Crystalline thin films Elaboration processes and applications

The physical chemistry of thin films is driving innovation and research, and is playing an increasingly important role in materials science. Among the many fields in which thin films are of interest, optics and photonics have made a major contribution to their development, particularly since the late 1970s, with the development of semiconductor materials. Many devices have been developed using thin films, including lasers, scintillators, waveguides and optical amplifiers. The deposition of thin films has also contributed to the functionalization of surfaces, by conferring certain properties: antireflection, photochromism, electrochromism, photocatalysis, photoluminescence or electroluminescence... More recently, the emergence of integrated optics has opened up new prospects, such as the multiplication of active and passive optical functions on a single substrate by structuring materials in the form of thin films using "top down" or "bottom up" approaches.

These thin films can be amorphous, crystalline or ceramic, and can be semiconducting or dielectric. For the sake of brevity, this article deals only with dielectric crystalline thin films, with thicknesses ranging from a few microns to a few hundred microns, which offer new and complementary application prospects to those offered by massive crystals.

We begin with an overview of the dielectric thin films currently being developed for optical and optoelectronic applications, and present the state of the art of the main techniques used to produce them. We then turn our attention to the Liquid Phase Epitaxy (LPE) technique, one of the most suitable methods for producing high-quality, micrometer-thick crystalline thin films. After defining the concept of epitaxy and the principle of the LPE technique, we present the different mechanisms involved in epitaxial growth, and introduce a few thermodynamic and kinetic considerations to better understand the phenomena involved. We then take a more experimental approach to the physical chemistry involved in synthesizing crystalline thin films of various materials of optical interest, based on fluorides on the one hand, and oxides on the other. The difficulties inherent in growing these materials, the importance of the substrates and solvents used, and the influence of experimental parameters are then discussed. We also highlight the crucial importance of the cleaning, shaping and polishing stages of epitaxial thin films, before discussing various possibilities for "post-processing" such samples, to optimize some of their properties according to the applications targeted. A non-exhaustive list of the various characterization techniques that can be used to characterize these samples is then presented. Finally, we present several examples of applications and devices based on rare-earth...