Seok Kil Han

PHASE FORMATION AND FERROELECTRICITY OF SOL-GEL DERIVED (PB, LA)TIO3 THIN FILMS

Ferroelectric lead lanthanum titanate (PLT) thin films with various lanthanum concentration were fabricated by sol-gel spin-on process onto Pt/Ti/SiO2/Si substrates. We investigated the crystal structure, microstructure, and dielectric and ferroelectric properties of PLT films according to lanthanum doping concentration. The films annealed at 600 °C for 30 min have the single perovskite phase having only tetragonal or pseudocubic structure. The film microstructures were changed according to the doping concentration of La. The PLT films showed the typical polarization-electric field hysteresis loops and capacitance-voltage characteristics representing the ferroelectric switching property. The coercive field and remanent polarization decreased due to the phase transformation from ferroelectric to paraelectric phase with increasing La concentration. The effective dielectric constant and the leakage current density increased according to the increase of La content.

Electrical Properties of Lead Lanthanum Titanate Thin-Film Capacitors Prepared by Sol-Gel Method

High-quality (Pb1-x Lax )Ti1-x /4O3 (x=0.05) [PLT] thin films were fabricated successfully by sol-gel processing onto Pt/Ti/SiO2/Si and p-Si substrates. The optimum route and the conditions for preparing homogeneous and crack-free thin films were systematically investigated. Ferroelectric perovskite phase was observed in the PLT thin films annealed at 600° C for 30 min. We studied the electrical properties of the PLT film capacitors with the metal-ferroelectric-metal (MFM) and the metal-ferroelectric-semiconductor (MFS) structure for use in ferroelectric memories. The dielectric constant and dissipation factor measured for MFM capacitor were 225 and 0.018 at 100 kHz, respectively. The remanent polarization (P r) and coercive field (E c) of the PLT film were 8.5 µC/cm2 and 110 kV/cm, respectively. Also the film with 600 nm thickness showed current density as low as 10-6 A/cm2 at the electric field of 200 kV/cm, which corresponds to the electric resistivity of ~1012 (Ωcm) and represents excellent insulating properties. The MFS capacitor showed ferroelectric switching properties in C-V characteristics and the memory window of the hysteresis loop was about 1 V in the operation of ±10 V.

Microwave multipole lowpass and bandpass filters fabricated by high-T/sub c/ superconducting thin films

The design, fabrication, and measurement of 7-pole microstrip lowpass filters and 4-pole parallel coupled-line bandpass filters using superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ epitaxial thin films are presented. The YBa/sub 2/Cu/sub 3/O/sub 7-x/ films were grown in situ at 770/spl deg/C and 200 mTorr oxygen partial pressure in a pulsed laser deposition system. The 7-pole microstrip-line lowpass filter and 4-pole parallel coupled-line bandpass filter patterned on the laser ablated YBCO films with dimension of 20 cm/spl times/10 cm/spl times/0.5 mmt were generated using photolithographic and wet etching processes. The complete devices packaged in a 3-component type brass test-fixture were characterized. The results showed the center frequency of 11.2 GHz with 6% bandwith and less than 1 dB insertion loss(IL) for the case of bandpass filters below 77 K.<<ETX>>

Characteristics of high power capability for high temperature superconducting microwave multiplexer

A high temperature superconducting (HTS) multiplexer consisting of two branch line hybrids and two identical parallel-coupled-line type bandpass filters was fabricated with a center frequency of 10.9 GHz and a bandwidth of 100 MHz. We used single sided YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) epitaxial thin film on MgO substrates prepared by using in situ pulsed laser deposition. Electric connection between adjacent multiplexers was easily done with a microstrip transmission line at the terminations so that the size and weight of the multiplexer could be significantly reduced. The multiplexer showed approximately 0.8 dB insertion loss and 100 MHz bandwidth in a range of -10 dBm to 30 dBm. We observed the third order intercept at 94 dBm and the fifth order intercept point at 86 dBm by using intermodulation distortion (IMD) measurements at T=80 K. Our results demonstrate the possibility of realizing a narrow band HTS multiplexer for high performance.

HTS Multipole Bandpass Filters with Arbitrary Parallel Coupled-Line Length

We report the design, fabrication, and characterization of new high-temperature superconducting (HTS) multipole bandpass filters (BPF) with arbitrary coupled-line length. We fabricated the 2-pole, 4-pole, and 6-pole BPFs using superconducting YBCO epitaxial thin films grown on MgO (100) substrates. The measured insertion losses were -0.5 dB and -1.0 dB at the center frequencies of 12 GHz and 11.5 GHz for the 2-pole and 4-pole filters, respectively. For 6-pole filter, the insertion loss measurements showed no obvious characteristics of a bandpass filter that was qualitatively agreed with the simulation results.

Microwave bandpass filters with closed loop resonators and coupled lines using YBCO thin films

Abstract We report the characteristics of a high- T c superconducting (HTS) 4-pole bandpass filter (BPF) with center frequency of 3.9 GHz. YBa 2 Cu 3 O 7−x films were grown on MgO substrate (20×20×5 mm 3 ) by using a hollow cathode sputtering method. To improve the stopband characteristics and to reduce the physical size of a device pattern, we designed a new type microwave bandpass filter consisting of closed-loop resonators and parallel coupled-lines. In resonator structure and in connections between input/output ports and resonators, the stepped impedance lines and the microstrip coupled-lines are used, respectively. The measured microwave responses of HTS 4-pole bandpass filters were in a good agreement with the simulation result except a small shift of center frequency.

Measurement on electrical tunability of microstrip line resonators using YBa 2 Cu 3 O 7-δ /SrTiO 3 /MgO multilayer thin films

We have fabricated YBa2Cu3O7-(delta ) (YBCO) superconducting line resonator and tunable line resonator on ferroelectric Sr0.5Ba0.5TiO3 (SBTO) buffered MgO(100) substrate and discussed the frequency shift mechanism of superconductor as a function of temperature and bias voltage, respectively. The resonators were designed using superconducting YBCO epitaxial thin films. Optimized resonator shown the resonant frequency of 10 GHz at 77 K. The YBCO films were grown in situ by pulsed laser deposition technique at 750 degree(s)C and oxygen partial pressure of 200 mTorr. The resonators have linear microstrip line separated by a gap of 5 micrometers and 0.5 mm, respectively. A gap is intentionally introduced to generate mainly a capacitive series reactance. The equivalence circuit of line resonator is a II network consisted of three capacitances. As the series capacitance C12 of SBTO ferroelectric thin films was changed by a bias voltage applied on the strip conductors including the gap, resonance frequency was shifted about 20 MHz from the unbiased center frequency of 10 GHz. The variation of resonance peak could be explained by a serial capacitance model. To find a central frequency mechanism depending on temperature, we fit the raw data using f(T)/f(10 K) and simple power law model. The shifting of the resonant frequencies due to temperature was fit to a two-fluid model, BCS theory and empirical formula. Also the surface impedance of superconducting YBCO films as a function of temperature at 10 GHz has been estimated by a transmission line method.

Miniaturized microstrip multipole low-pass filters using high-temperature superconducting YBa2Cu3O7-x thin films

We have designed the microstrip-type multipole (7-pole and 9-pole) lowpass filters consisting of both transmission lines and open stubs. The filters were fabricated on high temperature superconducting (HTS) YBa2Cu3O7-(delta ) (YBCO) thin films grown on MgO(100) substrates by pulsed laser deposition. For 7-pole lowpass filter, the measured insertion losses were within 0.5 dB, and up to 8 GHz the passband shows very flat with ripples of less than 0.05 dB. For 9-pole filter, we observed the insertion loss of 5.0 dB and the ripples of 0.64 dB. The skirt became steep and the off-band rejection increased large as the number of poles increases.

Modification of superconducting bandpass filter structures using different image impedances

A new bandpass filter design is presented which uses arbitrary image impedance of the superconducting thin film coupled lines. In order to investigate the performance of different superconducting bandpass filters with different image impedances, we have selected and designed 4-pole Chebyshev 50 (Omega) and 75 (Omega) coupled line superconducting bandpass filters which have the same passband of 11.7 approximately 12.4 GHz. The measured result of the 75 (Omega) coupled line superconducting bandpass filter agrees with the result of the 50 (Omega) coupled line superconducting bandpass filter. The technique used in this paper to change the filter structure is particularly attractive because of its application to improve the power-handling ability of superconducting passive device and its possibility to reduce the size of superconducting thin film microwave device which has the limitation of superconducting thin film size.