Joo Youl Huh

Time-resolved analysis of the set process in an electrical phase-change memory device

An experimental investigation was carried out on the kinetic nature of the set process in a phase change memory device by combined analyses of set voltage wave forms and time-resolved low-field resistances. As it turned out, the progress of a set process may be measured in terms of three characteristic times in sequence i.e., threshold switching time tth, incubation time for crystallization tinc, and complete set time tset. These characteristic times are supposed to demarcate, in some measure, different stages of crystallization in the memory material during a set process. Each of these times has a strong dependence on input pulse voltage and particularly threshold switching time tth was found to have an exponentially decaying dependence. The latter may be related to the decreasing capacitance of an amorphous phase-change material with approaching threshold switching.

Thermoelectric PbTe thin film for superresolution optical data storage

To find its practical use in ultrahigh density optical data storage, superresolution (SR) technique needs a material that can render a high SR capability at no cost of durability against repeated readout and write. Thermoelectric materials appear to be promising candidates due to their capability of yielding phase-change-free thermo-optic changes. A feasibility study was carried out with PbTe for its large thermoelectric coefficient and high stability over a wide temperature range as a crystalline single phase. Under exposure to pulsed red light, the material was found to display positive, yet completely reversible changes of optical transmittance regardless of laser power, fulfilling basic requirements for SR readout and write. The material was also shown to have a high endurance against repeated static laser heating of up to 106–107 cycles tested. A read only memory disk with a PbTe SR layer led to the carrier to noise ratio value of 47dB at 3.5mW for 0.25μm pit; below the optical resolution limit (∼0.2...

Influence of firing ambience on fire-through silver contact metallization for crystalline silicon solar cells

Screen-printed Ag thick-film metallization is used in the photovoltaic industry for the front-side emitter contacts of crystalline silicon solar cells owing to its cost-effectiveness and high throughput. In order to obtain a better understanding for the formation of Ag crystallites at the paste/Si interface and the correlation between the electrical properties and microstructure of the contact, the firing treatment was carried out under various oxygen partial pressures (Po2) and the contact resistance was determined by a transfer length method (TLM) measurement. The present study results demonstrate strong dependences of the Ag crystallite formation and the contact resistance on Po2 in the firing ambience.

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.

Effect of bismuth on the growth kinetics of intermetallic compounds in Sn-3.5Ag solder joints: A growth kinetic model

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150°C and 180°C. The Cu6Sn5 layer growth was significantly enhanced, but the Cu3Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu6Sn5+Cu3Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints was accompanied by the accumulation of Bi ahead of the Cu6Sn5 layer that resulted in the formation of a liquid layer at the Cu6Sn5/solder interface. A kinetic model was developed for the planar growth of the Cu6Sn5 and Cu3Sn layers in the solder joints, accounting for the existence of interfacial reaction barriers. Predictions from the kinetic model showed that the experimental results could be well explained by the hypothesis that the formation of a Bi-rich liquid layer at the Cu6Sn5/solder interface reduces the interfacial reaction barrier at the interface.

Effects of Current-injection Firing with Ag Paste in a Boron Emitter

A high contact resistance for screen-printed contacts was observed when a conventional Ag paste was used on a boron emitter. The results of this study suggest that electron injection during firing is one of the processes that contribute to a lower contact resistance. Larger quantities of Ag precipitates formed upon electron injection into the boron emitter, which was confirmed by observing Ag crystallite or dendrite structures on the boron and by measuring the contact resistance between the boron emitter and the Ag bulk. The electron-injected sample had approximately 10000 times lower contact resistance than an untreated sample. The contact resistance of the electron-injected sample was 0.021 mΩ∙cm2 under optimal conditions, which is lower than that of conventional p-type silicon solar cells. Thus, electron injection can effectively lower contact resistance when using Ag paste in n-type silicon solar cells. During the cooling in the firing process, dissolved Ag ions in the glass layer are formed as dendrites or crystallites/particles. The dendrites are formed earlier than others via electrochemical migration under electron injection conditions. Then, crystallites and particles are formed via a silicon etching reaction. Thus, Ag ions that are not formed as dendrites will form as crystallites or particles.

Fabrication of 100nm Sized Patterns on a Non-Planar Substrate by Using Nanoimprinting Lithography

A faithful pattern transferring onto a non-planar substrate was demonstrated by nano-imprinting technique. Uniform pressing of a flexible template onto a substrate was important for the faithful pattern transferring. Both the UV-based and thermal imprinting techniques were used to transfer patterns of 200nm sized features to the non-planar substrates such as outer wall of rod and inner surface of cylinder and it could be used for nano-devices such as lab on a chip.

Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates

To simulate the growth of Ni3Sn4 phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu1−xNix)6Sn5/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)3Sn4 and Ni3Sn4 layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)3Sn4 layer was formed at the (Cu,Ni)6Sn5/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)3Sn4 layer growing predominantly at the (Ni,Cu)3Sn4/Ni interface by fast diffusion of Sn across the (Ni,Cu)3Sn4 layer. It is proposed that the accelerated growth of the (Ni,Cu)3Sn4 and Ni3Sn4 layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.

Particle size effects on the coherent phase equilibria of binary nanoparticles

AbstractThe phase stability of isolated, radially symmetric nanoparticles of a binary system that exhibits a miscibility gap was analyzed by constructing coherent phase diagrams which account for both the surface stress (