Positive charge generation in SiO2 by electron-impact emission of trapped electrons

Abstract

Experiments are presented that firmly establish a common defect center for both positive and negative charges generated in thermally grown thin silicon oxide films and indicate the bridging oxygen vacancy as its probable origin (which is the well-established paramagnetic E′ center in crystalline and fused quartz and optical fibers). The positive oxide charge arises when one of the bound electrons at an initially neutral bridging oxygen vacancy, [(Si-O)3≡Si⋅⋅Si≡(O-Si)3], is emitted (released) by electron impact, VO0+e−*(kinetic energy≥7 eV)→VO++2e−. The measured electron impact threshold energy is 7 eV. An equal amount of negative oxide charge is generated at the same location (centroid about 9 nm from the SiO2/Si interface) by electron capture into the initially neutral oxygen vacancy, VO0+e−→VO−, whose peak density of charged state is located at 1 eV below the SiO2 conduction-band edge. The probable oxygen vacancy origin is deduced from an increase in the density of the shallow trap [VO−] and a higher positive charge generation rate at the deeper trap, d[VO+]/dt, in samples annealed in oxygen-deficient ambient (nitrogen and silicon gate) at 1000 °C, and from comparing the measured energies, (Ec−1 eV and Ec−7 eV), with the bound-state energies calculated by Fowler, Robinson, and associates. This origin is further supported by the oxide-electric-field dependence of the electron-impact emission coefficient of trapped electron at VO+, αnn=3500 cm−1 exp(−33 MV cm−1/EOX), and the electron capture cross section at a Coulombic VO+, σn∝EOX−n (n=2–4 and EOX=0.6–7.0 MV/cm); and by the steady-state balance between impact emission and thermal capture, which limits the buildup of positive oxide charge during electron injection into the oxide.