Crystal chemistry and electrical properties of the delafossite structure

Over the past few decades, the field of transparent conducting oxides has undergone tremendous advances. With the rapid growth of optoelectronic applications related to display technologies, traditional materials such as Sn-doped indium oxide (ITO) are now widely used as transparent electrodes. In addition, with the advent of p-type transparent conductors, through the transparent pn-junction building block, a wide range of functional transparent optoelectronic devices have been demonstrated including UV-emitting diodes, UV-detectors, and transparent thin film transistors. This paper will highlight the unique characteristics of oxide materials based on the delafossite structure with a focus on the interrelationship between the chemistry, crystal structure, process conditions, and electrical and optical properties. The delafossite structure (ABO2) is characterized by a layer of linearly coordinated A cations stacked between edge-shared octahedral layers (BO6). The A-site cation is comprised of Pt, Pd, Ag, or Cu ions nominally in a monovalent state. The B-site cation can consist of most trivalent transition metals, group III elements, rare earths, or charge compensated pairs (e.g. B2+/B4+). This layered structure leads to highly anisotropic physical properties. The crystal chemistry of the delafossite structure will be discussed in reference to phase stability, the stability of dopants, and the important physical properties such as the conductivity and optical transparency.