Our study focuses on the treatment of Si-rich SiGe surfaces (Si1-xGex x=0.25) with aqueous (NH4)2S. We varied the concentration of (NH4)2S in solution with and without added acid and characterized the surfaces using X-ray Photoelectron Spectroscopy (XPS). Immersions in (NH4)2S alone oxidized the surface and did not deposit a sulfide layer. Instead the surface oxidized as a function of the (NH4)2S concentration. In order to remove the oxide and slow down or stop the oxidation, small volumes of HF and HCl were added to the mixture. Addition of halogen acids dropped the pH of the solution from 10 to 8 and sulfides were detected with XPS in the S 2p region. Sulfur coverage increased with increasing concentrations of HF and HCl, however oxides increased as well. The simultaneous deposition of sulfur and oxygen suggests that oxidized sulfur, such as sulfates and thiosulfates, deposited on the surface.
In addition to chemical characterization of the surface, metal-insulator-semiconductor capacitors (MISCAPs) were fabricated after three different surface treatments: 1) clean and dry (control), 2) cleaning and immersion in (NH4)2S and 3) cleaning and immersion in (NH4)2S with added acid. Capacitance-voltage measurements were obtained for all three surface treatments. The density of interface defects (Dit) was calculated for each process from conductance and capacitance data. Processes that yielded the highest sulfur coverage (ammonium sulfide with added acid) showed a reduction in Dit (1.4 x 1012 cm-2 eV-1) compared to samples that were cleaned and dried only (6.4 x 1012 cm-2 eV-1) and samples treated with aqueous ammonium sulfide (4.4 x 1012 cm-2 eV-1).