**1. Introduction**

NiTiO3 (NTO) is a technologically relevant material proven suitable within photo catalysis, high-*k* dielectrics and non-volatile memory [1–5]. In addition, ab initio calculations predict ferroelectricity and crystal structure dependent weak ferromagnetism, potentially suitable for sensor, microwave and spintronic applications [6]. At elevated temperatures, NTO adopts the corundum structure (space group *R*−3*<sup>c</sup>*, Figure 1a) with random distribution of cations [7]. This disordered phase has so far been unquenchable, as cooling induces ordering of the two di fferent cations in alternating layers perpendicular to the crystallographic *c* axis. As ordering occurs the *c* glide plane is lost, and the structure is known as the ilmenite structure (space group *R*−3, Figure 1b) [8]. This is also the case for ilmenite (FeTiO3) itself, which additionally has been proven to adopt the related and technologically interesting LiNbO3 structure (space group *R*3*<sup>c</sup>*, Figure 1c) at high temperature and pressure [9]. In the *R*3*c* structure, the two di fferent cations alternate perfectly, both parallel and perpendicular to the *c* axis. Distinguishing between the three di fferent symmetries by the position of the reflections using X-ray di ffraction is di fficult. Unit cell values obtained from density functional theory calculations on *R*−3*<sup>c</sup>* and *R*3*c* are virtually the same [10], and e ffects from strain or o ff-stoichiometry are likely to be much larger. However, determining whether the symmetry is *R*−3 or *R*−3*c*/*R*3*<sup>c</sup>* is easy, as the di ffraction pattern from *R*−3 has reflections from ordering not allowed with the *c* glide plane of *R*−3*c*/*R*3*<sup>c</sup>* due to extinction rules. While there are no reports of bulk NTO with *R*3*c* structure, the phase was claimed to have been obtained by Varga et. al on α-Al2O3 substrates as thin films made by pulsed-laser deposition [10]. Even though the symmetry was not unambiguously confirmed, some degree of lattice polarization was

observed, and possibly weak ferromagnetism, indicating the presence of the LiNbO3 structure [11–13]. Ferroelectricity and weak ferromagnetism have indeed been observed in bulk samples of NTO as well [14], but the report did not discuss the crystal symmetry of the samples.

**Figure 1.** Schematic showing the similarity between the unit cells of space groups *R*−3*<sup>c</sup>* (**a**), *R*−3 (**b**) and *R*3*c* (**c**), viewed along [110]. The *R*3*c* structure (**c**) is shifted up one cation layer to emphasize the relation to the two other structures.

Thin films of NTO have previously been made by various deposition techniques, including aerosol-assisted CVD [2], sol-gel methods [3,15–18], dip-coating [19] and RF-sputtering [4,5], as well as with the previously mentioned pulsed laser deposition [10–12]. Especially for the latter technique, compositional control seems to be challenging. In addition, all methods require elevated temperatures (>500 ◦C) either during deposition or by post-deposition annealing. While amorphous films might be desirable for some applications, a well-defined crystallinity and orientation is usually required to make use of the material. The atomic layer deposition (ALD) technique offers an alternative route for deposition of epitaxial thin films, provided careful selection of substrates [20,21], at relatively low temperatures (typically <400 ◦C). We have previously reported details about the deposition of NTO by ALD [22]. As deposited films on Si(100) with a 1:1 ratio between Ni and Ti (within 1%, as measured by X-ray fluorescence), all showed a preferred (001) orientation in the deposition temperature range 175–275 ◦C. No sign of the ordering reflections belonging to the *R*−3 symmetry were visible at any temperature, neither before nor after annealing. Given the difficulty of obtaining the *R*3*c* phase in bulk, the films deposited on Si(100) were assumed to have the disordered *R*−3*<sup>c</sup>* symmetry. However, from the limited amount of data the *R*3*c* symmetry cannot be ruled out. In the work presented here, we show the possibility to control the orientation and crystallinity of deposited NTO films with the use of various single-crystal substrates. As with the previously reported films deposited on Si(100), the films discussed here are assumed to have the *R*−3*<sup>c</sup>* symmetry whenever the ordering reflections are not present. The ability to control the phase and orientation of these films is interesting with respect to catalysis. In addition, by using ALD it should be easy to incorporate other elements. Doping of NiTiO3 has already been shown to improve solar water splitting efficiency [23], as well as inducing ferromagnetism in coexistence with ferroelectricity [24].

#### **2. Materials and Methods**

The films were deposited in an F-120 Sat reactor (ASM Microchemistry Ltd., Helsinki, Finland) using the precursors Ni(acac)2 (nickel acetylacetonate, 95%, Sigma-Aldrich, St. Louis, MO, USA) in combination with O3 (produced in a BMT Messtechnik GMBH ozone generator from 99.6% O2, AGA), and TTIP (Titanium(IV) tetraisopropoxide, 97%, Sigma-Aldrich, St. Louis, MO, USA) in combination with deionized water. A pulsing and purging sequence of (2 s Ni(acac)2–1.5 s purge–3 s O3–2 s purge) + (0.5 s TTIP–1 s purge–2 s H2O–3 s purge) were used for all depositions, as described more thoroughly in our prior work on deposition of NTO [22]. The substrates used were 3 × 3 cm<sup>2</sup> single crystals of Si(100), for thickness and composition determination, as well as α-Al2O3(001) (referred to simply as Al2O3 from now on), LaAlO3(100) (LAO), SrTiO3(100) (STO) and MgO(100) for structural analysis. All dust was blown clean of substrates using pressurized air before being placed in the reaction chamber, and subjected to 15 s of O3 immediately before deposition. The films grown on Al2O3 and MgO were deposited in the same experiment with a nominal thickness of 165 nm (measured on Si(100) substrates). Likewise, the films grown on STO and LAO were deposited in the same experiment with a nominal thickness of 130 nm (measured on Si(100) substrates). All depositions were performed at 250 ◦C. Annealing was undertaken in air by rapid thermal processing (RTP) in an MTI Corporation OTF-1200X furnace (Richmond, VA, USA). The heating program consisted of a 15 min. ramp from room temperature to 650 ◦C, followed by a dwell time of 15 min., and subsequent cooling in the furnace to room temperature, for a typical duration of 30 min.

The film thicknesses were measured by spectroscopic ellipsometry on Si(100) substrates, using a J.A. Woollam (Lincoln, Dearborn, MI, USA) alpha-SE ellipsometer. The CompleteEASE software package (version 4.92, J.A. Woollam, Lincoln, Dearborn, MI, USA) was used to fit a Cauchy function to the obtained data in the 390–900 nm wavelength range. Measurements on several positions on the 3·3 cm<sup>2</sup> Si substrates were averaged (std.dev. ~3 nm) used to indicate a "nominal thickness" for the films on single crystalline oxide substrates. The cationic composition was determined by X-ray fluorescence (XRF) measurements on the Si substrates using a Philips (Almelo, The Netherlands) PW2400 spectrometer and the UniQuant analysis software (version 2, Omega Data Systems, Veldhoven, The Netherlands). All θ-2θ X-ray diffraction (XRD) was performed on a Bruker AXS (Karlsruhe, Germany) D8 Discover diffractometer in Bragg–Brentano configuration with Cu Kα radiation. The diffractometer was equipped with a Ge(111) monochromator and a LynxEye strip detector. Reciprocal space maps and ϕ scans were collected using a PANalytical Empyrean diffractometer, equipped with a Cu Kα source powered at 45 kV/40 mA, a hybrid monochromator and a PIXcel3D detector (PANalytical, Almelo, The Netherlands). Atomic force microscopy (AFM) was performed with a Park Systems (Santa Clara, CA, USA) XE-70 AFM, equipped with a PPP-CONSTCR tip (Nanosensors, Neuchâtel, Switzerland) in contact mode.

Literature values for NTO in this paper refer to the PDF# 01-075-3757, ICDD. The listed cell parameters of the unit cell, having the ilmenite structure (*R*−3), are *a* = 5.0321 Å and *c* = 13.7924 Å.
