Energy and spatial distribution of an electron trapping center in the MOS insulator

Abstract

The energy and spatial distributions of electrons trapped in the silicon‐dioxide layer of a metal‐oxide‐semiconductor structure have been determined. Visible–near‐uv trap photodepopulation techniques have been used measure these electron distributions through a series of reductions of the oxide layer thickness by chemical etching. Electron trapping centers which could be emptied by visible–near‐uv light were distributed uniformly throughout the oxide volume, with a volume density of order 10¹⁴ cm⁻³. These trapping centers produced a bell‐shaped distribution of localized electrons states within the SiO₂ band gap. The width of this distribution was 0.42 eV (FWHM), and the peak was located 2.4–2.5 eV below the SiO₂ conduction edge. The peak location (trap depth) increased systematically as the oxide layer was reduced in thickness. Trapped electrons which could not be released by visible–near‐uv light were determined to have an average concentration of 10¹⁵–10¹⁶ cm⁻³. These electrons had a significant excess concentration in the vicinity of the SiO₂‐Si and SiO₂‐Al interfaces (10¹⁷ cm⁻³). The midfilm concentration of these trapped electrons was approximately 2×10¹⁵ cm⁻³.