Growth of BaTiO3/SrTiO3 superlattices by injection MOCVD

Abstract

BaTiO₃ (BTO) and SrTiO₃ (STO) and Ba_xSr_1-xTiO₃ (x=0–1) (BST) thin films have been epitaxially grown on LaAlO₃ and SrTiO₃:Nb at a substrate temperature of 800°C using a new liquid source delivery technique called injection MOCVD. A X-ray study evidenced FWHMs of 0.16° and 0.45° for SrTiO₃ and BaTiO₃ respectively.

In a next step the feasibility of BaTiO₃/SrTiO₃ superlattices was studied. The multilayers obtained were epitaxially grown on LaAlO₃ as well as on SrTiO₃:Nb. The structural properties were studied using X-ray diffraction as well as XPS, proving the low interface roughness of 1nm. The XPS study also confirmed the absence of carbon contamination in the film.     

Thick film fabrication of ferroelectric phase shifter materials

Abstract

Various composites of BSTO combined with other nonelectrically active oxide ceramics have been formulated. In general, the composites have adjustable electronic properties which can be tailored for use in phased array antennas and other phase shifting devices. The dielectric constant and the loss tangents have been reduced to enhance the overall impedance matching and thereby lowering the insertion loss of the device. In addition, the overall tunability, the change in the dielectric constant with applied voltage, is maintained at a sufficiently high level. The thickness limitation of the bulk materials is around 3–4 mils which can be used up to approximately 15 GHz. In order to increase the operating frequencies of the phase shifters, thick films were fabricated using non-aqueous tape-casting. The tapes were electrically characterized and compared to bulk ceramics. Also, laminated stacks of high dielectric constant and low dielectric constant alternating layers were fabricated.     

Epitaxial behavior and interface structures of BSTO thin films

Abstract

Ferroelectric Ba_(1-x)Sr_xTiO₃ (x = 0.5 and 0.25) thin films were grown on (001) LaAlO₃ by using pulsed laser ablation. Extensive x-ray diffraction and selected area electron diffraction reveal that the as-grown films were (001) oriented with a good in plane relationship of <100>_BSTO // <100>_LAO. Rutherford Backscattering Spectroscopy ion-channeling studies suggested that the films had excellent epitaxial quality and crystallinity with an ion beam minimum yield χ_min of only 2.6%. Atomically sharp interfaces were seen by cross-sectional high-resolution electron microscopy, indicated that the density of misfit dislocations was consistent with the lattice mismatch from the theoretical calculation.     

Microwave Dielectric Properties of (111) Oriented Ferroelectric (Ba1 − xSrx)TiO3 Thin Films

Epitaxial (111) oriented ferroelectric (Ba_{1 ? x}Sr_x)TiO₃ (BST) films were deposited on MgO (111) single crystals using pulsed laser deposition. Structural properties of BST films were investigated using X-ray diffractometer. The dielectric properties of BST films were investigated under the dc bias field of 0–40 V using interdigital capacitors (IDT) fabricated by photolithography and etching process. The small signal dielectric properties of BST films were calculated by modified conformal mapping both the measured data using an impedance gain/phase analyzer and the reflection coefficient data measured using a HP 8510C vector network analyzer in 0.05–10 GHz at room temperature. The IDT capacitor based on (111) oriented BST film exhibits about 40% of capacitance change with an dc bias of 40 V which value is somewhat smaller than that of the IDT device based on (001) oriented BST film. But the dielectric quality factor value is about twice that of the device based on (001) oriented BST film.     

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.     

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.     

Modification on PTCR behavior of (Sr0.2Ba0.8)TiO3 materials by post-heat treatment after microwave sintering

Abstract

Effect of post-sintering treatment on PTCR behavior of (Sr_0.2Ba_0.8)TiO₃ materials prepared by microwave-sintering (ms) process was compared to that prepared by rapid thermal sintering (RTS) process. The microwave-sintering process needed only 1130°C-40 min to effectively densify (Sr_0.2Ba_0.8)TiO₃ materials. The grain size was around 6 μm and PTCR characteristics was around ρ_max/ρ_min≈ 10^1.75, with T_c = 50°C. Lowering the cooling rate after sintering substantially increases the resistivity jump (ρ_max/ρ_min) from 10² to 10^3.4, without altering the microstructure. The annealing at 1250°C for 2 h markedly increased the resistivity jump to (ρ_max/ρ_min)≈10⁶. On the other hand, the rapid thermal sintering (RTS) process required 1320°C-30 min to fully develop the good microstructure (～15 μm) and PTCR property (ρ_max/ρ_min ～ 10^3.0). Post-sintering process, including cooling rate control and annealing, did not improve the electrical properties of these samples, that is ascribed to the slow-cooling rate characteristics of RTS-process for a temperature lower than 800°C.     

Characterization of Pb(Zr,Ti)O3 thin films on SOI prepared by excimer laser deposition

Abstract

Ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films were prepared by pulsed excimer laser deposition on Silicon-on-Insulator (SOI) substrates with and without an electrode. Their properties can be improved by rapid thermal annealing, based on the structural and interfacial characteristics analysis by X-ray diffraction, Rutherford backscattering spectroscopy and automatic spreading resistance measurements. The thin films were revealed of to be polycrystalline perovskite structure with mainly ?100? and ?110? orientations; the crystallite size and the structure are dependent on the annealing time. The PZT thin films did not interact with the top silicon layers of SOI, and the composition was on the tetragonal side of the morphotropic phase boundary in the PbTiO₃-PbZrO₃ phase diagram.     

Novel diffusion control process using ultra thin buffer layer for MFIS memory

Abstract

MFIS structures having excellent clear interfaces and well-crystallized ferroelectric layer were successfully fabricated by a newly developed ultra thin metal buffer layer process on SiO₂/Si. We examined the effect of sputtered Zr or ZrO₂ ultra thin films as a buffer layer for Pb_xLa_1?xTiO₃ (PLT) growth. TEM observation revealed that the buffer layer formation process in which Zr oxidized after the metal deposition had advantages to produce MFIS structures. This method is also superior for the crystallization and the control of the orientation of PLT thin film on amorphous SiO₂. Especially, for buffer layer thicknesses below 10 nm, preferred c-axis oriented PLT thin films were grown. The I-V characteristics of MFIS-FET fabricated by the proposed method showed a clear memory window due to the remanent polarization of the ferroelectric thin film. This process is the most attractive candidate for realizing MFIS structure memory.     

Abnormal resistivity-temperature characteristics of semiconducting (Ba1-x Pb x )TiO3 materials prepared by microwave sintering

Abstract

(Ba_0.7Pb_0.3)TiO₃ and (Ba_0.4Pb_0.6)TiO₃ materials possessing double critical temperature (T_c ) and single high-T_c in resistivity-temperature (ρ-T) behavior, respectively, were obtained by microwave sintering at 1050°C for 5 minutes. These characteristics were ascribed to the existence of the dual phases with core-shell structure. The cooling-rate control modified the relative magnitude of low-T_c and high-T_c resistivity jumps, without altering the T_c -values. It occurred via the change in relative proportion of the core and shell phases.     

Partial Electronic Conductivity and Chemical Diffusivity of Li<Subscript>3<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>2/3−<Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> Solid Solution

Partial electronic conductivity and chemical diffusivity of Li have been measured on the system of Li_3xLa_2/3–xTiO₃ (LLT) with x = 0.13, a prospective Li⁺ electrolyte, against oxygen activity in the range of 10^–22 < a_O2 < 0.21 at 557, 610 and 663C, respectively by an ion-blocking polarization technique. It is found that the electronic conductivity of LLT, which in air is essentially an ionic conductor, varies as a_O2^–1/4 to render it mixed-conductive in reducing atmospheres, say, in a_O2 < 10^–12. The chemical diffusivity of component Li also increases from a value of the order of 10^–8 cm²/s in air atmosphere up to a maximum on the order of 10^–3 cm²/s as the electronic conductivity increases with decreasing oxygen activity. This is attributed to the variation of the electronic transference number and the thermodynamic factor with oxygen activity. The latter has been evaluated to be on the order of 10–10³.     

Preparation of (PbxBa1-x)TiO3 thin films by MOCVD using an aerosol-assisted liquid delivery system

Abstract

Thin films of PbTiO₃, BaTiO₃ and (Pb_xBa_1-x)TiO₃ (PBT) have been prepared by metal-organic chemical vapor deposition using a horizontal reactor with an aerosol-assisted liquid delivery system. Structural and electrical properties have been investigated as a function of the lead content x. First results on PBT thin films grown on platinized silicon substrates show, for x < 0.8, an increasing tetragonal distortion of the lattice cell (c/a >1), and accompanying ferroelectric behavior which is similar to the bulk material. For smaller lead content (x < 0.8) no ferroelectric behavior is established and a small tetragonal distortion of opposite type (c/a <1) is observed. This distortion is attributed to a thermally induced tensile film stress and may be responsible for the suppression of the ferroelectric phase transition.     

Thermal Conductivity of Nano ZnO Doped CaFe2O4

The research objective of this study was to examine whether Zn was an effective doping element for thermal conductivity. Ca_1-xZn_xFe₂O₄ (x = 0.0–0.5) were synthesized by solid state reaction method. The XRD results showed that all samples were mixed phase of CaFe₂O₄ and ZnFe₂O₄. The structure of Ca_1-xZn_xFe₂O₄ (x = 0.0–0.5) belonged to a group of an orthorhombic system (space group: Pbnm). It was observed that all the samples of Ca_1-xZn_xFe₂O₄ (x = 0.0–0.5) had positive Seebeck coefficient as shown on p-type semiconductor behavior. Thus thermal conductivity tended to decrease with increasing x value. The Ca_0.6Zn_0.4Fe₂O₄ showed lowest thermal conductivity of 6.52 W m^?1 K^?1 at 473 K, which was lower than 50.81% of CaFe₂O₄. These results suggested that Zn was an effective doping element for improving the thermal conductivity of Ca_1-xZn_xFe₂O₄.     

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.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Effects of molten salt synthesis (MSS) parameters on the morphology of Sr3Ti2O7 and SrTiO3 seed crystals

A two-step molten salt synthesis process was utilized to fabricate Sr₃Ti₂O₇ and SrTiO_3. High aspect ratio SrTiO₃ seed crystals were developed by optimizing processing conditions such as temperature, salt-to-oxide ratio, and flux type in a systematic fashion. Sr₃Ti₂O₇ seeds were synthesized at temperatures ranging from 1050–1350°C, using salt-to-oxide ratios of 3:1, 1:1, and 1:3, and various salt types, including NaCl, KCl, and a 1:1 combination of NaCl and KCl. Sr₃Ti₂O₇ seeds synthesized at 1250°C with a 1:1 salt-to-oxide ratio in 100% NaCl salt resulted in a majority of higher aspect ratio platelets and elongated platelets as opposed to lower aspect ratio cubic-like and tetragonal-like morphologies. The seeds were 10–40 μm in length with aspect ratios of highly elongated platelets as high as 25:1. A second MSS step was used to synthesize SrTiO₃ seeds of the proper composition by TiO₂ addition to the Sr₃Ti₂O₇ seeds and heat treatment at 1100°C. These studies showed that highly anisotropic SrTiO₃ seeds could be produced at 1250°C using a 1:1 salt-to-oxide ratio in 100% NaCl flux. XRD studies of the resulting SrTiO₃ seeds revealed that the increase in aspect ratio for these particular seeds also resulted in the enhancement of (200) peaks, which are of major interest for texturing of PMN-PT.     

Influence of Post-Annealing on the Characteristics of Pb(Zr,Ti)O3 Thin Films Deposited by Liquid Delivery MOCVD Using a Cocktail Solution

Ferroelectric PZT thin films were deposited by liquid delivery MOCVD using a cocktail solution. The cocktail solution consisted of Pb(METHD)₂, Zr(METHD)₄ and Ti(MPD)(METHD)₂ diluted with ethylcyclohexane. The films deposited on Pt/Ti/SiO₂/Si at a substrate temperature of 500°C consisted of PZT, PbO and PbPt_x, and showed poor properties. However, after annealing at 450°C in air for thirty minutes, the PbPt_x phase disappeared while the volume of the PbO phase increased. The hysteresis properties were also improved by annealing at 450°C. After annealing at 600°C in air for thirty minutes, the PbPt_x and the PbO phases disappeared perfectly and the PZT thin films showed good hysteresis properties with the remanent polarization of 30 μC/cm² and the coercive field of 88 kV/cm.