Electron trapping in SiO₂ due to electron-beam deposition of aluminum

Electron trapping in the SiO2 layer of n-channel polycrystalline silicon-SiO2-silicon field-effect transistors with electron-beam-evaporated aluminum was studied. The increased electron trapping was attributed to the x rays generated when the electron beam impinged on the aluminum target. Traps with low-field capture cross sections greater than 10-13 cm2 are associated with the x-ray-induced positively charged centers, while traps with low-field capture cross sections of about 1×10-15 cm2 are associated with the x-ray-induced neutral centers. For the silicon-gate devices, both traps could be effectively reduced by annealing in dry forming gas at 550 °C for 20 min. As reported earlier, the capture cross section of the positively charged traps has a strong field dependence of approximately E-3ox and is approximately independent of temperature. The field dependence of the capture cross section of the neutral traps is much weaker, with roughly a σ =σ0 exp(-bEox) dependence, where σ0=1.6×10-15 cm2 and b=7.35×10-7 cm/V. A possible origin of these neutral traps is displaced bonds forming polarization potential wells.