Thin-film Coating System for Alkali Antimonide Photocathode Performance Enhancement

Alkali antimonide photocathodes are commonly used in high current photoinjectors for electron cooling at nuclear physics facilities like RHIC and EIC. They have ultra-high quantum efficiency (QE) and relatively low requirements for growth. The lifetime of the photocathode can be extremely reduced or destroyed in a non-UHV vacuum environment with even low doses of residual gases in the chamber. Consequently, the transport and storage of alkali antimonide photocathodes is eminently difficult. Encapsulating the cathode with a thin layer of a 2D material has a great potential to protect the cathode, however the stringent requirement for the growth of the material, such as hBN or graphene, is likely to damage the cathode without an innovative way for the thin-film deposition. A new mechanism to produce the thin film while assuring the integrity of the photocathode is needed. Euclid Techlabs (PI: Dr. Ao Liu) and BNL (PI: Dr. Mengjia Gaowei) together propose to develop a stepwise encapsulation method using an alkali intercalation process. We propose to use different deposition techniques including thermal evaporation and sputter to achieve intercalation and entrapment of alkali atoms into an hBN or graphene covered Antimony base to form the alkali antimonide cathode under the hBN or graphene layer, which now serves as the encapsulation layer. Our proposed method allows for the thin film of 2D materials to be transferred in an ambient lab environment like in a wet or dry transfer, as an alternative way to our proposed growth of the 2D material using the same sputtering chamber. In Phase I, we will develop the deposition recipe for the alkali intercalation and hBN and graphene wet transfer, investigate an enhancement mechanism for the hBN growth on the Antimony base in parallel, and do material characterization for the final product from the process, such as SEM and EDX, AFM, and XPS if applicable. We will be able to use and upgrade a dedicated UHV-compatible sputtering chamber for photocathode encapsulation to avoid cross-contamination. The system will be tested at Euclid before being shipped to BNL for installation. The materials will be characterized using LEEM/LEED at BNL. The photocathode growth and encapsulation method we are proposing can become a new standardized way to prepare the high-performance cathodes for daily operations of nuclear physics facilities such as RHIC and EIC. Users will benefit from the encapsulated photocathodes and their unaltered QE. More robust cathodes with longer lifetime will become available for heavy-duty experiments.