Density functional Theory (DFT) allows for accurate prediction of electronic, structural, and energetic properties of materials by solving the electronic structure problem. When applied to point defects and interfaces, DFT reveals their impact on band structure, charge redistribution, and local atomic configurations. Defects and interfaces play crucial roles in determining the properties and functionalities of semiconductors and novel quantum materials. Therefore, their identification and characterization are essential for developing advanced electronic, optoelectronic, and energy-related applications. In this presentation, we illustrate the use of DFT to understand two problems related to rare-earth pnictides, a class of materials that are promising as source and detection of terahertz radiation, and as hosts of non-trivial topological electronic structures. Rare-earth pnictides are compensated semimetals in the rock salt crystal structure and can be epitaxially grown on III-V semiconductor substrates. They can be made as embedded nanoparticles in the III-V matrix or as epitaxial thin films with very sharp and abrupt interfaces. Their electronic structure has been subject to debate largely due to the presence of 4f electrons. We will discuss the results of DFT calculations for the electronic structure of ErAs nanoparticles in III-Vs, predicting the stability of nanoparticles with respect to shape and size, and deriving a direct correlation between the electron density and atomic density associated with excess metal atoms at the interface. We will also address of the electronic structure of RE-V thin films and their behaviour under epitaxial strain and film thickness. We will show how Rare-earth pnictides change from a trivial semimetal to a non-trivial topological insulator upon quantum confinement in thin films instead of an expected normal semiconductor state.
