Investigation of Lithiated Tin (Sn/Li Alloy) Thin Film as a Low Work Function Termination on Silicon/Diamond Substrates

Abstract

Abstract Body: Low work function (WF) materials are necessary for energy storage and conversion devices. Scientific literature is full of explorations made in this field with new and emerging technologies applied to different elements with the purpose of creating low work function materials in the bulk and low dimensional regime. Alkali metals and their alloys with tin, cobalt, indium, etc. have been researched very well in batteries and other energy devices. Alkali metals have been shown to effectively aid in reducing the WF of many metals including Pb, Indium, tin, etc. Particular focus has been laid on Sn/Li alloy system where a work function dip up to 1.25 eV has been shown for a stoichiometric ratio of Sn2Li5[1][2]. Of course, this has been demonstrated to occur in the bulk alloys. However, with the purpose of achieving a similar goal in low dimensions (thin films), we are demonstrating for the first time the formation of a thin film of low work function Sn/Li alloy, formed using physical vapour deposition technique (E-Beam and Thermal Evaporation) at Ultra High Vacuum (UHV) conditions. The experiment utilises the high-end NanoESCA facility at the University of Bristol, UK, where the in situ deposition and the characterisation using X – Ray photoemission spectroscopy (XPS), Ultra-Voilet photoemission spectroscopy (UPS), spot profile low energy electron diffraction (Spa-LEED) techniques is carried out.[3]. In the initial experiments, a single crystal Si substrate was heated at a temperature of 800 oC for the removal of contaminants and Sn was deposited using the E beam deposition while Li was deposited using thermal evaporation method on Si substrate. Si surface was found to be terminated with the oxygen atoms. This should be the reason for the further lowering of WF as oxygen has been found to improve the sticking coefficient between the terminating metal species and the substrate surface. In the beginning, the O terminated Si surface was seen to possess a WF of 4.65 eV and a characteristic 2×1 LEED surface pattern which was then lost due to the disordered Sn and Li layers on the surface. However, Sn 3d peak was seen to shift towards the lower binding energy (B.E.) characteristic of the formation of Sn/Li alloy[4]. The WF was seen to lower to 4.15 eV and the surface was seen to become ordered after going through a series of annealing steps i.e., at 200 oC and 300 oC for 1 hour each and then 350 oC for 2 hours. After the 350 oC anneal, the LEED pattern started emerging. However, the Sn 3d peak was shifted to higher B.E. again which demands further analysis in order to find the optimised stoichiometric ratio between S/Li for ultra-low work function demonstration. [1] K. P. Loh, X. N. Xie, S. W. Yang, J. S. Pan, and P. Wu, “A spectroscopic study of the negative electron affinity of cesium oxide-coated diamond (111) and theoretical calculation of the surface density-of-states on oxygenated diamond (111),” Diam. Relat. Mater., vol. 11, no. 7, pp. 1379–1384, Jul. 2002, doi: 10.1016/S0925-9635(02)00014-6. [2] B. B. Alchagirov, R. K. Arkhestov, and F. F. Dyshekova, “Electron work function in alloys with alkali metals,” Tech. Phys., vol. 57, no. 11, pp. 1541–1546, 2012, doi: 10.1134/S1063784212110023. [3] G. Wan, M. Cattelan, and N. A. Fox, “Electronic structure tunability of diamonds by surface functionalization,” J. Phys. Chem. C, 2019, doi: 10.1021/acs.jpcc.8b11232. [4] D.-T. Shieh, J. Yin, K. Yamamoto, M. Wada, S. Tanase, and T. Sakai, “Surface Characterization on Lithium Insertion/Deinsertion Process for Sputter-Deposited AgSn Thin-Film Electrodes by XPS,” J. Electrochem. Soc., vol. 153, no. 1, p. A106, Dec. 2006, doi: 10.1149/1.2133711.