Surface Cleaning of SiGe(100) and Passivation of Ge(100) with Aqeuous Ammonium Sulfide

As scaling of microelectronic devices continues, germanium and silicon-germanium are promising materials for integration into silicon-based technology. Germanium and silicon-germanium based field-effect transistors can reduce leakage current and consume less power by means of improved carrier mobility and a narrower band gap, as opposed to traditional electronics. However, silicon-germanium surfaces are more challenging to work with because the topmost surface layer is comprised of germanium oxides that complicate cleaning and passivation processes. Here we seek to obtain a smooth, oxide free SiGe surface and hope to garner an understanding of the best conditions to effectively remove surface oxides. Additionally, we hope to further our understanding of aqueous ammonium sulfide passivation processes on Ge surfaces that will prevent re-oxidation of the surface for applications towards modern field-effect transistors (FETs).

            The first portion of this work focuses on the removal of oxides from the SiGe surface with 50 and 75% Ge molar ratios using a variety of aqueous processes. These processes include SC-1, HCl, HF, and HCl/HF mixtures. The SiGe surface was characterized using X-ray photoelectron Spectroscopy (XPS) after each chemical treatment. Film thicknesses were measured using spectroscopic ellipsometry. Immersion in HCl showed similar results to as shipped samples and was ineffective in removing surface oxides. Aqueous HF, however, removed both surface oxides and carbon contamination.  SC-1 treatments selectively reduced the Ge in the SiGe film and left the surface enriched in Si, which oxidized as confirmed by XPS.  If the Ge could be completely removed from the top-most layer, the SiO₂ layer that forms could be used to passivate the SiGe surface and avoid problematic Ge oxides. Ellipsometry results showed that SC-1 solutions (NH₄OH:H₂O₂:H₂O, 1:1:100 v/v) etched 1 nm of the SiGe film, which was then oxidized. Ge 2p to Si 2p XPS peak ratios confirmed that increasing concentrations of SC-1 depleted the Ge content on the surface from 0.8 to 0.4, confirming the enrichment of the surface in Si. Ge to Si peak area ratios as a function of the SC-1 dilution factor showed a non-linear trend for SiGe 50% surfaces.

            Passivation of Ge with aqueous (NH₄)₂S was also studied. Because Ge oxides are unstable and water soluble, it is advantageous to remove them and deposit a sulfur layer to prevent further oxidation when exposed to air. Surface passivation was studied as a function of deposition time, solution concentration and the addition of H₂O₂. Passivation of the surface was ineffective at times below 20 min.  Changing the concentration of (NH₄)₂S also had no noticeable effect on the surface based on XPS. Furthermore, at high ammonium sulfide concentrations the addition of H₂O₂ did not affect the passivation process. The addition of  H₂O₂ to dilute (NH₄)₂S showed poor passivation due to oxidation of the surface by H₂O_2.