Seunghyun Jang

Epitaxial stabilization of artificial hexagonal GdMnO3 thin films and their magnetic properties

The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.

Characterization of an Oxide Trap Leading to Random Telegraph Noise in Gate-Induced Drain Leakage Current of DRAM Cell Transistors

An accurate method for extracting the depth and the energy level of an oxide trap from random telegraph noise (RTN) in the gate-induced drain leakage (GIDL) current of a metal-oxide-semiconductor field-effect transistor (MOSFET) is developed, which correctly accounts for variation in surface potential and Coulomb energy. The technique employs trap capture and emission times defined from the characteristics of GIDL. Ignoring this variation in surface potential leads to an error of up to 116% in trap depth for 80-nm technology generation MOSFETs. RTN amplitude as a function of MOSFET drain-gate voltage is also investigated.

Electric-field-controlled directional motion of ferroelectric domain walls in multiferroic BiFeO3 films

We describe the directional ferroelectric domain wall motion in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A structural analysis of the film shows that a strain gradient is developed in our film, which creates a symmetry breaking in a ferroelectric double-well potential. The asymmetric double-well potential can cause ferroelectric domain walls to move sideways with preferred directionality under a vertical electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient.

Step bunching-induced vertical lattice mismatch and crystallographic tilt in vicinal BiFeO3(001) films

Epitaxial (001) BiFeO3 thin films grown on vicinal SrTiO3 substrates are under large anisotropic stress from the substrates. The variations of the crystallographic tilt angle and the c lattice constant, caused by the lattice mismatch, along the film thickness were analyzed quantitatively using the x-ray diffraction technique. By generalizing the Nagai model, we estimated how step bunching resulted in the vertical lattice mismatch between adjacent BiFeO3 layers, which induced the strain relaxation and crystallographic tilt. The step bunching was confirmed by the increased terrace width on the BiFeO3 surface.

Investigation of Gate Etch Damage at Metal/High-

Plasma damage on a high-k/SiO2 dielectric at a gate edge during a dry etch process is investigated. The damage was observed to generate slow oxide traps, causing a random telegraph noise (RTN) in a gate edge direct tunneling current. Through the analysis of the RTN, the distribution of the oxide traps in the high-k/SiO2 dielectric was obtained, and the plasma-damage-induced oxide traps were found to be distributed over a wide area of the high-k/SiO2 sidewall at the gate edge region.

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Epitaxial thin films of hexagonal ErMnO3 fabricated on Pt(111)/Al2O3(0001) and yttria-stabilized zirconia(111) substrates exhibited both ferroelectric character and magnetic ordering at low temperatures. As the temperature was reduced, the ErMnO3 films first showed antiferromagnetism. At lower temperatures, the films deposited at lower oxygen partial pressures exhibited spin glass behavior. This re-entrant spin glass behavior was attributed to competition between an antiferromagnetic interaction in the hexagonal geometry and a ferromagnetic interaction caused by a change in Mn valence induced by excess electrons from the oxygen vacancies.Epitaxial thin films of hexagonal ErMnO3 fabricated on Pt(111)/Al2O3(0001) and yttria-stabilized zirconia(111) substrates exhibited both ferroelectric character and magnetic ordering at low temperatures. As the temperature was reduced, the ErMnO3 films first showed antiferromagnetism. At lower temperatures, the films deposited at lower oxygen partial pressures exhibited spin glass behavior. This re-entrant spin glass behavior was attributed to competition between an antiferromagnetic interaction in the hexagonal geometry and a ferromagnetic interaction caused by a change in Mn valence induced by excess electrons from the oxygen vacancies.

Gate Dielectric Stack Through Random Telegraph Noise in Gate Edge Direct Tunneling Current

In this brief, we have investigated the program disturb characteristics caused by drain-induced barrier lowering (DIBL) in a 32-nm nand Flash memory device. It was found that the VTH shift of the (N + 2)th erased state cell is larger than that of the (N + 1)th erased state cell if it is assumed that the channel of the Nth cell is cut off. It is revealed that the cut off is caused by a cell-to-cell coupling effect that is becoming more severe in the development of high-density Flash memory arrays.

Oxygen vacancy induced re-entrant spin glass behavior in multiferroic ErMnO3 thin films

We present the results of an optical method of quantitatively estimating the spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films. The two in-plane (a-b plane) spin exchange integrals J1 (intratrimer Mn-Mn interaction) and J2 (intertrimer Mn-Mn interaction) are deduced from the magnon scattering peak wavenumbers. We found that J2 decreases systematically when the R ionic radius increases, while J1 is nearly independent of R ionic radius, contrary to the expectation in single crystals. We show that the R dependence of J1 could be understood in terms of the stress in the thin films. Our result indicates that the stress has stronger effect on the atomic displacement of the intratrimer Mn-Mn distance than the intertrimer Mn-Mn distance.

DIBL-Induced Program Disturb Characteristics in 32-nm NAND Flash Memory Array

In this paper, extraction method of electron band mobility in amorphous silicon thin-film transistor (a-Si TFT) is presented. First, we propose the mobility model considering the traps of amorphous silicon and the vertical field dependent mobility degradation. Then we calculate the ratio of effective mobility to band mobility by considering the traps. After that, the vertical field dependent mobility degradation is applied to the model using fitting parameters. Through this process, 13 cm2 V-1 s-1 of band mobility is extracted in our devices. Experimentally extracted electron band mobility of a-Si TFTs would be useful to technology computer aided design (TCAD) and simulation program with integrated circuit emphasis (SPICE).

Spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films

We investigated a slow oxide trap causing a random telegraph noise (RTN) within a gate edge overlap region in an inversion mode at metal/high-k dielectric nMOSFETs. The oxide trap was observed to generate RTN only in gate leakage current (Ig RTN) without in drain current (Id RTN) in the inversion mode. Through the analysis of the RTN dependence on drain, source, and body bias, we found that the oxide trap in the dielectric exists not within the channel area but within the gate edge overlap region, resulting in the absence of Id RTN.