A Field Guide to Materials with Axis-Dependent Conduction Polarity

Most electronic materials uniformly exhibit either dominant p- or n-type conduction along all crystallographic directions. However, there is an emerging class of materials in which either dominant p- or n-type conduction is observed depending on the crystallographic direction. This phenomenon is referred to as axis-dependent conduction polarity (ADCP) and has been experimentally detected across many different classes of materials. In this Review, we start by establishing the band structure origins and signatures of this phenomenon. Then, we thoroughly detail each material in which ADCP has been observed. We provide a chemical understanding of why ADCP occurs in these materials and show that closely related isostructural and isoelectronic phases will also exhibit this effect. This zoological survey of ADCP materials will serve as an important reference as electronic device concepts exploiting this effect arise.