Dielectric Properties of Mg₂TiO₄-Doped Ca_0.65Sr_0.35Zr_0.65Ti_0.35O₃ with High Withstand Voltage and Low Loss

Abstract

Mg₂TiO₄-doped Ca_0.65Sr_0.35Zr_0.65Ti_0.35O₃ (CSZT) thin films with different Mg₂TiO₄ concentrations were deposited on the LaNiO₃(LNO)/p-Si substrate using radio-frequency magnetron sputtering technology. The dielectric response of the prepared x% Mg₂TiO₄-CSZT thin films with frequency, voltage, and temperature was systematically studied. The tanδ and leakage current density of CSZT thin films were reduced effectively by introducing Mg₂TiO₄ content. The prepared 6% Mg₂TiO₄-CSZT thin film, due to its low loss (tanδ ~0.01 at 1 MHz), satisfied temperature stability (TCC ~−68 ppm/°C, from −55 °C to 205 °C), high withstand voltage (>160 V), and small leakage current density (about 3.34 × 10⁻⁶ A/cm² at operating voltage of 160 V). This may be useful for capacitor materials in the next generation of portable electronic systems.

1. Introduction

Thin film capacitors are considered one of the most strategic technologies for future integral capacitors to meet the needs of miniaturized electronic equipment [1,2,3,4]. Among them, the thin film chip capacitors are recommended as candidate products in the field of handheld products. It is agreed that the dielectrics with satisfied stability in frequency, voltage, and temperature are still the key capacitor materials in the next generation portable electronic systems. Linear dielectric thin films are expected to be candidate materials for the next generation of integrated capacitor materials due to their high dielectric breakdown strength, low loss, and high stability in frequency and temperature [5]. However, traditional linear dielectrics such as SiO₂, Al₂O₃, and Ta₂O₅, etc., are limited to achieve a high specific capacitance due to their low dielectric constant [6,7,8] (ε ~3.9 for SiO₂, ~7 for Al₂O₃, ~26 for Ta₂O₅).

For decades, different approaches have been carried out to develop high dielectric response performance dielectric thin films. It is notable that the high dielectric permittivity of 55 was achieved in (Ca_0.7Sr_0.3)(Zr_0.8Ti_0.2)O₃ thin films by Lee et al. [9]. The loss of (Ca_0.7Sr_0.3)(Zr_0.8Ti_0.2)O₃ thin films was 0.015 at 1 MHz, and the leakage current density reach to 5 × 10⁻⁶ A/cm² at voltage of 12 V [10]. The results show that the (Ca_0.7Sr_0.3)(Zr_0.8Ti_0.2)O₃ thin film could be one of the most potential materials in the field of advanced capacitors. Similar research was reported in the Ca_0.65Sr_0.35Zr_0.65Ti_0.35O₃(CSZT) thin films on Pt/SiO₂/Si substrate by our group [11]. At room temperature, the dielectric permittivity, ε_r ~56.4, low tanδ ~0.003 at 1 MHz and leakage current density ~1.1 × 10⁻⁷ A/cm² at the bias voltage of 80 V were achieved. Further investigations reveal that the loss of the CSZT thin films on p-type Si substrate is higher than that on Pt/SiO₂/Si substrate and the TCC (temperature coefficient of capacitance) increased from +23.3 ppm/°C to +135 ppm/°C. For a practical integral capacitor application, p-type Si substrate is a more flexible choice. The development and improvement of the dielectric properties of CSZT thin films deposited on p-type Si substrate are necessary.

LaNiO₃ (LNO) is a type of metallic oxide with a pseudo-cubic structure and is often used as a buffer layer to eliminate the interfacial dead layer between the ferroelectric thin film and the Pt electrode [12,13,14]. In addition, LNO was reported with the capability of contributing to the preferential orientation growth in lots of ABO₃ thin films [15,16]. Preferential orientation has been proven as an important approach to enhance the performance of the thin films. Therefore, the LNO layer was also used in this work as a buffer layer between the CSZT and p-type silicon substrate.

In present work, the dielectric properties of Mg₂TiO₄-doped CSZT thin films are reported. Previous studies have reported that non-ferroelectric materials, such as MgO, MgTiO₃, Mg₂TiO₄, Mg₂SiO₄, BaTi₄O₉, etc., are helpful to reduce the dielectric loss [17,18,19,20,21,22,23,24]. The x% Mg₂TiO₄-CSZT thin films were deposited on LNO/p-Si by radio frequency magnetron sputtering technology and the LNO was deposited as buffer between the CSZT thin film and the p-type silicon substrate. The work aimed to obtain a reliable high performance thin film for capacitor application which not only has a moderate dielectric permittivity and low loss, but also small TCC in wide temperature range.

2. Experiments

In this work, sol-gel process was employed to synthesis x% Mg₂TiO₄ doped CSZT and LNO powders, respectively. The targets used to prepare x% Mg₂TiO₄ doped CSZT thin film and LNO buffer layer were made of the x% Mg₂TiO₄ doped CSZT and LNO powders, respectively. The Ca(CH₃COOH)₂·2H₂O (Aladdin, 98%), Sr(NO₃)₂ (Chron, 99.5%), Ti(OC₄H₉)₄ (Aladdin, 98.5%), Zr(NO₃)₄·5H₂O (Chron, 99.28%), and Mg(CH₃COO)₂·4H₂O (Greagent, 99.0%), weighted according to the x mol% Mg₂TiO₄ - Ca_0.65Sr_0.35Zr_0.65Ti_0.35O₃ stoichiometry, were used to synthesize x% Mg₂TiO₄ doped CSZT precursor sols. The LaNiO₃ stoichiometric La(NO₃)₃·6H₂O (Chron, 99.00%) and Ni(CH₃COO)₂ ·4H₂O (Chron, 98.00%) were weighted to synthesize LNO precursor sol. After the sols changed into wet gels, the wet gels were dried into xerogels by rotating evaporation at 80 °C. Then, the x% Mg₂TiO₄ doped CSZT xerogels and LNO xerogel were calcined 3 h at 1350 °C and 800 °C to form the x% Mg₂TiO₄ doped CSZT and LNO ceramic powdesr, respectively. Here, x% = 0, 2, 4, 6, and 8 in mol%, respectively. The deposition process of LNO buffer layer and x% Mg₂TiO₄-CSZT thin films were carried out on p-type Si substrates using a Radio Frequency Magnetron Sputtering system (RFMS, Sky Technology Development, JGP560D, Shenyang, China). The specific process parameters of x%-Mg₂TiO₄ CSZT films and LNO buffer layer are shown in

Table 1. The sputtering process parameters of x% Mg₂TiO₄-CSZT thin films and LNO buffer layer.

Target	x% Mg2TiO4-CSZT	LNO
Substrate distance	4 cm	4 cm
Substrate rotation rate	10–20 r/min	10–20 r/min
Base vacuum	4.0 × 10−4 Pa	4.0 × 10−4 Pa
Sputtering temperature	575 °C	550 °C
Sputtering time	600 min	30 min
Sputtering power	70 W	40 W
Sputtering gas pressure	2 Pa	2 Pa
Sputtering atmosphere ratio	Ar/O2 = 4:3	Ar/O2 = 4:1

. All the prepared thin films were conventional thermal annealed (CTA) in air at 700 °C for 180 min to obtain well-crystallized grains. A 0.3 mm diameter Au topped circular electrode was prepared on the CSZT layer by electroplating.

The crystal phase of x% Mg₂TiO₄-CSZT thin films were measured using X ray-diffraction (DX-2700, Dandong, China) with Cu Kα radiation obtained under a Ni filter (Range: 2θ = 20°~70°, Step angle: 0.2°/step, Sampling time: 0.5 s/step, Tube voltage: 40 kV, Tube current: 30 mA). The thickness of the prepared thin films was investigated by field emission scanning electron microscope (Thermo Scientific, Helios G4 UC, Waltham, MA, USA) with an accelerating voltage of 15 kV. The capacitance–frequency curve (C-f curve) from 10 kHz to 1 MHz and the capacitance-voltage curve (C-V curve) measured by using a multi-frequency Inductance-Capacitance-Resistance (LCR) meter (Agilent, HP4294A, Santa Clara, CA, USA), for C-V curve measurement, the thin films were measured with 100 kHz and a dielectric spectrum oscillation voltage of 500 mV at room temperature. The leakage current density of the prepared thin films was measured by using a Radiant Precision Workstation (Radiant, Precision Multiferroic II, Albuquerque, NM, USA) at room temperature. The temperature-related properties were carried on a heating probe stage (Linkam, HFS600E-PB4, Surrey, UK) with a temperature rising rate of 3 °C/min.

3. Results and Discussion

The XRD patterns of x% Mg₂TiO₄-CSZT thin films measured at room temperature are shown in Figure 1. The thin films were deposited on LNO/p-Si and annealed at 700 °C for 3 h. All the x% Mg₂TiO₄-CSZT thin films show the significant (l0l)-preferred orientation growth characteristic (according to PDF#35-0645). Clearly, the (l0l)-preferred orientated LNO buffer layer plays a key role here. Among them, the 0%, 2%, and 4%Mg₂TiO₄-CSZT thin film show the only perovskite phase. The 8% Mg₂TiO₄-CSZT thin film displays clear second phase diffraction peaks at ~43° and ~74° (marked with •). The two peaks are associated with the diffraction peaks of MgO (according to PDF# 87-0653). It suggests that Mg₂TiO₄-CSZT is limited solid solution and small amount of MgO separated out from the 8%Mg₂TiO₄ doped CSZT solid solution [25]. Furthermore, compared with pure CSZT thin film sample, the orientation of LNO buffer layers are weakened due to the growth of Mg₂TiO₄ doped CSZT thin film on them. As a result, the LNO (021) diffraction peaks could be observed in the Mg₂TiO₄ doped CSZT thin film XRD patterns. As a close observation, all the LNO (101) and (202) peaks (sited ~23°and ~47°) of those Mg₂TiO₄-CSZT thin film samples shift to left (see Figure 1). It is likely that the Mg₂TiO₄ doped CSZT interpenetrated the LNO layer at the interface, resulting in the orientation degree of LNO reducing, and more LNO diffraction peaks could be observed.

Figure 2 shows cross-section morphology of the prepared thin films by SEM micrographs. All the thin films show dense, crack free, and uniform microstructure. The columnar crystalline grain of x% Mg₂TiO₄-CSZT grown on the LNO layer is clearly visible. Additionally, it could be noticed that the thicknesses of the thin films are different even though they were prepared in the same time. The thicknesses and growth rates of the thin films were listed in

Table 2. The thickness and the growth rate of the x% Mg₂TiO₄-CSZT/LNO thin films.

x (%)	0	2	4	6	8
Thickness of the x% Mg2TiO4-CSZT/LNO layer (nm)	972 (±2)	732 (±1)	934 (±3)	995 (±3)	1137 (±2)
Growth rate of x% Mg2TiO4-CSZT films (nm/min)	1.40	1.09	1.41	1.49	1.71

. The growth rate of the x% Mg₂TiO₄-CSZT thin films increases significantly with the increase in the Mg₂TiO₄ content when the content is more than 2%.

The dependence of DC voltage and frequency to dielectric permittivity and dielectric loss were measured at room temperature and shown in Figure 3. Both dielectric permittivity-frequency curve and the dielectric permittivity-DC voltage curve display the frequency independent (see Figure 3a) and operating voltage independent (see Figure 3b) characteristics for all the prepared thin films. The results indicate that all the prepared thin films behave with a typical linear dielectric response, even the Mg₂TiO₄ doped CSZT thin films. Additionally, small amounts of Mg₂TiO₄ leads to a small increase in dielectric permittivity; the dielectric permittivity will reduce if the Mg₂TiO₄ content is higher than 4%.

Figure 3c shows the dielectric loss-frequency curve of all the thin films. The dielectric loss of all the thin films is no mre than 0.0175 at 1 MHz. Figure 3d shows the DC field dependence of the dielectric loss (tanδ) of all the thin films. The measured signal is an AC signal with an amplitude of 0.5 V and a frequency of 100 kHz. The DC voltage ranges from −40 V to 40 V. The tanδ of all the thin films is no more than 0.006 in the measured range. The tanδ valus change rarely with the increase in voltage value, indicating the typical linear dielectric response feature which is consistent with the result of Figure 3b. For the thin film x = 6%, the dielectric permittivity ε_r is around 50 at 1 MHz and tanδ is ~0.01 at 1 MHz which is lower than reported reference [10].

The dielectric response-temperature curve of x% Mg₂TiO₄-CSZT thin films is shown in Figure 4. The measured temperature ranges from −55 °C to 205 °C. Compared with the 0% Mg₂TiO₄–CSZT thin film, the dielectric permittivity of 6% Mg₂TiO₄-CSZT thin film shows the smallest temperature changing rate, the TCC value, ~−68 ppm/°C among all the prepared thin films. The TCC values of all the thin films ranging from −55 °C to 205 °C are around −135 ppm/°C, −98 ppm/°C, −78 ppm/°C, −68 ppm/°C, and −97 ppm/°C for x = 0, 2, 4, 6, and 8, respectively. The results reveal that moderate Mg₂TiO₄ are helpful to improve dielectric response-temperature characteristic. Meanwhile, the tanδ of the x% Mg₂TiO₄-CSZT thin films increase with the incerasing of temperature mildly below 100 °C, then the increase becomes significant and reaches 0.02 at 160 °C. For the thin films with x = 6 and 8, the tanδ values are still smaller than 0.01 at 120 °C. Compared with the other linear dielectrics reported in the previous work (see Figure 5), 6% Mg₂TiO₄-CSZT thin film show ideal dielectric permittivity temperature stability, TCC value is second only to that of SiO₂. It is noticed that 6% Mg₂TiO₄-CSZT thin film keeps small temperature changing rate, TCC value (~−68) in a wide temperature range (−55 °C~205 °C).

Figure 6 shows the leakage current density of the prepared x% Mg₂TiO₄-CSZT thin films at room temperature. The measured voltage ranges from −160 V to 160 V. At the voltage of 160 V, 8% Mg₂TiO₄-CSZT thin film has the lowest leakage current density (1.61 × 10⁻⁶ A/cm² at ±160 V), the leakage current density of 6% Mg₂TiO₄-CSZT thin film is 3.34 × 10⁻⁶ A/cm² at ±160 V) which is lower than reported in reference [9] and [10]. The results indicate that the proper Mg₂TiO₄ introduction can decrease the leakage currents effectively.

4. Conclusions

Investigations on the dielectric properties of x% Mg₂TiO₄-doped Ca_0.65Sr_0.35Zr_0.65Ti_0.35O₃ thin films showed that the addition of Mg₂TiO₄ can effectively reduce the dielectric loss and leakage current density of CSZT thin films. The prepared 6% Mg₂TiO₄-CSZT thin film exhibits a satisfying comprehensive performance of ε_r ~50.00 at 1 MHz, tanδ ~0.01 at 1 MHz, TCC ~−68 ppm/°C from −55 °C to 205 °C, and about 3.34 × 10⁻⁶ A/cm² at operating voltage of 160 V compared with the reported linear dielectrics given in reference [26,27], and thus could be a suitable capacitor material in the next generation portable electronic systems.