Electronic Properties of Cesium Oxide Thin Films

In the recent years research on surface production H^- ion has been strongly motivated by the need of intense neutral H beams in controlled-fusion and Strategic Defense Initiative applications. Low work function surfaces are of great importance for high-efficiency H^- productions. Cesium-oxide thin films are one of the most important low-work-function materials. The electronic properties of cesium-oxygen-composite thin films deposited on metal (molybdenum) and semiconductor (silicon) substrates are investigated. The surface is characterized using Auger electron spectroscopy (AES), ultraviolet photoelectron spectroscopy (UPS), work function probe and thermal desorption spectroscopy (TDS). Experiments are performed in an ultra -high vacuum (UHV) surface analysis system. Cs/O thin film deposition on Si(100) is accomplished by depositing a monolayer of cesium (using a 20 eV low energy cesium ion gun) and followed by exposure to O _2 gas, which results in a minimum work function of 0.9 eV. Various cesium oxide species are formed during the oxidation of cesiated Si(100), and the minimum work function is due to the formation of a special Cs-O-Si(100) surface structure that is related to the reconstructed Si(100) and Cs_2O_2 . We also studied a 0.5 μm coating on a molybdenum substrate produced by thermal decomposition of Cs_2CO_3 at 920 K. Work function of 0.9 eV is achieved with a sample temperature of 470 K, due to a significant increase of the bulk conductivity. This low work function is maintained for many hours under UHV condition and the sample is completely stable in an atomic hydrogen environment.