Atomic Layer Deposition of Iridium Using a Tricarbonyl Cyclopropenyl Precursor and Oxygen

Abstract

Atomic layer deposition (ALD) is an advanced technology that can be used to deposit extremely thin and conformal films of iridium (Ir). However, ALD techniques for Ir coating are not well-developed. In particular, new Ir precursors with high reactivity at a suitable low temperature are essentially required. In this study, we report a novel ALD precursor with improved reactivity by introducing a cyclopropenyl ligand. Tricarbonyl (1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl iridium (C18H27IrO3 or TICP) is used as an ALD precursor with molecular O2 as a reactant. Ir films are grown by ALD on a Si substrate at deposition temperatures ranging from 200 to 325 °C, and an ALD window of 250–275 °C and self-limiting growth at a rate of 0.52 Å cycle–1 at 250 °C are observed. The negligible O impurity content (<2 at. %) and low resistivity (13 μΩ cm) indicate that pure metallic Ir films are formed. The differential delay of nucleation depending on the substrate surface is explained in terms of the dominant surface functional group, indicating possible application of the current ALD process toward area-selective deposition of Ir. Density functional theory calculations show that the adsorption of the Ir precursor is feasible on Si and Ru but is unfavorable on hydroxyl-terminated SiO2. Ru is adopted as the seed layer for conformal deposition on a SiO2 trench, and a step coverage of ∼100% is obtained. Finally, an Ir thin film grown on a three-dimensional titanium substrate shows overpotentials (at 10 mA cm–2) of ∼65 mV for the hydrogen evolution reaction and ∼336 mV for the oxygen evolution reaction in an acid electrolyte, which suggest its potential application as a water-splitting catalyst.