Epitaxial Na KSb(111) Growth Reveals Dispersive Surface States Via ARPES

Researchers are striving to unlock the full potential of highly efficient spin-polarized electron emitters, and a new study sheds light on the electronic structure of the promising Na KSb photocathode. N. Yu. Solovova (Rzhanov Institute of Semiconductor Physics), V. A. Golyashov, and S. V. Eremeev, alongside S. Yu. Priobrazhenskii (Ioffe Institute) et al., have achieved the first crystalline epitaxial growth of Na KSb films, a crucial step towards understanding and optimising these materials. By utilising angle-resolved photoemission spectroscopy, the team directly investigated the electronic structure, revealing dispersive surface states and demonstrating preserved crystalline order after activation , findings that pave the way for rationally designing improved multialkali photocathodes and advancing spintronic technologies.

This breakthrough, detailed in a recent publication, addresses a long-standing challenge in materials science, the difficulty of growing ordered, crystalline structures of multi-alkali antimonides. The research team employed chemical vapor deposition (CVD) on a graphene-coated silicon carbide (SiC(0001)) substrate to cultivate these films, enabling a detailed investigation of their electronic structure. These findings provide fundamental insights into the electronic behaviour of this material, clarifying the origins of its unique properties and paving the way for targeted improvements. This preservation of crystalline order during activation is particularly significant, as it allows for rational design and optimization of photocathode properties. The ability to control the electronic band structure through epitaxial growth opens exciting possibilities for engineering spin polarization, potentially surpassing the performance of existing materials like gallium arsenide (GaAs(Cs,O)). This achievement is not merely incremental; it represents a fundamental shift in our ability to fabricate and understand these crucial materials.

Experiments demonstrate that the developed epitaxial technique is a viable route towards creating high-brightness photoinjectors, where the conservation of transverse momentum from an ordered surface is essential for generating electron beams with exceptionally low mean transverse energy. The research team achieved this breakthrough using chemical vapor deposition (CVD) on a graphene-coated silicon carbide (SiC(0001)) substrate, enabling direct investigation of the material’s electronic structure via angle-resolved photoemission spectroscopy (ARPES). This innovative approach overcomes limitations of previous studies reliant on first-principles calculations and indirect spectroscopic methods lacking momentum resolution. Alkali metals, cesium, sodium, and potassium, were evaporated from SAES-type dispensers, then re-evaporated onto the sample surface to initiate the chemical reaction, while antimony was directly deposited from an effusion cell. High-purity, semi-insulating 6H-SiC wafers with a (0001) ±0.25° Si-face orientation were utilized, undergoing chemical-mechanical polishing (CMP) to ensure a pristine surface for epitaxial graphene growth. To monitor film growth, the study harnessed photocurrent measurements using 515nm and 650nm lasers, providing real-time feedback on the evaporation rates of sodium, potassium, cesium, and antimony. ARPES and X-ray photoelectron spectroscopy (XPS) were then performed using a SPECS GmbH ProvenX-ARPES system, employing He Iα light (hν = 21.22 eV) for ARPES and focused monochromatic AlKα radiation (hν = 1486. The research team employed chemical vapor deposition (CVD) on a graphene-coated silicon carbide (SiC(0001)) substrate to cultivate these high-quality films, paving the way for detailed electronic structure investigations. This breakthrough directly addresses a long-standing gap in understanding the electronic properties of alkali antimonides, materials crucial for efficient spin-polarized electron emission. These calculations and spectroscopic data corroborate the existence of unique electronic behaviours at the material’s surface, offering insights into its photoemission characteristics. ARPES data clearly demonstrates the presence of distinct surface states, providing a detailed map of the material’s electronic band structure and its potential for spin-polarized electron emission. This achievement represents the first epitaxial growth of this material, enabling detailed investigation of its electronic structure using angle-resolved photoemission spectroscopy (ARPES). Furthermore, the study demonstrates that the crystalline order of the films is maintained even after activation with cesium and antimony, crucial steps in creating efficient photocathodes.
This preservation of order is significant as it allows for further investigation into the material’s properties and potential for improved performance in applications like electron sources for accelerators. The authors acknowledge limitations in the stoichiometric control of the films, noting variations in the sodium, potassium, and antimony ratios across samples. Future research will focus on spin-resolved ARPES measurements to explore the spin texture of electronic states and its impact on spin polarization, potentially leading to the rational design of more effective photocathodes.