LaNiO₃ (LNO) has attracted much attention in recent years not only due to its metallic property at all temperatures among rare earth nickelate, but also owing to its alternative to Pt to serve as bottom electrode of ferroelectric thin film capacitors^[1,2]. LNO has the perovskite structure with a pseudo-cubic lattice parameter of 0.384 nm^[3]. It has been reported that the use of LNO bottom electrode can improve the performance of the ferroelectric thin film device such as fatigue characteristics and leakage current characteristics due to the mutual diffusion between Pt electrode and ferroelectric thin film^[4,5]. At the same time, the orientation of LNO seed layer can induce the oriented growth of ferroelectric thin films^[6]. And the epitaxial strain has been proved to enhance the remanent polarization and increase the Curie temperature in ferroelectric epitaxy^[7,8].

Synthesis of LNO thin films have been reported by a variety of techniques including physical vapor deposition (radio frequency magnetron sputtering (RF-MS)^[9], pulsed laser deposition (PLD)^[10], molecular beam epitaxy (MBE)^[11]) and chemical solution deposition (CSD)^[12]. High-quality epitaxial films can be obtained by physical vapor deposition, but these processes are associated with expensive and complex equipment, vacuum environment, limited deposition area and high costs^[13]. While in the traditional CSD process, metallic organic precursors with high activity are used as reaction sources to generate various oligomers through hydrolysis. These oligomers, which contain metal ions, with the proper viscosity, are easy to rotate and uniformly coating, and can be made into ceramic materials by combustion of organic matter at high temperatures^[14]. Although this method needs simple equipment and low cost, it cannot meet the requirements of the epitaxial growth of thin films. Moreover, many functional compound materials cannot be deposited because many metal precursors react violently with water to form metal hydroxides and precipitate out of the solution even before coating the solution on the substrates. The degree of hydrolysis can be in question owing to the differences in chemical reactivity among the metals used in the solution^[15]. Thus, it is necessary to find a synthetic method which can prepare epitaxial films like the physical method while having the advantages of the chemical method.

Polymer assisted deposition is a new chemical solution method developed in 2004^[16]. This technique has been widely used for the epitaxial growth of thin films in different fields such as oxides^[17], nitrides^[18], carbides^[19], etc^[20]. The key to the successful of epitaxial growth of thin films is the use of polyethyleneimine (PEI) with functional -NH₂ groups to bind metal that serves both to coordinate, stabilize cations and maintain an even distribution of the metal cations in solution^[21,22]. PEI can not only encapsulate the metal to prevent chemical reaction, but also control the viscosity of the precursor by adjusting the molecular weight of PEI^[23]. Therefore, the solution has adequate viscosity to coat on the substrate evenly. After the decomposition of PEI, metal cations can be released orderly on the monocrystalline substrate and epitaxial crystallization occurs^[16]. Compared with the physical vapor deposition, PAD requires no vacuum, which has the advantages of simple operation, simple equipment and low cost. Compared with the Sol-Gel method, PAD avoids the hydrolysis and condensation reaction of precursors, so it is easier to obtain stable precursors with precise stoichiometric ratio. The PAD method offers several advantages over CSD method, such as epitaxial growth, no need of ageing and easy control of viscosity.

This study presents a new attempt to prepare LNO epitaxial film by PAD. Various analyses on the structure and chemical composition of the conductive LNO thin film were carried out. It is proved that the crystallization occurs at crystallization temperature after the decomposition of PEI which results in the epitaxial growth of LNO thin film.

Epitaxial LNO thin films were fabricated on (001) oriented SrTiO₃ (STO) single-crystal substrate by PAD. The precursor for the growth of LNO film was obtained by the following steps. Lanthanum acetate hydrate (La(NO₃)₃·6H₂O, 99.99%, Aladdin Industrial Medicines Co.Ltd) and nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 98%, China National Industrial Medicines Co.Ltd) as sources of metal were dissolved in deionized H₂O. Both polyethyleneimine (PEI, 50% (in mass) in H₂O, Aladdin Industrial Medicines Co.Ltd) and ethylenediaminetetraacetic acid (EDTA, 99%, Aladdin Industrial Medicines Co.Ltd) were used to complex the metal cations. In detail, EDTA was 1 : 1 molar ratio with metal ion, and PEI was incorporated into the solution in a 1:1 mass ratio with EDTA. After stirring for 24 h, a transparent blue solution with reasonable viscosity was achieved. The solution concentration was 0.15 mol/L. The precursor solution was spin-coated onto STO (001) substrate at 5000 r/min for 30 s to obtain LNO wet film. The wet film was heated at 700 ℃ for 30 min in air to form a dense film.

Crystallinity of the film and the heteroepitaxial relationships between the film and the substrate were measured by high resolution X-ray diffractometer (HRXRD, D8 Discover, Bruker, Germany). Atomic force microscope (AFM, FM-Nanoview6800, FSM-Precision, China) was used to characterize surface roughness of the film. The thickness and lattice parameter of the film were characterized by the high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan). Chemical composition of films and oxidation state of elements were detected by X-ray photoelectron spectroscope (XPS, ESCAlab-250, Thermo Fisher, England). The resistivity of the LNO film was performed from 10 K to 300 K using a four-probe technique by a physical property measurement system (PPMS, PPMS-9T, Quantum Design, USA).

Fig. 1(a) presents the θ-2θ scan of LNO film on STO (001). It is obvious that only (001) peak of the film and substrate can be seen, indicating that the LNO film exhibits substrate orientation. The calculated lattice parameter obtained from the position of (002) reflection is 0.3838 nm, which is well consistent with the theoretical value 0.384 nm^[3]. Fig. 1(b) shows the rocking-curve of the (002) peak. A full width at half-maximum (FWHM) value is 0.38°, suggesting that the LNO film fabricated by PAD has good crystallization quality. Additionally, to further confirm the heteroepitaxial growth of LNO film on STO substrate, XRD φ-scans of reflections of LNO (202) and STO (202) (Fig. 1(c)) were carried out. It can be seen from the φ-scans that there are four peaks separated by 90°, showing that the LNO thin film is cubic-on-cubic grown on STO substrate. The heteroepitaxial relationships between the LNO film and the STO substrate can be described as (001)LNO||(001)STO and [101]LNO||[101]STO. As shown in Fig. 1(d), the RMS roughness of LNO film is 0.67 nm, indicating that the surface is smooth. Fig. 1(e) shows the cross-sectional HRTEM image of LNO film on STO substrate. And the thickness of the film can be measured as ~25 nm. The detailed epitaxial interface structure between the LNO thin film and the STO substrate through the HRTEM is exhibited in Fig. 1(f). The excellent epitaxial growth could be attributed to the relatively small lattice mismatch ~1.7% between the film (a_LNO=0.384 nm measured through the (001) lattice plane spacing) and the substrate (a_STO=0.391 nm).

To show the epitaxial growth of LNO thin film on STO substrate, the wet film coated by LNO precursor was tested by in-situ high temperature XRD measurement. It can be seen from Fig. 2 that there is no obvious change in the XRD pattern in the low temperature zone. With the increase of the temperature up to 500 ℃, PEI gradually decomposes and at the end of the pyrolysis there are no other peaks observed except for the substrate. Until the beginning of the crystallization at 700 ℃, the (002) orientation peak of LNO film appears. In other words, crystallization of the film occurs after the decomposition temperature of the PEI polymer. The complete degradation of PEI is above 550 ℃ by Thermogravimetric Analysis^[22]. That means that the high decomposition temperature of PEI prevents the formation of the film below this temperature which is basically consistent with the above in-situ XRD results. After the decomposition of PEI, metal cations can be released orderly on the monocrystalline substrate and epitaxial crystallization occurs.

Fig. 3 shows the XPS survey spectrum and La3d, Ni2p and O1s spectra for the film fabricated by PAD. It can be seen from Fig. 3(a) that only the peak of La, Ni, O and C elements can be found in the survey spectrum. Except for the surface adsorbed carbon, there is no detectable impurity elements. Fig. 3(b) gives the XPS spectrum of La with high resolution which presents the binding energy of La3d_3/2 between 854.3 and 850.1 eV and La3d_5/2 between 837.5 and 833.2 eV (standard value: La3d_3/2~ 853.0 eV; La3d_5/2~836.0 eV)^[24,25]. Fig. 3(c) represents the narrow scan of Ni2p. The Ni2p_3/2 spin-orbit is located at 854.4 eV accompanied by a satellite peak locating at higher binding energy of 863.4 eV. The chemical valence state of Ni ion can be judged from Ni2p_1/2 bonding energy peak^[26,27]. The Ni2p_1/2 usually has a single peak for Ni³⁺, while a double peak for Ni²⁺. Additionally, the existence of Ni²⁺ degenerates the conductive properties of LNO films. In Fig. 3(c), the Ni2p_1/2 spin-orbit with a single peak at 871.7 eV was observed. It can be inferred that the oxidation state of nickel ion in the LNO film prepared by PAD is +3, which is consistent well with the good conductive property described as follows. Fig. 3(d) shows the narrow scan of O1s. The only peak at binding energy of 528.9 eV is considered to be the lattice oxygen in LNO. This indicates that LNO thin film annealed in air do not have oxygen vacancies which means that LaNiO₃ fabricated by PAD is stoichiometric.

Fig. 4 shows the resistivity from 10 to 300 K of the LNO film on STO substrate with a thickness of 25 nm. The LNO film measured by a standard four-probe method presents metallic resistivity versus temperature behavior which means that the resistivity increases by increasing temperature. The LNO thin film has a room temperature resistivity of 160 μΩ·cm. Table 1 summarizes the room-temperature resistivity of LNO thin films deposited by different methods on single crystal substrate.

It should be noted that the LNO thin film deposited by PAD is epitaxial growth as compared with chemical solution deposition, and has a low resistivity comparable to that fabricated by physical vapor deposition^[28,29,30].

In summary, we have successfully fabricated epitaxial LaNiO₃ thin film on SrTiO₃ substrate through polymer assisted deposition. XRD and HRTEM results reveal that the film has good crystallinity and epitaxial quality. Additionally, the in-situ XRD measurement has been carried out to elucidate the epitaxial growth process of LNO thin film. The resistivity result shows all metallic property with good conductive property from 10 to 300 K. The successful growth of the LNO thin film provides a new approach in preparing epitaxial functional thin film material by using low-cost PAD.