Enhanced Surface Passivation of Subnanometer Silicon Dioxide Films by Superacidic Treatments

Abstract

Subnanometer-scale silicon dioxide (SiO2) films are frequently present before, during, and after silicon device processing, yet they offer minimal surface passivation and can detrimentally impact subsequent processing steps. Here we develop a process whereby the surface passivation of nanometer and subnanometer SiO2 films is enhanced by up to 2 orders of magnitude by a simple room temperature treatment using the superacid bis(trifluoromethane)sulfonimide (TFSA, sometimes TFSI). By accurately modeling the effective lifetime curves corresponding to the superacid treated SiO2 samples, we have determined that the enhanced passivation is mainly due to a reduction in the interface defect density (Dit) at the Si/SiO2 interface, with a minor contribution also arising from the presence of negative charge. X-ray photoelectron spectroscopy of the treated SiO2 films reveals the presence of fluorine, and this, along with hydrogen, is a strong candidate for the chemical passivation of defects at the Si/SiO2 interface. Post treatment, the SiO2 films show short time scale electronic instability, whereby a degradation and then recovery are observed over a period of 1–10 h which is attributed to variations in the Dit, as determined from our analysis of the injection-dependent lifetime data. Following the instability period, the surface passivation remains relatively stable for days. Nuclear magnetic resonance measurements of superacid-based solutions reveal that electron-donating solvents should be avoided, as they exacerbate surface passivation instabilities. The results presented demonstrate that simple strategies can be used to enhance the passivation properties of ultrathin films greatly, which in the age of nanotechnology could offer benefits to device performance in a range of applications including solar cells and batteries.