The atomic layer deposition of ultrahigh-k oxides




Le, Thanh Tung (Ph. D. in chemical engineering)

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Various high-k oxides have been studied and employed in memory devices. As devices scale towards the sub-20 nm and even 10 nm scale, however, ultrahigh-k oxides are needed to meet the required electrical thresholds in the capacitors of devices such as NAND and especially DRAM. Perovskite oxides, such as BaTiO₃ (BTO) and SrTiO₃ (STO), stand out for their bulk crystals’ exceptionally high k’s. An alloy of these perovskite oxides, Ba [subscript x] Sr [subscript 1-x] TiO₃ (BST), exhibits this ultrahigh-k behavior at room temperature as bulk crystals when x ~ 0.7. Atomic layer deposition stood out as a technique to potentially deposit conformal films on the 3-D surfaces of next-generation memory devices. We set out to study this material and aimed to study the high-k behaviors of BST as thin films. BST thin films were grown first on STO (001) substrates using ALD. The close lattice match between BST and STO meant that the thin films were grown epitaxially. Films were grown at thicknesses below 20 nm. As the BST films were very thin and strained to the substrate, they exhibited lower k’s than those shown by bulk crystals. As the films were strained to the substrates and the lattice constants hence changed, an x of 0.7 did not result in the maximum k achieved. The leakage current of the films were high due to the negligible band offset between BST and STO. BST thin films were then grown on Zintl-templated Ge (001) substrates using ALD. The Zintl template, which consisted of 0.5 ML of Ba, allowed BST films to be grown epitaxially on Ge substrates. Films grown on Ge substrates exhibited high bulk k’s but low overall k’s due to the influence of the film/substrate interface. Leakage currents were also high due to the small conduction band offset between the film and the substrate. Oxygen annealing and the insertion of Al₂O₃ capping layers were carried out to potentially remedy the high leakages.


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