High field time dependent charge injection in SiO2

Thin films (d ≤ 100 nm) of SiO₂ are grown by dry oxidation on n-silicon. By evaporation of gold electrodes MOS structures are prepared. Applying a positive voltage to the metal yields a distinct charge injection for fields E ≥ 6 MV/cm. After application of a step voltage the time dependent injection currents show a pronounced peak for E ≥ 8 MV/cm. We find a time to peak t_p= 0.5 s for E = 8.5 MV/cm and d = 97 nm. t_p decreases exponentially with increasing voltage, i.e. the normal SCLC theory with g = const. is not applicable. A Poole-Frenkel like field dependent mobility appears. Under the condition E = const. t_p increases with the sample thickness. Temperature dependent measurements reveal a thermally activated mobility. Using the capacitance voltage (CV) technique it turns out that after field application at first positive charges are found in the insulator. With increasing pulse length negative carriers are injected from the silicon surface. This charge injection causes states at the Si/SiO₂ interface. After the pulse the positive carriers decay logarithmically in time whereas the negative charges are trapped for longer times.