Charge injection and trapping in Al2O3 gate insulators

Previous efforts to effectively investigate and model the high-field charging phenomenon in pyrolytic Al2O3 have been thwarted by the difficulty of separating the injection and trapping phenomena. Characterizing the injection is extremely difficult because the interface electric field varies as charge accumulates in traps, and the varying field produces rapidly varying currents, making it equally difficult to ascertain trapping behavior. In this work high-field electron injection and photoinjection into Al2O3 are investigated using new experimental techniques which maintain constant the interface fields and injection currents, thereby permitting independent determination of injection and trapping characteristics. It is shown that the electron trapping in Al2O3 MIS structures is strongly localized near the Si-Al2O3 and metal-Al2O3 interface regions. Comparisons of electron injection from the metal and from silicon show that the hypothetical native SiO2 layer at the Si-Al2O3 interface either is nonexistent or has no appreciable effect on injection properties. Studies of field and temperature dependence provide evidence that the electron injection occurs by trap-assisted tunneling.