Translartion. Region: Russians Fedetion –
Source: Novosibirsk State University – Novosibirsk State University –
2nd year Master’s student Faculty of Physics, Novosibirsk State University Nadezhda Solovyova studies the conditions for the formation of monocrystalline films of multi-alkali compounds of antimony and bismuth. These materials are used as photocathodes – effective sources of free electrons and important elements of various photoelectron converters, allowing the registration of weak light signals, down to single photons.
Photocathodes are devices that operate on the principle of external photoelectric effect. They allow to obtain a beam of electrons with the necessary characteristics for various applications: in electron microscopes, for accelerators, colliders, night vision systems, etc.
GaAs, GaN, and various alkali metal composites combined with antimony (Sb) are used as photocathode materials. Today, photocathode materials have a natural surface disorder, which affects their quantum efficiency, electron beam brightness, and other characteristics. Multialkali compounds (Cs3Sb, Na2KSb, K2CsSb) have been used as electron sources since the 1930s. In 2022, the Rzhanov Institute of Semiconductor Physics SB RAS discovered the possibility of emitting spin-polarized electrons from a multialkali Na2KSb/Cs3Sb photocathode. This discovery sparked a surge of interest in this and other multialkali materials. And at the moment, one of the important issues is the possibility of obtaining crystalline ordered films of these compounds. The answer to this question requires the development of epitaxial growth technology for multialkali materials.
— Despite the fact that multi-alkali materials have long been used to manufacture photocathodes, single-crystal films of multi-alkali compounds have not been obtained before, so it was not possible to study their electronic structure using experimental methods; only calculations existed. Currently, we have a method that will allow us to study the electronic structure, and now our task is to obtain crystalline ordered compounds. They are needed to control the characteristics of photocathode materials. For example, the growth of photocathode materials on a crystalline ordered substrate can allow us to change the ratios of lattice constants and temperature expansion coefficients, which can increase the spin polarization of photosimulated electrons, as well as affect other characteristics. For example, the diffusion length can change or the quantum efficiency of photocathode materials can increase. And, what is important, we will have the opportunity to control their properties during the growth process, — explained Nadezhda Solovyova.
At present, a method for growing epitaxial Cs3Sb films on the 3C-SiC(001) surface has been proposed in the literature. Experiments on epitaxial growth of multi-alkali antimony compounds (Na2KSb, K2CsSb, etc.) on various substrates have not been described in the literature.
The study of such materials should improve the characteristics of photocathodes, but in addition to the practical task, this study also sets a fundamental one: obtaining crystalline ordered films in order to experimentally study their electronic structure.
Nadezhda Solova’s project “Study of the conditions for the formation of single-crystal films of multi-alkali compounds of Sb and Bi” was among the winners of the youth competition of scientific research projects “X-ray, synchrotron, neutron methods of interdisciplinary research” last year.
As part of the project, the young researcher is tasked with obtaining such films and determining how replacing antimony with bismuth will affect their properties. It is assumed that the use of single-crystal films of solid solutions of Cs3BixSb1-x will shift the working range of photocathodes to the infrared region and affect the spin polarization of photoemitted electrons (?). At the moment, Nadezhda Solovyova, under the supervision of Vladimir Golyashov, PhD, research fellow at the Institute of Physical Problems of the Siberian Branch of the Russian Academy of Sciences, has obtained quite interesting results: for the first time, it was possible to obtain single-crystal films of Na2KSb.
— We were able not only to obtain crystalline ordered films, but also to obtain the first experimental measurement of the electronic structure using angle-resolved photoelectron spectroscopy. We did not find any publications about such work in scientific journals. Now we have extensive work ahead of us to decipher the obtained structure, perform the necessary calculations and publish our results, but the first steps have already been taken, and they have proven to be quite productive, — said Nadezhda Solovyova.
The films were grown using two methods. First, in collaboration with the Novosibirsk enterprise ZAO Ekran-FEP, the scientists followed the method used in the production of electron-optical converters and obtained the first crystalline-ordered films. However, they were not satisfied with the fact that under such conditions the growth of photocathodes was difficult to control due to the excess pressure of alkali metals.
Another growth technique was fully implemented in the laboratory of the A.V. Rzhanov Institute of Semiconductor Physics SB RAS, and was closer to molecular beam epitaxy:
— Our laboratory has conditions that allow us to create molecular sources from which film growth will be determined by the ratio of several flows, and there will be no excess of a certain chemical element and the stoichiometric composition of the film will be determined by the ratio of these flows. Having tested both methods, we came to the conclusion that new sources for the growth of such materials need to be created. The laboratory setup is currently being improved, — said Nadezhda Solovyova.
All compounds studied by Nadezhda Solova are extremely unstable in the atmosphere, so their study requires the development of in-situ ultrahigh-vacuum photoelectron spectroscopy methods. Preparation of atomically clean substrate surfaces, synthesis and epitaxial growth of (Na,K,Cs)3(Sb,Bi), as well as their subsequent analysis are carried out directly in the ultrahigh-vacuum chambers of the SPECS Proven-X ARPES photoelectron spectroscopy facility at the ISP SB RAS. It implements such methods for analyzing the surface of solids as angle-resolved photoelectron spectroscopy (ARPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XRD), and low-energy electron diffraction (LEED). LEED allows one to quickly determine whether the structure of the grown films is crystalline, polycrystalline, or amorphous. XPS is used to study the stoichiometric composition of films, their thickness and the chemical state of atoms on the surface.
— The electronic structure of the grown monocrystalline films is studied using angle-resolved photoelectron spectroscopy (ARPS). However, XPS, ARPES and RFD can be fully realized only when using monochromatic ultraviolet and soft X-ray radiation sources based on synchrotrons. Therefore, if our project shows high potential for further research, we will continue working at the SKIF synchrotron under construction in Novosibirsk. One of the suitable stations where similar measurements can be implemented will be station 1-6-2 “Electronic Structure”, — explained Nadezhda Solovyova.
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