Structural, optical, dielectric and electrical studies on RF sputtered nanocrystalline Zr doped MgTiO₃ thin films

Highlights

• Nanocrystalline MZT film capacitors were fabricated using RF magnetron sputtering.

• Highly (1 1 0) crystal orientation is observed in pure O₂ plasma.

• MZT Nyquist plots show two semicircular arcs with nonzero high frequency intercept.

• DC conductivity with frequency follows Jonscher law with an activation energy 0.32 eV.

• Obtained MZT properties are suitable for integrated, CMOS and optical devices.

Abstract

Mg(Zr_0.05Ti_0.95)O₃ (MZT) thin films have been deposited onto amorphous SiO₂ and platinized silicon (Pt/TiO₂/SiO₂/Si) substrates by RF magnetron sputtering at different oxygen mixing percentages (OMP). The effect of OMP and post annealing temperature at 600 °C on crystal orientation, surface morphology, optical, electrical and dielectric properties of crystallined MZT films are investigated. A change of preferred orientation from (1 0 4) to (1 1 0) has been observed with increasing OMP from 0% to 100%. The preferred orientation growth has been explained on the basis of Lotgering model by calculating the orientation factor. The optical properties such as refractive index and bandgap were strongly influenced with O₂ content. It is also observed that the refractive index and packing density decreases with increasing OMP, leading to an enhancement in the optical bandgap from 4.19 to 4.52 eV. The dielectric constant and dielectric loss of the films ranged between 5.0–15.69 and 0.082–0.0135. The MZT thin films deposited at 100% OMP exhibited the high dielectric constant and low loss tangent. Further, the DC conductivity with frequency follows Jonscher law and the activation energy found to be 0.32 eV for MZT films deposited in pure oxygen atmosphere. Nyquist plots of MZT films revealed two semi circular arcs and are explained on the basis of equivalent circuit model. A series of two parallel RC combination in series fit with experimental data. The I–V characteristics reveals that the leakage current density decreases from 1.136 × 10⁻⁸ to 4.69 × 10⁻⁹ A/cm² for OMP, 0–100%, respectively for electric field strength of 200 kV/cm.

Introduction

Recently, dielectric thin films have attracted much attention due to their diverse applications in integrated optic and microwave devices. In integrated optic applications, dielectric thin films are very useful since dielectric materials offer various functions, such as optical switching, modulation and coupling in optical communications [1]. Moreover, thin film fabrication technology makes the integration of different devices feasible. The recent rapid growths of microwave communications require dielectric materials, which have a high dielectric constant, low dielectric loss, and better thermal stability in microwave frequencies [2]. The continuing miniaturization of microwave devices demands dielectric materials in a thin film form for integration to other devices [3].

MgTiO₃ (MTO) is one of the leading dielectric materials for microwave frequency applications due to promising microwave dielectric properties in bulk form. It possesses high dielectric constant (ε_r ∼ 17), a high quality factor (Q × f ∼ 160,000 GHz) and a negative temperature coefficient of resonant frequency value (τ_f ∼ −50 ppm/°C) [2], [4]. It is interesting to note that the MTO also exhibits similar properties even in the thin film form [5]. In addition to its excellent dielectric properties, MTO thin films also exhibit good optical properties, which find applications in antireflecting coatings, wave guides, and high permittivity insulating coating in electronics devices and as protective coatings [5], [6]. Further, dielectric materials in a thin film form can be fabricated at low processing temperatures and, thus they can be integrated into monolithic integrated devices [7]. Lee and Choi [8] reported the growth of MgTiO₃ thin films by the sol–gel process as a buffer layer for epitaxial LiNbO₃ growth on the c-plane Al₂O₃ substrates for application to integrated optic devices. Furthermore, these MgTiO₃ films have been also used as a cladding layer for LiNbO₃ thin film optical waveguide. Surendran et al. [9] reported that substitution of Zn²⁺ and Ni²⁺ into Mg²⁺ site significantly increased the dielectric permittivity and reduces the losses and the corresponding values were tan δ ≈ 1.1 × 10⁻², 1.9 × 10⁻² for 5 wt% Ni and Zn substitution, respectively. Ho and Huang [10] studied the effect of crystal orientation on the photoluminescence spectra of pure MTO thin films deposited on P-type Si(1 1 1) oriented substrates. They reported that at 800 °C annealing temperature, MTO grain growth occurred along (0 0 3) orientation at an argon to an oxygen ratio of 60/40 sccm. However, MgTiO₃ thin films grown by PLD method on c-axis oriented Al₂O₃ substrates are highly oriented along (0 0 3) direction and exhibited very high dielectric constant 24 at 1 MHz [11]. Recently, Huang et al. [12] reported the electrical properties of the MgTiO₃ based metal–insulator–metal (MIM) devices at various temperatures. Consequently, the MIM capacitor fabricated at 200 °C with 210 nm thick MgTiO₃ shows very high density of capacitance ≈ 1.2 nF/μm² and low leakage current 1.51 × 10⁻⁹ A/cm² at 5 V. The present authors have reported the growth and characterization of (Mg_0.95Co_0.05)TiO₃ (MCT) thin films under different oxygen mixing percentage (OMP). The (1 1 0) oriented growth of MCT films at higher OMP (75%) and improved dielectric properties (ε_r ∼ 12.2), low leakage currents (J = 1.14 × 10⁻⁹ A/cm²) and losses (tan δ = 1.2 × 10⁻³) in 5 at.% Co containing MCT thin films deposited by RF magnetron sputtering [13].

Tseng [14] reported Zr = 0.05 substitution in B site of MTO and exhibited a dielectric constant of ∼18.1, a Q × f product of 380,000 GHz and a temperature coefficient of resonant frequency ∼−50 ppm/°C. Due to these excellent properties of MZT ceramics in bulk form, they also studied the optical and electrical properties of Mg(Zr_0.05Ti_0.95)O₃ thin films grown on Si substrates by sol–gel method. They reported the optical bandgap ∼3.7 eV and ε_r ∼ 9 for the 800 °C annealed crystallined thin films [15]. Further, the above films exhibit MgTi₂O₅ as a secondary phase. However, to the best of the authors, knowledge there is no study on RF sputtered Zr doped MTO thin films. Sputtering has attracted great attention due to its simplicity and unique features it offers such as low temperature crystallinity, large throughput, uniform and higher rate of depositions, control over stoichiometry, and it is a cost-effective process. Hence, in the preset study, the effect of processing parameters such as oxygen mixing percentage (OMP), annealing temperature on the crystal orientation, optical, electrical and dielectric properties of Mg(Zr_0.05Ti_0.95)O₃ thin films are studied systematically.