Structural Features and Mutual Influence of the Layers in PZT–LNO–SiOx–Si and PZT–LNO–Si Compositions

The phase composition and specific features of the microstructures of layers in the Pb(Zr_0.52Ti_0.48)O₃–LaNiO₃–Si and Pb(Zr_0.52Ti_0.48)O₃–LaNiO₃–SiO_x–Si compositions grown by chemical vapor deposition from solutions have been investigated by high-resolution transmission electron microscopy, electron diffraction, and energy-dispersive analysis. The influence of the structure of the lower LaNiO₃ electrode on the structure and properties of ferroelectric lead zirconate titanate films with a perovskite structure is studied. It is shown that the misoriented porous polycrystalline structure of the lower electrode leads to violation of the columnarity of perovskite grains. The electrical parameters are slightly deteriorated in comparison with a conventional platinum electrode. The structures of the thin films with a silicate sublayer under the LaNiO₃ electrode and without it are compared.

Lead zirconate titanate (Pb(Zr0.52Ti0.48)O3, PZT) films are widely used to design integrated ferroelectric devices, such as nonvolatile storage devices and various micromechanics devices based on the direct and inverse piezoelectric effects, pyroelectric effect, and some other nonlinear properties of active dielectrics [[1]–[5]]. The basic element of such devices is a ferroelectric capacitor, which is a ferroelectric (PZT) layer with adjacent metal electrodes. Noble metals and their oxides (Pt, Ir, IrOx, Ru, and RuOx) are generally used as electrodes. These materials, on the one hand, prevent chemical interaction between neighboring layers during high-temperature treatment during film crystallization and, on the other hand, have close lattice parameters and provide the formation of a columnar structure of perovskite grains due to the preferred process of heterogeneous nucleation at the interface [[6]]. This structure ensures the best electrical parameters, because it is comprised of perovskite grains grown through the entire depth with pronounced texture and barely having any transverse (with respect to the electric field direction) boundaries in the PZT layer.

Application of metals as a material for the lower electrode has several considerable drawbacks. For example, platinum (a widespread material) is characterized by very low adhesion to silicon oxide, even when using an adhesive layer (e.g., Ti or TiO2), and can hardly be subjected to plasma etching. This structure cannot be formed directly on silicon due to the silicification reaction and on the surfaces of metal substrates, which is often required, e.g., in energy-saving devices based on the piezoelectric effect, flexible electronics devices, etc. [[8]]. Along with technological limitations, the metal electrode is impermeable for oxygen-vacancy migration, which leads to the occurrence of space charges and degradation of properties (for example, induces fatigue) [[10]].

A good alternative is the use of oxide electrodes [[10]], an interesting representative of which is LaNiO3 (LNO) with a pseudocubic perovskite structure with a close-to-PZT lattice parameter (3.84 Å for LNO and 4.04 Å for PZT) [[11]–[13]]. An important fact is that these films can be grown by chemical vapor deposition from solutions, i.e., in a single technological process by changing the initial film-forming solutions during the successive multilayer deposition. An important problem is to determine the possibility and specific features of the structure formation implying direct deposition on silicon substrates.

It is known that the crystal structure and, accordingly, properties of a ferroelectric PZT layer are determined to a great extent by the lower-electrode structure [[6], [11]]. In addition, the structures of all layers are significantly affected by mechanical stress due to the difference in the thermal expansion coefficients and lattice mismatch, as well as the phase transitions and transformations [[2]].

We investigated the structural features of the layers in PZT–LNO compositions grown by chemical vapor deposition from solutions directly on silicon and using a buffer silicon silicate layer.

PZT–LNO–Si and PZT–LNO–SiOx–Si samples (x = 1–2) were prepared by chemical vapor deposition from solutions on KDB-10 Si(100) (p-Si:B with a resistivity of 10 Ω cm) substrates 150 mm in diameter. The film-forming LNO solution was obtained using the corresponding amounts of lanthanum acetate La(CH3COO)3 and nickel acetate tetrahydrate Ni(CH3COO)3 ⋅ 4H2O in iced acetic acid CH3COOH with a concentration of 0.2 M. The La : Ni molar ratio was 1 : 1. The film-forming PZT solution was prepared using zirconium isopropylate monosolvate Zr[O(CH3)2CH]4(CH3)2CHOH, titanium tetraisopropoxide Ti[O(CH3)2CH]4, and dehydrated lead acetate Pb(CH3COO)2 obtained by solid-phase synthesis according to the technique [[14]]. The film-forming silicate solution was prepared by cohydrolysis and polycondensation of a mixture of alkoxysilanes (methyltriethoxysilane (99%) and tetraethoxysilane (99.999%) in a molar ratio of 3 : 2) in a medium of organic solvents (isopropyl alcohol, ethanol, 2-methoxyethanol, 99.5%) and water in the presence of 0.002 mol of HNO3 with subsequent addition of 42 wt % surfactant (Brij L4, C12H25(OCH2OCH2)4OH, molar mass 362 g/mol) according to the technique [[15]].

A buffer silicate layer was preliminarily deposited on one substrate by centrifugation of a film-forming solution. After the deposition, the silicate film was annealed in air at 200°C for 5 min and then at 650°C for 20 min.

To obtain the LNO layer, the film-forming solution was layer-by-layer deposited on silicon substrates (seven sublayers). After the deposition, each sublayer was dried in an IR furnace (T ~ 200°C, 5 min) with subsequent crystallization annealing at T = 650°C for 5 min. To form the PZT layer, precursors were multiply deposited, with each layer dried in an IR furnace at 200°C for 5 min and at 400°C for 10 min. The final crystallization of the entire film was carried out at T = 650°C for 15 min.

The compositions obtained were studied by transmission electron microscopy (TEM), z-contrast scanning transmission electron microscopy (STEM), electron diffraction, and energy-dispersive analysis (using an FEI Tecnai Osiris microscope equipped with an array of silicon detectors for ultrafast elemental mapping at an accelerating voltage of 200 kV). Cross cuts of the compositions for TEM were prepared using focused ion beams in the column of an FEI Scios scanning electron microscope.

The capacitance–voltage characteristics were measured using a complex (MDC, United States) composed of an Agilent 4284A LCR meter and a Model 802–150 mercury probe. The scan voltage was ±10 V, the measurement-signal amplitude was 25 mV, and the frequency was 100 kHz.

The dielectric hysteresis loops P(E) were measured by an AixACCT-TF 2000 specialized complex (AixACCT, Germany) at a scan voltage of ±10 V and a frequency of 100 Hz.

Panoramic images of the cross cuts of the PZT–LNO–SiOx–Si and PZT–LNO–Si compositions are presented in Fig. 1. The corresponding layer thicknesses measured from the electron microscopy images are listed in Table 1.

Graph: Fig. 1. Structures of the cross cuts of the (a, b) PZT–LNO–SiOx–Si and (c, d) PZT–LNO–Si compositions: (a, c) TEM images and (b, d) the corresponding schematic of the structure.

Layer thicknesses in the compositions

PZT layers in both compositions are characterized by pronounced oriented growth of grains from the heterogeneous LNO–PZT interface. The majority of grains have a columnar structure and grow throughout the entire film; however, smaller grains, which have not reached the upper PZT film boundary, are also observed. PZT grains have different thicknesses (a typical value is 30–90 nm for both samples), in contrast to classical compositions on platinum, which are characterized by grains of identical thickness in the range of 150–200 nm [[7]].

As can be seen in Fig. 1, the LNO layer has a porous polycrystalline structure. The pore size is 10–20 nm. The mean grain size dmean is 21.1 ± 8.5 nm for the sample with the silicate layer and 26.4 ± 12.9 nm for the sample without it. Individual sublayers deposited successively during the lower-electrode preparation can clearly be distinguished (especially in the first sample), because grains are located within a sublayer or even have a smaller size and do not grow into neighboring sublayers, and pores are arranged in rows, mainly at the sublayer interfaces.

A comparison of the LNO layer structures in both samples shows that the observed network of pores is more pronounced in the PZT–LNO–SiOx–Si sample, for which the surface pore density is 9 pores per 100 nm2 on average, whereas for the PZT–LNO–Si sample this quantity is smaller by a factor of almost 2 (5 pores per 100 nm2).

In the sample with an electrode formed directly on silicon, one can distinguish a row of pores at the bottom boundary of the LNO layer, which are located directly at the LNO–Si interface. In the sample with an LNO layer formed on silicate, this porous interlayer is not observed. Apparently, silicate facilitates the nucleation of LNO grains and is characterized by higher adhesion to oxide; however, further LNO crystallization on the porous sublayer leads to the formation of a porous structure with smaller grains.

An analysis of the electron diffraction pattern (Fig. 2a) showed that the film is crystallized with the formation of the perovskite phase with a lattice period a = 4.036 Å (sp. gr. P4mm, according to the PCPDFWIN v. 1.30 database, no. 33-084). We should also note that there is no metastable non-ferroelectric pyrochlore phase, which is generally present in the form of nanoscale inclusions at the annealing temperature T = 650°C, as was reported previously in [[6], [16]].

Graph: Fig. 2. (Color online) Phase compositions of the PZT and LNO layers: (a) electron diffraction pattern of the PZT layer, (b) electron diffraction pattern of the LNO layer, and (c, d) HREM image and the corresponding Fourier transform for a selected portion with the [001] zone axis of an LNO crystallite (PZT–LNO–SiOx–Si sample).

Interplanar spacings measured from the electron diffraction pattern (Fig. 2b) correspond to the cubic stoichiometric LaNiO3 phase (sp. gr. Pmm). However, the tetragonal La2NiO4 phase (sp. gr. I4/mmm) with similar interplanar spacings should not be excluded from the consideration. Electron diffraction has not yielded unambiguous information about the layer phase composition because of the poor accuracy in determining interplanar spacings. The application of high-resolution electron microscopy (HREM) and analysis of the corresponding Fourier transforms are more efficient in our case. To identify the phase composition, we performed Fourier transform of the high-resolution image of an individual crystallite of the LNO layer (Figs. 2c, 2d), which proved the presence of the cubic LaNiO3 phase (sp. gr. Pmm).

Let us consider the PZT–LNO interface structure in more detail. Figure 3 presents bright- and dark-field TEM images of grains at the interface for both samples. Note that large PZT grains have generally a specific shape with a narrow base, which expands while approaching the upper film boundary (Fig. 3a). This shape is caused by the nucleation of PZT grains on small LNO crystallites (Fig. 3b), which may occur both at individual layer crystallites (Figs. 3a, 3c) and at a group of LNO crystallites with similar orientation (Fig. 3d). The similar orientation of individual groups of LNO grains is also pronounced in the electron diffraction pattern of this layer (Fig. 2b), which confirms the significant orienting influence of the lower electrode on the PZT film. It is noteworthy that, since the LNO layer in the PZT–LNO–Si sample is less porous, crystallites can grow beyond the original sublayer and have a larger size on the whole (as was shown previously).

Graph: Fig. 3. (Color online) Structure of the PZT–LNO interface (indicated by white arrows): (a, b) dark-field TEM image of columnar PZT grains grown on LNO crystallites in the PZT–LNO–SiOx–Si sample, (c) dark-field TEM image of a columnar PZT grain grown on an LNO crystallite in the PZT–LNO–Si sample, and (d) HREM image of the base of a PZT grain grown on similarly oriented LNO grains (dotted lines indicate LNO layer grains).

An increase in the size of LNO crystallites in the PZT–LNO–Si sample leads to an increase in the area (oriented in a certain way) for nucleation of the perovskite PZT phase. This facilitates, in turn, the formation of columnar perovskite grains with a wider base, as is illustrated by the dark-field image (Fig. 3c). A direct dependence of the PZT grain size on the LNO grain size was also observed in [[17]].

The presence of many PZT grains that have not grown to the upper layer boundary is also related to the fineness and random orientation of the LNO layer: a large amount of crystallites are formed during the nucleation and then compete with each other for the possibility of growing, suppressing the growth of least favorable nuclei. Thus, the size, morphology, and orientation of the structural elements of the lower electrode affect directly the PZT film structure.

Figure 4 shows TEM and HREM images of the LNO–Si and LNO–SiOx interfaces. In the former case, the lower electrode is formed on the buffer silicate layer designed for stress relaxation in the heterostructure. However, it is obvious that, along with this function, silicate somewhat facilitates the fineness of the LNO structure and the increase in its porosity. Pores in the silicate layer mainly have a circular cross section, without connecting channels, which, taking into account the randomness of their positions, is indicative of their isolation character and sphericity. The pore size is 3–15 nm. A specific feature of the layer structure is the absence of pores in the region of about 9 nm thick near the Si–SiOx interface. In the PZT–LNO–Si sample, a thin (5 nm) silicon oxide layer formed as a result of thermal effect is observed at the LNO–Si interface. It is generally present on the silicon wafer surface (Fig. 4b).

Graph: Fig. 4. TEM images of the interfaces: (a) LNO–SiOx (PZT–LNO–SiOx–Si sample) and (b) LNO–Si (PZT–LNO–Si sample).

It is known [[6], [16]] that diffusion processes are significantly enhanced during the crystallization of the PZT layer on platinum at T = 650°C. As a result, some elements may penetrate the neighboring layers at a large depth. To estimate the homogeneity of the sample chemical composition after annealing, we obtained elemental distribution maps for each layer (Fig. 5). Titanium diffusion from the PZT film to the LNO layer is observed in both samples. The counterpropagating lanthanum diffusion from LNO to PZT is less intense. The counterpropagating diffusion of elements was also observed in [[12]]; however, in contrast to our experiment, diffusion of nickel, lead, and zirconium (along with lanthanum and titanium) was revealed, and a smeared interface between layers with mutual intergrowth of PZT and LNO grains was observed. It should also be noted that nickel is nonuniformly distributed in the LNO layer.

Graph: Fig. 5. (Color online) Distribution of chemical elements in the cross cut of the PZT–LNO–SiOx–Si sample: (a) z-contrast STEM image and (b) La, (c) Ni, (d) Pb, (e) Zr, (f) Ti, and (g) O distribution maps in the layers.

The measured electrical properties (Fig. 6) are characteristic of PZT films. The remanent polarization corresponds to 26 and –27.4 µC/cm2 for the PZT–LNO–SiOx–Si sample and 29.4 and –31.0 µC/cm2 for the PZT–LNO–Si sample. In contrast to the structures formed on a platinum electrode, the dielectric hysteresis loops has a less pronounced saturation portion, especially for the structure without a buffer silicate layer, which may indicate leakages in such structures [[18]]. The maximum permittivity values are 788 and 1019 for the PZT–LNO–SiOx–Si and PZT–LNO–Si samples, respectively.

Graph: Fig. 6. Ferroelectric properties of the (a, b) PZT–LNO–SiOx–Si and (c, d) PZT–LNO–Si samples: (a, c) hysteresis loops and (b, d) permittivity.

The specific features of the microstructure of Pb(ZrxTi1–x)O3–LaNiO3–Si and Pb(ZrxTi1–x)O3–LaNiO3–SiOx–Si compositions grown by chemical vapor deposition from solutions were investigated by TEM, STEM, electron diffraction, and energy-dispersive analysis. It was shown that the lead zirconate titanate layer in both compositions is crystallized heterogeneously from the substrate with the formation of a columnar perovskite phase.

A polycrystalline porous structure is formed in the LNO layer (LaNiO3 phase); it is characetized by a grain size of 21.1 ± 8.5 and 26.4 ± 12.9 nm and a pore density of 9 (per 100 nm2) and 5 (per 100 nm2) for the PZT–LNO–SiOx–Si and PZT–LNO–Si samples, respectively. It was shown that the size and orientation of crystallites in the lower LNO electrode determine the PZT grain morphology. An increase in the grain size in the LNO layer facilitates the growth of columnar perovskite crystals with a wide base. The presence of a porous silicate sublayer affects the formation of the LNO structure: one can observe an increase in the porosity and grain fineness in the LNO layer. The porous interlayer at the LNO–SiOx interface is absent (in contrast to the PZT–LNO–Si sample). It was established that diffusion processes are enhanced during the annealing of the compositions at T = 650°C; these processes induce counterpropagating titanium diffusion from the PZT layer to the lower layers and lanthanum diffusion from the LNO layer to the PZT film. Nonuniform nickel distribution in the LNO layer was also observed. Thus, the TEM analysis revealed a strong influence of the LNO structure on the size and morphology of perovskite grains in the PZT layer.

This study was supported by the Ministry of Science and Higher Education of the Russian Federation within a State assignment for the Federal Scientific Research Centre "Crystallography and Photonics" of the Russian Academy of Sciences in the part concerning the structural investigations and within the project part of State assignment for research contract no. 11.2259.2017/4.6 in the part concerning the preparation of film samples.

The structural investigations were performed using equipment of the Shared Equipment Center of the Federal Scientific Research Centre "Crystallography and Photonics" (Russian Academy of Sciences). Film samples were prepared at the Russian Technological University "MIREA."

Translated by A. Sin'kov