  The charge transport mechanism of electron via traps in amorphous SiO$_2$ has
been studied. Electron transport is limited by phonon-assisted tunneling
between traps. Thermal and optical trap energies $W_\mathrm{t}=1.6$ eV,
$W_\mathrm{opt}=3.2$ eV, respectively, were determined. Charge flowing leads to
oxygen vacancies generation, and the leakage current increases due to the
increase of charge trap density. Long-time annealing at high temperatures
decreased the leakage current to initial values due to oxygen vacancies
recombination with interstitial oxygen. It is found that the oxygen vacancies
act as electron traps in SiO$_2$.
