Exploring the Effects of Order vs. Disorder using Three Cations: the example of Al-Mg-Si-N

Wurtzite structured AlN is used in ultraviolet light emitting devices and high electron mobility transistors, as well as in high temperature ceramic applications. If we replace pairs of Al atoms in AlN with Mg and Si, while fulfilling local charge neutrality around each N atom, MgSiN₂ will be formed. This so-called II-IV nitride forms in an orthorhombic wurtzite-derived crystal structure and it has been shown that MgSiN₂ has a large in-direct band gap that is similar in size to the band gap in AlN. It is, however, possible to imagine that, rather than forming an ordered compound, Mg and Si atoms randomly occupies the cationic sublattice of the wurtzite crystal structure. Such a structure has a higher free energy while also significantly reduces the size of the band gap compared to ordered MgSiN₂. In this study, we have investigated the questions related to the structure of alloyed Al-Mg-Si-N systems from both experiment and first principles density functional calculations, where the composition has been varied from pure AlN to pure MgSiN₂. Especially, we have investigated how order and disorder in these types of systems impact the electronic properties.