Metallic films: conductivity properties vs. work function: new opportunities for accelerator and other technological applications
Aula Bruno Touschek (LNF INFN)
Aula Bruno Touschek
Technological activities to design, manufacture and test new accelerating devices using different materials and methods are under way all over the world. The main goal of these researches is to increase the accelerating gradients minimizing the probability of RF breakdown. Among the possible options, experimental results point out that relatively thick metallic coatings of Transition Metal (TM) atoms may improve the properties of standard materials like copper, in particular, the breakdown rate. In the framework of the researches performed by the INFN/DEMETRA experiment, I will present and discuss experimental data of Mo films grown on different substrates with a resistivity < 100-150 μΩ cm.
Mo films are multiphase metallic systems with a not negligible contribution of disordered oxide phases: transparent and insulating phases such as MoO3 or metallic phases such as MoO2, both of high-interest also for technological applications. The interplay of nano- and micrometer-scale factors is typically at the origin of the properties and the macroscopic behavior of TM films, so that the capability to probe morphology and phase distribution of these complex systems at multiple length scale is mandatory. Different chemical and structural factors may affect the properties of Mo films and, in particular, the work function. As a consequence of the relationship between defects and work function, many transition metal (TM) oxides tend to have decreased work functions near a metal/metal-oxide interface, a behavior useful to tune the work function and the field emission, if the TM oxide and the metallic substrate is properly selected.
The field emission from a metal surface has been continuously studied for a century and a general consensus exists regarding the correlation between the emitted current and the applied electric field. However, recently has been pointed out for the L-band RF gun at the Argonne Wakefield Accelerator facility that a localized field emission could be probably affected by global parameters.