An international research group has discovered the core mechanism that limits the indium (In) content in indium gallium nitride (InGaN) thin films, a key material for making blue LEDs. This is a scanning transmission electron microscopy image of the atomic ordering in an (In, Ga)N monolayer: single atomic column containing only indium (In) atoms (shown by higher intensity on the image), followed by two containing only gallium (Ga) atoms. Courtesy of IKZ Berlin. Independent of growth conditions, indium concentrations have never exceeded 25 to 30 percent. In an effort to push the indium content to the limit, scientists grew single atomic layers of indium nitride (InN) on gallium nitride (GaN). Using an atomic resolution transmission electron microscope (TEM), in situ reflection high-energy electron diffraction (RHEED), and other advanced characterization methods, researchers found that as soon as the indium content reached around 25 percent, the atoms within the InGaN monolayer arranged themselves in a regular pattern consisting of a single atomic column of indium alternating with two atomic columns of gallium (Ga) atoms. Comprehensive theoretical calculations revealed that the atomic ordering was induced by a particular surface reconstruction; that is, indium atoms were bonded with four neighboring atoms, instead of the expected three. This led to stronger bonds between indium and nitrogen atoms. Based on the observed chemical ordering, researchers surmised that higher temperatures could be used during the growth of indium. Also, the ordering could make for a better-quality material. Researchers presumed that, on the other hand, the ordering set the limit of the indium content at 25 percent and that this limit could not be overcome under realistic growth conditions. This is a top view of the surface reconstruction. Courtesy of IKZ Berlin. “Apparently, a technological bottleneck hampers all the attempts to shift the emission from the green toward the yellow and the red regions of the spectra," said Tobias Schulz, scientist at the Leibniz Institute for Crystal Growth. "Therefore, new original pathways are urgently required to overcome these fundamental limitations — for example, growth of InGaN films on high-quality InGaN pseudo-substrates that would reduce the strain in the growing layer.” The discovery of ordering could help to overcome well-known limitations of the InGaN material system, such as localization of charge carriers due to fluctuations in the chemical composition of the alloy. Results suggest that growing stable, ordered InGaN alloys with the fixed composition at high temperatures could improve the optical properties of devices. The work was a result of a collaboration among the Leibniz Institute for Crystal Growth, the Max Planck Institute for Iron Research, the Paul Drude Institute for Solid State Electronics, the Institute of High-Pressure Physics, and the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics. The research was published in Physical Review Materials (doi: 10.1103/PhysRevMaterials.2.011601).