Impact of Wet Treatments on the Electrical Performance of Ge_0.9Sn_0.1-Based p-MOS Capacitors

We have investigated the impact of different wet treatments on the electrical performances of germanium–tin (GeSn)-based p-MOS capacitors with 10% Sn. Atomic force microscopy (AFM) showed the presence of Sn droplets for the degreased Ge0.9Sn0.1 surface, which were removed by HCl, HF, and HF:HCl treatments. On the other hand, (NH4)2S and NH4OH treatments were not fully able to remove these droplets. X-ray photoelectron spectroscopy (XPS) measurements confirmed AFM results and highlighted the efficiency of HF, HCl, and HF:HCl treatments in removing Ge and Sn native oxide, which was not the case with (NH4)2S and NH4OH. Nevertheless, XPS showed a reoxidation of the Ge0.9Sn0.1 surfaces a few minutes only after HF, HCl, and HF:HCl wet treatments. Therefore, another approach was tested. It consisted in using (NH4)2S to protect Ge0.9Sn0.1 surfaces from immediate reoxidation by creating a Ge0.9Sn0.1-S monolayer. Chemical depth profiles of Ge0.9Sn0.1/Al2O3 stacks were investigated using parallel angle resolved XPS (pAR-XPS), indicating a high quality interface when the Ge0.9Sn0.1 surface is cleaned previously by HF and then (NH4)2S. There was notably a lack of Sn or Ge diffusion into the Al2O3 layer. C–V characteristics combined with a custom-analytical model yielded a low interface trap density (Dit).