Origin of the apparent delocalization of the conduction band in a high-mobility amorphous semiconductor

In this paper, we show that the apparent delocalization of the conduction band reported from first-principles simulations for the high-mobility amorphous oxide semiconductor $\text{InGaZn}{\text{O}}_4$ (a-IGZO) is an artifact induced by the periodic conditions imposed to the model. Given a sufficiently large unit-cell dimension (over 40 Å), the conduction band becomes localized. Such a model size is up to four times the size of commonly used models for the study of a-IGZO. This finding challenges the analyses done so far on the nature of the defects and on the interpretation of numerous electrical measurements. In particular, we re-interpret the meaning of the computed effective mass reported so far in literature. Our finding also applies to materials such as SiZnSnO, ZnSnO, InZnSnO, In₂O₃ or InAlZnO₄ whose models have been reported to display a fully delocalized conduction band in the amorphous phase. (paper)