Can the structure of amorphous indium gallium zinc oxide be described in terms of a few polyhedral motifs?

The coordination polyhedra around the cations are the building blocks of ionic solids. In context of amorphous InGaZn oxide (a-IGZO), even though the coordination polyhedra are irregularly arranged, it will be beneficial to identify them. In this work, we address the questions, (a) is it possible to classify all the polyhedra that occur in a-IGZO into only a few distinct groups? and (b) are these the same polyhedral motifs as those observed in the crystalline indium gallium zinc oxide (c-IGZO) or other related crystalline oxides of indium, gallium and zinc? Therefore, in this first principles based study, a large number (ten) of equivalent samples of a-IGZO were prepared by ab initio melt-and-quench molecular dynamics, so that several distinct samples of the amorphous landscape are obtained. The combination of all these structures thus obtained is a better representation of a real a-IGZO sample. For the ten samples containing 360 cations, we propose a simpler and more accurate method for determining the coordination number of each polyhedron, which was verified by charge density plots. Based on a method of comparing bond angles between metal and oxygen atoms, the identified polyhedra were matched to the polyhedral motifs present in the related crystalline systems, such as, InGaZnO4, In2O3, Ga2O3 and ZnO. We find, the a-IGZO primarily consists of the following polyhedra: a tetrahedron from space group 199 and an octahedron from space group 206 of In2O3; a tetrahedron from space group 12 and an octahedron from space group 167 of Ga2O3; a tetrahedron from space group 186 of ZnO; zinc and gallium trigonal bipyramids from c-IGZO; and one zinc 4-fold, one zinc 5-fold and one indium 5-fold coordination polyhedra that occur only in the amorphous phase. We are able to reduce the description of structure from 360 to 10 groups of polyhedra. The benefits of this identification could be enormous.