In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ structures was calculated as a function of the InN content, $\mathrm{x,}$ 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 ${\mathrm{In}}_{\text{0.19}}{\mathrm{Ga}}_{\text{0.89}}\mathrm{N}$ 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 spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of “quantum dotlike In-rich” or “phase separated” regions, as commonly proposed.

In this paper we describe the use of electron backscatter diffraction (EBSD) mapping and electron channeling contrast imaging—in the scanning electron microscope—to study tilt, atomic steps and dislocations in epitaxial $\mathrm{GaN}$ thin films. We show results from a series of $\mathrm{GaN}$ thin films of increasing thickness and from a just coalesced epitaxial laterally overgrown $\mathrm{GaN}$ thin film. From our results we deduce that EBSD may be used to measure orientation changes of the order of 0.02°, in $\mathrm{GaN}$ thin films. As EBSD has a spatial resolution of $\approx 20\mathrm{nm}$, this means we have a powerful technique with which to quantitatively map surface tilt. We also demonstrate that electron channeling contrast images may be used to image tilt, atomic steps, and threading dislocations in $\mathrm{GaN}$ thin films.

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120\ifmmode^\circ\else\textdegree\fi{} to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.

Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower $(\le 5$ keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a step change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.

We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10^-4 and 0.01° respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 10^10 cm^-2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistors (HEMTs) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated