MOCVD preparation of oxide films

In recent years, the possibility to grow High Temperature Superconducting(HTS) or ferroelectric oxide films by MOCVD techniques has been demonstrated by several authors. These oxide layers (essentially YBa2Cu3O7, BaTiO3, SrTiO3, ...) can be used in the field of microelectronics (memories, microwave, antennas, squids, bolometers, ...) but also, with an emerging interest today, in high current devices (wires, tapes, ...). For all these applications MOCVD can be attractive, if the growth process can be sufficiently controlled in order to ensure a good homogeneity and reproducibility in the produced layers; but also if high growth rates can be reached. tinder these conditions, the advantages of MOCVD are manifold : good growth control, deposition on non-planar objects, rather inexpensive set-up compatible with an industrial environment. Nevertheless, during a long time,the lack of suitable precursor materials (for Barium essentially) has been detrimental for the rapid development of MOCVD and, despite several important developments in the chemistry of novel precursors [2,3], only limited evaporation rates and a poor stability can be reached today. Most of the metalorganic precursors used belong to the -diketonate family, with an extensive use of Y(tmhd)3, Ba(tmhd)2 and Cu(tmhd)2. The precursors for yttrium and copper have reasonable volatility and stability at moderate temperatures (around 100 degrees C). Only Ba(tmhd)2 has to be heated to temperatures higher than 200 degrees C, which affects its long term vaporisation stability. Oligomerisation can occur which decreases volatility, leading to a compositional shift in the gas phase and in the film during oxide deposition. The evaporation temperature for Barium must therefore be very precisely controlled and kept relatively low, thus reducing the maximum available Barium partial pressure into the deposition zone and limiting the growth rate by mass transport towards the substrate. In order to increase the stability of Chemical Vapour reactions and to improve the growth rate in the deposition process, alternative MOCVD techniques have though been developed in the last ears. These processes are largely described in the present paper and carefully analysed in terms of Chemical reaction pathways and experimental parameter dependence. Their fundamental principle is based on the evaporation of Mixed Liquid Sources, where the metalorganic precursors are associated with a suitable solvent and conditioned in small droplets with a controlled size. The main advantage of the Mixed Liquid Source (MLS) MOCVD, against conventional MOCVD, is that metal-organic precursors are exposed to elevated temperatures only during the short time necessary for their evaporation. The composition control and the reproducibility of the process are therefore substantially improved. Furthermore, the Mixed Liquid Source CVD process, due to the possibility to transport a large amount of precursors to the preheating zone, yields higher partial pressures of the reacting species in the gas phase and, consequently, gives rise to an improved growth rate. The dominating technique is actually computer-controlled injection MOCVD. This technique has been used for the synthesis of various functional oxides and for the growth of multilayered nanostructures. - Amorphous or heteroepitaxial oxides YBa2Cu3O7-x, PrBa2Cu3O7-x, BaTiO3, SrTiO3, Ba(1-x)SrxTiO3, MgO, CeO2, Ta2O5, La(1-x)MnO3, La(1-x)(ou Nd)SrxMnO3, Y2O3, Al2O3, LaAlO3, SiO2, TiO2, ZrO2(Y), TiN, AlN, TiAlN... - Multilayers, heterostructures and nanostructures : (YB2Cu3O7-x/PrBa2Cu3O7-x)(n), Al2O3/CeO2/YBa2Cu3O7-x, Ba(1-x)SrxTiO3/YBa2Cu3O7-x, (Ta2O5/SiO2)(n), (BaTiO3/SrTiO3)(n), (La(1-x)MnO3/CeO2)(n) ...