Controllable fabrication of Cu<sub>2</sub>O porous nanostructured films by negative bias deposition method

Abstract

As one of the most common two kinds of copper oxides, cuprous oxide (Cu2O) is an important p-type transition metal oxide semiconductor material. Due to the advantages of low-cost, non-toxicity and abundant copper sources and the potential applications in the fields of gas sensors, solar cells and photocatalysts, thin films of Cu2O have attracted great interest of researchers. To enhance the performances of the above Cu2O-based surface-sensitive devices and materials, the researchers tend to prepare Cu2O thin films of porous or even nanoporous structures. However, there is still no effective method available for the controllable fabrication of Cu2O porous nanostructured films (or porous nanostructure-films, short for PNFs), which owns not only the common features of porous thin films but also the unique properties of nanosize building units. By using a radio- frequency balanced magnetron sputtering (MS) deposition system, in this paper, Cu2O PNFs were prepared on clean glass slides by applying different negative bias voltage during film deposition. After the preparation, a field-emission scanning electron microscope (FESEM), a grazing-incidence X-ray diffractometer (GIXRD) and an ultraviolet-visible (UV-Vis) spectrophotometer were applied subsequently for the detailed characterizations of surface morphology, texture and optical property respectively. It was observed that the as-prepared Cu2O PNFs exhibited flexible porosities and nanosize building units, which were greatly dependent on the substrate negative bias voltage. In particular, when the substrate bias voltage was kept at −50 or −150 V, the as-prepared Cu2O PNFs both demonstrated intriguing triangular pyramid-like nanostructures with distinct edges and corners on the porous film surface. Further, the side view FESEM images and the out-of-plane GIXRD spectra demonstrated a columnar growth of the Cu2O PNFs with a notable preferential orientation of (111). The optical testing results showed that the band gap of the Cu2O PNFs obtained at different negative bias voltages was tunable between 2.0 and 2.35 eV, which demonstrated a little red or blue shift relative to that of bulk Cu2O (2.17 eV). It is expected that the traditional ion bombardment and re-sputtering theories are not suitable for the explanation of the above bias voltage effects. This is because the traditional ion bombardment and re-sputtering theories were proposed to account for the bias deposition in an unbalanced magnetron sputtering (MS) system rather than the present balanced MS system. Further, the experimentally observed non-linearly changed density or porosity of the Cu2O PNFs with the bias voltage at relatively low values and the common even surface at relatively high values confirmed this viewpoint. Based on the above findings and analysis, a selectively preferential deposition of material atoms or molecules on the film surface during the negative bias deposition was proposed. That is, when the substrate is negatively biased, the tip- charging effect of electrons would occur on the nanoscale rough surface of the substrate or the depositing film. The resulting electric field near the substrate or film surface is non-uniform and could be regarded as an assembly of many electric fields of particle or tip charges. As a consequence, the sputtered atoms or molecules would be acted by two kinds of Coulomb forces: one is the attractive force originating from the electric fields of the closest particles or tips when traveling near the substrate, and the other is the repulsive force coming from the surrounding particles or tips with the same kind of charges after depositing on one particle or tip. For the former force, it would lead to a preferential deposition of sputtered atoms or molecules on the particles or tips and meanwhile provide an additional kinetic energy for the deposited atoms or molecules to migrate or diffuse around; for the later force, the constraint effect of the surrounding electric fields of particle or tip charges would hinder the migration or diffusion of deposited atoms or molecules on the film surface. It is thus expected that the tip charging effect would lead to a columnar growth of films, and the contradictory of the above two forces would influence or even determine the final surface morphology of films which depends on the value of substrate bias voltage.