Dielectric Properties and Impedance Spectroscopy of RF Sputtered Nanocrystalline (Mg0.95Zn0.05)TiO3 Films for Electronic Applications

Oxide thin lms attracted signicant attention because of their favorable responses, especially dielectric and stable electrical properties. In this study, nanocrystalline (Mg 0.95 Zn 0.05 )TiO 3 (MZT) lms are deposited on Pt/TiO 2 /SiO 2 /Si and quartz substrates by radio-frequency (RF) reactive magnetron sputtering. The effect of deposition gas ratio, i.e., the proportion of oxygen and argon (O 2 /Ar) on broad band and microwave dielectric properties and impedance spectroscopy of MZT lms are investigated. The dielectric properties showed a wide range of variation with the different O 2 /Ar ratios. The MZT lm deposited in a pure oxygen environment showed the best dielectric properties, and the plasma survived which shows that the target is not fully oxidized. The impedance spectroscopy of the lm deposited under a pure oxygen environment exhibited the single semi-circular arc. The Ag/ MZT/Pt thin lms capacitors exhibited the best dielectric constant with low loss tangent. The microwave dielectric constant measured at different spot frequencies in the GHz range displayed inferior dielectric response as compared to their bulk counterparts. An equivalent circuit model to explain the Nyquist plots of MZT lms are worked out. The observed electrical and dielectric characteristics of the thin lms suggest interesting applications in dielectric mirror and integrated circuits.


Introduction
The current trend in the global telecommunication industry is drastically favoring miniaturization technologies. Advanced research in dielectric thin-lm manufacturing plays a pivotal role in miniaturizing many microwave and integrated optical devices [1,2]. Such devices often require high-quality antire ection coatings, the manufacturing of which requires dielectric thin lms having superior optical properties [3]. Signi cant advancements in thin-lm capacitor technology can be expected through improvements in high dielectric constant materials, whereby capacitors with increased charge retention (due to low dielectric loss) and high charge density can be easily achieved.
MgTiO 3 based thin lms nd extensive applications compared to other material types due to their excellent dielectric, optical and electrical properties. To improve the utility of MgTiO 3 thin lms many alterations in the preparation technique, including varying the ionic substitution of A and B sites of MgTiO 3 are reported. Still, no reports are available on the electrical and dielectric properties of the lms of (Mg 0.95 Zn 0.05 )TiO 3 (MZT). Various research groups have elaborated their efforts to improve the quality factor of MgTiO 3 based ceramics, but reports involving the thin lm form of this composition are scarce [4,7]. In our previous investigation, we have studied the role of Zn on the dielectric properties of MgTiO 3 ceramics and optimized the best composition as MZT in the bulk form [8]. In the current study, an attempt has been made to prepare MZT thin lms using radio frequency (RF) magnetron sputtering. To study the role of nano crystallinity on the structural and electrical properties and compare them with the bulk parameters. The effect of oxygen and argon ratio (O 2 /Ar) during the deposition of Zn doped MgTiO 3 thin lms is studied thoroughly. The correlation between O 2 /Ar ratio induced structural modi cation and the corresponding changes in microstructural, dielectric, and electrical properties of MZT thin lms are investigated. Further, impedance spectroscopy is used to determine the contribution of grains and grain boundaries to capacitance, resistance, and conductivity of MZT thin lms.

Experimental Details
The MZT thin lms were deposited onto platinized silicon (Pt/TiO 2 /SiO 2 /Si) substrates at room temperature using RF reactive magnetron sputtering. The sputtering chamber was evacuated to a base pressure of 0.001 Pa while keeping the substrate to the target distance at 4 cm. Pre-sputtering was performed on the target in the Ar atmosphere for 10 min to clean the target's surface. The processing gas was a mixture of highly pure oxygen (99.999%) and argon (99.999%). The RF power (55 W) and the sputtering pressure (3 Pa) were maintained constant throughout the deposition process. Oxygen and argon (O 2 /Ar) ratio of (0

Characterization Details
The MZT thin lms' crystal structure was analyzed by using an X-ray diffractometer (XRD) (M/s Rigaku, TTRAX-III 18 kW). CuKα radiation (λ = 1.5406 Å) in parallel beam con guration was used. The annealed The MZT lms deposited at room temperature are amorphous with a strong glassy background. It is known that oxide lms have a natural tendency to grow in an amorphous state as there is no external energy provided to facilitate the crystallinity. Also, the lattice mismatch between the substrate and MZT lm is very high. Therefore to induce crystallinity, MZT lms were annealed at 600 o C for 1 h. Figure 1 shows the XRD patterns of the annealed MZT lms deposited under different O 2 /Ar ratios on platinized silicon substrates. All the MZT lms showed partial crystallinity with an amorphous background even after annealing. A very small and broad re ex of the MgTiO 3 phase is observed in the lms deposited in pure argon atmosphere ((0:1) O 2 /Ar).
The broadening of the peak indicates small grain size and more signi cant in uence from inter-grain boundaries. An increase in O 2 in O 2 /Ar ratio led to an increase in the peaks' amplitude. Some minor secondary peaks (Mg 2 TiO 4 ) are observed up to the O 2 /Ar ratio (3:1). The appearance of (Mg 2 TiO 4 ) is impressive as there would be a competition between MZT and Mg 2 TiO 4 . Mg 2 TiO 4 required lower energies than MZT, and MgTi 2 O 5 is entirely suppressed as it forms naturally during the deposition process. The improvement in the crystallinity with an increase in the O 2 ratio may be due to the variations in the ions' momentum during the deposition process. Slow deposition rate in the oxygen environment enhances surface diffusion, which can cause better adhesion, nucleation, and crystallinity [9]. Further, it is worth noting that even in the presence of pure oxygen, we could get an excellent quality thin lm indicating that the target is not oxidized entirely even in the presence of reactive gas. The highly intense and sharp re exes of the MgTiO 3

Dielectric properties
The microwave dielectric properties of MZT thin lms are measured using a split post dielectric resonator method at the spot frequencies 5, 10, and 15 GHz and are listed in Table 1. To measure the dielectric characteristics of MZT lms at microwave frequencies the resonance is adopted which gives the better accuracy. It is observed that with an increase in the O 2 percentage the dielectric response of the lms is enhanced. Further, with an increase in the measurement frequency, the dielectric response is diminished due to the reduction in the contribution from the polarization mechanisms. The improvement of dielectric properties of MZT lms with higher O 2 in the O 2 /Ar ratio can be correlated to reducing oxygen vacancies, development in grain size, and crystallinity of the lms [13]. The lms deposited under low O 2 in the O 2 /Ar ratio exhibited high dielectric losses, which might be due to oxygen vacancies and partial crystallinity. The measured dielectric response is almost in comparison with a bulk counterpart with an inferior loss tangent. The bulk MZT ceramics exhibit lower loss tangents due to the reduction in the grain boundary and strain as they were processed at high sintering temperatures. The dielectric properties of the MZT lms were measured over the frequency range of 1 kHz − 1 MHz and are displayed in Fig. 3. It is clear that at lower frequencies, the dielectric constant (ε r) is very high due to the contribution from all four types of polarization mechanisms [14]. As the measurement frequency increases, the dipoles cannot align with oscillations of frequency signal which reduces the dielectric constant whereas the loss tangent of MZT lms is lower which is the inherent nature of this lm. The

Impedance spectroscopy
The complex impedance spectroscopy (Nyquist plots) is a bene cial technique used to complete the electrical properties of the polycrystalline ceramics, which include electrode and the grain and grain boundaries contributions. The Nyquist plot is well known for the representation of grain and grain boundary response of polycrystalline ceramics [15]. The Nyquist plot usually consists of two adjacent semicircles. The lower frequency semicircle is due to the grain boundary's contribution, and the grain effect is responsible for the semicircle observed in the higher frequency range [16]. Figure 4 shows the Nyquist plots of the MZT lms deposited under different O 2 /Ar ratios. All the lms exhibited a semi-circular behavior representing the grain and grain boundary response. The experimental impedance data are tted (shown as "calculated" in Fig. 4) with an equivalent circuit consisting of a resistor (R) and capacitor (C), with the help of ZsimpWin 3.20 software (M/s Echem software, Ann Arbor, Michigan, USA). A series of two sub-circuits composing of resistor and capacitor connected in parallel is well tted for all the lms, which simulate the impedance contribution of grain (Cg, Rg) and grain boundary (Rgb, Cgb), respectively, and the obtained tted values are listed in Table 2. It is observed that the grain boundary effect is minimized, whereas the grain effect is enhanced with an increase in O 2 concentration in the O 2 /Ar ratio which lead to higher dielectric constant and lower loss tangent. This might be due to the lms' large grain size, as seen in the AFM images (Fig. 2).  To see the in uence of temperature on the impedance, the lm capacitor fabricated under (1:0) O 2 /Ar is exposed to different temperatures, and Nyquist plots are shown in Fig. 5a & 5b. It is observed that only one semi-circular arc appears up to 433 K. But, from 478 K, a second semi-circular arc started appearing in the low-frequency range. The low-frequency arcs at higher temperatures are due to grain boundary response. All the measured data (msd) are tted (shown as "cal" in Fig. 5a & 5b), and obtained values are listed in Table 3. It is observed that the value of Rg decreases with the temperature rise, demonstrating the negative temperature coe cient of resistance of these lms. This is probably caused by the enhanced movements of defects and charge carriers present in the grain interior and interfacial region of the grains [17] due to higher thermal energy.  6.438 x 10 -11 6.341 x 10 -11 6.372  Figure 6 shows the frequency dependence of the real part of impedance (Z′) at different temperatures for MZT thin lm, deposited under (1:0) O 2 /Ar. It is seen that the value of Z′ decreases with a rise in both temperature and frequency. In the low-frequency region, Z′ shows the sigmoidal variation with frequency. Further increase in frequency, the value of Z′ decreases and attains a constant value irrespective of temperature for all the lms. The reduction in the value of Z′ with the increase in frequency indicates a slow dynamic relaxation process in the lm, which may be attributed to the space charge carriers' mobility. The decrease of Z′ with a rise in temperature indicates an enhancement in the conduction process [18].
The frequency dependence of the imaginary part of impedance Z″ studied at different temperatures, and the results are shown in Fig. 7. A single peak is observed above 100 kHz up to temperature 433 K. From 478 K onward another small peak appears in the low-frequency region. This peak is correlated to the grain boundary contribution, while the other prominent peak in the high-frequency area is associated with the grain response. This peak moves towards higher frequencies with a temperature rise, and a broadening in the curves is observed with a reduction in peak height. The expansion of the peak with an increase in temperature suggests a temperature-dependent electrical relaxation phenomenon present in the sample. The decline in Z″ value with temperature indicates an enhancement in a loss in the lm's resistive response [19].

Conclusions
Nanocrystalline thin lms of MZT ceramics are deposited onto platinized silicon and quartz substrates using RF magnetron sputtering. The XRD patterns con rms that phase pure MZT dielectric thin lms are deposited under pure oxygen plasma. The dielectric constant and loss tangent values are progressively improved with an increase in O 2 in the deposition gas. The maximum values of ε r~ 12.72 are obtained for MZT thin lm deposited under (1:0) O 2 /Ar due to the higher crystallinity and increased grain size. The impedance behavior of the MZT lms is analyzed thoroughly using the Nyquist plot. An equivalent circuit model is well tted for the lms. The observed dielectric characteristics of the thin lms suggest interesting applications in dielectric mirror and integrated circuits.