Germanium and silicon germanium are attractive channel materials for transistors. Germanium has a smaller band gap and higher hole mobility than silicon, and varying the Ge concentration is one means of tuning the properties of SiGe alloys. Although these materials have many potential benefits, they are challenging to work with. The native oxide, GeO₂, is water soluble and therefore cannot protect the surface during aqueous processes. One option is to deposit a layer that chemically passivates the surface after cleaning to protect it from reoxidation. Sulfur could passivate the surface due to its size and valency. Since the dominant cleaning technology is wet, aqueous ammonium sulfide could be a viable option. While ammonium sulfide processes deposit sub-monolayer coverages of sulfur on germanium, the surface reactions on SiGe surfaces are vastly different.

Our study focuses on the treatment of Si-rich SiGe surfaces (Si_1-xGe_x 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 (D_it) 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 D_it (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).