Morphological control of band offsets for transparent bipolar heterojunctions: The Bädeker diode

Cuprous iodide is an important transparent p-type semiconductor, it has an illustrious history, and has recently enjoyed a renaissance. In this work, we investigate the electronic band structure and absolute electron energies of the three major known phases of CuI, zincblende (F43m), wurtzite (P63mc), and rock salt (Fm-3m). We also consider CdO, an important n-type transparent semiconductor in these three phases, in order to assess the possibility of constructing transparent bipolar heterojunctions from the two materials. We calculate the relative stability of all three phases of both materials, demonstrating that all three phases should be accessible through non-equilibrium growth techniques. We then calculate the band structures of all six phases, as well as the absolute electron energies (ionization potentials and electron affinities). Thus, we are able to construct energy band alignment diagrams between the materials in the three different structures. These diagrams reveal that one can achieve type-I, type-II, or type-III offsets depending on the crystal structure of the materials. We consider possible applications opened up by these findings.