Atomic Layer Deposition of Rare Earth Oxides
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The principles of Atomic Layer Deposition (ALD) for thin film growth are briefly introduced, emphasizing the aspects of a self-limiting mechanism. Binary rare earth oxide (REO) thin films have been grown by ALD using various precursor approaches, which are discussed starting with the β-diketonates (thd-complexes) which require ozone to form the oxide. The focus of this review is on the most recent developments in the precursor chemistry, viz. the use of precursors which coordinate to the trivalent RE ions through carbon or nitrogen and react with water at reasonable temperatures, generally below 350°C, to produce the oxide. The growth rates obtained with RE-cyclopentadienyl complexes together with water as oxygen source are significantly higher than those observed in the conventional β-diketonate/ozone system, e.g., for Y2O3 using (CpMe)3Y/H2O and Y(thd)3/O3 the observed growth rates were 1.2–1.3 and 0.23Å cycle–1, respectively. Furthermore, the resulting REO films from the organometallics and nitrogen-coordinated precursors have lower carbon and hydrogen impurity levels and good electrical characteristics. However, poor thermal stability of the novel carbon- or nitrogen-coordinated precursors may restrict their use as ALD precursors.
In addition to the desired ALD-mode reactions, other reactions may occur causing some deleterious effects on the REO films. In particular, the large and basic La3+ ion tends to adsorb and react with environmental water and carbon dioxide to form hydroxide and carbonate phases, respectively. Generally, dielectric properties may be improved by annealing, e.g., the permittivity for as-deposited Gd2O3 film was 10.4 but annealing in oxygen at 700°C raised it to 15.4.
The ALD processes for the binary REOs form the basis for depositing multi-component RE-containing films. Besides ternary oxides with the perovskite structure, e.g., LaAlO3 and LaGaO3, phases with two REOs can be processed as solid solutions. An example thereof is YScO3 having permittivity above 15 and remaining amorphous up to 800°C to 1000°C, depending on the ALD process selected, thus offering a very attractive alternative to the existing high-κ gate dielectrics.
Keywords71.55.-i; 72.80.Sk; 73.20.At; 75.47.Lx; 77.55.+f
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