Novel iron-based oxide, hybrid and superlattice thin films - Versatility of atomic/molecular layer deposition

Atomic and molecular layer deposition (ALD and MLD) thin-film techniques may potentially yield novel materials not attained by other synthesis techniques. In particular, the superior feature common for both ALD and MLD is the precise control of the layer thicknesses which makes them well-suited for the fabrication of nanoscale thin films and superlattice (SL) structures. The aim of this work was to develop ALD/MLD processes for different iron oxide-based materials that could exhibit interesting magnetic, optical and redox properties. The work involved three types of thin film materials: iron oxides, iron-organic hybrids and oxide-hybrid superlattices. The requirements for the iron precursors were strongly exclusionary in order to be applicable to all the three process types. The suitable precursors found were cyclopentadienyl iron(II) dicarbonyl dimer and iron(III) chloride, which reacted with water to form iron oxide thin films. In addition to the common magnetite phase, a simple ALD process for the extremely rare ε-Fe2O3 phase could be developed. The ε-Fe2O3 phase is known for its enormous coercive field (20 kOe) and multiferroic properties. Stable iron-organic hybrid thin films were successfully deposited with hydroquinone, 4-aminophenol and terephthalic acid as the organic precursors. Among the new hybrid thin films, the crystalline iron terephthalate (Fe-TP) thin films were studied in more detail for their chemical structure. Iron was found to be at the trivalent state and bonded to the terephthalate entities so that the bonding for the carboxylates is of the bidentate chelation type. The SL structures were successfully deposited for both of the iron oxides. The optical properties and optical band gap values were determined for the plain ε-Fe2O3 and Fe-TP films and for their superlattices. In addition, the magnetic characteristics of ε-Fe2O3 thin films were analyzed. For the ε-Fe2O3 films the coercive field was determined to be 1.6 kOe, the transmittance was less than 50 % in the visible range and the indirect optical band gap was estimated to be 2.0 eV. The hybrid films were nearly transparent, having a band gap of 3.0 eV. In the SL thin films, a sudden increase in the band gap was observed when the ε-Fe2O3 layer thickness was decreased below ~2 nm, indicating towards a quantum confinement effect.