Radiation Induced Hole Transport and Electron Tunnel Injection in SiO2 Films

The transport and trapping of holes generated by strongly absorbed UV light has been observed in SiO2. Hole photocurrents decay with time under small applied fields because the accumulation of trapped holes in the light-absorbing region counteracts the applied field. However, with increasing applied fields a steady-state hole photocurrent has been observed for the first time. The quantum efficiency for hole collection saturates at 100% as the applied field is increased. A new mechanism for radiation-induced currents has been observed: The trapping of holes near the Si-SiO2 interface enhances Fowler-Nordheim tunneling, producing electron-injection currents nearly two orders of magnitude larger than the photo-current. Upon cessation of the radiation these field-induced currents decay slowly with time as trapped holes are gradually annihilated, giving the appearance of very slow carriers being swept out of the oxide. Hole photocurrents increase with field, saturating at about 3 MV/cm. This is interpreted as a decrease in electron-hole pair recombination with increasing field. A qualitative model involving hole transport and trapping near both interfaces satisfactorily explains the experimental results. The trapping and injection phenomena are shown to be extremely sensitive to high-temperature processing parameters and, therefore, provide sensitive tools for investigation and optimization of radiation hardness.