Robust heterostructured photocathodes based on InGaN for bright electron beam sources

Ratnakar Palai, University of Puerto Rico (Principal Investigator)

Luca Cultrera, Brookhaven National Laboratory (Co-Investigator)

Abstract:

Photocathodes are very important in accelerator physics as they generate high brightness and spin-polarized electron beams that allow scientists to unravel mysteries about fundamental particle interactions and to produce intense X-ray beams (synchrotron radiation) for material characterization. Photocathodes can be also used to investigate the dynamics of ultrafast physical processes in material science by producing electrons used to generate X-ray beams in free electron lasers (FELs) or by directly using them in ultra-fast electron diffraction and microscopy. During the last few decades, photoinjectors have been demonstrating unparalleled performance for the generation of high-brightness beams and the number of applications enabled by this technology is continuously expanding. Advancement in photocathode research played a pivotal role in the last decade enabling photoelectron sources to operate close to the limits imposed by the photoemission process. Photocathode sources are one of the key components of the facilities that provide electrons in particle accelerators and many other research experiments. Development and application of new photocathode materials are central to the enduring success and continuing promise of accelerator technology. There is a great demand for robust photocathodes that can improve the beam brightness at the electron source, support electron bunch production at the highest repetition rates, and produce high-brightness spin-polarized electrons. Typically, a high efficiency, a long operational time, low thermal emittance, and fast response time are special requirements for accelerator applications. We also need the photocathode to be robust against any residual gases present during the operations. Further improvement on electron beam brightness could be possible from additional advancement in the design of new electron guns capable of supporting higher accelerating gradients and/or from superior photocathode materials that can produce electron beams with increased efficiency, lower emittances, and longer operation lifetime compatible with moderate vacuum requirements. Solving these issues required exploring new photocathode materials and approaches beyond the current paradigm of photocathode materials. Different semiconductor materials, such as CsTe, K2CsSb, and GaAs have been explored to as suitable photocathode sources. GaAs is the only photocathode materials at the moment which is able to produce highly spin-polarized electrons. However, the complex activation with alternating fluxes of Cs and O₂ and the very strict requirement of stable extreme-high vacuum (XHV) conditions are required to ensure a long lifetime. On the other hand, III-nitride (GaN and InGaN) photocathodes show enormous potential as a future novel electron source for particle accelerators due to the observation of high quantum efficiency, fast response time, and robustness (no degradation under the residual gases during the operation) and spin-polarized electron emission. The importance of developing new materials and engineered structures that can be used to produce electron beams with superior properties has been stressed by the scientific community in DoE’s recent reports on “Report of the Basic Energy Sciences Workshop on the Future of Electron Sources”.    

III-nitrides have very sharp optical emission in a wide wavelength range, from ultraviolet (UV) through visible to infrared (IR). InGaN alloy will allow the excitation of the photocathodes with visible light, which has a significant advantage compared to the photocathodes that require a high-power ultra-violet source. The main objective of this project is to study InGaN and InGaN:RE thin films with Cs or Cs₂Te activating heterostructures to enhance photoemission. The proposed project will also provide a comprehensive understanding of the photoemission processes in III-nitrides and RE-doped III-nitrides (InGaN:RE) semiconductors. The project will be focused on understanding the effect of RE-doping on the growth mechanism, spin polarization efficiency, and photoemission properties of III-nitrides. The thin films of III-nitrides will be grown at the PI’s lab at the University of Puerto Rico using the state-of-the-art molecular beam epitaxy (MBE). The fabrication of heterostructure with Cs or Cs₂Te activation will be carried out at the Co-PI’s Lab at Brookhaven National Laboratory (BNL) for photoemission and spin polarization studies.

A robust photocathode with high quantum efficiency, long lifetime, and fast response time will offer a high-brightness electron source for particle accelerators, which will have a profound impact on accelerator physics. The project aims to foster the participation of underrepresented Hispanic US citizens and women in science education by involving UPR students. The project will involve one graduate student and will provide training in thin film deposition and photocathode fabrication and characterization.  The PhD student will work with BNL scientists, which will provide the student with a highly interactive and innovative research and educational experience. The outcomes of the project will be disseminated by delivering public seminars, presenting at international conferences, publishing in scientific journals, and submitting results for PhD or MS degrees at UPR-RP.