Eun-hong Lee

A new low voltage fast SONOS memory with high-k dielectric

Abstract The comparison of simulated write/erase characteristics of silicon–oxide–nitride–oxide–silicon (SONOS) nonvolatile memory with different oxides SiO 2 , Al 2 O 3 and ZrO 2 as a top dielectric was made. We demonstrate, that an application of high- k dielectrics allows to decrease the write/erase programming voltage amplitude or programming time from 1 ms to 10 μs. The ZrO 2 suppresses parasitic electron injection from polysilicon gate. Also the design of SONOS memory based on high- k dielectrics is promising for terabit scale using hot carriers injection EEPROM and DRAM memory.

Tunability and loss tangent of ferroelectric films in superconducting resonator

Using a high-temperature superconducting resonator, measurements of the dielectric properties of ferroelectric films at microwave frequency ( GHz) are performed. The variation in dielectric constant, , of film under the influence of an applied dc voltage, , at liquid-nitrogen temperature affects the value of the capacitance, C, and loss tangent, , of the ferroelectric capacitor. In order to evaluate the values of C and of the capacitor, we used the resonator which has a planar ferroelectric capacitor in the break of the microstrip line. The couplings of each microwave input and output port are designed in the resonator, including the bias circuits of the zero-field points. ferroelectric capacitors with values of the capacitance of about 0.5 pF and less than 0.01 successfully control the tunability of the resonance frequency in the resonator with a factor of .

Construction and measurements of HTS DC SQUID electronic gradiometer to be used in NDE systems

We have designed a non-destructive evaluation system by using an electronic gradiometer constructed with two DC SQUID magnetometers. Typical flux modulation of the SQUIDs used in this work was about 30 /spl mu/V and the noise at 10 Hz was about 14 /spl mu//spl Phi//sub 0///spl radic/Hz. The system includes a non-magnetic stainless steel dewar and a set of coaxial exciting coils, which was used to induce an eddy current in the test piece. The sample motion stage and the SQUIDs were computer controlled and the output data from the electronic gradiometer was obtained by using Labview software.

Fabrication of high quality YBa/sub 2/Cu/sub 3/O/sub y/ thin films using pulsed laser deposition

High quality YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) thin films for directly coupled DC-SQUIDs were fabricated by pulsed laser deposition. Several critical parameters have been optimized through systematic studies. The best quality YBCO thin films on MgO and on LaAlO/sub 3/ substrates were made under the O/sub 2/ pressure of 400/spl sim/500 mTorr at the substrate temperature of 780/spl sim/800/spl deg/C and target to substrate distance of 50/spl sim/60 mm. Films showing a T/sub c/ above 91 K and J/sub c/ above 2/spl times/10/sup 6/ A/cm/sup 2/ at 77 K were routinely obtained. Extensive AFM and x-ray diffraction studies have been conducted for morphological and structural analyses. The directly coupled DC-SQUIDs were fabricated from the YBCO thin films deposited on SrTiO/sup 3/ bicrystals under the optimized conditions. Measurement of 2I/sub c/ and swing voltage gives values of 420 /spl mu/A and 30 /spl mu/V at 77 K, respectively.

Characteristics of YBCO step-edge junctions fabricated by a chemical etching process

Abstract High- T c step-edge junctions have successfully been fabricated on chemically etched MgO substrate steps. Epitaxial thin films of YBa 2 Cu 3 O 7−δ have been prepared utilizing an off-axis RF magnetron sputtering method. AFM observations were made to investigate surface morphology of the etched substrate and YBCO films. The microstructures of the YBCO films were also observed by HRTEM, which clearly showed a 40° titled grain boundary at the concave side of the step. The voltage-current characteristics, magnetic field dependence of the critical current, microwave and millimeter-wave response of the SEJs were investigated. The response of millimeter-wave ( f ;=94 GHz ) irradiations on the SEJ clearly showed well-defined constant voltage steps of 194 μV. In addition, the characteristics of DC-SQUID made of step-edge junctions were also observed at 67 K.

Directly coupled DC-SQUIDs of YBCO step-edge junctions fabricated by a chemical etching process operating at 77 K

High T/sub c/ directly coupled DC-SQUIDs have been successfully fabricated on chemically etched MgO substrate steps. The chemical etching was performed in a mixed acid solution of H/sub 3/PO/sub 4/ and H/sub 2/SO/sub 4/ for the best control of etched surface and roughness. YBCO thin films were deposited epitaxially on the step-edged MgO substrate by a KrF laser ablation method. Characteristics of the directly coupled DC-SQUID have been studied following the patterning and fabrication of the device. The chemically etched steps show sharper edges at the bottom of the step as well as the top unlike those made by ion milling. AFM and Raman Spectroscopy studies on the YBCO thin film deposited and patterned on chemically etched show no sign of appreciable degradation. The result is a good quality junction even at a relatively low step angle. Directly coupled DC-SQUIDs that are fabricated with this process show sweeping voltages of 160 /spl mu/V at 4.2 K, and 6 /spl mu/V at 77 K, respectively.

Josephson junction and d.c.-SQUID made on step-edge substrate☆

Abstract We have fabricated step-edge junctions (SEJs) using YBCO films deposited on MgO substrates. The substrate step-edge produced at desired places were prepared by a chemical solution as one of simple fabrication techniques for Josephson junctions. The microstructures of the YBCO films were observed by HRTEM, which clearly showed a 40° tilted grain boundary at the concave side of the step. The I–V characteristics, millimeter-wave response of the SEJs were investigated. The response of millimeter-wave radiation ( ƒ = 94 GHz ) to the SEJs exhibited Shapiro steps of 194 μV interval. The traditional behavior in terms of the voltage versus magnetic flux curves was clearly observed in the d.c.-SQUIDs fabricated with the SEJs.

Properties of high-T/sub c/ ramp-edge junctions with a Ga-doped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// barrier

We have fabricated high-T/sub c/ ramp-edge junctions with a Ga-doped YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) barrier in the trilayer geometry of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///YBa/sub 2/Cu/sub 2.79/Ga/sub 0.21/O/sub 7-/spl delta///YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// on LaAlO/sub 3/ single crystals. Interface resistances of the junctions were drastically reduced by using in-situ RF plasma cleaning treatment. The cross-sectional images of the interface of the junctions were analyzed by high resolution transmission electron microscopy. The temperature dependences of critical currents and junction resistances were consistent with the behavior predicted by the conventional proximity effect. The critical currents of the Ga-doped junctions were less sensitive to the variation of the barrier thickness compared to those of the other junctions. The increase of the barrier resistivity by Ga-doping resulted in an enhancement of the I/sub c/R/sub n/ values, up to 320 /spl mu/V at 60 K.

Growth of YBCO Thin Films on Si(100) with Double Buffer Layers

We have successfully grown high quality c-axis oriented YBa2Cu3O7-y(YBCO) thin films on Si(100) substrates with double buffer layers of SrTiO3 and YSZ. The double buffer layers and YBCO films were deposited by electron beam evaporation and off-axis rf sputtering, respectively. The double buffer layers epitaxially grown on Si effectively prevented the interdiffusion between Si and the YBCO films and enhanced the quality of YBCO thin films. Substrate temperature and thickness of buffer layers were critical in obtaining epitaxial buffer layers and high quality YBCO films. The YBCO thin films showed a smooth surface morphology and did not show any microcracks in spite of its thickness of more than 220 nm. This would be probably due to the very slow deposition rate and a good combination of buffer layers.