Hyun-Seok Lee

Origin of nonlinear optical characteristics of crystalline Ge-Sb-Te thin films for possible superresolution effects

An experiment was conducted to investigate the origin of the nonlinear optical characteristics of crystalline Ge2Sb2Te5 and Ge-doped SbTe thin films which have been demonstrated to have the capability of a superresolution readout. By means of the real-time measurement of the electrical resistance combined with concurrent optical reflectance/transmittance measurement during pulsed laser irradiation of varying power, it was found that the absorption coefficients of these films decrease with increasing laser power, accompanied most probably by an increase in the carrier concentration. This finding may be understood in light of the saturable absorption due to band filling by thermally assisted photoexcited carriers. This has been previously proposed as an important cause of the superresolution effect of PbTe, which belongs to the chalcogenide degenerate semiconductors such as the GeSbTe materials under study.

Dielectric and Piezoelectric Properties of Low-Temperature-Sintering Pb(Mg1/2W1/2)O3-Pb(Ni1/3Nb2/3)O3-Pb(Zr,Ti)O3 Ceramics for Multilayer Piezoelectric Actuator

In this study, in order to develop low-temperature-sintering ceramics for a multilayer piezoelectric actuator, Pb(Mg1/2W1/2)O3-Pb(Ni1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMW-PNN-PZT) ceramics were fabricated using Li2CO3 and CaCO3 as sintering aids and their dielectric and piezoelectric properties were investigated in relation to the amount of Bi substitution. Piezoelectric actuators require both a high piezoelectric constant (d33) and a high electromechanical coupling factor (kp). Accordingly, in this study, Bi was substituted at the Pb site of basic composition PMW-PNN-PZT ceramics for the enhancement of kp and d33. With increasing Bi substitution, density gradually increased. At the sintering temperature of 900°C, the electromechanical coupling factor (kp) and piezoelectric constant (d33) of PMW-PNN-PZT composition ceramics with 1.0 mol % Bi substitution as well as 0.2 wt % Li2CO3 and 0.25 wt % CaCO3 showed the optimal value of 0.618 and 473 pC/N, respectively, for multilayer piezoelectric actuator application.

Investigation on the role of nitrogen in crystallization of Sb-rich phase change materials

To better understand the role of nitrogen (N) during crystallization of Sb-rich phase change materials, a study was conducted using Sb and Sb70Te30 as host materials of N. Crystallization of the as-sputtered Sb–N films of varying N content was examined to reveal that Sb–N bonds are formed in the as-sputtered states, enhancing amorphous phase stability increasingly with N content. Crystallization appeared to proceed with irreversible dissociation of these bonds to form N2 molecules that may then exist stably during the subsequent memory operations. N2 molecules are considered to play as growth-retarding agents as demonstrated with memory operations of N-doped Sb70Te30.

Microstructural and optical analysis of superresolution phenomena due to Ge2Sb2Te5 thin films at blue light regime

Superresolution (SR) phenomena due to Ge2Sb2Te5 films were examined by combined analysis of the transmission electron microscopy (TEM) microstructures of the laser-irradiated films and the results from dynamic and static tests using blue lasers. A new finding was made that comprises a complementary case of the classical SR readout by Ge2Sb2Te5 film; an amorphous band instead of a closed aperture of melt in the crystalline background forms behind a moving laser but still produces a high SR signal. A complete carrier-to-noise-ratio curve of a SR-read-only memory employing Ge2Sb2Te5 may be derived from a nonlinear optical effect, specifically thermally assisted saturable absorption.

Measurement of ultrasonic surface wave velocities in silicon crystals and its comparison with finite element analysis

The velocities of leaky surface acoustic waves (SAWs) have been measured using a large-aperture line-focused transducer for three silicon single crystals (namely, Si-100, Si-110, and Si-111). While rotating the specimen, velocities were measured as a function of propagation direction. The slowness curves of the SAWs show good agreement with the crystal structures of the specimens. Finite element method (FEM) simulation has also been carried out for silicon crystals and the results were compared with the measurement results. The FEM results can explain the measured ones. This method has high potential for determining crystal structures.

Piezoelectric and Dielectric Characteristics of Low Temperature Sintering Pb(Mg 1/2 W 1/2 )O 3 -Pb(Mn 1/3 Nb 2/3 )O 3 -Pb(Zr 1/2 Ti 1/2 )O 3 Ceramics with the Substitution of Pb(Mg 1/2 W 1/2 )O 3

In this study, in order to develop low temperature sintering piezoelectric ceramics for LTCC (low-temperature co-fired ceramic) multilayer piezoelectric actuator, PMW-PMN-PZT ceramics using 0.2 wt% Li2CO3 and 0.25 wt% CaCO3 as sintering aids were investigated according to the variation of PMW substitution. The composition ceramics could be sintered at 900degC by adding sintering aids. As the amount of PMW substitution increased, the crystalline structure of PMW-PMN-PZT ceramics moved gradually from tetragonal phase to rhombohedral phase, and MPB (morphotrophic phase boundary) region appeared at 2 mol% PMW substitution. At the sintering temperature of 900degC, the density, electromechanical coupling factor (kp), mechanical quality factor (Qm), dielectric constant (epsivtau), piezoelectric constant (d33) and Curie temperature (Tc) of 2 mol% PMW substituted PMW-PMN-PZT ceramics showed the optimal values of 7.88 g/cm3, 0.58, 1002, 1264, 352 pC/N and 336degC, respectively, for LTCC multilayer piezoelectric actuator application.

Microstructure and Ferroelectric Properties of Low Temperature Sintering PMN-PNN-PZT Ceramics with Sintering Temperature

In this study, in order to develop the low temperature sintering multilayer piezoelectric actuator, PMN-PNN-PZT system ceramics were manufactured and their microstructure, ferroelectric and piezoelectric properties were investigated. By increasing sintering temperature, remanent polarization was increased due to the increase of sinterability and grain size. However, coercive showed an opposite tendency to remanent polarization owing to the feasibility of domain wall motion. At the sintering temperature of , dielectric , electromechanical coupling , piezoelectric and mechanical quality showed the optimal value of 1095, 0.60, 363 and 1055, respectively, for multilayer piezoelectric actuator application.

Frequency–Temperature Characteristics of AT-Cut Crystal Resonator and Oscillator Utilizing Relaxator Ceramic as Load Capacitor

In this study, a simple AT-cut crystal resonator and an oscillator utilizing a Bi0.5(Na0.84K0.16)0.5TiO3 relaxator ceramic as a load capacitor were designed and their frequency–temperature characteristics were investigated. To modify the temperature dependence of the dielectric constant, Bi0.5(Na0.84K0.16)0.5TiO3 ceramics were manufactured with variations in the level of Sr substitution. At a composition with 6 mol % Sr substitution as a load capacitor, the frequency–temperature characteristics of the AT-cut crystal resonator and oscillator with a series load capacitance were varied similarly to those of cut orientation variation, and also compensated.

A Study on the Growth of a Ferroelectric Thin Film Using Ionized Cluster Beam Epitaxy Technique and the Application for Ulsi Fabrication

The dielectric properties of Ba 0.65 Sr 0.35 TiO 3 (BST) films deposited by an electron beam assisted ionized cluster beam epitaxy (ICBE) technique were investigated. Highly (110) oriented BST films having a thickness up to 1, um have been successfully grown on a Si (100) substrate at 400°C using the electron beam assisted ICBE system with a plasma O 2 source. It was found that the dielectric constant increases from 475 to 1191 with different process conditions. A BST film with a thickness of 500A deposited at a substrate temperature of 400°C has a dielectric constant of 1191 and a leakage current of about 2.37×10 -9 A/cm 2 . This shows the BST film can be applied to dielectrics of ULSI capacitors.