Structure, Microstructure, and Dielectric Response of Polycrystalline Sr_1-xZn_xTiO₃ Thin Films

Abstract

In a view of the research interest in the high-permittivity materials, continuous enhancement of the dielectric permittivity ε′ with Zn content was reported for conventionally prepared Sr_1-xZn_xTiO₃ ceramics with x up to 0.009, limited by the solubility of Zn on Sr site. Here, we use a sol-gel technique and a relatively low annealing temperature of 750 °C to prepare monophasic Sr_1-xZn_xTiO₃ thin films with higher x of 0.01, 0.05, and 0.10 on Pt/TiO₂/SiO₂/Si substrates. The incorporation of Zn on the Sr site is confirmed by the decrease of the lattice parameter, while the presence of Zn in the films is proven by energy dispersive spectroscopy. The film thickness is found to be ~330 nm by scanning electron microscopy, while the average grain size of 86–145 nm and roughness of 0.88–2.58 nm are defined using atomic force microscopy. ε′ measured on the films down to 10 K shows a decreasing trend with Zn content in contrast to that for weakly doped Sr_1-xZn_xTiO₃ ceramics. At the same time, the temperature dependence of the dissipation factor tanδ reveals a peak, which intensity and temperature increase with Zn content.

1. Introduction

Dielectric thin films are widely used in modern technology and engineering aiming to decrease the size and weight of devices, although the physical phenomena in films are much more complicated to study and understand than the behaviour of bulk materials. Among them, SrTiO₃-based compounds have been attracting considerable interest both for a wide range of applications, particularly in energy conversion and storage as well as tunable electronic devices, and for break-through insights from a fundamental point of view [1,2]. Bulk undoped strontium titanate (SrTiO₃—ST) is known as incipient ferroelectric or quantum paraelectric due to paraelectric state stabilization by quantum fluctuations at low temperatures [3]. At the same time, whenever an electric field [4] or strain [5] is applied, or dopants [6,7,8] are introduced into the ST lattice, large changes are evident in the compound physical properties. In the case of ST thin films, these changes can be significantly enhanced, resulting e.g., in ferroelectricity induced at room temperature in epitaxial ST films by DyScO₃ substrate lattice misfit strain [9]. In addition to the strain [5,9,10], dopants can also induce a variety of phases, ranging from dipolar glass and relaxor to ferroelectric, at isovalent substitution for Sr²⁺ by Ba²⁺ [6,11], Pb²⁺ [6], Ca²⁺ [12,13], and Mn²⁺ ions [8,14] as well as at heterovalent substitution for Sr²⁺ by Bi³⁺ [7,15,16,17], Y³⁺ [18,19,20], Dy³⁺ [21] and Gd³⁺ ions [22,23]. We would wish to note here, that since the perovskite structure of SrTiO₃ is tolerant to substitutional dopants, whereas interstitial doping is not likely to occur in SrTiO₃ [24], doping in this work means ionic substitution.

Regarding Sr-site Zn-substituted ST, no ferroelectric phase transition but low-temperature dielectric permittivity increase with Zn content was reported in weakly (up to 0.9%) doped Sr_1−xZn_xTiO₃ ceramics sintered at 1400 °C by Guo et al. [25]. On the other hand, the room-temperature dielectric permittivity was reported to decrease with increasing ZnTiO₃ content in sol-gel derived SrTiO₃/ZnTiO₃ heterostructures annealed just at 750 °C by Li et al. [26] thus omitting the problem of high-temperature Zn volatility [27]. However, no low-temperature dielectric characterization was reported so far on moderately Sr-site Zn-substituted ST films. Therefore, in this work, we performed a structural, compositional, microstructural as well as variable temperature dielectric characterisation of sol-gel-derived Sr_1−xZn_xTiO₃ thin films with x = 0.01, 0.05, and 0.10, deposited on Pt/TiO₂/SiO₂/Si substrates and annealed at 750 °C.

2. Materials and Methods

For deposition of Sr_1−xZn_xTiO₃, thin films with x = 0.01, 0.05, and 0.10, solutions with a concentration of about 0.2 M were prepared using strontium acetate C₄H₆O₄Sr (98%, abcr GmbH, Karlsruhe, Germany), tetra-n-butyl orthotitanate C₁₆H₃₆O₄Ti (98%, Merck KGaA, Darmstadt, Germany) and zinc acetate-2-hydrate C₄H₆O₄Zn×2H₂O (99.5%, Riedel-de Haën, Seelze, Germany) as starting precursors. Acetic acid C₂H₄O₂ (99.8%, Merck KGaA, Darmstadt, Germany), 1,2-propanediol C₃H₈O₂ (99.5%, Riedel-de Haën, Seelze, Germany) and absolute ethanol C₂H₆O (99.8%, Merck KGaA, Darmstadt, Germany) were used as solvents. Strontium acetate was initially dissolved into heated acetic acid (T ~ 60 °C) followed by the addition of zinc acetate-2-hydrate under constant stirring to form a transparent solution. After cooling to room temperature, the former solution was diluted with 1,2-propanediol and then titanium isopropoxide was added. The resultant solution was continuously stirred in a closed flask for 12 h, at the end of which ethanol was added as a final step. Using these transparent and homogeneous solutions, layers of Sr-site Zn-substituted SrTiO₃ were deposited on Pt/TiO₂/SiO₂/Si substrates (Inostek Inc., Seoul, Republic of Korea) by spin-coating at 4000 rpm for 30 s, using spin-coater KW-4A (Chemat Technology, Los Angeles, CA, USA). Before the deposition, the substrates were cleaned in boiling ethanol and dried on a hot plate. After the deposition of each wet layer on the substrate, they were heated on a hot plate at 350 °C for ~1 min to ensure the complete removal of the volatile species between the layers. After the complete deposition of 10 layers, they were annealed in air at 750 °C for 60 min with a heating/cooling rate of 5 °C/min.

The thin film crystal phase was analysed at room temperature using a Rigaku D/Max-B X-ray diffractometer (Rigaku, Tokyo, Japan), using Cu Kα radiation. The X-ray diffraction (XRD) data were recorded in 0.02° step mode with a scanning rate of 1°/min from 20° to 80° using Cu Ka radiation. The lattice parameters were calculated from the XRD peak positions in the range of 30–60° using Bragg’s law. Compositional analysis of the films was conducted using an energy dispersive spectroscopy (EDS) system (QUANTAX 75/80, Bruker, Ettlingen, Germany) in the top-view geometry under an acceleration voltage of 10 kV of a scanning electron microscope (SEM, Hitachi TM4000Plus, Tokyo, Japan) to reduce the substrate contribution. The thickness of the thin films was determined and their cross-sectional morphology was also observed using SEM (Hitachi S4100, Tokyo, Japan) but under the acceleration voltage of 25 kV. For the film roughness and average grain size determination, a modified commercial atomic force microscope (AFM, Multimode Nanoscope IIIa, Veeco, Santa Barbara, CA, USA) with conductive hard Si tip cantilevers was employed. The topography images were processed using WSxMbeta6_0 software. Dielectric spectroscopy measurements of Sr-site Zn-substituted ST films were performed using Au, sputtered through a mask onto the films, as top electrodes, and the substrate Pt layer as the bottom one. Complex dielectric permittivity, consisting of real part ε′ and imaginary part ε″, as well as the dissipation factor tanδ = ε″/ε′, were measured under an oscillation voltage of 50 mV at a frequency of 10 kHz, using a precision LCR-meter (HP 4284A, Hewlett Packard, Palo Alto, CA, USA). A He closed-cycle cryogenic system (Displex APD-Cryostat HC-2, Allentown, PA, USA) equipped with silicon diode temperature sensors and a digital temperature controller, Scientific Instruments Model 9650, was used for temperature variation in the range of 10–300 K. Part of preparation and characterization procedure was performed according to the methodology in Ref. [28].

3. Results and Discussion

XRD profiles of 1%, 5%, and 10% Sr-site Zn-substituted ST thin films on Pt/TiO₂/SiO₂/Si substrates reveal the cubic perovskite-related peaks corresponding to Pm-3m structure of the films (PDF#35-0734) and those from the substrate, particularly Pt layer, as presented in Figure 1a. With increasing Zn content, positions of the peaks related to the perovskite structure slightly shift toward higher 2θ values as displayed in Figure 1b for the (200) peak. Accordingly, the lattice parameter decreases with Zn content, as shown in the inset of Figure 1b. Such a decrease proves that at least the major part of Zn is incorporated onto the Sr site of the SrTiO₃ lattice, taking into account that Sr²⁺ ionic size is larger than that of Zn²⁺, while Zn²⁺ ionic size is larger than that of Ti⁴⁺ [29]. Therefore, the formation of Sr_1-xZn_xTiO₃ solid solution is reasonable to be supposed from the lattice parameter decrease, since for SrTi_1-xZn_xO_3-δ solid solution the lattice parameter should increase with Zn content.

EDS analysis of Sr_1-xZn_xTiO₃ thin films, presented in Figure 2a, clearly displays the Zn peak, which intensity increases with the x-value. According to the spectra semi-quantitative analysis, Zn concentrations in Sr-site Zn-substituted ST thin films increase together with nominal ones, while overall estimated elemental contents indicate the proximity of all the film compositions to the nominal ones. Just in the case of x = 0.10 the real substitution content looks to be slightly lower than the nominal one, in agreement with a lower decrement of the lattice parameter compared to that for x = 0.05. Figure 2b shows the SEM cross-sectional microstructure of Sr_1−xZn_xTiO₃ thin films with x = 0.01, 0.05, and 0.10 grown on platinized silicon substrates. The film thickness for Sr-site Zn-substituted ST films is about 330 nm on average as seen in

Table 1. Average thickness, grain size, root mean square (RMS) roughness, peak dielectric permittivity value, peak dissipation factor value, and temperature at 10 kHz for Sr_1-xZn_xTiO₃ thin films deposited on platinized Si substrates.

Zn Content, x	Average Film Thickness (nm)	Average Grain Size (nm)	RMS Roughness (nm)	Peak ε′	Peak tan δ	tan δ Peak Temperature (K)
0.01	345 ± 30	141 ± 38	2.58 ± 0.08	412	0.046	85
0.05	260 ± 10	145 ± 41	0.88 ± 0.23	277	0.078	101
0.10	375 ± 5	86 ± 22	1.26 ± 0.36	229	0.097	105

. Moreover, the morphology of all the films reveals several rounded and closely packed grains across the film thickness.

From the AFM topological images, shown in Figure 3, the root mean square (RMS) roughness of the films is estimated to be below 2.7 nm, implying the applicability of the films for reliable macroscopic dielectric characterisation. The films reveal also a dense and crack-free microstructure with an average grain size of about 141–145 nm for x = 0.01 and 0.05 decreasing to 86 nm for Zn content of 10%, as also shown in Table 1 together with the standard deviation values.

The ε′(T) dependence of Sr_1-xZn_xTiO₃ thin films is shown in Figure 4 for a frequency of 10 kHz revealing an increase upon cooling until a diffuse peak at about 50 K. The permittivity value, increasing to 412 in peak for x = 0.01 compared with that of 267 in peak for identically prepared undoped ST film [1,16], decreases monotonously with further increasing Zn content. The temperature dependence of dissipation factor tanδ of Sr-site Zn-substituted ST films shown in Figure 4 presents up to three peaks at ~30 K, ~95 K, and ~185 K. While edge tanδ peaks diminish, the middle peak increases with Zn content, revealing values from about 0.5% to about 1.0%, while its position shifts from 85 to 105 K at 10 kHz.

The key microstructural and dielectric response parameters of the studied Sr-site Zn-substituted ST thin films are listed in Table 1. They show that increasing Zn content in Sr_1-xZn_xTiO₃ films from 1% to 5% and further to 10%, leads to lower dielectric permittivity and higher peak losses. Therefore, the permittivity increase reported by Guo et al. for the Sr_1-xZn_xTiO₃ system [25] should be valid for low x-values only, whereas for moderately Sr-site Zn-substituted ST system the permittivity decreases towards that reported by Li et al. for SrTiO₃/ZnTiO₃ heterostructures [26].

A similar variation of the permittivity with dopant content originated from the off-centrality of smaller ions on large Sr sites was reported for Ca-, Bi-, Gd-, Dy-, and Y-substituted ST systems [12,15,18,19,21,22,23]. At low content of the off-central ions, they induce independent polar dipoles, which can enhance the dielectric permittivity of the material. However, when the off-central ion concentration increases, the polar dipoles start to interact with each other that inhibits their ability to be re-orientated by an external electric field and hence makes the dielectric permittivity lower. In the case of Sr_1-xCa_xTiO₃, the permittivity was reported to be the highest for Ca content x = 0.0107, further dropping with x increase toward 0.12 [12]. For Sr_1-1.5xBi_xTiO₃ ceramics, the highest permittivity was observed at Bi content x = 0.0067 [15], while for Sr_1-1.5xM_xTiO₃ ceramics with M = Gd, Dy, or Y, the highest permittivity was also observed for x as low as 0.01 [19,21,23]. Therefore, the dielectric behaviour of Sr_1-xZn_xTiO₃ films of this study is in the line with the permittivity variations of other ST-based systems.

4. Conclusions

Sr-site Zn substitution was successfully performed in about 330 nm thick and below 2.7 nm rough sol-gel derived Sr_1-xZn_xTiO₃ films with x = 0.01, 0.05, and 0.10, deposited on Pt/TiO₂/SiO₂/Si substrates, and found to have a significant effect on the dielectric response. The observation of the Zn peak by EDS and the lattice parameter decrease with Zn content confirmed the substitution for Sr²⁺ by Zn²⁺ ions. Due to such substitution, the relative permittivity was found to decrease in the whole range from 10 K to room temperature for Zn content exceeding 1%. Additionally, a peak, which intensity increases and position shifts to a higher temperature with Zn content, is observed in the temperature dependence of the dissipation factor. Thus, this study helps to fill the gap in the literature regarding low-temperature dielectric characterization on moderately Sr-site Zn-substituted ST, showing that this system behaves similarly to other ST-based systems with Sr-site substitution by smaller ions.