Towards a Model System for Lamellar Magnetism by Atomic Layer Deposition

The combination of the two well behaved binary oxide ALD processes, NiO (Ni(acac)2 + O3) and (TiO2 (TiOPr)4 + H2O), provide excellent means for depositing thin films of NixTi1−xOy with highly controllable stoichiometry. In particular, stoichiometric NiTiO3 was obtained for a 1:1 molar pulsing ratio of NiO:TiO2. An ALD window was observed between 175 and 275 ◦C for this composition, within which the growth rate remained unchanged at ∼43 pm/subcycle. The measured refractive index was 2.42 and a direct band gap of 2.27 eV was found. The described process yielded smooth (0.4–0.9 nm RMS roughness), crystalline er Deposition ate films of NiTiO3 as-deposited on Si (1 0 0) substrates, with a strained (0 0 1) orientation. Annealing at 650 ◦C for 15 min relaxed the strain and improved the crystallinity. © 2014 Elsevier B.V. All rights reserved. per diffe have n the ired uenti mate io of ry n ario )2 [1 2 [20 sors e ent re one e to t ain ivate 3–Fe t wor r of nts t lutio owar teriza uction l titanate, NiTiO3, belongs to the ilmenite family of mateh general formula MTiO3 (M = Mg, Mn, Fe, Co, Ni, Zn). enite type structure the Tiand M-atoms are ordered rnating layers along the hexagonal c-axis (space group ). If the Tiand M-atoms are randomly distributed this becomes isostructural to the corundum structure (space 8, R-3c). The cations can also be “perfectly dispersed” with atom surrounded by M-atoms and vice versa, yielding the ype structure (space group 161, R3c). The different cation ions may introduce magnetic ordering (R-3) [1] or multi(R3c) [2–4]. Ilmenite materials are further suggested used ave dielectrics [5], gas sensors [6], and catalysts [7]. When ly mixed with other materials, even more exotic propy emerge. For instance, solid solutions of Fe2O3–FeTiO3 f-reversed thermoremanent magnetization [8] and sigexchange bias [9]. Additionally, nickel titanate exhibits alytic properties [10], and is a potential highdielectric [11]. Nickel titanates have previously also been grown as by techniques such as sol–gel processes [11,12], aerosolhemical vapor deposition [10], pulsed laser deposition [4] frequency magnetron sputtering [13]. current work, we use the Atomic Layer Deposition (ALD) e to grow nickel titanate thin films. The technique has antages [14], one of which is the possibility to grow conlow tem of the well be betwee the des by seq binary the rat Bina using v Ni(dmg Ni(thd) precur in a rec with oz sors du low rem Mot in Fe2O curren membe represe solid so route t charac lms with excellent control of composition at relatively onding author. Tel. : +47 97668855. address: j.e. bratvold@kjemi.uio. no (J.E. Bratvold). 2. Experim The films istry Ltd.) u with O3 an oi.org/10.1016/j.apsusc. 2014.05. 092 /© 2014 Elsevier B.V. All rights reserved. atures. The film is grown by cycles of sequential pulses rent precursors, separated by purges of inert gas. In d systems each pulse represents self-limiting reactions gas phase and the surface. The cycle is repeated until thickness is achieved. Ternary materials are deposited al combination of the processes for deposition of the rials. The deposited stoichiometry is tuned by varying the binary processes. ickel oxide has previously been demonstrated by ALD us types of precursors: Ni(apo)2 [15], Ni(acac)2 [15], 5], Ni(cp)2 [16,17], Ni(MeCp)2 [18], Ni(EtCp)2 [19], ], Ni(dmamp)2 [21] and Ni(dmamb)2 [22]. The range of xplored for deposition of titanium is wide, as described view [23]. Here, we have chosen nickel acetylacetonate and titanium tetraisopropoxide with water as precurheir ease of handling, well documented processes, and s of residue in the deposited films. d by the exotic magnetic properties of nanolamellae TiO3 solid solutions of geological origin [24,25], the k focuses on ALD growth of NiTiO3 as an interesting the ilmenite class of materials. In this respect, NiTiO3 he first step towards synthetic Fe2O3–NiTiO3 analogous ns for detailed exploration. We present a robust ALD ds NiTiO3 thin films, along with structural and physical tion of the achieved samples.