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 Eox=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 SiO 2 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.