Metal organic chemical vapor deposition of functional fluoride phases

Abstract

Inorganic metal fluorides and oxide-fluorides have significant importance in the development of many new technologies, and are impacting various key points of modern life, that is, energy production and storage, microelectronics and photonics, catalysis, automotive, building, etc. In the past few years, fluoride materials have attracted a great and increasing interest due to their multifunctional properties, such as ferroelectricity, induced ferro/antiferromagnetism, thermal stability, high transparency and low phonon energy. Currently, the research on multiferroic materials is directed towards Bi-containing perovskite such as BiFeO3 or BiMnO3 and toward both hexagonal and orthorhombic rare-earth manganites such as HoMnO3 and TbMnO3. Among the few known ferroelectric fluoride crystals, the barium fluoride BaMF4 (M=Mg, Mn, Co, Ni, Zn) phases have recently caught substantial attention in view of their interesting and multifunctional properties. The BaMF4 multiferroics have been proposed as systems where is possible to incorporate both magnetism and ferroelectricity in the same phase. The goal of this work is to explore a new class of multiferroic non-oxide based materials, focusing on the class of BaMF4 systems, to widen the range of candidates for magnetoelectric device applications. In addition, fluoride compounds have compelling advantages for many optical applications due to their unique combination of low phonon energy, high UV absorption edge energy, and relatively weak crystal field. Compared with oxides, fluorides are considered to be efficient hosts for down-conversion (DC) and upconversion (UC) luminescence of rare earth (RE) ions due to their low phonon energies and optical transparency over a wide wavelength range. The choice of host materials is of great importance in designing lanthanide-based luminescent UC materials for efficient practical applications. Among the fluorides reported, rare earth (RE) doped NaYF4 and NaGdF4 are considered as the most efficient DC and UC host lattices, and have recently raised increasing attention. Many applications of RE-doped fluorides have been demonstrated, such as lasers, optical communications, display devices, and so on. On the other hand, trivalent RE-doped MF2 (with M=Sr, Ca, Ba, Cd) compounds are also considered a suitable material characterized by low energy phonons (usually less than 300 cm-1) and large transfer coefficients between the RE ions. In this typology of halide hosts, multiphonon relaxation is thought to be strongly suppressed and therefore efficient UC luminescence can be obtained. In this context, the present work discusses about some different rare-earth doped fluoride materials based on NaREF4 and MF2 host lattices, which possess physical properties of technological interest. In this study, Metal Organic Chemical Vapor Deposition (MOCVD) approaches have been used to fabricate the fluoride phases of interest in form of thin films. A systematic study of precursors suitable for the MOCVD growth of the fluoride films has been addressed. The synthesized films have been characterized from a structural, morphological and compositional point of view.