We have examined boron penetration from heavily boron-doped p(+)-polycrystalline-Si gate into the gate dielectric/Si-sidewall interface and into bulk Si of U-shaped trench metal-oxide-Si capacitor. We have found that the inclusion of a thin nitride layer atop the thermal gate oxide inhibits boron penetration from the boron-doped p(+)-polycrystalline-Si gate into the dielectric/Si-sidewall interface and into bulk Si. The incorporation of the nitride layer on top of the thermal oxide to produce a two-layer gate dielectric, however, is observed to result in a significantly weaker dielectric, which breaks down at relatively low electric fields. This is the first time that degradation in breakdown qualities of thermal oxides in the presence of nitride layers is reported. Interface state bands are observed by deep level transient spectroscopy. These bands cover most of the Si bandgap and are of comparable concentrations in capacitors with different gate dielectric compositions (e.g., bare oxide, nitride-on-oxide or oxynitride) irrespective of the polycrystalline-Si gate doping (e.g., boron-doped p(+) or phosphorus-doped n(+)). These energy bands are argued to be independent of gate dopant atoms as well as independent of nitrogen atoms coming from the oxynitride in the gate dielectric. (C) 2003 Elsevier Science Ltd. All rights reserved.