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The Center for Bright Beams, A National Science Foundation Science and Technology Center

Reduction of surface roughness emittance of Cs3Sb photocathodes grown via codeposition on single crystal substrates

Alice Galdi,  Jan Balajka,  William J. I. DeBenedetti,  Luca Cultrera, Ivan V. Bazarov,  Melissa A. Hines, and Jared M. Maxson

Imag of sample growth
(a1) 500 nm ×500 nm and (a2) 1𝜇m ×1𝜇m STM images of the sample grown on Al2O3; (b1) and (b2) for the sample grown on TiO2(001); (c1) and (c2) for a sample of the same series grown on SiC(100); (d1) and (d2) for a second growth on SiC(100), transferred in a different experimental run. All images are plotted with the same color scale. Typical scan parameters are Vsample = 0.85–1 V and I = 60–200 pA. (a3), (b3), (c3), and (d3) Apparent height histograms corresponding to the STM images above.

In this paper we fabricated thin films of the high efficiency photocathode material Cs3Sb on different single-crystalline substrates. These samples are extremely air sensitive and have to be kept in very low residual gas pressure to maintain their properties. We transferred our samples using a special vacuum suitcase to a scanning tunneling microscope, always in very strict vacuum conditions, capable of measuring the surface of conducting samples with atomic resolution, to obtain a map of the surface of our samples and study how the choice of the crystalline material could change the morphology of the film. Using the maps of the film surface we could estimate how the asperities on the photocathode surface would increase the momentum spread of the photoemitted electrons in a high electric field, thus reducing the brightness of electron beams. We discovered that large island size and reduced roughness in samples deposited on SiC substrates reduce this spurious effect by a factor of 17.

The CBB Beam Production objective is to develop photocathodes for high peak-intensity beam generation with < 5 meV MTE. Our results show that lower effective MTEs (momentum spread of the electrons) can be achieved thanks to the choice of single crystalline substrates that promote the growth of smoother photocathodes with large islands.

 

High peak-intensity beams are crucial for single-shot ultrafast electron microscopy, capable of capturing the structure of large molecules evolving on very small time-scale with a single electron pulse, and to achieve more efficient free-electron lasers that can emit high intensity radiation in the x-ray range.

 

Cs3Sb and related compounds have been used for a long time without a real study of the growth mechanisms and how they affect the nanostructure of the photocathode materials. This study brings us closer to synthesizing these technologically important compounds in ordered form. This would allow better performances, more controlled properties and the opportunity to achieve a better theoretical understanding of the photoemission process. 

Reference:

A. Galdi et al., “Reduction of surface roughness emittance of Cs3Sb photocathodes grown via codeposition on single crystal substrates,” Appl. Phys. Lett., vol. 118, no. 24, p. 244101, Jun. 2021, doi: 10.1063/5.0053186. [Online]. Available: https://aip.scitation.org/doi/10.1063/5.0053186