Behavior of ultrathin Al2O3 films in very high electric fields: Scanning tunneling microscope-induced void formation and dielectric breakdown

We report on the use of high electric fields applied via scanning tunneling microscopy (STM) under ultrahigh vacuum conditions to induce voids and die lectric breakdown in ultrathin gamma'-Al2O3 films (similar to7 Angstrom thi ck) grown on Ni3Al (111) substrates. Voltage pulses (bias voltages of 0.1-6 V, either polarity) are applied to a specific location on the oxide surfac e with the feedback current loop operative to prevent tip/sample physical c ontact. Subsequent STM constant current imaging reveals the effects of high field on surface composition and topography. The results show that dielect ric breakdown occurs at a field of 12.3 +/-1.0 MV/cm. Breakdown is marked b y the creation of features with apparent size of 8-25 nm high and 100-250 n m wide in the constant current STM images, and by the loss of the insulatin g nature of the oxide as deduced from current/voltage spectroscopy. Constan t height scanning indicates that these features are caused by electronic ch anges in the oxide and not by mass transport. At lower electric fields, sma ll voids that are 0.2-0.8 nm deep can be created. STM images taken at a 2 v s 0. 1 V gap voltage show that these voids are located at the oxide/metal i nterface, where they grow into the metal. Repeated application of a lower f ield within the void region leads to dielectric breakdown, even though the same field does not induce breakdown in regions away from the void. The res ults provide direct evidence of the ability of localized defects to decreas e the barrier to dielectric breakdown in an ordered oxide film. (C) 2001 Am erican Vacuum Society.