Structural twinning-induced insulating phase in CrN (111) films

Electronic states of a correlated material can be effectively modified by structural variations delivered from a single-crystal substrate. In this paper, we show that the CrN films grown on MgO (001) substrates have a (001) orientation, whereas the CrN films on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) substrates are oriented along the (111) direction parallel to the surface normal. Transport properties of CrN films are remarkably different depending on crystallographic orientations. The critical thickness for the metal-insulator transition in CrN 111 films is significantly larger than that of CrN 001 films. In contrast to CrN 001 films without apparent defects, scanning transmission electron microscopy results reveal that CrN 111 films exhibit strain-induced structural defects, e.g., the periodic horizontal twinning domains, resulting in an increased electron scattering facilitating an insulating state. Understanding the key parameters that determine the electronic properties of ultrathin conductive layers is highly desirable for future technological applications.