Leakage currents in Al2O3/HfO2 multilayer high-k stacks and their modification by post-deposition annealing steps

Leakage currents of Al2O3/HfO2 multilayer stacks deposited by atomic layer deposition in the low voltage region (0 ÷ ±4V) are investigated in dependence on Al2O3 sublayers thickness and the post-deposition annealing in two ambients (O2, N2,). The stacks with 5cy of Al2O3 show highest current, compared to structures with lower (2cy) and higher (10cy) Al2O3 content. Oxygen treatment does not affect significantly the leakage current at low voltages, while annealing in nitrogen definitely increases it. The current follows power law dependence, Vn, with n varying in the interval of about 0.6 to ∼1. The application of ±12V pulses shift the leakage curves towards higher or lower current values depending on the polarity of the pulse. Leakage curves after the pulse stress are described either with Vn or (V-Vt)n relations depending on both pulse and measurement bias polarity. The results are found consistent with a model similar to the collective transport in array of small metal dots model in which the current is limited by the available sites for charge carriers to move on.