Propriétés électriques et optiques de couches minces de WS2 et MoS2 en vue d'applications photovoltaïques

Single crystals of layered semiconductors such as WS2 and MoS2 have already proven their efficiency as active elements in photovoltaic cells. Due to their high optical absorption coefficient in the visible range, these materials could be used in the form of thin films in photovoltaic devices. In this work, we explore the potential of the sputtering technique to prepare semiconducting films of WS2 and MoS2. The influence of thermal treatments on the films is also examined. The study focuses on the electrical and optical properties of the films. They are determined by conventional conductivity, Hall effect, photoconductivity and reflection-transmission measurements. With the help of scanning probe microscopes like the scanning tunneling microscope (STM) or the atomic force microscope (AFM), the local electrical and photovoltaic properties are measured down to the nanometer scale and the relationships with the properties measured over macroscopic distances are established. Reactive sputtering (with H2S reactive gas) from a WS2 or a MoS2 target yields polycristalline WSx (0.7 < x < 1.95) and MoSx (0.9 < x < 2.1) films, at deposition temperatures between 70° and 600°C. The structural analyses, based on transmission electron microscopy (TEM), X-ray diffraction (XRD) and STM, reveal a maximum grain size of 20–30 nm, as well as a high density of stacking faults. An original STM study of the initial growth of sputtered films shows a three-dimensional growth mode including some spiral growth. The grains grow in the shape of trigonal pyramids, with step heights of 0.6 nm corresponding to the thickness of one single WS2 molecular layer. In the sputtered films, the variation of the electrical conductivity versus the temperature shows a typical semiconductor behavior with an activation energy of up to 90 meV at room temperature (RT). The high carrier concentrations (n > 1025 m-3) indicate an important level of doping and the Hall mobilities (µH < 0.1 × 10-4 m2V-1s-1) are smaller than those measured in single crystals against (µH ≈ 200 × 10-4 m2V-1s-1). STM current-voltage (I-V) spectroscopy on the sputtered film surface is typical of a degenerate semiconductor or of a high density of surface states. The films are therefore not suitable for the preparation of junctions. In order to improve the semiconducting properties of the film, a new preparation method is developed. The fabrication process, which can easily be scaled up, produces WS2 and MoS2 films with electronic properties close to those of single crystals. Firstly, an amorphous WS3-4 film is sputter-deposited at low temperature (0°C) on a substrate coated with a thin (10 nm) layer of nickel or cobalt. Secondly, the film is annealed for one hour under an argon flow between 750°C and 950°C. By predepositing Ni or Co the cristallinity, the grain size and the texture of the films are spectacularly improved. Elements such as Ni or Co are hereafter named crystallization promoter. After annealing, they remain i