Lanthanum Aluminate Substrates Research Articles

Investigations of PbxSr1-xTiO3 Thin Films and Ceramics for Microelectronic Applications

ABSTRACTWe have investigated electrical and optical properties of the lead strontium titanate {(PbxSr1-x)TiO3 or PST} ceramic and dielectric properties of the thin films of PST at low and high frequencies. (PbxSr1-x)TiO3 compositions with × ≤ 0.4 are paraelectric at room temperature and exhibit ferroelectric phase transition below room temperature. Only one phase transition in the PST system (compared to three in BaxSr1-xTiO3) was recorded. The studies indicated that PST has potential for tunable microwave devices in the paraelectric phase. In the present studies, Pb0.3Sr0.7TiO3 (PST30) ceramic was prepared by the conventional solid-state reaction method and thin films of PST were prepared by sol-gel technique. Structural, microstructural, dielectric, and Raman measurements were performed on these samples. Sharp phase transition was observed in case of the ceramic by dielectric and Raman measurements at 283 K. Raman measurements revealed well-pronounced soft-mode behavior below the Curie temperature in PST ceramic. The thin film of PST deposited on lanthanum aluminate substrate was highly (100) oriented and showed dielectric maxima at ∼280 K, which was close to that in case of the bulk. Eight element coupled micro-strip phase shifters (CMPS) was fabricated on the PST film and tested in the frequency range of 15-17 GHz. The average figure of merit of 49 °/dB for PST30 film in the Ku band at 533 kV/cm suggests the potentiality of these films for high frequency tunable dielectric devices.

Fabrication of Tl-2212 superconducting thin films by a two-step procedure with post-annealing in a closed container

Highquality Tl2Ba2CaCu2Oy(Tl2212) thin films were prepared on s ingle crystal(001) lanthanum aluminate (LaAlO3) substrates by a twostep method. Tlfree amor phous precursor films were first deposited by a pulsed laser deposition techniqu e and then the thalliation, crystallization and oriented growth of the films wer e obtained in the process of annealing treatment in a sealed high-temperature st eel capsule at temperature of 720—850℃ . Xray diffraction reflections reveal th e existence of only Tl2212 phase with nearly perfect caxis orientation. The fu ll width at half maximum of ω scan of (0012) peak was 072°. Scanning electro n m icrographs of the thin films show that the surfaces of the films are smooth. The zeroresistance transition temperature TC0 is 1062K.

Stabilizing the Tl-1223 phase by doping with tellurium

Te-doping of (Tl0.54Pb0.5−xTex)(Sr0.85Ba0.15)2Ca2Cu3Oy superconductors with x ranging from 0 to 0.1 has been investigated. Small amounts of Te (x < 0.05) strongly favoured the formation of the Tl-1223 phase in bulk samples, leading to phase-pure material. The concentration of the Tl-1212 phase was below the detection limit of the x-ray diffraction. The critical temperatures of bulk phase Tl-1223 for x = 0.02 were 120 K. The transition widths narrowed and the critical current densities for bulk phase material increased upon addition of small amounts of Te. At higher doping levels, Ba2CuTeO6 appeared as a secondary phase but the remaining material was still high-purity Tl-1223 phase. (Tl0.54Pb0.48Te0.02)(Sr0.85Ba0.15)2Ca2Cu3Oy films on lanthanum aluminate substrates prepared by screen printing of the precursor and followed by ex situ thallination, lead to highly c-axis oriented and ab-aligned samples with critical temperatures Tc(0) of 117 K and transition widths ΔT(90–10%) of 1 K. Critical current densities of 1.15 × 1010 A m−2 were achieved for 1.5 µm thick films in self-field at 77 K. The critical current densities at 0.8 T and 77 K were 7.4 × 109 A m−2 for H||ab and 0.7 × 10−9 A m−2 for H||c.

Double sided high critical current density HgBa2CaCu2O6+ films on lanthanum aluminate and sapphire substrates

Double sided c-axis oriented HgBa2CaCu2O6+δ (Hg-1212) thin films have been prepared on lanthanum aluminate as well as on CeO2 buffered r-plane sapphire substrates using an improved Tl–Hg cation-exchange process. The films on LaAlO3 exhibited critical current densities (Jc) of up to 7 MA/cm2, and films on sapphire substrates showed Jc values of up to 5 MA/cm2 in a self-field. The superconducting transition temperatures (Tc) were in the range of 121–124 K for films on LaAlO3 substrates and 118–122 K for films on sapphire substrates. Microwave surface resistances (Rs) of about 2 mΩ at 17.86 GHz and 77 K for films on lanthanum aluminate substrates have been obtained. Films on sapphire substrates showed slightly higher Rs values of 3–6 mΩ, probably due to a microstructure which is more disturbed by defects.

Microstructure control in the growth of large area Tl-2212 thin films

Large area high temperature superconducting thin films are needed for the implementation of a range of passive microwave devices. We have been investigating the critical processing issues that control the surface resistance values in 2 inch films grown on lanthanum aluminate substrates, including the possibility of batch processing wafers in a vertical geometry. We have shown that the microstructure of vertically-processed films shows a systematic variation in microstructure from top to bottom which can seriously degrade the uniformity of the superconducting properties. We have also used electron backscattered diffraction to analyze the fine-scale mosaic structure of thin films of varying thickness and processing conditions to show that the best surface resistance values are not found in films with the sharpest epitaxial relationship between the substrate and film. We propose an explanation for this counterintuitive observation based on how the films can relieve stresses generated during processing.

Superconducting Hg-Ba-Ca-Cu-O thin films on lanthanum aluminate and sapphire substrates

Epitaxial c-axis-oriented HgBa/sub 2/CaCu/sub 2/O/sub 6+/spl delta// (Hg-1212) thin films have been prepared on lanthanum aluminate as well as on CeO/sub 2/ buffered r-plane sapphire substrates using a Tl-Hg cation-exchange process. Films on LaAlO/sub 3/ exhibited superconducting transition temperatures (T/sub c/) up to 122 K and critical current densities in the range of 2.2-3.1 MA/cm/sup 2/ at 77 K and under self-field. For the microwave surface resistance (R/sub s/) values of /spl sim/4 m/spl Omega/ at 17.86 GHz and 77 K have been obtained. In comparison, films on sapphire substrates with T/sub c/ values up to 118 K showed critical current densities in the range of 0.7-1.0 MA/cm/sup 2/ and R/sub s/ values of /spl sim/7 m/spl Omega/. With further optimization of processing an improvement of superconducting properties of Hg-1212 films on sapphire substrates is expected.

Response of the electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 epitaxial films to biaxial compressive mechanical (001) or (110) strains

The behavior of the electrical resistivity and magnetoresistance of 40-to 120-nm-thick La0.67Ca0.33MnO3 films grown on differently oriented lanthanum aluminate substrates was studied. The cell volume in thin (40 nm) La0.67Ca0.33MnO3 films grown coherently on (001)LaAlO3 was found to be substantially smaller. Mechanical stress relaxation in biaxially strained La0.67Ca0.33MnO3 films is accompanied by an increase in the cell volume. The temperatures at which the electrical resistivity and magnetoresistance in biaxially strained La0.67Ca0.33MnO3 films were maximum can differ by 60–70 K from those observed in bulk single crystals.

Improvement in electrical characteristics of graded manganese doped barium strontium titanate thin films

Highly (100) textured graded manganese (Mn) doped Ba0.5Sr0.5TiO3 [BST (50/50)] thin films were deposited on lanthanum aluminate substrates using sol-gel technique. We have demonstrated that the graded acceptor doping is a promising technique to reduce the temperature coefficient of capacitance (TCC), loss tangent, and leakage current of BST thin films. In the temperature range between 175 and 260 K the reported TCC of Mn graded BST (50/50) films is less than 5.55×10−4/K, which is comparable to the best capacitors known so far. The lower temperature coefficient of the capacitance of the Mn graded films has been argued to be due to the induced compositional heterogeneity resulting into a distribution of the Curie temperature.

Surface resistance measurements of surface and interface sides of YBa2Cu3O7 films on sapphire and LaAlO3

We have measured the surface resistance of YBCO superconducting thin films deposited on sapphire and lanthanum aluminate substrates at the film side and at the interface between the substrate and the film. The measured difference in the Rs values of the interface and the film sides was between 30% and 60% for the YBCO films on LAO in the temperature range 15–80 K. For the Ag-doped films on sapphire the difference in Rs monotonically varied from 10% to 65% when temperature increased from 30 K to 80 K.

Precise microwave characterisation of YBa2Cu3O7−δ films on sapphire and lanthanum aluminate substrates

We have precisely measured the surface resistance (R S )of high quality YBCO thin films, deposited on Lanthanum Aluminate and Silver doped YBCO thin films on Cesium buffered Sapphire substrates using the Hakki-Coleman dielectric resonator at frequency of 10 GHz as a function of temperature from 25 K to 85 K. We have also studied the microwave power dependence of YBCO films on both substrates and the frequency dependence of films on LAO. Precise microwave characterisation of HTS films is needed for selection of best films for mobile phone base station receivers.

(Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oy superconducting thick films with high in-field critical current densities

Superconducting (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oy films with a thickness between 1 and 2 μm are prepared by screen printing the precursor material on monocrystalline lanthanum aluminate substrates followed by ex situ thallination. Texture analysis confirmed that highly c-axis oriented samples with critical temperatures Tc(0) between 117 and 119 K and with transition widths ΔT(90–10%) of 1 K are achieved. The critical current densities in self-field are above 1.2 × 1010 A m−2 at 77 K, and the in-field behaviour is excellent, even without the introduction of artificial pinning centres. jc of 4 × 109 A m−2 at 77 K and 6 T (H || ab) and of 1 × 109 A m−2 at 1 T for H || c are observed.

Polycrystalline (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oy superconducting thick films with high critical current densities

Superconducting (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oy films were fabricated on lanthanum aluminate substrates by screen printing of the precursor material followed by ex situ thallination. The thickness of the films varied between 1.3 and 2.6 μm. X-ray diffraction and optical microscopy showed that the films were highly textured. Samples with critical temperatures Tc(0) between 116 and 120 K and with transition widths ΔT(90−10%) of 1 K have been fabricated. Critical current densities between 0.6 and 0.9 MA cm−2 were measured.

On the anisotropy of dielectric permittivity in single crystal lanthanum aluminate substrates

The dielectric properties of LaAlO3 single crystal plates were studied by microwave techniques in a frequency range from 20 to 28 GHz. The measurements were performed using a resonator representing crossed round (cylindrical) and radial evanescent waveguides with the polarization-degenerate fundamental oscillation mode. Various single crystal samples, including those obtained from different manufacturers, exhibit anisotropy in the (100) crystal plane, whereby the permittivity varies within Δe=0.1–0.9.

Highly Textured Chemical Solution Deposited Ba 0.5 Sr 0.5 Ti 1−x Mn x O 3 (x ∼⃒ 0 to 5 at %) Thin Films For Microwave Dielectric Applications

We have optimized the process methodology to grow highly (100) oriented barium strontium titanate (Ba x Sr 1 m x TiO 3 ) (x = 0.5 and 0.6) thin films on lattice-matched (100) lanthanum aluminate (LAO) substrates by sol-gel technique. These films exhibited excellent phase shift and insertion loss characteristics at microwave frequencies. The work has been extended to study the effect of manganese (Mn) doping on the nature of epitaxial growth and electrical behavior of (100) oriented Ba 0.5 Sr 0.5 TiO 3 (BST) thin films. The degree of texturing and quality of the in-plane epitaxy of BST thin films on lanthanum aluminate (LAO) was found to be improved for upto 3 at% Mn doping. These films were characterized in terms of their electrical properties and dielectric behavior at low (1kHz-1MHz) and microwave frequencies. We have fabricated eight-element coupled microstrip phase shifters (CMPS) and tested them in terms of their degree of phase shift and insertion loss characteristics. The phase shift increases from 239 ° (undoped) to 337 ° with 3 at% Mn doping. However, the insertion loss also increases (5.4 to 9.9 dB respectively) with the increase in dopant concentration so that effective s factor (defined as phase shift/insertion loss) does not improve significantly and remains in the range of 33-44 °/dB. The observed electrical properties are correlated with the structure and microstructure of the Mn doped BST films.

Off Axis Growth of Strontium Titanate Films with High Dielectric Constant Tuning and Low Loss

AbstractWe have measured the nonlinear dielectric properties of strontium titanate (STO) thin films grown on neodymium gallate (NGO) and lanthanum aluminate (LAO) substrates. The films prepared by off-axis pulsed laser deposition were characterized by their dielectric constant and loss tangent at 1 MHz and 2 GHz, and from room temperature down to 4 K. The resulting films show significant variations of dielectric properties with position of the substrates with respect to the plume axis. STO films on LAO substrates show low loss and high dielectric constant in regions near the plume axis. On the other hand, STO on NGO shows this effect only on the films grown far from the plume axis. We also obtained a figure of merit from the relative variation of the dielectric constant divided by the loss tangent in the presence of a DC electric field up to +/- 4 V/μm. Careful mapping of the plume crossection allowed us to improve the quality and reproducibility of the dielectric films, obtaining a best figure of merit at 2 GHz and 4 K close to 100 for NGO substrate but only well off axis.

High-Q adjustable superconducting high-frequency circuit

The possibility of creating a high-Q (Q ∼ 105) superconducting HF circuit mechanically adjustable in a broad frequency range was studied. The device comprises two planar single-coil structures based on YBaCuO superconducting films. The component coils were deposited onto lanthanum aluminate, neodymium gallate, or strontium titanate substrates.

In-plane magnetic field dependence of intrinsic Josephson junctions in Tl-Ba-Ca-Cu-O thin films

We have fabricated intrinsic Josephson bridges using Tl-Ba-Ca-Cu-O thin films grown on vicinal lanthanum aluminate substrates. Measurements of the lock-in transition allow us align a magnetic field of up to 4 T with the copper oxide planes to better than 0.3/spl deg/. This allows us to study the dynamics of Josephson fluxons in the films. For fields above 1.4 T and at low currents we observe Josephson flux creep. Above a field-dependent threshold current we observe free flow of the fluxons with a flux-flow resistance which is proportional to field. The fluxons reach a field-independent velocity before the jump to the quasiparticle branch occurs. Our results suggest that the development of a sub-mm-wave oscillator using these bridges will require sub-micron patterning.

Application of solution deposition to fabricate Y-Ba-Cu-O coated conductor

This paper demonstrates the successful application of solution deposition techniques to deposit buffer layers and YBCO films on flexible metal substrates, thus making coated conductor fabrication scalable and cost effective. We present results obtained using strontium titanate buffer layers. These buffer layers, processed on single crystal substrates using solution precursors and heat treated in a reducing atmosphere are highly textured and have a uniform smooth surface. YBCO films, 0.3-0.5 mm thick, deposited on these buffered single crystals using the trifluoroacetate process shows good superconducting properties. YBCO films processed on strontium titanate buffered lanthanum aluminate substrates have an onset of transition around 90-91 K and critical current densities of the order of 10/sup 5/-10/sup 6/ A/cm/sup 2/ at 77 K and self-field. These processing techniques have been extended to textured nickel substrates. These results offer promise for the development of a scalable, solution-based processing route to deposit buffer layers and YBCO films on flexible metallic textured tapes.

94-K Superconducting Yttrium Barium Cuprate Films Prepared by Spray Pyrolysis of Methacrylate Solutions

High-Tc YBa2Cu3O7 films 2–6 μm in thickness were deposited onto lanthanum aluminate substrates by spray pyrolysis of methacrylate solutions, and their properties were studied. By optimizing the deposition conditions (substrate material, solution concentration, and firing schedule), single-phase, nearly stoichiometric films well adherent to the substrate were obtained, with Tc = 94 K and ΔT = 9 K.

Process optimization and characterization of chemical solution deposited highly textured BaxSr1-xTiO3 (x = 0.5–0.6) thin films

In the present work the process methodology was optimized to synthesize highly textured (100) BST(50/50) and BST(60/40) thin films on single crystalline lanthanum aluminate substrates. These films were characterized in terms of their phase formation behavior, microstructure evolution and dielectric properties. We have also fabricated eight coupled micro-strip phase shifters (CMPS). An insertion loss of 8.43 dB, phase shift in the order of 320° (20–340V) and κ factor ∼38.0 was achieved in the BST(60/40) films.