(Invited) Reaction of Aqueous Tetramethylammonium Sulfide on SiGe(100) 25% as a Function of pH

Abstract

Large improvements in the performance of CMOS (complementary metal oxide semiconductor) devices are possible with nonplanar architectures, such as finFET (fin field effect transistors) and NW-FET (nanowire field effect transistors), combined with semiconductors that have higher charge mobilities than Si. Silicon germanium (SiGe) alloys are especially useful channel materials because the carrier mobility and bandgap can be tuned by changing the percentage of Ge. Integrating SiGe into CMOS technology requires an understanding of the similarities and differences between the way Si and Ge react. Tetramethylammonium sulfide (TMAS) is an aprotic source of sulfide ions (S2-). Clean SiGe(100) 25% reacted with aqueous TMAS ([(CH3)4N]2S) forming covalent Ge-S bonds based on x-ray photoelectron spectroscopy (XPS). Lowering the pH of the solution by adding aqueous HF and HCl increased the S coverage. The highest S/elemental Ge molar ratio of 0.158 was calculated based on XPS data after treatment in TMAS at a pH of 2.0-2.5. The Ge-S bond is broken and S start to desorb from the surface at 450 K based on temperature programmed desorption spectroscopy. During TMAS solution treatment, the SiGe 25% surface was oxidized, as shown by the increased O/Si molar ratio calculated from XPS data compared to a control surface. Reducing the pH of the aqueous TMAS solution reduces the amount of oxide that forms. The surface oxidation is likely due to the formation of oxidizing species in solution such as thiosulfate (S2O3 2-). The comproportionation between thiosulfate and sulfide at lower pH can explain the reason for less surface oxidation when the surface is treated with the low pH TMAS solution.