Modification of Trap Distributions in Anodic Aluminum Tunnel Barriers

The trap distribution in thin (d=4.3 nm) aluminum oxide films was modified electrochemically and subsequently quantified in a solid-state device. A combined potentiodynamic/potentiostatic experiment enabled preparation of samples showing different trap distributions with negligible thickness variation. Potentiodynamic oxide formation at a scan rate of 100 mV s-1 was used to form the anodic oxide with many defects. Subsequent potentiostatic polarization allowed a systematic reduction of the number of defects by electromigrative annealing. These oxides were used as a dielectric barrier in a metal-insulator-metal multilayer system with a silver top electrode. The current response of these samples to voltage pulses was investigated over a wide range in current (100 nA-1 A) and time (1 µs-100 s). A separation of Debye charging, dielectric relaxation, and steady-state tunnel current was possible in the time domain. The dielectric relaxation showed a very strong dependence on the previous electromigrative annealing time being approximately 85 times higher for a short annealing time of 0.2 s as compared to the longest annealing time of 100 s. The steady-state tunnel current displayed a much weaker dependence of only a factor of 1.6 and changed mainly during the initial annealing step from 0.2 s to 1 s. In this way it is possible to prepare anodic oxide films of constant film thickness with variable trap site densities. These films have identical tunneling properties but display extremely different relaxation behaviors.