Multielement composition with both structural order and chemical disorder are known as high entropy alloys (HEAs). Such materials exhibit unexpected mechanical and catalytic properties. However, the effect of composition and the structure of HEAs on their functional properties including the optical ones is still not well studied. Therefore, this work focuses on the development of HEA thin films using a Cantor alloy (FeCrMnNiCo) base, modified by varying a sixth element (Pt, Al, and Ti) concentrations to evaluate the changes in the film's structure and microstructure. The films were deposited via DC magnetron co-sputtering, which provided control over stoichiometry and film morphology. Then, the structural, electrical, and optical properties were characterized using X-ray diffraction, high-resolution transmission electron microscopy, resistivity measurement, and optical reflection measurements. Moreover, the interaction of the films with coherent light was also examined, revealing their nonlinear optical response to photons of visible and near-infrared range. In details, the structural analysis shows abundant nanotwins in the initial Cantor (CrMnFeCoNi) and CrMnFeCoNiPt films, both of which possessed a single fcc crystalline structure. However, CrMnFeCoNiAl films transitioned from a single fcc phase to a duplex fcc + bcc phase structure, eventually stabilizing as a single bcc structure. Such duplex fcc+bcc phase exhibited a low degree of nanotwins with larger grains of each phase. In contrast, CrMnFeCoNiTi films displayed an amorphous structure at various Ti contents. The study also advances the understanding of structure-related functional properties of HEAs and sets the stage for their future utilization in non-linear optics and photonics.
The development of solar cells with a low ecological footprint requires the optimization of semiconductor thin films having different and complementary optoelectronic properties: transparent electrode (TCO: transparent conductive oxide), window, buffer and absorber layers. Oxide semiconductors have two crucial advantages for this purpose: they can be developed using low-cost techniques such as ultrasonic spray pyrolysis (USP), and their optoelectronic properties can be extensively modulated through the chemical composition to adapt them to each of the layers. ZnMgAlO thin films have been developed by USP by optimizing the most impacting growth parameters: concentrations and pH of the precursors solution, substrate temperature, flow rate, nozzle speed, dwell time and shaping air pressure. In addition, a post-deposition annealing process has been developed by optimizing the annealing temperature, duration and nitrogen flow. The thickness can be easily adjusted by the deposition parameters from 50 nm to 700 nm and films investigated in the present study have a thickness of 200 nm. X-ray diffraction (XRD) analysis evidenced single-phase wurtzite structure with a strong preferred <002> orientation. A grain size varying from 40 nm to 70 nm was achieved, thus being in the upper range of the state of the art. Optical transmission results are well correlated with XRD data and indicate the bandgap increases linearly with the magnesium composition, an Urbach energy of about 70 meV, and a transparency of more than 90 % in the visible region. A Mg-modulable electrical resistivity of 10−2 Ω · cm to 102 Ω · cm was obtained with a mobility of approximately 5 cm2V−1s−1. The incorporation of aluminum into ZnMgO was achieved for the first time by USP and making possible to adjust the resistivity for application as a TCO. These results show the full potential of ZnMgAlO for application in all-oxide solar cells.
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