Is Cu(3–x)P a Semiconductor, a Metal, or a Semimetal?

[Image: see text] Despite the recent surge in interest in Cu(3–x)P for catalysis, batteries, and plasmonics, the electronic nature of Cu(3–x)P remains unclear. Some studies have shown evidence of semiconducting behavior, whereas others have argued that Cu(3–x)P is a metallic compound. Here, we attempt to resolve this dilemma on the basis of combinatorial thin-film experiments, electronic structure calculations, and semiclassical Boltzmann transport theory. We find strong evidence that stoichiometric, defect-free Cu(3)P is an intrinsic semimetal, i.e., a material with a small overlap between the valence and the conduction band. On the other hand, experimentally realizable Cu(3–x)P films are always p-type semimetals natively doped by copper vacancies regardless of x. It is not implausible that Cu(3–x)P samples with very small characteristic sizes (such as small nanoparticles) are semiconductors due to quantum confinement effects that result in the opening of a band gap. We observe high hole mobilities (276 cm(2)/(V s)) in Cu(3–x)P films at low temperatures, pointing to low ionized impurity scattering rates in spite of a high doping density. We report an optical effect equivalent to the Burstein–Moss shift, and we assign an infrared absorption peak to bulk interband transitions rather than to a surface plasmon resonance. From a materials processing perspective, this study demonstrates the suitability of reactive sputter deposition for detailed high-throughput studies of emerging metal phosphides.