Liquid-phase atomic layer deposition of crystalline hematite without post-growth annealing

Abstract

Owing to its cost-effectiveness and low-energy consumption, solution-processing of functional metal oxide films has been extensively studied over the past few decades. However, direct-solution routes to crystalline hematite (α-Fe₂O₃) films without post-heat treatment have not been explored yet. This is because the solution-processing of these films involves the preferential formation of other iron oxides, hydroxides, and oxyhydroxides during the conventional precipitation reactions. Herein, we developed a direct-growth route for the preparation of crystalline α-Fe₂O₃ using the spin spray (SPS) technique. We carried out the liquid phase-atomic layer deposition (LP-ALD) of α-Fe₂O₃. The mechanism involved the surface adsorption of Fe²⁺ followed by the oxidization and formation of Fe³⁺–O²⁻ bonds. The LP-ALD approach facilitated the low-temperature direct growth of highly crystalline, dense, and uniform α-Fe₂O₃ films at 95 °C with a high deposition rate of 6.3 nm min⁻¹. The deposition temperature and rate are much lower and higher than those offered by cutting edge gas-phase ALD techniques (230 °C and 1.3 × 10⁻² nm min⁻¹), respectively. Furthermore, the effects of the growth conditions on the formation and microstructure of the products were investigated in detail in order to elucidate the LP-ALD mechanism.