XRD component is shown to have the same indium content as the pseudomorphic component, from a consideration of the effect of strain on the c- and a-lattice constants. The composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS channeling angular scans also show that the region closer to the GaN/InGaN interface is nearly pseudomorphic to the GaN substrate, whereas the surface region is almost fully relaxed.

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Even in good quality undoped GaN samples, as assessed by the intense excitonic emission, the yellow band is present. This band has been attributed either to a shallow donor to deep double donor pair recombination [1], to a deep donor to a shallow acceptor [2] or to a shallow donor and a deep state [3]. However, its origin is not yet clear. We present data on time resolved spectroscopy compared with steady state results. These results indicate that there is no difference in band shape between steady state and time resolved spectra at all temperatures. However, in some samples there is an increase in intensity of the yellow band. It is concluded that besides a fast emission, due to prompt excitation of the centre, an indirect path from a trap 13.7 meV below the shallow donor is responsible for the long component of the decay and the intensity increase. An emission with a lifetime of ca. 300 ms is also present with a maximum at 2.35 eV.

We investigate strain and composition of epitaxial single layers of wurtzite InxGa1−xN (0<x<0.25) grown by MOCVD on top of GaN/Al203 substrates. It is shown that significant inaccuracies may arise in composition assessments if strain in InxGa1−xN/GaN heterostructures is not properly taken into account. Rutherford backscattering spectrometry (RBS) measures composition, free from the effects of strain and with depth resolution. Using X-ray diffraction (XRD) we measure both a- and c- parameters of the strained wurtzite films. By measuring both lattice parameters and solving Hooke's equation, a good estimation for composition can be obtained from XRD data. The agreement between RBS and XRD data for composition allows reliable values for perpendicular (εzz) and parallel strain components ( (εxx) to be determined. RBS and depth resolved cathodoluminescence (CL) measurements further indicate that the indium content is not uniform over depth in some samples. This effect occurs for the most strained layers, suggesting that strain is the driving force for compositional pulling.