ABSTRACT
In this paper we perform angular resolved annular-dark field (ADF) scanning-transmission electron microscopy (STEM) to study the scattered intensity in an InGaN layer buried in GaN as a function of the scattering angle. We achieved angular resolution with a motorized iris aperture in front of the ADF detector. Using this setup, we investigated how the intensities measured in various angular ranges agree with multislice simulations in the frozen-lattice approximation. We observed a strong influence of relaxation induced surface-strain fields on the ADF intensity, measured its angular characteristics and compared the result with simulations. To assess the agreement of the measured intensity with simulations, we evaluated the specimen thickness in GaN and the indium concentration in InGaN for each angular interval by comparing the measured intensities with simulations. The thickness was strongly overestimated for scattering angles below 40mrad and also the evaluated indium concentration varies with the considered angular range. Using simulations, we investigated which angular ranges show a high sensitivity to variations of the thickness and which intervals strongly depend on the indium concentration. By combining two angular intervals, the indium concentration and the specimen thickness were determined simultaneously, which has potential advantages over the usual quantification method. It is shown that inelastic scattering, surface contamination and mistilt can have an influence on the measured intensity, especially at lower scattering angles below 30-50mrad, which might explain the observed difference between the frozen lattice simulation and the experiment.
ABSTRACT
The electronic properties of quaternary AlInGaN devices significantly depend on the homogeneity of the alloy. The identification of compositional fluctuations or verification of random-alloy distribution is hence of grave importance. Here, a comprehensive multiprobe study of composition and compositional homogeneity is presented, investigating AlInGaN layers with indium concentrations ranging from 0 to 17at% and aluminium concentrations between 0 and 39 at% employing high-angle annular dark field scanning electron microscopy (HAADF STEM), energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT). EDX mappings reveal distributions of local concentrations which are in good agreement with random alloy atomic distributions. This was hence investigated with HAADF STEM by comparison with theoretical random alloy expectations using statistical tests. To validate the performance of these tests, HAADF STEM image simulations were carried out for the case of a random-alloy distribution of atoms and for the case of In-rich clusters with nanometer dimensions. The investigated samples, which were grown by metal-organic vapor phase epitaxy (MOVPE), were thereby found to be homogeneous on this nanometer scale. Analysis of reconstructions obtained from APT measurements yielded matching results. Though HAADF STEM only allows for the reduction of possible combinations of indium and aluminium concentrations to the proximity of isolines in the two-dimensional composition space. The observed ranges of composition are in good agreement with the EDX and APT results within the respective precisions.
