Undergraduate Research at Jefferson Lab

Temperature Effect on Bialkali-Antimonide Photocathode Grown on Niobium

Student: Edgar Abarca-Morales

School: National Autonomous University of Mexico

Mentored By: Matthew Poelker

Alkali photocathodes have been widely used in many opto-electronic devices since the remarkable discovery of the highly efficient cesium-antimony photocathode by Görlich in 1936. The development of multi-alkali antimony photocathodes is integral to the accelerator community due to their applications in high brightness photoinjectors of free electron lasers and LINAC's. Moreover, there is a special interest in the fabrication of this kind of photocathode using superconducting substrates (such as niobium) for use in superconducting radio frequency (SRF) electron guns. In this study, bialkali-antimonide photocathode was fabricated on Nb and Ta substrates and evaluated for QE as a function of temperature. Deposition of antimony and co-deposition of alkalis was made over the substrate with controlled temperature, time and partial pressure, while monitoring the photocurrent induced by a 532nm laser targeting the wafer. The alkali co-deposition was terminated when reaching the maximum photocurrent and the 1/e lifetime was measured. QE was evaluated for different substrate temperatures. A maximum QE of 6% was achieved (300K, 532 nm) with a corresponding 1/e lifetime of 9 days. It was found that QE slightly increases at 343K and drops considerably (without becoming null) at 77K. The bandgap energy of semiconductors (like CsK₂Sb) is expected to vary as a function of temperature. When a semiconductor is heated (cooled), the bandgap energy is expected to decrease (increase), and correspondingly QE is expected to increase (decrease). Our results show QE dependence on temperature consistent with a changing band gap energy, but also suggest a QE reduction resulting from unwanted chemical processes that serve to "dirty" the photocathode surface. These first measurements are encouraging, illustrating the potential benefit of using the bialkali-antimonide photocathode on a niobium substrate inside an SRF gun. However, further research in the Nb-Sb system should be made due to the fact that at certain temperatures Sb-alloys may be formed.