Byung Ki Cheong

Bio: Byung Ki Cheong is an academic researcher from Korea Institute of Science and Technology. The author has contributed to research in topics: Amorphous solid & Thin film. The author has an hindex of 11, co-authored 30 publications receiving 928 citations.

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

Abstract: Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorpho...

Thermal conductivity of phase-change material Ge2Sb2Te5

Abstract: The thermal conductivity of thin films of the phase-change material Ge2Sb2Te5 is measured in the temperature range of 27°C

Nanometer-scale order in amorphous Ge2Sb2Te5 analyzed by fluctuation electron microscopy

Abstract: The phase change material Ge2Sb2Te5 is widely investigated for use in nonvolatile memories. It has been reported that the crystallization speed depends on the thermal history, indicating that structural differences exist between amorphous states. The authors apply fluctuation electron microscopy to quantify differences in the nanometer-scale structural order between several amorphous states of Ge2Sb2Te5. All as-deposited films are found to contain ordered regions. Thermal annealing below the crystallization threshold increases the nanoscale order, and such samples crystallize slightly more rapidly. The authors hypothesize that the nanoscale ordered regions act as the nuclei for crystallization, with the largest regions being the most significant.

A Study on the Scalability of a Selector Device Using Threshold Switching in Pt/GeSe/Pt

Abstract: We performed a study on the scalability of Ovonic Threshold Switching (OTS) devices using an amorphous chalcogenide material, Ge0.4Se0.6. As the cell size decreased, the maximum driving current was estimated to be over 3 × 10 7 A/cm 2 , surpassing the state of the art devices based on crystalline Si. However, the threshold voltage (VTH), the holding voltage (VH), and the holding current (IH) were observed to increase laying challenges to be resolved for developing non-destructive and low-power consuming selector devices. VTH was found to be reduced by decreasing the thickness of GeSe film until 40 nm, below which it started to saturate. This might be associated with the Schottky barrier formed at the interface between the amorphous semiconductor and the metal electrode.

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

Abstract: 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.

Phase Change Memory

Abstract: In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal properties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling properties of PCM are illustrated with recent experimental results using special device test structures and novel material synthesis. Factors affecting the reliability of PCM are discussed.

Nanoscale thermal transport. II. 2003–2012

Abstract: A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

Phase-change memory

Abstract: A phase-change memory element with side-wall contacts is disclosed, which has a bottom electrode. A non-metallic layer is formed on the electrode, exposing the periphery of the top surface of the electrode. A first electrical contact is on the non-metallic layer to connect the electrode. A dielectric layer is on and covering the first electrical contact. A second electrical contact is on the dielectric layer. An opening is to pass through the second electrical contact, the dielectric layer, and the first electrical contact and preferably separated from the electrode by the non-metallic layer. A phase-change material is to occupy one portion of the opening, wherein the first and second electrical contacts interface the phase-change material at the side-walls of the phase-change material. A second non-metallic layer may be formed on the second electrical contact. A top electrode contacts the top surface of the outstanding terminal of the second electrical contact.

Optically reconfigurable metasurfaces and photonic devices based on phase change materials

Abstract: Photonic components with adjustable parameters, such as variable-focal-length lenses or spectral filters, which can change functionality upon optical stimulation, could offer numerous useful applications. Tuning of such components is conventionally achieved by either micro- or nanomechanical actuation of their constituent parts, by stretching or by heating. Here, we report a novel approach for making reconfigurable optical components that are created with light in a non-volatile and reversible fashion. Such components are written, erased and rewritten as two-dimensional binary or greyscale patterns into a nanoscale film of phase-change material by inducing a refractive-index-changing phase transition with tailored trains of femtosecond pulses. We combine germanium–antimony–tellurium-based films with a diffraction-limited resolution optical writing process to demonstrate a variety of devices: visible-range reconfigurable bichromatic and multi-focus Fresnel zone plates, a super-oscillatory lens with subwavelength focus, a greyscale hologram, and a dielectric metamaterial with on-demand reflection and transmission resonances.

Resonant bonding in crystalline phase-change materials

Abstract: The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.