title: Electron emission from GaAs photocathodes using conventional and Li-enhanced activation procedures
Creators: name: Herbert, Maximilian
Publisher: name: UNSPECIFIED
Year: 2022

description: Gallium Arsenide has been in use as photocathode material for many years, exclusively providing photo-emitted spin-polarized electrons for a multitude of applications. The negative electron affinity (NEA) surface layer that is required for electron photo-emission close to the band gap of the semiconductor commonly consists of Cs in combination with oxygen or nitrogen trifluoride. The optimization of this surface layer is a central challenge for the operation of GaAs photocathodes, since the layer dictates the performance of GaAs photocathodes and is prone to decay. It is of great interest to enhance the surface layer robustness in order to slow the decay, increasing the performance and operational uptime of GaAs photo-electron sources. This dissertation presents several experiments aimed at improving GaAs photocathode effectiveness and surface layer robustness by investigating the activation process that is used to deposit the NEA layer. To this end, the parameters of the process were scrutinized and the effects of introducing Li during the procedure were studied at the Institut für Kernphysik (IKP) of the Technische Universität Darmstadt (TUDa) for Cs in combination with oxygen, and at the Center for Injectors and Sources (CIS) of the Thomas Jefferson National Accelerator Facility for Cs in combination with nitrogen trifluoride. At the IKP, the test stetup Photo-CATCH was used to conduct activations with Cs and oxygen. The resulting surface layers were studied with respect to quantum efficiency and decay, with a focus on the amount and ratio of ingredients introduced during activation. Based on these parameters, a co-deposition activation scheme has been optimized. This served as basis for the first successful proof-of-principle of an automated activation procedure for GaAs photocathodes. Using bulk-GaAs cathodes, typical quantum efficiencies between 7 and 10 % for manual and between 4 and 6 % for automated activation have been established at wavelengths of 780 to 785 nm with an anode bias voltage of 102 V. An enhancement of the photocathode performance by introduction of Li during the activation process has also been demonstrated, yielding a significant increase in surface layer lifetime by a factor of 7. Additionally, the effects of anode bias variation on both quantum efficiency and lifetime have been investigated. A change in dependency of the quantum efficiency on the anode bias voltage was observed for Li-enhanced surface layers, hinting at a connection between the height of the surface Schottky barrier and the electric potential present at the photocathode surface. At the CIS, the performance of bulk-GaAs activated with Cs and nitrogen trifluoride was investigated and compared in a test chamber, yielding quantum efficiencies between 4 and 7 % at a wavelength of 773 nm and an anode bias voltage of 282 V. Through the addition of Li, the lifetime could be extended by a factor of 2. The first operational use of a Li-enhanced photocathode in the DC photo-electron source of the UITF at a wavelength of 780 nm with an operating voltage of -144 kV, producing a beam current of 100 µA, demonstrated a superior surface layer lifetime, reproducing the increase by a factor of 2 observed in the test chamber. Additionally, an increase in quantum efficiency by a factor of 1.1 was observed.