A MOS capacitor is fabricated on  p-type Si (Silicon) where the metal work function is 4.1 eV and electron affinity of Si is 4.0 eV. $E_C-E_F=0.9$ eV, where $E_C$ and $E_F$ are the conduction band minimum and the Fermi energy levels of Si, respectively. Oxide $\epsilon _r=3.9$, $\epsilon _0=8.85\times 10^{-14}$ F/cm, oxide thickness $t_ox=0.1\mu m$ and electronic charge $q=1.6\times 10^{-19}$ C. If the measured flat band voltage of this capacitor is -1 V, then the magnitude of the fixed charge at the oxide-semiconductor interface, in $nC/cm^2$, is ___________.