Ba0.6Sr0.4TiO3 Films Research Articles

Large in-plane permittivity of Ba0.6Sr0.4TiO3 thin films crystallized using excimer laser annealing at 300 °C

We demonstrated a way to fabricate the crystalline Ba0.6Sr0.4TiO3 (BST) thin films using excimer laser annealing technique on the amorphous BST thin films fabricated by sol-gel process. The grain size of the laser-annealed films is larger than that of the conventionally thermal-annealed films. However, an uncrystallized, amorphous layer was observed near the film/substrate interface due to the limited laser absorption depth. The uncrystallized layer has a critical influence on out-of-plane dielectric property of BST films. The significant difference of the relative dielectric permittivity (εr) between in-plane (1383) and out-of-plane (184) directions is observed.

Dielectric properties of sol–gel synthesized SrTiO3/(Ba0.7Sr0.3)TiO3 and SrTiO3/Ba(Zr0.3Ti0.7)O3 thin film heterostructures

The paper presents synthesis of Ba0.7Sr0.3TiO3 (BST), BaZr0.3Ti0.7O3 (BZT) and SrTiO3 (ST) thin films and their heterostructures using modified Pechini method. The La0.7Sr0.3MnO3 has been used as a conducting bottom layer to form metal ferroelectric metal capacitor. The thin films are spin coated on SiO2/n-Si(100) substrates. The thin films thus deposited are characterized for crystal structure, morphology, dielectric, complex impedance and admittance properties. Deposition of surface layer ST is observed to reduce loss tangent tan δ of BST and BZT thin films, still maintaining equivalent magnitude of figure of merit γ. The results on dielectric properties are analyzed in terms of the Maxwell–Wagner model and Koop’s phenomenological theory.

In-plane dielectric properties of epitaxial Ba0.7Sr0.3TiO3 thin films grown on GaAs for tunable device application

We have epitaxially deposited ferroelectric Ba0.7Sr0.3TiO3 (BST) thin films grown on GaAs substrate via SrTiO3 buffer layer by laser molecular beam epitaxy. Structural characteristics of the heterostructure were measured by various techniques. The in-plane dielectric properties of the heteroepitaxial structure under different applying frequency were investigated from −190 to 90 °C, indicating Curie temperature of the BST film to be around 52 °C. At room temperature, the dielectric constant of the heterostructure under moderate dc bias field can be tuned by more than 30% and K factor used for frequency agile materials is found to be close to 8. Our results offer the possibility to combine frequency agile electronics of ferroelectric titanate with the high-performance microwave capabilities of GaAs for room temperature tunable device application.

Strain profile and polarization enhancement in Ba0.5Sr0.5TiO3 thin films

AbstractThe sensitivity of spontaneous polarization to epitaxial strain for both 10 and 50 nm thick Ba0.5Sr0.5TiO3 (BSTO) ferroelectric thin films has been studied. Crystal truncation rod (CTR) profiles in the 00L directions at different wavelengths, and grazing incidence diffraction (GID) in the 0K0 direction on a single crystal have been recorded. Modeling of the CTR data gives a detailed picture of the strain and provides clear evidence of the film out‐of‐plane expansion at the surface, an increase of the polarization, as well as a contraction at the interface. GID data confirm the fitting of the CTR, showing an in‐plane expansion of the BSTO film at the interface and a contraction at the surface.

The properties of Ba0.5Sr0.5TiO3 thin film prepared by RF magnetron sputtering from powder target

The Ba0.5Sr0.5TiO3 (BST) thin film with the thickness of 400 nm deposited from powder target is prepared by the radio-frequency magnetron sputtering technique. The deposition rate of BST film is estimated to be 45 nm/min, which is very fast for ferroelectric materials. The dielectric properties of the as-prepared BST thin film are demonstrated. High dielectric tunability up to 42.7% and low dielectric loss small to 0.01 are achieved at a low applied voltage of 5 V. The results demonstrate that the RF magnetron sputtering from powder target is a versatile, novel technique for the deposition of high-quality ferroelectric thin films.

Significant polarization variation near room temperature of Ba0.65Sr0.35TiO3 thin films for pyroelectric energy harvesting

Ba0.65Sr0.35TiO3 (BST) thin films with self-buffered layers were prepared on Pt(111)/Ti/SiO2/Si substrates by chemical solution deposition. The effect of self-buffered layer on the crystal orientation of BST film was studied. Polarization variation with room temperature and harvested energy density of BST film were also investigated. It is found that the (100) of the BST film with self-buffer layer is more intense than those of the BST film without self-buffer layer. For the BST film with 550°C annealed self-buffer layer, the polarization variation reaches to 5.2μC/cm2 from 77 to 2°C, and the harvested energy density is calculated to be 128mJ/cm3 based on the polarization variation using the Ericsson cycles.

Improved properties of barium strontium titanate thin films grown on copper foils by pulsed laser deposition using a self-buffered layer

Ba0.6Sr0.4TiO3 (BST) films were deposited by pulsed laser deposition on copper foils with low-temperature self-buffered layers. The deposition conditions included a low oxygen partial pressure and a temperature of 700 °C to crystallize the films without the formation of secondary phases and substrate oxidation. The results from x-ray diffraction and scanning electron microscopy indicated that the microstructure of the BST films strongly depended on the growth temperature. The use of the self-buffered layer improved the dielectric properties of the deposited BST films. The leakage current density of the BST films on the copper foil was 4.4 × 10−9 A cm−2 and 3.3 × 10−6 A cm−2 with and without the self-buffered layer, respectively. The ferroelectric hysteresis loop for the BST thin film with buffer layer was slim, in contrast to the distorted loop observed for the film without the buffer layer. The permittivity (7 0 0) and dielectric loss tangent (0.013) of the BST film on the copper foil with self-buffered layer at room temperature were comparable to those of the film on metal and single-crystal substrates.

Temperature dependence of rapidly thermally annealed Ba0.6Sr0.4TiO3 thin film fabricated on platinized Si substrate

Abstract Sol-gel prepared Ba0.6Sr0.4TiO3 (BST) thin films were prepared on platinized silicon substrate and annealed by rapid thermal annealing at temperatures ranging from 650 °C to 850 °C. X-ray diffraction, atomic force microscopy and electrical measurements were used to investigate the temperature dependence of the structural and physical properties of Ba0.6Sr0.4TiO3 thin film. The dielectric constants of BST samples annealed at 700 °C and 750 °C, measured at zero-bias electric field, are 166 and 193, respectively. It is found that all the prepared BST films show Ohmic-like conduction at low voltages except for the 650 °C annealed BST sample, for which Ohmic conduction covers the whole measured voltage range; BST films annealed at 700 °C, 750 °C and 800 °C demonstrate Poole–Frenkel emission conduction at high voltages, while BST film annealed at 850 °C presents Schottky emission behavior.

Preparation and dielectric properties of BST films with high tunability

Six-layer Ba0.6Sr0.4TiO3 (BST) films are prepared by an improved sol-gel method. The effects of various preheating methods on morphology and dielectric properties are studied. XRD results reveal that all BST films preferentially grow along the (110) direction and have cubic perovskite structures, and that preheating can obviously enhance crystallization. XPS results show that the preheating can also decrease surface non-perovskite structure and cause perovskite structure to increase markedly. AFM results display that as the preheating number increases, BST films show significant improvement on morphology with surface roughness reducing. The dielectric property measurement exhibits that tunability is remarkably increased with the increase of preheating number. BST film with all layers preheated shows that its tunability exceeds 55% at 20V bias voltage, while BST film with odd number layers preheated presents a moderate capacitance, lower dielectric loss, approximately 50% tunability at 20V bias voltage and significant increase in figure of merit (FOM) with the bias voltage. The excellent combination of dielectric properties can satisfy the application demand of microwave tunable devices. Also, the relevant mechanism of high tunability is discussed.

Microwave properties of epitaxial (111)-oriented Ba0.6Sr0.4TiO3 thin films on Al2O3(0001) up to 40 GHz

Perovskite Ba0.6Sr0.4TiO3 (BST) thin films have been grown on Al2O3(0001) substrates without/with inserting an ultrathin TiOx seeding layer by rf magnetron sputtering. X-ray diffraction and pole figure studies reveal that the film with the TiOx layer (12-Å-thick) is highly oriented along the (111) direction and exhibits a good in-plane relationship of BST(111)∥Al2O3(0001). The high frequency dielectric measurements demonstrate that the complex permittivity (ε=ε′−jε″) is well described by a Curie–von Scheidler dispersion with an exponent of 0.40. The resulting epitaxial BST films show high permittivity (∼428) and tunability (∼41%, at 300 kV/cm and 40 GHz) and their microwave properties (1–40 GHz) potentially could be made suitable for tunable devices.

Comparison of BST film microwave tunable devices based on (100) and (111) MgO substrates

We have increased the figure-of-merit (FOM) of a (Ba,Sr)TiO₃ (BST) film microwave tunable device by approximately three times for MgO(111) compared with a MgO(100) substrate at a frequency range of 20 GHz. Differences in permittivity and tunability in a BST film may be closely related to the difference in the film strain. The ratio of calculated permittivities of BST(100) and BST(111) films nearly corresponds to that of the FOM in the microwave range, which was rather unexpected because a higher permittivity leads to both larger tunability and dielectric loss in ferroelectrics. From a series of results, it is suggested that there are additional influences of orientation (other than the direct influence of strain itself) on the tunable properties in BST films especially in the high-frequency region.

Effects of Ultrathin TiOx Seeding Layer on Crystalline Orientation and Electrical Properties of Sputtered (Ba,Sr)TiO3 Thin Films

A series of ultrathin TiOx (0–12 nm) seeded (Ba,Sr)TiO3 (BST) thin films have been prepared on Pt (111)/TiOx/SiO2/Si substrates by RF-magnetron sputtering. Experimental results show that an ultrathin TiOx layer acts as an initial template for the BST films in the first nucleation stage, resulting in a significant influence on crystalline orientation and electrical characteristics of the resultant BST films. Interestingly, highly (111) oriented BST film [α(111)∼97%] with 5-nm-thick TiOx seeding layer exhibits a shifted Curie temperature (Tc=275 K) and an enhanced tunability of 61.16% at 400 kV/cm, when compared with (001)-oriented BST without TiOx layer (Tc=250 K and tunability=50.05%).

MICROWAVE-INDUCED NANOCRYSTAL LINES ON THE Ba0.6Sr0.4TiO3 THIN FILMS SURFACE

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin films were deposited on Pt/Ti/SiO2/Si (100) substrates by radio-frequency magnetron sputtering. Nanocrystal lines on the BST films surface were obtained by microwave electrical field irradiation annealing. Atomic force microscopy (AFM) results reveal that microwave annealing can give rise to certain self-assembled nanocrystal lines on the BST films surface, and the length and the width of the nanocrystal lines are dependent on annealing time. After the films further annealed for 20 min or more, the nanocrystal lines broke up into spherical nanocrystals of about 25 nm in diameter. The directions of BST nanocrystal lines are determined by microwave electrical field. X-ray diffraction (XRD) shows that the microwave-annealed BST films are (110) preferred orientation. Specular X-ray reflectivity (XRR) measurement shows that the surfaces of the microwave-annealed BST thin films exhibit anisotropy.

THICKNESS DEPENDENT PROPERTIES OF BARIUM STRONTIUM TITANATE THIN-FILM VARACTORS

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin-films with large dielectric tunability as high as 5.8:1 were obtained using a large area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large area processed BST thin film varactors. Varactors with 0.6 μm thick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 1280 tuned to 220 at 20 V, at an electric field of 333 kV/cm. The leakage current through the BST film was below 1 nA up to 10 V dc bias. The Quality factor (Q) exceeded 500 at relatively low 10 V dc bias for both 0.6 and 1 μm thick BST varactors.

Dielectric tunability and electro-optic effect of Ba0.5Sr0.5TiO3 thin films

We elucidated the relationship between the dielectric tunability and the electro-optic (EO) effect. A new measurement system for EO effect and dielectric permittivity (εr) was developed to investigate the accurate correlation between EO and dielectric properties of epitaxial barium strontium titanate, Ba0.5Sr0.5TiO3 (BST) thin films grown on SrTiO3 (STO) substrates. The BST films had a strained lattice with a large lattice parameter along the thickness direction. Small birefringence was induced in the as-deposited film by the alignment of the c-axis in the film plane. The tunability of dielectric permittivity calculated from the complex voltage and current with planer electrodes reached to 53.1%, and the tunability of birefringence by EO effect was 0.6%. The birefringence change from EO effect was much lower than the tunability of dielectric permittivity. Therefore, there is no strong correlation between the dielectric tunability and EO effect, and the materials with high tunability do not always exhibit high EO effect. Finally, we discussed how to obtain the materials with high EO effect considering these results.

Effects of Sol Viscosity on Properties of BST Thin Films Prepared by Sol-Gel Method

A series of BST(Ba0.65Sr0.35TiO3) sol with different viscosity were prepared by using Ba(CH3COOH)2, Sr(CH3COOH)2 and Ti(OC4H9)4 as raw materials. The BST thin films were fabricated by sol-gel method and spin-coating process on Si(100) substrates. The effect of sol viscosity on crystallization, microstructure and dielectric properties of BST thin films were analyzed by using X-ray diffractometry, scanning electron microscopy and impedance phase analyzer. The results show that the BST films coated by sol with viscosity of 3.0cp and annealed at 750°C for 1h are basically non-crystalline. The crystallization degree increases with the increase of sol viscosity at the same annealing temperature. The ABO3 perovskite structure is formed when the sol viscosity increases to 3.5cp. With further increasing sol viscosity to 6.0cp, the crystal grains of the film grow well and range very compact, the surface become smooth, and the films have optimal dielectric properties.

Reduction in Operation Voltage of Silicon Ring Optical Modulator Using High-k (Ba,Sr)TiO3 Cladding Layer

Electric-field drive Si ring optical modulators with a (Ba,Sr)TiO3 (BST) cladding layer have been fabricated. A lateral electric field is applied across the Si ring using the side Pt electrodes, and the optical output from the drop port is measured. It is estimated that an operation voltage of less than 10 V is possible when the relative dielectric constant of the cladding layer is larger than 150 and the quality factor is 5×104. The operation voltage of the fabricated optical modulator with a BST cladding layer is reduced to <1/8 relative to that of the SiO2 and Si3N4 cladding layers. One reason for the still high operation voltage (125 V for 75% modulation) is the low quality factor, which is limited by the light propagation loss of the BST film. It is also estimated that an operation voltage of less than 10 V is achievable when the propagation loss of the cladding layer is improved to less than 28 dB/cm.

Structure and Properties of BST/BZT/BST Multilayer Film

利用脉冲激光沉积法在LaNiO3/LaAlO3（001）基片上生长了Ba0.6Sr0.4TiO3（BST）和Ba（Zr0.2Ti0.8）O3（BZT）单层薄膜,以及Ba（Zr0.2Ti0.8）O3/Ba0.6Sr0.4TiO3/Ba（Zr0.2Ti0.8）O3（BZT/BST/BZT）多层薄膜.X射线衍射（XRD）分析发现,BST、BZT和LNO薄膜都具有高度的（00l）取向.原子力显微镜（AFM）显示三种样品表面光滑无裂纹,晶粒尺寸和表面粗糙度相当.电容测试表明,相对BST、BZT单层薄膜,多层薄膜具有最大的品质因数42.07.表明多层薄膜在微波应用中具有很大的潜力.

Dielectric Tunable Properties of Ba&lt;sub&gt;0.6&lt;/sub&gt;Sr&lt;sub&gt;0.4&lt;/sub&gt;TiO&lt;sub&gt;3&lt;/sub&gt; Thin Films with and without LSCO Buffer Layer

Barium strontium titanate Ba0.6Sr0.4TiO3 (BST) thin films were fabricated by pulsed laser deposition (PLD) technique on Pt/TiO2/SiO2/Si substrate with and without high (100) oriented LSCO buffer layer. Crystal structure and surface morphology of BST thin films with and without LSCO buffer layer were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1M Hz and at room temperature. It was found that the LSCO buffer layer was beneficial for BST films to decrease surface roughness, dielectric loss and to increase grain size, dielectric constant, tunability and figure of merit (FOM). The influence of LSCO buffer layer on the microstructure, dielectric and tunable properties of BST thin films were analyzed. At 1M Hz, the dielectric constants of BST films with and without LSCO buffer layer are 684 and 592, respectively. The tunability of BST thin film with LSCO buffer layer was about 65.48%, which was higher than that (about 41.84%) of BST thin film without LSCO buffer layer.

Perfectly (001)- and (111)-Oriented (Ba,Sr)TiO3Thin Films Sputtered on Pt/TiOx/SiO2/Si Without Buffer Layers

We report the growth of (001)- and (111)-oriented (Ba,Sr)TiO3 (BST) films on Pt/TiOx/SiO2/Si substrates by RF magnetron sputtering without buffer layers. Interestingly, it was observed that the crystallographic orientation of prepared films strongly depended on O2/(Ar+O2) mixed ratios (OMR) during deposition. The samples with OMR=0% showed a perfect (001) orientation, while those with OMR ranging from 10% to 50% showed a preferential (111) orientation. The perfectly (111)-oriented BST film deposited with OMR=30% exhibited a broader grain size distribution than the (001) textured BST film. At ±400 kV/cm, high tunability of 52% and 68% can be achieved for (001)- and (111)-oriented BST films, respectively.