Nanocrystalline copper sulfide and copper selenide thin films with p-type metallic behavior

Copper chalcogenide materials are interesting for multiple applications due to the feasibility of suiting their optical absorption and electrical conduction by the creation of copper vacancies. Here, CuxS and CuxSe nanocrystalline films with p-type conductivity have been obtained by heating evaporated copper layers of various thicknesses with elemental sulfur or selenium, at temperatures ranging from 250 to 400 °C. These preparation parameters determine the composition and the crystalline structure of the samples, which in turn control their morphology, optical and electrical properties. Thus, the surface roughness increases with the mean crystallite size, whereas the hole concentration increases as the copper atomic proportion (or x value) decreases. Owing to the high carrier densities achieved, around 1022 cm−3, the samples show a metallic behavior with plasmonic absorption in the near infrared and electrical transport dominated by phonon scattering. Apart from such common behavior, some differences have been established between the sulfide and selenide films. One is the superior thermal stability of hexagonal CuS, present in all the temperature range, with respect to hexagonal CuSe, which evolves to cubic Cu1.8Se above 300 °C. Other is about the bandgap, wider for the sulfide than selenide samples.