9.2.3 MBE Processes

This process essentially involves evaporation of source material in a UHV environment and then deposition of the evaporated flux onto the substrate. Vacuum evaporation is a low energy process as compared to sputtering because energy of species reaching the substrate is very low, of the order of 0.5 eV. For deposition of oxides, reactive evaporation is a very important method. Reactive evaporation involves use of a reactive gas, O₂, to maintain the correct oxygen stoichiometry of the film material while using elemental sources and also while using a compound source to compensate the loss of oxygen during the evaporation. The critical factors involved in the evaporation processes are vacuum level and the background oxygen pressure (pO₂) which are instrumental in controlling the impurity level and stoichiometry of the film. Among one of the early efforts on reactive evaporation, deposition of TiO₂ and Si₂O₃ was successfully demonstrated.²⁵ Use of active oxygen has been shown to improve the film quality considerably. Epitaxial YBCO films have been deposited by MBE in the presence of an RF plasma discharge to generate reactive oxygen species at low temperatures (< 500°C) and improvement in film properties was observed^.26 Epitaxial DyBa₂Cu₃O_6+x films were grown by MBE and it was shown that by controlling the local nucleation rate and lateral growth rate, it was possible to achieve layer by layer growth.⁶ These processes can produce films with controlled stoichiometry and better epitaxy but at the cost of low deposition rates (of the order of 5-10 nm/m). Although these methods are complex in nature and expensive, they offer advantage of in-situ growth monitoring by RHEED/LEED.