Computational approach to modeling electronic properties of titanium oxynitride systems

The study presents the use of a novel on-lattice sampling approach to generate titanium oxynitride (TiNxOy)structures with potential applications in photovoltaic and water splitting. This approach presents a simple routeto overcome challenges with structure-generating tools like Cluster Approach to Statistical Mechanics (CASM),and Ab initio Random Structure Search (AIRSS), CASM faces difficulty in generating ternary structures with largeunit cells. With AIRSS, there is an increase in probability of sampling amorphous sample spaces with increasednumber of atoms in the unit cell. Here an on-lattice sampling approach was used to model the electronic structureof TiNxOy as a function of composition. We present results for Ti2N2O, Ti5N4O4 and Ti7N4O8, with 33 %, 50 %and 67 % N replaced by O via substitution relative to titanium nitride (TiN), respectively. Koopmans theoremwas used correct the Kohn-Sham Density Functional Theory (KS-DFT) bandgaps with corresponding values of2.68 eV, 3.03 eV, and 3.65 eV for 33, 50 and 67 % O doping respectively. The projected density of states (PDOS)plot for TiN shows that the Fermi level is dominated by the 3d atomic orbitals of Ti, confirming pure TiN’smetallicity. The valence bands of TiNxOy structures were dominated by 2p orbitals of O at lower energy levels,but they were dominated by 2p orbitals of N at energies close to the valence band maximum (VBM). The conductionbands were dominated by the 3d atomic orbitals of Ti, with the bandgap increasing with O compositionleading to creation of shallow trap states near the VBM, which negatively impacts carrier mobility. In conclusion,the on-lattice sampling approach is an effective tool to generate highly crystalline structures of large unit cells,also keeping O substitution for N below 33 % as seen in Ti2N2O is crucial for avoiding shallow traps in TiNxOystructures.