Electrical characteristics of multilayered HfO₂-Al₂O₃ charge trapping stacks deposited by ALD

Electrical and charge trapping properties of atomic layer deposited HfO₂-Al₂O₃ multilayer stacks with two different Al₂O₃ sublayer thicknesses were investigated regarding their implementation in charge trapping non-volatile memories. The effect of post deposition annealing in oxygen at 600°C is also studied. The decreasing Al₂O₃ thickness increases the stack's dielectric constant and the density of the initial positive oxide charge. The initial oxide charge increases after annealing to ∼6×10¹² cm^-2 and changes its sign to negative for the stacks with thicker Al₂O₃. The annealing enhances the dielectric constant of the stacks and reduces their thickness preserving the amorphous status. Nevertheless the annealing is not beneficial for the stacks with thicker Al₂O₃ as it considerably increases leakage currents. Conduction mechanisms in stacks were considered in terms of hopping conduction at low electric fields, and Fowler- Nordheim tunnelling, Schottky emission and Poole-Frenkel effect at higher ones. Maximum memory windows of about 12 and 16V were obtained for the as-grown structures with higher and lower Al₂O₃ content, respectively. In latter case additional improvement (the memory window increase up to 23V) is achieved by the annealing.