Thermal reemission of trapped electrons in SiO2

Abstract

Electron trapping by neutral trap centers in SiO₂ was studied at 77 K and at room temperature, using n‐channel silicon‐gate IGFET structures. The electrons were injected in the dark using the forward‐bias pulsed injection method. The results show that electron trapping by the neutral centers was one to two orders of magnitude more efficient at 77 K than at room temperature; which may be compared with the previously reported electron trapping by Coulomb‐attractive centers where the capture cross sections at room temperature and at 77 K were about the same. For injected electron concentrations of less than 10¹⁶ cm⁻², more than 90% of the electron trapping at 77 K was due to shallow‐level centers where the captured electrons were thermally reemitted as the samples were warmed to room temperature. The concentrations of these shallow‐level traps in dry, wet, and HCl oxides were about the same, regardless of whether the aluminum evaporation was by electron‐beam or by rf heating in a tantalum boat. The capture cross section of these traps at 77 K was estimated to be about 10⁻¹⁵ cm² at E_ox=1×10⁶ V/cm, decreasing slowly with increases in the oxide field. Thermally stimulated reemission measurements were made by monitoring the gate voltage shifts at constant channel conductance. Analyses of the results indicated a broad energy distribution for these shallow‐level traps, with a peak at 300±50 meV below the conduction‐band edge of SiO₂ and a half‐width of about 200 meV. A small portion of the enhanced trapping at 77 K was attributable to deep‐level centers where the captured electrons were not thermally reemitted at room temperature.