F. Sweeney

Structural and optical properties of InGaN/GaN layers close to the critical layer thickness

In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectr...

Characterisation of nitride thin films by electron backscattered diffraction

Abstract In this paper, we describe the technique of electron backscattered diffraction (EBSD) and illustrate its use in the characterisation of nitride thin films by describing our results from a silicon-doped 3 μm thick GaN epilayer grown on a sapphire substrate misoriented by 10° towards the m-plane (10-10). We show that the EBSD technique may be used to reveal the relative orientation of an epilayer with respect to its substrate (a 90° rotation between the GaN epilayer and sapphire substrate is observed) and to determine its tilt (the GaN epilayer was found to be tilted by 12±3° towards [10-10] GaN ).

Depth resolved studies of indium content and strain in InGaN layers

A depth resolved study of optical and structural properties in wurtzite InGaN/GaN bilayers grown by MOCVD on sapphire substrates is reported. Depth resolved cathodoluminescence (CL), Rutherford backscattering spectrometry (RBS) and high resolution X-ray diffraction (HRXRD) were used to gain an insight into the composition and strain depth profiles. It is found that both quantities can vary considerably over depth. Two representative samples are discussed. The first shows a CL peak shift to the blue when the electron beam energy is increased. Such behaviour conforms to the In/Ga profile derived from RBS, where a linear decrease of the In mole fraction from the near surface (0.20) down to the near GaN/InGaN interface (0.14) region was found. The other sample discussed shows no depth variations of composition. However, the strain changes from nearly pseudomorphic, close the GaN interface, to an almost relaxed state close to the surface. This discrete variation of strain over depth, originates a double XRD and CL peak related to InGaN.

Characterization of nitride thin films by electron backscatter diffraction.

Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light‐emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality.

Characterisation of nitride thin films by electron backscatter diffraction and electron channelling contrast imaging

Abstract In the present paper the authors describe the use of electron backscatter diffraction (EBSD) mapping and electron channelling contrast imaging (in the scanning electron microscope) to study tilt, strain, atomic steps and dislocations in epitaxial GaN thin films. Results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film are shown. From the results it is deduced that EBSD may be used to measure orientation changes of the order of 0·02° and strain changes of order 2 × 10−4 in GaN thin films. It is also demonstrated that channelling contrast in electron channelling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films. In addition the authors will consider the results of the first many-beam dynamical simulations of EBSD patterns from GaN thin films, in which the intensity distributions in the experimental patterns are accurately reproduced.

Electron Backscattered Diffraction Patterns from Cooled Gallium Nitride Thin Films

The acquisition of electron backscattered diffraction (EBSD) (or Kikuchi diffraction) patterns in the scanning electron microscope is proving to be a useful technique with which to probe the structural properties of nitride thin films. In this paper we show that if a sample is cooled the patterns improve dramatically, an increase in intensity of the Kikuchi lines and a decrease in the intensity of the diffuse background is observed. Kikuchi lines from higher order planes become visible and the HOLZ rings become better defined. Such cooled patterns yield more information on the sample, particularly on non-centrosymmetric planes, from which the polarity of the nitride thin film under investigation may be deduced.

Strain and Compositional Analysis of InGaN/GaN Layers

We investigate strain and composition of epitaxial single layers of wurtzite InxGa 1−x N (0 2 0 3 substrates. It is shown that significant inaccuracies may arise in composition assessments if strain in InxGa 1−x N/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 Hookes 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 (e zz ) and parallel strain components ( (e xx ) 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.

Characterization of Nitride Thin Films by Electron Backscatter Diffraction and Electron Channeling Contrast Imaging

In the present paper the authors describe the use of electron backscatter diffraction (EBSD) mapping and electron channelling contrast imaging (in the scanning electron microscope) to study tilt, strain, atomic steps and dislocations in epitaxial GaN thin films. Results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film are shown. From the results it is deduced that EBSD may be used to measure orientation changes of the order of 0·02° and strain changes of order 2 × 10−4 in GaN thin films. It is also demonstrated that channelling contrast in electron channelling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films. In addition the authors will consider the results of the first many-beam dynamical simulations of EBSD patterns from GaN thin films, in which the intensity distributions in the experimental patterns are accurately reproduced.

Probing nitride thin films in 3-dimensions using a variable energy electron beam

In this paper we illustrate the application of electron beam techniques to the measurement of strain, defect and alloy concentrations in nitride thin films. We present brief comparative studies of CL spectra of AlGaN and InGaN epilayers and EBSD patterns obtained from two silicon-doped 3 mum thick GaN epilayers grown on an on-axis (0001) sapphire substrate and a sapphire substrate misoriented by 10 degrees toward the m-plane (10 (1) over bar0).