Structure of Ferroelectric (Ba,Sr)TiO3 Thin Films for Tunable Microwave Devices

Ferroelectric (Ba,Sr)TiO₃ films have been deposited on (001) MgO single crystals by a pulsed laser deposition with oxygen background while heating the substrates. Deposited BST films exhibit epitaxial growth along (001), which are confirmed by x-ray diffraction measurement. Structure of (Ba,Sr)TiO₃ along in-plane and surface normal direction have been investigated and found to have a tetragonal distortion depend on the deposition conditions, such as oxygen pressure. Lattice parameter decreases with increasing oxygen pressure, and tetragonallity (c/a) changes from 1.005 to 0.997 as oxygen pressure increase. Interestingly, energy gap measured by FTIR decreases with decreasing oxygen pressure until it reach a certain oxygen pressure, then increases again with increasing oxygen pressure. Furthermore, microwave properties of devices measured by a HP 8510C vector network analyzer from 0.045–20 GHz suggest that the least distorted films exhibit a larger dielectric constant changes with dc bias field.     

Optimum Reflection-Type Phase Shifter Using (Ba,Sr)TiO3 Thin Film

In this paper, in order to obtain a large differential phase shift with a little change in applied voltage, a ferroelectric reflective load circuit has been designed on top of barium strontium titanate (Ba,Sr)TiO₃ [BST] thin film. The design of the ferroelectric reflection-type phase shifter is based on a reflection theory of terminating circuit, which has a reflection-type analogue phase shifter with two ports terminated in symmetric phase-controllable reflective networks. To achieve large amounts of phase shift in low bias-voltage range, the effects of change of capacitance and transmission line connected with two coupled ports of a 3-dB 90° branch-line hybrid coupler have been investigated. A large phase shift with a small capacitance change in the parallel terminating circuit has been demonstrated in the paper.     

Calculation of the intrinsic dead layers thicknesses from Au/Ba0.5Sr0.5TiO3/Pt thin film capacitors

Ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films were prepared on Pt/Ti/SiO₂/Si substrates by the sol-gel process. The films were spin-coated at 2000 rpm for 30 secs and then pyrolysed for 5 mins at the temperature of 350^∘C. This coating procedure was repeated for 3, 4, 5 and 6 times to obtain BST films with different thicknesses. After coating the films with the desired repetition times, the films were finally annealed in a conventional furnace at temperatures ranging from 600^∘C to 800^∘C with a 50^∘C interval in between. The films obtained with an annealing procedure of 750^∘C were polycrystalline with the presence of an impurity BaCO₃ phase. The capacitance and leakage current were measured and used to extract information on the metal-BST interface. With the series capacitance model and modified Schottky emission equation, the thickness of the dead layers for Au/BST and Pt/BST interfaces were calculated to be less than 6 nm and 5 nm, respectively.     

Pulsed Laser Deposited Ba(Mg1/3Ta2/3)O3 Microwave Dielectric Thin Films

Ba(Mg_1/3Ta_2/3)O₃ (BMT) microwave dielectric thin films were successfully synthesized by a modified pulsed laser deposition (PLD) process, which includes low temperature (200°C) deposition and high temperature (>500°C) annealing. Crystalline structured BMT thin films were obtained when the PLD-deposited films were post-annealed at a temperature higher than 500°C in oxygen atmosphere. The characteristics of BMT thin film, including crystallinity, grain size, film roughness, and dielectric properties were improved with annealing temperature, achieving dielectric constant K = 23.5 and dissipation factor tan δ = 0.015 (at 1 MHz) for the 800°C-annealed films.     

Growth,Microwave Properties and Microstructure of Ba0.05Sr0.95TiO3 Thin Films

Charged defects were found to have a significant influence over the microwave properties of Ba_0.05Sr_0.95TiO₃ thin films.     

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Pulsed Laser Deposition of Ba0.6Sr0.4TiO3 Thin Films and Their Optical Properties

ABSTRACT

Barium strontium titanate [Ba_0.6Sr_0.4TiO₃ or BST (60/40)] thin films were deposited on MgO (100) substrates using pulsed laser deposition. X-ray diffraction (XRD) measurements revealed that the BST thin films had epitaxially grown on the MgO (100) substrates. The surface morphology of the thin films was observed using an atomic force microscope and the grain size was found to be about 100–150 nm. The surface roughness was around 4.9 nm for a 250 nm thick film. The optical transmittance of the BST thin film was measured using a transmission mode ellipsometer. The BST/MgO configuration was highly transparent in the visible region. The optical band gap energy of the BST film, calculated by applying the Tauc relation, was 3.56 eV. Optical waveguide characteristics of the BST (60/40) thin film were determined using a prism coupler. The electro-optic (E-O) properties were measured at 632.8 nm wavelength using a phase modulation detection method. The BST film exhibited a predominately quadratic E-O behavior and the quadratic E-O coefficient was found to be 0.58 × 10^{? 17} m²/V².     

STRESS CONTROLLED FERROELECTRIC (BA0.5Sr0.5)TiO3 THIN FILMS PREPARED ON GD2O3/SRTiO3(001) BY A PULSED LASER DEPOSITION

ABSTRACT

Stress controlled epitaxial ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited on Gd₂O₃/SrTiO₃ by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd₂O₃ buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd₂O₃ were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd₂O₃ layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.     

Measurement of Microwave Dielectric Properties of (Sr,Ba)Nb2O6 Thin Films

(001) oriented (Sr,Ba)Nb₂O₆ (SBN) thin films were deposited on MgO (001) single crystal substrates by the pulsed laser deposition method. Structural properties of SBN films were investigated using X-ray diffractometer. The microwave dielectric properties of SBN films were examined by calculating the scattering parameter obtained using a HP 8510C vector network analyzer with the frequency range 0.5–20 GHz at room temperature under the dc bias field of 0–80 kV/cm for interdigital capacitors (IDT) and coplanar waveguide (CPW) device based on SBN/MgO layer structure. Thick metal electrode patterns were fabricated by dc sputtering deposition, photolithography and etching process. The IDT device based on (001) oriented SBN films exhibited about 40% capacitance tunability with an electric field change of 80 kV/cm at room temperature, and the dielectric quality factor was about 20 at 12 GHz with no dc bias.     

MOCVD of (Ba,Sr)TiO3: Nucleation and Growth

(Ba_0.7Sr_0.3)TiO₃ and SrTiO₃ thin films were deposited on Pt electrodes in a planetary multi-wafer MOCVD reactor. The nucleation behavior and the size of the stable nuclei were investigated by different SPM techniques. Characteristic differences were observed for different deposition temperatures, i.e. a homogeneous nucleation of small BST grains on the larger Pt grains at 565°C and a dominating nucleation at the grain boundaries at 655°C. The micro structural evolution after further film growth was investigated by HRTEM and revealed randomly oriented grains (typical inplane size 10–20 nm) with a high density of twins at 565°C and (100)-oriented defect free grains of only slightly increased size at 655°C. For SrTiO₃ the inplane grain size was increased, however, the (100) texture was less perfect. As the electrical properties like permittivity and also leakage current depend on film thickness the final discussions of the electrical properties are based on thickness series (5 nm–100 nm films) and evaluated within the phenomenological dead layer model.     

Coplanar Waveguide Using Ferroelectric Thin Oxide Film: Dielectric Constant

Coplanar waveguide (CPW) transmission lines were fabricated on thin ferroelectric Ba_{1 – x}Sr_xTiO₃ films for tunable microwave applications. The growth of the ferroelectric oxide films was accomplished by a pulsed laser deposition with a partial oxygen background. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.045–20 GHz with –40–40 V of dc bias. A large phase shift angle of 120 at 10 GHz was observed from the CPW (gap = 4m, length = 3 mm) with a 40 V of dc bias change. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameter by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film exhibits a gap dependency; dielectric constant (990–830) decreases with increasing gap size (4–19 m, respectively). By adjusting the filling factors of the film, a constant dielectric constant of BST film is found to be 810 ± 5.     

Annealing Effects on Structural and Dielectric Properties of Tunable BZT Thin Films

Ba(Zr, Ti)O₃ thin films have attracted great attention in recent years for their potential use in DRAMs and MCMs due to their high dielectric constant and relatively low leakage current. However, their tunable dielectric properties were rarely investigated and the corresponding potential for tunable microwave applications was seldom reported. In this paper, we present the tunable dielectric behavior of BZT thin films deposited by RF magnetron sputtering from a Ba(Zr_0.3Ti_0.7)O₃ ceramic target on MgO single crystal substrates. The composition, thickness and crystallinity of the thin films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The dielectric constant and loss tangent were measured as a function of electric field (0–7 kV/mm) and temperature (–140 to +160°C) at frequencies up to 1 MHz, using interdigital capacitors (IDC) with Au electrodes on thin films. By optimizing the preparation process, a tunability {defined as = [ (0) – (E_max)]/ (0)} of 76% at E_max = 7 kV/mm and a low loss tangent of 0.0078 can be achieved. In addition, the influence of annealing temperature on the dielectric properties of the thin films is also discussed.     

Dielectric properties of ferroelectric (Ba0.6Sr0.4)TiO3 thick films prepared by tape-casting

Thick BST films have been fabricated by a tape casting and firing method. Dielectric constants of BST films are changed from 5700 to 7000 at 1 MHz after focused beam annealing. Furthermore, surface morphologies and depth profile of chemistry have been altered after annealing. Especially, Sr atoms diffuse out to the surface, while Ba atoms diffuse into the center. The possibility of the surface alteration of the thick films have been clearly demonstrated in this study, which may applied for the integration of ferroelectrics and other dielectrics and/or conductors for low cost microwave tunable devices.     

Growth of (Ba,Sr)TiO 3 Thin Films by MOCVD: Stoichiometry Effects

(Ba x Sr 1 m x )TiO 3 thin films were deposited in a planetary multi-wafer MOCVD reactor combined with a liquid delivery system using 0.35 molar solutions of Ba(thd) 2 and Sr(thd) 2 and a 0.4 molar solution of Ti(O-i-Pr) 2 (thd) 2 . The film growth on Pt-(111) was investigated within a wide parameter field, e.g., the deposition temperature was varied between 560C and 650C, which yields films with microstructures ranging from randomly oriented polycrystalline to perfectly (100)-textured columnar structures. Special emphasis is given to film stoichiometry: starting with (Ba 0.7 Sr 0.3 )TiO 3 the Group-II/Ti content was varied from 0.9 to 1.1 and the Ba content was reduced to the limit of pure SrTiO 3 films. The electrical properties of MIM structures were investigated after deposition of Pt top electrodes. The nominal thickness of the films was varied between 5 and 100 nm and permittivity and leakage current both are shown to depend strongly on the film thickness. These dependencies on the film thickness are analyzed within the phenomenological dead layer model. The dependence of the electrical properties on stoichiometry are discussed in detail.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Interface potential barrier height and leakage current behavior of Pt/(Ba,Sr)TiO3/Pt capacitors fabricated by sputtering process

Abstract

Variations of the leakage current behaviors and interface potential barrier height (φ _B ) of rf-sputter deposited (Ba, Sr)TiO₃ (BST) thin films, with thickness ranging from 20nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. φ _B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N₂ atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the φ _B from about 2.4 eV to 1.6eV due to the oxidation. φ _B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current and the thicker films show a mixed characteristics and bulk and interface limited currents although the mechanism is not clearly understood at this moment.     

Preparation and electrical properties of Ni/(Ba,Sr)TiO3 PTC composite with low resistivity

Ni/ (Ba,Sr)TiO₃ PTC composite of low resistivity was fabricated by a solid state route. A mildly reducing sintering atmosphere was employed to avoid the oxidation of nickel. Metallic nickel is the main chemical state after sintering in the mildly reducing sintering atmosphere. With the increase in nickel amount, the room-temperature resistivity declines and the PTC effect worsens. The quantum mechanical tunneling effect at the Ni–(Ba_,Sr)TiO₃ interface is presumably the prime factor in the deterioration of the PTC effect. PbO–B₂O₃–ZnO–SiO₂ glass was added to modify the interface between nickel and (Ba,Sr)TiO₃ ceramics. The intergranular phase introduced by the glass has an amorphous structure and exists at the interfaces and triple junctions of (Ba,Sr)TiO₃ grains and nickel grains. No obvious diffusion occurs at the interface between crystalline (Ba,Sr)TiO₃ grain and the intergranular phase. Also the added-glass improves the distribution of metal phase. The proper glass addition screens interfacial electron tunneling effect and improves the composite electrical properties. An abundance of the intergranular phase due to excess glass will, however, result in high room-temperature resistivity. The influences of nickel amount and glass amount on the microstructure evolution and electrical properties were analyzed.     

Microwave Dielectric Properties of Ferroelectric BaTiO3 Thin Film

The dielectric properties of c-axis epitaxial BaTiO₃ thin film on LaAlO₃ are investigated at frequencies of 0.5–30 GHz. For the measurements, interdigital capacitors with the Au/Ti electrode configurations of five fingers pairs that are 15 m wide and spaced 2 m apart are prepared by photolithography and lift-off patterning. Finger length varies from 20 to 80 m. The capacitance of epitaxial BaTiO₃ films exhibited no frequency dependence up to 10 GHz with the exception of slightly upward tendency of capacitance in BaTiO₃ film with a finger length of 80 m due to the self resonant frequency at 20 GHz. The Q-factors of the capacitors, defined as Q = 1/CR, are decreased up to 10 GHz with increased frequency. At 10 GHz, the BaTiO₃ film has a tunability [defined as k(V) = [C(0)–C(V)]C(0)] of 1.5% at 15 V, a loss tangent of 0.2 at room temperature. The small tunability can be interpreted as a result of in-plane compressive stress of BaTiO₃ film exhibiting large dielectric anisotropy. For the improvement of tunability and dielectric loss in the interdigital BaTiO₃ capacitor, the tetragonality (c/a) of epitaxial BaTiO₃ film and design of interdigital capacitor should be modified.     

Microwave Properties of Mn Doped (Ba1 - x,Srx)TiO3Thin Films for Tunable Phase Shifter

Ferroelectric Mn doped Ba_0.5Sr_0.5TiO₃ (Mn-BST) films with/without BaTiO₃ (BT) buffer layer have been grown on (001) MgO substrates by a pulsed laser deposition to investigate electrical tunability at microwave frequencies. Structural properties and surface morphologies of the films were investigated using an X-ray diffractometer and a scanning electron microscope, respectively. Microwave dielectric properties of Mn-BST thin films with BT buffer were studied for reduction of dielectric loss and improvement of electrical tunability. Distributed analog phase shifters have been designed and fabricated on Mn-BST films with/without BT buffer layer to understand microwave dielectric properties. The differential phase shift of the phase shifter fabricated on Mn-BST film was 22° at 10 GHz with 80 V of applied dc bias voltage. In comparison, phase shifter fabricated on Mn-BST/BT multilayers exhibit 41° of differential phase shift at the same condition. This suggests that a BT buffer layer is for microwave tunable device applications. The phase shifter fabricated on Mn-BST/BT multilayers exhibit a low insertion loss (S₂₁) of ?1.1 dB, and a low return loss (S₁₁) of ?14 dB with a bias voltage of 80 V.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.