Silicon surface preparation for two-dimensional dopant characterization

Most two-dimensional (2D) dopant characterization techniques deal with near surface carrier concentration or surface potential measurements using a device cross-section. Thus, a reproducible and well-characterized surface condition is essential for accurate measurements. We report on an X-ray photoelectron spectroscopy study of surface preparation techniques commonly used in 2D dopant characterization: (i) hydrogen termination, (ii) colloidal silica polishing, (iii) low temperature silicon oxidation. Although the presented results may be applied to any other technique, in this report we concentrate on scanning capacitance microscopy (SCM). For high quality SCM a thin, uniform, and charge-free insulating film has to be formed on top of the silicon sample. Two phenomena may have a critical impact on accuracy of SCM: (i) leakage through the insulating layer and (ii) surface charging which may cause unpredictable variations in flat-band voltage. We found that samples polished by colloidal silica have about one monolayer of oxidized silicon. If the polishing is followed by baking at 200 °C in the air then 1 to 2 monolayers of silicon are oxidized and an ultra-thin insulating layer of (4–6) Å is formed. A significant leakage through the silicon oxide layer, which may alter SCM measurements, is expected in both cases. Silicon oxidation under UV irradiation in ozone ambient improves insulating property of silicon oxide film. Interface equivalent to ca. (8–15) Å of silicon dioxide is reproducibly formed by this technique. Importantly, all tested methods of silicon surface preparation are dopant independent. No noticeable surface charging has been observed for any studied method of silicon surface preparation. For silicon oxidized by UV-generated ozone the position of the ${\mathrm{Si\hspace{0.17em}2p}}_{\mathrm{3/2}}$ core-level relative to the Fermi level changes systematically with dopant type and concentration. The binding energy is higher for n-type silicon and lower for p-type doping. Such behavior is expected for silicon with a relatively low density of surface electronic states and hence unpinned Fermi level. We conclude that silicon oxidation by UV-generated ozone is a promising method of silicon surface preparation for high quality SCM.