First-Principles Studies of The Role of Cation Disorder on The Electronic and Optical Properties of Photoactive ZnTiN₂

The recently synthesized wurtzite-derived nitride semiconductor ZnTiN₂ exhibits promising properties for photoelectrochemical (PEC) applications, including an optically measured band gap appropriate for PEC reactions, high potential for integration with high-performing semiconductors, and the formation of self-passivating surface layers under operating conditions. Similar to other heterovalent ternary nitrides, experimentally synthesized ZnTiN₂ thin film features a large concentration of antisite defects, or cation disorder, which can have significant effects on its electronic and optoelectronic properties, and which has been used to rationalize the large difference between experimental absorption onset around 2 eV and theoretical fundamental band gap of cation-ordered ZnTiN₂ of 3.4 eV. Using state-of-the-art first-principles density functional theory calculations with non-empirical optimally-tuned hybrid functionals on nearly one hundred large supercells, we compute the energetics, electronic structure, and optoelectronic properties of cation-disordered ZnTiN₂. Energetically preferable and undesirable local atomic arrangements are identified. We also demonstrate that cation disorder-induced local charge imbalance broadens the valence and conduction bands near band edges, leading to a reduction in band gap relative to the cation-ordered phase, a mechanism that can be extended to the family of multivalent ternary nitrides. Accounting for cation disorder, our calculated optical absorptions agree well with experiments. Our work reveals the nature of cation disorder and its influence in ZnTiN₂ in light of possible solar energy applications.<br/><br/>This work is supported by the Liquid Sunlight Alliance, a DOE Energy Innovation Hub; computational resources provided by NERSC.