Electronic and optical properties of metallic nitride: A comparative study between the MN (M = Al, Ga, In, Tl) monolayers

The electronic and the optical properties of metallic nitride (MN) monolayers are studied using a DFT formalism. In most of these monolayers, the electron density of the metallic atoms is much higher than that of the nitride atoms, and ionic, covalent, and metallic bonds are found in M-N bonds, resulting in fascinating electronic and optical properties. The optical band gap is varied from almost 0.0 to 3.0 eV for the MN monolayers depending on the bond type between the metallic and the nitride atoms, as well as the contribution of the type of orbitals around the Fermi energy. The optical properties such as the dielectric function, the excitation spectra, the refractive index, the reflectivity, and the optical conductivity of MN monolayers are calculated. The excitation energy and static dielectric constant are found to be inversely proportional to the band gap at low photon energy, and the refractive index at energy approaching zero for the incident light is found to be 2.302, 1.93, 1.37, and 1.306 for TlN, InN, GaN, and AlN monolayers, respectively, in the case of parallel electric field. The MN monolayers with a large band gap have good visible light functionality, while the MN monolayers with a lower band gap are found to be active in the infrared region. Furthermore, it is shown that the optical properties of MN monolayers show a strong anisotropy with respect to the polarization of the incoming light. It is found that the optical conductivity of TlN is twice that of InN, GaN, and AlN at low energy. Our results for the optical properties of MN monolayers show that they could be beneficial in optoelectronic device applications.