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 in film in the form of NiSx or CoSx droplets. Films obtained by this sputtering/annealing process show large (1–5µm) and thick (50–200 nm) grains with their c axis oriented perpendicular to the substrate. Optical reflection and transmission coefficients are similar to those measured on WS2 single crystals, with excitonic absorption peaks of same intensity at 1.94 and 2.36 eV. The films are photoconductive when illuminated with photons whose energy is superior to 1.35 eV, which corresponds to the indirect bandgap of WS2. The conductivity is of p type with a carrier concentration of about 1023 m-3 and a Hall mobility of 5–10 × 10-4 m2V-1s-1at RT. The Hall mobility is thermally activated with an activation energy of 60–90 meV between 200 et 320K. The transport properties (mobility, photoconductivity) are mainly controlled by the potential barriers at grain boundaries. STM I-V spectroscopy with and without illumination shows that the flat (002) surfaces of the WS2 cristallites in the films have a low density of surface states, similarly to the single crystal (002) surfaces. Measurements using an AFM mounted with a conductive tip indicate that the NiSx phases in the film are metallic while the WS2 grain edges are typical of a degenerate semiconductor. In order to characterize more quantitatively the local electronic properties of the films an innovative characterization technique is introduced. A lattice of triangular gold electrodes, each electrode having a typical area of 0.2 µm2, is evaporated on the p-type WS2 film. With the help of an AFM, the current-voltage characteristics of the contacts between the gold electrodes and the WS2 film are measured. The electrodes deposited on flat WS2 crystallites form rectifying diodes with the underlying grains. Barrier heights of 0.56–0.74 eV and diode ideality factors between 1.15 and 2 are determined. Under illumination, open-circuit voltages of up to 500 mV can be measured on some contacts. The photodiodes collect all carriers photo-excited on a surface of about 20 µm2, i.e. the typical size of the WS2 cristallites. The various experiments performed on the micro-contacts clarify the respective roles of the cristallites and the grain boundaries in the macroscopic measurements (photoconductivity, Hall effect, diodes). The use of crystallization promoters during the annealing of WS3-4 films therefore proves fruitful for the preparation of semiconducting films. The WS2 crystallites inside these films are suitable for the preparation of solid-state photodiodes, what is a first step towards the realization of photovoltaic devices based on WS2 or MoS2 thin films.

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