BST Films Research Articles

Ultra-High Tunability of <inline-formula> <tex-math notation="LaTeX">$\text{Ba}_{(2/3)}\text{Sr}_{(1/3)}\text{TiO}_{3}$</tex-math> </inline-formula>-Based Capacitors Under Low Electric Fields

We report the high-frequency electrical characterization of $\text{Ba}_{2/3}\text{Sr}_{1/3}\text{TiO}_{3}$ (BST) thin films exhibiting a very high dielectric tunability with low losses at 2.45 GHz under very low applied electrical fields. BST layers were integrated in out-of-plane Metal-Insulator-Metal (MIM) devices with optimized Ir/MgO(100) bottom electrodes. The high frequency properties of BST films with thicknesses of 200 nm, 450 nm and 1450 nm were thoroughly investigated in the 100 MHz–10 GHz domain and exhibit extremely high capacitance tuning abilities of 82%, 81% and 70% respectively, under applied voltages as low as 10 V. MIM devices responses show the onset of acoustic resonances associated with the BST electrostrictive behavior under an electric field. By combining high tunability with low resistive losses under low applied voltages, these devices are opening promising avenues for their integration in high-performance tunable devices in the microwave domain and particularly at 2.45 GHz, corresponding to the widely used ISM (industrial, scientific and medical) frequency band.

Structures and dielectric properties of multilayer BST/ST/BST and ST/BST/ST sandwich structures

ABSTRACTMultilayer thin films with sandwich structures of BST/ST/BST and ST/BST/ST were designed and prepared by sol-gel method on Pt/Ti/SiO2/Si substrates, and their structures and dielectric properties have been studied. Pure BST and ST films were also studied for comparison. X-ray diffraction reveals that all films grow along (110) orientation and show polycrystalline cubic perovskite structures. SEM shows that the BST/ST/BST film exhibits smaller grain size and better uniformity than the pure BST film. BST/ST/BST and ST/BST/ST films almost keep significantly high tunable capabilities of 40–50% at low applied voltage of ±10 V like the pure BST film, but show lower dielectric losses, resulting in higher figure of merit (FOM). Related mechanisms on such dielectric properties are analyzed by resistor-capacitor (RC) equivalent circuit models. Such sandwich structure design is a novel route for BST films to fulfill the tunable microwave application.

Modification of electrical properties of Au/n-type InP Schottky diode with a high-k Ba0.6Sr0.4TiO3 interlayer

Au/Ba0.6Sr0.4TiO3 (BST)/n-InP metal/insulator/semiconductor (MIS) Schottky diodes have been analyzed by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The surface morphology of the BST films on InP is fairly smooth. The Au/BST/n-InP MIS Schottky diode shows better rectification ratio and low leakage current compared to the conventional Au/n-InP metal-semiconductor (MS) Schottky diode. Higher barrier height is achieved for the MIS Schottky diode compared to the MS Schottky diode. The Norde and Cheung's methods are employed to determine the barrier height, ideality factor and series resistance. The interface state density (NSS) is determined from the forward bias I-V data for both the MS and MIS Schottky diodes. Results reveal that the NSS of the MIS Schottky diode is lower than that of the MS Schottky diode. The Poole-Frenkel emission is found dominating the reverse current in both Au/n-InP MS and Au/BST/n-InP MIS Schottky diodes, indicating the presence of structural defects and trap levels in the dielectric film.

Characterization of Ba0.55Sr0.45TiO3 films as light and temperature sensors and its implementation on automatic drying system model

abstractThe growth of Ba0.55Sr0.45TiO3 films on p-type silicon substrate with depletion and enhancement treatments have been conducted in this research. The aims were to examine film sensitivity as light sensor and value range, resolution, acuracy level, and their hysteresis as temperature sensor. The films were annealed at 800, 850, and 900 °C for 15 hours. In this work, enhancement BST of 850 °C was the best quality film and utilized as light and temperature sensors. Its implementation has been successfully conducted on ATMega8535 microcontroller-based automatic drying system model by exploiting the working principle of the BST films as automatic switch.

High tunable dielectric properties of Ce and Mg alternately doped Ba0.6Sr0.4TiO3 films

For barium strontium titanate (Ba0.6Ti0.4TiO3, BST) films used in tunable microwave devices, they must have excellent structural characteristics and outstanding combination of dielectric properties i.e., a low loss tangent over the range of operating direct current (DC) bias voltages, a moderate dielectric constant for impedance matching purpose, a large variation in the dielectric constant with applied dc bias (high tunability, in particular high tunability at low applied dc bias), etc. To achieve such a high objective, the following two great improvements are carried out. A normal sol-gel method is modified to prepare multilayer BST films layer by layer. Each multilayer BST film is composed of six layers, where each layer is preheated at 600 ℃, thus the layers from the first layer to the sixth layer are successively preheated once to six times. Thus each BST film is smooth and dense, and contains almost no organic residues. On the other hand, as a new doped mode, Ce/Mn alternate doping is performed. For every six layer-BST films, when the odd number layers are doped with Ce, then the even number layers are doped with Mg, vice versa. The above two improvements result from the fact that Ce doping, Mg doping and Y and Mn alternate doping could make BST thin films significantly improve the dielectric tunability, reduce the dielectric loss, and improve the combination of dielectric properties, respectively. According to the above two improvements, 1 mol% Ce and 3 mol% Mg alternately doped BST thin films are prepared on Pt/Ti/SiO2/Si wafers (substrates). The prepared BST films are denoted by the doped element as follows: Ce/Mg/Ce/Mg/Ce/Mg with Ce doped BST layer is referred to as the first layer (for short Ce/Mg) and Mg/Ce/Mg/Ce/Mg/Ce with Mg doped BST layer as the first layer (Mg/Ce), and the structure and dielectric properties of the films are studied. X-ray diffraction results show that two films present cubic perovskite structures, mainly grow along (110) crystal face, and show strong crystallization. SEM results indicate that the surface morphologies of two films are greatly improved, and Ce or Mg doped BST layer as the first layer can be well matched with the substrate. The surface of the Ce/Mg film is more uniform and denser with slightly smaller grains and weaker crystallization. XPS results demonstrate that the non-perovskite structures on the surfaces of two films are significantly reduced. Each of the two films has high tunability at low applied dc bias and high figure of merit (FOM). The Mg/Ce film shows more stable combination of dielectric properties in a high frequency range. The Ce/Mg film shows more excellent combination of dielectric properties and higher dielectric strength in a low frequency range, where when the testing frequency is 100 kHz, 10 V, 20 V and 40 V applied dc bias voltages correspond to tunabilities of 47.4%, 63.6% and 71.8%, and FOMs of 71.8%, and 27.1, 77.5 and 86.5, respectively. Such good dielectric properties can fully satisfy the requirements for tunable microwave device applications. The relevant mechanisms are also analyzed.

Thickness-dependent Electrical and Piezoelectric Properties of Lead-Free Ferroelectric Ba0.8Sr0.2TiO3 Thin Films

The thickness dependent of electrical and piezoelectric properties of lead-free ferroelectric Ba0.8Sr0.2TiO3 thin films is reported. Ba0.8Sr0.2TiO3 (BST 80/20) thin films for various thickness, ranging from 150 nm to 550 nm, were prepared by high-frequency reactive sputtering of a ceramic target in an oxygen atmosphere on p-type Si substrates. Memory windows and effective dielectric constant of the BST film in Au/BST/Si thin film capacitors is found to increase with the increasing thickness of the film. Domain structure, domain switching and hysteresis loops of the BST 80/20 thin film were investigate via the piezoresponse force microscopy. Complete domain switching and strong piezoresponse are found in the ferroelectric BST film. The piezoelectric coefficient ( ) and the remnant piezoelectric response (ΔPR) of BST 80/20 films is found to increase with the thickness of the film.

Dielectric properties of BaMg1∕3Nb2∕3O3 doped Ba0.45Sr0.55Tio3 thin films for tunable microwave applications

Ba(Mg[Formula: see text]Nb[Formula: see text]O3 (BMN) doped and undoped Ba[Formula: see text]Sr[Formula: see text]TiO3 (BST) thin films were deposited via radio frequency magnetron sputtering on Pt/TiO2/SiO2/Al2O3 substrates. The surface morphology and chemical state analyses of the films have shown that the BMN doped BST film has a smoother surface with reduced oxygen vacancy, resulting in an improved insulating properties of the BST film. Dielectric tunability, loss, and leakage current (LC) of the undoped and BMN doped BST thin films were studied. The BMN dopant has remarkably reduced the dielectric loss ([Formula: see text]38%) with no significant effect on the tunability of the BST film, leading to an increase in figure of merit (FOM). This is attributed to the opposing behavior of large Mg[Formula: see text] whose detrimental effect on tunability is partially compensated by small Nb[Formula: see text] as the two substitute Ti[Formula: see text] in the BST. The coupling between [Formula: see text] and V[Formula: see text] charged defects suppresses the dielectric loss in the film by cutting electrons from hopping between Ti ions. The LC of the films was investigated in the temperature range of 300–450[Formula: see text]K. A reduced LC measured for the BMN doped BST film was correlated to the formation of defect dipoles from [Formula: see text], V[Formula: see text] and Nb[Formula: see text] charged defects. The carrier transport properties of the films were analyzed in light of Schottky thermionic emission (SE) and Poole–Frenkel (PF) emission mechanisms. The result indicated that while the carrier transport mechanism in the undoped film is interface limited (SE), the conduction in the BMN doped film was dominated by bulk processes (PF). The change of the conduction mechanism from SE to PF as a result of BMN doping is attributed to the presence of uncoupled Nb[Formula: see text] sitting as a positive trap center at the shallow donor level of the BST.

Influence of Aliovalent Doping on Dielectric Properties of Ba0.6Sr0.4TiO3 Thin Film for Voltage Tunable Applications

Ba0.6Sr0.4TiO3 thin film with aliovalent doping of acceptor nature at ‘A’ lattice sites of ABO3 structure were prepared by a metal organic solution deposition (MOSD) technique on low loss quartz substrate. Oxide thin film having perovskite structure, namely, lanthanum nicklate (LaNiO3), also deposited using chemical solution deposition technique (CSD), was used as conductive bottom electrode. Solution properties and deposition process parameters were optimized for crack-free, uniform and smooth, pure and doped BST films on LNO electrode. The structural and microstructural analyses of the film were done using X-ray diffraction (XRD) and atomic force microscopy (AFM). Dielectric properties of the deposited film were studied using C-V and C-F electrical characterizations. The results indicate high tunability of 46%, very low loss tangent of 0.0065 with corresponding figure of merits of 71 for 2% K doped BST films at 1 MHz. The considerable improvement of dielectric properties of doped films compare to pure BST film is due to combined effect of perovskite lattice matched conductive bottom electrode and optimum doping level. These films had moderate dielectric constant, very low loss with good tunability which is suitable for tunable microwave applications.

Fabrication and characterization of (110)-oriented (Ba0.5,Sr0.5)TiO3 thin films using PdO//Pd buffer layer

A new orientation control method for depositing (110)-oriented perovskite thin films was demonstrated using a (101)PdO//(111)Pd buffer layer on (100)Si substrates, which is widely used in integrated ferroelectric and dielectric applications. The (101)-oriented PdO was obtained by oxidizing (111)Pd films deposited on a (111)Pt/TiOx/SiO2/(100)Si substrate and (110)-oriented perovskites including SrRuO3 and (Ba0.5,Sr0.5)TiO3 (BST) were obtained on (101)PdO by the RF magnetron sputtering method. Although the BST films deposited at 500 °C were not crystallized, the BST films deposited above 600 °C are fully crystallized and the (110)-orientation is dominant. The (110)-oriented BST thin film showed good dielectric property with high dielectric constant (>600) at 0 kV/cm and high tunability (>50% at ±200 kV/cm). Comparing this dielectric property with those of other oriented BST films, both the dielectric constant and the tunability tend to decrease in the order of (111)-, (100)-, and (110)-preferred orientations.

Dielectric microwave properties of Si-integrated pulsed laser deposited (Ba, Sr)TiO3 thin films up to 110 GHz

Ba0.6Sr0.4TiO3 thin films with a thickness of 339 nm are deposited directly on the high resistivity silicon through pulsed laser deposition. Coplanar waveguides with a slot width of 4.5 μm are designed to extract the complex permittivity of ferroelectric thin film in the frequency range from 1 GHz to 110 GHz. A fast three-dimensional (3D) finite element method (FEM) model is proposed to implement the permittivity extraction based on the propagation-constant matching, i.e., narrowing the difference between measured and simulated propagation-constants by adjusting the changeable permittivity in the fast 3D FEM model. In order to reduce the calculation overhead, the quasi transverse electromagnetic mode and conformal mapping analysis are introduced to realize the adjusting. The relative difference between measured and simulated propagation-constants is defined to describe the precision of the result. Experimental results show that the relative difference is less than 1.1%. The relative dielectric permittivity of BST films equals 332.6 at 1 GHz and reduces to 240.1 at 110 GHz. The loss tangent is about 17.5% at 110 GHz.

Tailoring the room temperature ferroelectric/paraelectric state in polycrystalline (Ba0.70Sr0.30)TiO3 thin films for silicon compatible integration

Polycrystalline (Ba0.70Sr0.30)TiO3 (BST) thin films were prepared by ion beam sputtering and postdeposition annealing method, and their crystallization kinetics was monitored in situ. It was found that 550°C is a critical temperature for the BST׳s perovskite phase formation, below which a long annealing time is needed to obtain good perovskite structure because of a slow crystallization process, while generating much lower inhomogeneous and thermoelastic strains. As a consequence, the ferroelectric/paraelectric state formation in BST films, being strain sensitive, was purposefully tailored through simple kinetics instead of pure thermodynamics. A long time annealing at 530°C induced a room temperature (RT) ferroelectric state, whereas the annealing at 550°C and above promoted much faster perovskite crystallization and tailored the film at RT in its paraelectric state, thereby giving rise to two distinct dielectric behaviors. These findings provide important implications for simply controllable state growth of perovskite thin films with desired electrical properties so as to meet their respective requirements in different application fields.

Structural and physical properties of Ba0.6Sr0.4TiO3 film prepared by pulsed laser deposition in a mixture of argon and oxygen

In order to investigate the effect of argon introduction on the structural and physical properties of Ba 0.6 Sr 0.4 TiO 3 (BST) films, epitaxial BST films have been grown by pulsed laser deposition (PLD) on (001) oriented Nb - SrTiO 3 (STON) substrates in the ambient gas of pure oxygen and the mixture of argon and oxygen, respectively. The atomic force microscopy (AFM) reveals that the sample deposited in the mixture of argon and oxygen has a bigger grain size than that deposited in pure oxygen. The dielectric properties of two BST films have been investigated using the Pt /BST/STON parallel plate capacitors. The tunability and the figure of merit (FOM) of the BST film deposited in the mixture gas are about 41% and 7.07 at a DC bias voltage of 8 V at room temperature, which are better than 32% and 4.51, for BST film deposited in pure oxygen. On account of the introduction of argon, the leakage current density of the BST film deposited in the mixture gas has fallen by almost five times, and the leakage current mechanism is both in accordance with ohmic conductive mechanism.

Lead-free piezoelectric thin films for RoHS devices

This paper reports a study of Ba0.9Sr0.1TiO3 and BaTi0.98Sn0.02O3 thin films elaborated by a sol–gel route and deposited on Pt/Ti/SiO2/Si substrates. These films were annealed at 950°C and exhibited a polycrystalline structure. The AFM analysis revealed an important difference in the microstructure of the BST and BTS films. Our BTS film showed no significant piezoelectric properties as its coefficients were d33=5pC/N and 4pm/V for the direct and reverse measurements. Under our optimized annealing conditions, we gave the evidence that ferroelectric Ba0.9Sr0.1TiO3 could be a good candidate to replace PZT in RoHS microelectronic devices, therefore for green applications where lead must be prohibited, and that, despite the factor 2 or 3 between coefficients measured on PZT and lead-free films.

Influence of Space Charge Layers on the Dielectric Properties of Ba0.6 Sr0.4TiO3 Nanostructured Thin Films

Barium strontium titanate (Ba0.6Sr0.4TiO3) nanostructured thin films have been deposited on platinized silicon substrates by the sol-gel method. The as-fired films were found to be amorphous, which crystallize to cubic phase after annealing at 550°C in air for one hour. The low-frequency dielectric responses of the BST films were measured as functions of frequency range from 1 kHz to 1MHz. The thickness dependence dielectric constant of the BST thin films were measured in the temperature range from −150°C to 150°C at 100 kHz and discussed in the light of an interfacial dead layer. All the samples showed a diffuse type phase transitions. Both the dielectric constant and loss tangent showed anomaly peaks at about 10°C, which corresponds to a tetragonal ferroelectric-to-cubic paraelectric phase transition.

Tunable Phase Shifter Based on Inkjet-Printed Ferroelectric MIM Varactors

Abstract This paper presents a new method for the fabrication of tunable multilayer ferroelectric components based on inkjet printing. Inkjet printing is a low-cost technology for selective film fabrication and has high potential for the preparation of tunable dielectric layers for radio frequency and microwave applications. With this technology, tunable metal-insulator-metal (MIM) capacitor is fabricated, that is composed of inkjet-printed Barium-Strontium-Titanate striplines and photo-lithographically structured gold electrodes. Compared to coplanar capacitors, such MIM varactors require significantly lower DC-voltage for tuning. By applying 20 V across a 1 μm-thick BST film, a tunability of 33% is achieved at 8 GHz and tunability of 60% by applying 50 V. To demonstrate the field of application of this MIM varactor, a tunable phase shifter is designed and fabricated at 8 GHz. A phase shift of 143° and a figure of merit (FoM) of 28°/dB are achieved by applying maximum 50 V tuning voltage.

Design of an Instantaneous-Wideband Frequency Reconfigurable Microstrip Antenna Based on <formula formulatype="inline"><tex Notation="TeX">$({\rm Ba}, {\rm Sr}) {\rm TiO} _{3}$</tex></formula>/MgO Composite Thin Films

A novel frequency reconfigurable microstrip antenna with a wide instantaneous operation band based on BST/MgO composite films is proposed and experimentally demonstrated. In our design, the frequency reconfigurability is achieved by altering the permittivity of BST film, and an extra Fabry-Perot resonant structure is adopted to broaden the instantaneous operation band. As a consequence, a wide instantaneous band that is nearly 26% greater is obtained in measurement, which means that the working band of our antenna can be tuned from 9.9-13.5 GHz to 10.9-14.2 GHz. This instantaneous wideband feature makes our frequency reconfigurable microstrip antenna more attractive and practical in wireless communication systems.

Secondary effects in wide frequency range measurements of the pyroelectric coefficient ofBa0.6Sr0.4TiO3andPbZr0.2Ti0.8O3epitaxial layers

We describe measurements of the pyroelectric coefficient of epitaxial layers of ${\mathrm{Ba}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{TiO}}_{3}$ (BST) and ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ (PZT) using a modulated laser as the heat source in the frequency range 1 Hz to 10 MHz. The pyroelectric coefficient is also measured as a function of applied static electric field and for films grown on ${\mathrm{SrTiO}}_{3}$, ${\mathrm{DyScO}}_{3}$, and ${\mathrm{GdScO}}_{3}$ substrates. The heat diffusion equation is solved in cylindrical coordinates for the temperature field in a multilayer sample heated by a Gaussian-shaped laser beam. The secondary contribution to the pyroelectric effect caused by piezoelectric effects and in-plane thermal expansion is revealed by the difference between the pyroelectric coefficients at high and low frequencies. The secondary pyroelectric effect has the same dependence on applied field as the pyroelectric coefficient. The magnitude of the secondary effect changes with heating frequency because of changing mechanical conditions, but the pyroelectric coefficient has no other frequency dependence between 1 Hz and 10 MHz. The secondary effect is approximately 15% and 20% of the total pyroelectric response for PZT and BST films, respectively.

Effect of doped concentration on dielectric properties of Mn and Y alternately doped BST films

Mn and Y alternately doped Ba0.6Sr0.4TiO3 (Mn/YBST) films were prepared by improved sol–gel method on Pt/Ti/SiO2/Si substrates. The effect of different doped concentrations (0.5%, 1.0%, 1.5%, 2.0% and 2.5%) on crystallization and dielectric properties of BST films was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and HP4294A. All films grow along (110) orientation, possessed dense nodular microstructure, exhibited decreased grain size and improved morphologies with smaller average roughness. As a result, the doping behavior changes from donor doping to acceptor doping and again to donor doping and the dielectric constants and the dielectric losses tend to decrease with increasing doped concentration. 1.5% corresponds to the optimized dielectric properties with the dielectric losses of about 0.007 and the figure of merit (FOM) of 38.83 at 100kHz. Also, the related mechanisms and the dielectric properties at microwave frequencies are discussed.

EFFECT OF THICKNESS ON MICROSTRUCTURE, DIELECTRIC AND OPTICAL PROPERTIES OF SINGLE LAYER Ba0.6Sr0.4TiO3 THIN FILM

Single layered Ba 0.6 Sr 0.4 TiO 3 (BST) thin films were prepared on stainless steel (304) and quartz substrates by solution method. The microstructure, grain size, surface morphology and thickness of the films were reported on the basis of X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and UV-visible spectrometer. Variation in thickness influences the microstructure of the films. The single layered thin film had uniform crack-free surface morphology. The low frequency dielectric constants for the films of thicknesses 663, 476 and 451 nm were found to be 1246, 859 and 703, respectively at room temperature. The dielectric loss values for different thicknesses were found to be 0.238%, 0.170% and 0.120% for 100 kHz and 0.043%, 0.029 % and 0.028% for 1 kHz. The dielectric properties changed significantly with thickness of the film as well as with frequency. The tunability of the single layered BST film increased with film thickness. The refractive index, bandgap and thickness of the single layered thin film were calculated by using envelope method and Tauc's relation from the UV-visible transmission spectrum. The bandgap increases with the film thickness. These results show that this single layered film will be a potential material for tunable devices application.

Influence of Film Thickness on Dielectric Properties of Y and Mn Alternately Doped BST Films

By adjusting film thickness, smooth and dense Mn and Y alternately doped Ba0.6Sr0.4TiO3 (Mn/Y-BST) films have been designed and prepared by improved sol-gel method, and the structure and dielectric properties of the films have been studied. X-ray diffraction (XRD) reveals that all films grow along (110) orientation, doped Mn2+ ions or Y3+ ions show acceptor doping, and thicker film shows stronger crystallization. Atomic force microscope (AFM) shows the films are relatively smooth without obvious cracks or pinholes. Scanning electronic microscope (SEM) indicates that from substrate to surface the films show nodular microstructures to columnar ones without obvious interface among nodular microstructure layers. Compared to pure BST film or Mn or Y doped BST films, all alternately doped films show markedly improved structures and dielectric properties, which is strongly dependent on film thickness. 6-layer Mn/Y-BST film shows best structure and combination of dielectric properties with dielectric loss of 1.18%, tunability of 38% and figure of merit (FOM) of 32.2 at 100 kHz.