Point defects create exotic properties in materials such as defect-induced luminescence in wide-bandgap semiconductors, magnetism in nonmagnetic materials, single-photon emission from semiconductors, etc. In this article, oxygen defect formation in metallic TiN and semiconducting rock salt-(Al,Sc)N is investigated with a combination of first-principles density functional theory, synchrotron-based x-ray absorption spectroscopy (XAS) analysis, and scanning transmission electron microscopy–energy-dispersive x-ray spectroscopy mapping. Modeling results show that oxygen in TiN and rock salt-(Al,Sc)N prefers to be in the defect complex of substitutional and interstitial oxygen (nON + Oi) types. While in TiN, the preferential interstitial sites of oxygen in ON + Oi are at the tetrahedral site, in rock salt-(Al,Sc)N, a split interstitial site along the [111] direction was found to be energetically preferable. Simulations performed as a function of the oxygen partial pressure show that under experimental growth conditions, four oxygen atoms at the substitutional sites of nitrogen (4ON), along with four Ti atoms, decorate around an interstitial oxygen atom at the tetrahedral site (Oi) in the energetically favored configuration. However, in rock salt-(Al,Sc)N, n in nON + Oi was found to vary from two to four depending on the oxygen partial pressure. Theoretical predictions agree well with the experimentally obtained XAS results. These results are not only important for a fundamental understanding of oxygen impurity defect behavior in rock salt nitride materials but will also help in the development of epitaxial metal/semiconductor superlattices with efficient thermionic properties.
We discuss the microstructural origin of enhanced radial growth in magnesium (Mg) doped gallium nitride (GaN) nanorods (NRs) using electron microscopy and first-principles Density Functional Theory calculations. Experimentally, we find the Mg incorporation increases surface coverage of the grown samples and the height of NRs decreases as a consequence of an increase radial growth rate. We also observed the coalescence of NRs becomes prominent and the critical height of coalescence decreases with the increase in Mg concentration. From first-principles calculations, we find the surface free energy of Mg doped surface reduces with increasing Mg concentration in the samples. The calculations further suggests a reduction in the diffusion barrier of Ga adatoms along [1120] on the side wall surface of the NRs, possibly the primary reason for the observed enhancement in the radial growth. * sanjaynayak@jncasr.ac.in † smsprasad@jncasr.ac.in
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