Ki-Soo Sohn

Temperature dependent leakage currents in polycrystalline silicon thin film transistors

We have studied the origins of the leakage current in polycrystalline silicon (poly-Si) thin film transistors. Temperature dependent transfer characteristics were measured as a function of drain voltage. Three kinds of leakage current were introduced to explain the experimental results. The leakage current may arise from the generation current at very low drain voltage, and may result in the same activation energy between leakage current and conductivity of undoped poly-Si. The leakage current may be due to the thermionic field emission via grain boundary defects in the intermediate drain voltage region. At high drain voltage and high negative gate voltage, the leakage current may result from the field enhanced tunneling of electrons in the valence band to the conduction band via grain boundary traps.

Structural, electronic, and magnetic properties of thin V/Cu film

Abstract We studied the electronic and magnetic properties of the surface and interface of V/Cu alloys by means of the ab initio full-potential linear muffin-tin orbital (FP-LMTO) method. One monolayer (ML) V adsorbed on the Cu (1 0 0) and Cu (1 1 1) substrates is non-magnetic in contrast to the V MLs on Ag and Fe, which are magnetic. And also, the V ML embedded in bulk Cu are non-magnetic. In the 1-ML V sandwich of Cu / V (1 1 1)/ Cu , although there is the very weak spin-polarization in the ferrimagnetic phase, the energy of the magnetic band-splitting turned out to be very low. We did not find the magnetic order of 1-ML V (1 0 0) embedded in bulk Ni, either. In summary, our result for the 1– 3 ML V embedded in bulk Cu provided no evidence for long-range magnetic ordering, either in-plane or out-of-plane. Our thin film calculations are compared with other theoretical results and with data measured by angle-resolved ultraviolet photoemission spectroscopy (ARUPS).

Barriers for diffusion and interactions with hydrogen in palladium

Abstract We investigated theoretically relaxations of a few surface layers of palladium (0 0 1) by a calculation of the total energy based on the real-space tight-binding framework. We discussed the behaviors of H atoms in vacuum, on the (0 0 1) surface and in the bulk. The diffusion barrier height of H atoms in Pd depends on the H concentration. For H occupations of 1 3 and 1 3 on surface layers and on subsurface layers, respectively, the change of activation energy barriers is the most insensitive. We obtained metal-H binding energies and diffusion energies as x increases in PdHx, which are in agreement with experimental data of Nernst and Harada. We also obtained electronic structures of adsorbed H and absorbed H by the recursion method.

Charge transfer in core-level shifts at (110) surfaces of compound semiconductors

The charge transfers in compound semiconductors are estimated for the zincblend (110) surface, based on the point-charge model. It is found that the charge transfers at the (110) surface are larger than those in the bulk in contrast to the earlier results. It is, therefore, expected that surface appears to be more ionic than the bulk. The estimated results contain information about the charge distribution and ionicity, which will be helpful to understand the electronic properties at the surface such as the surface binding energy, the surface polarization and surface excitons.

Optical properties of a multiple quantum well in a HgTe/CdTe system

A finite HgTe/CdTe superlattice with different dielectric media on either side of the surfaces is investigated by taking into account the wavefunction overlap between the interface states and plasmons. The unit cell of the finite HgTe/CdTe superlattice consists of two electron-like states and a heavy-hole-like state in HgTe and two light-hole-like states in CdTe. Using the random-phase approximation added with some assumptions, we have studied the density - density correlation function by considering the interface state with the wavefunctions overlapping with the electron-like states, the light-hole-like states and the heavy-hole-like states. We have calculated the collective excitation spectra of the intrasubband and the intersubband for both the bulk plasmons and the surface plasmons as a function of the number of unit cells. The Raman intensities due to bulk and surface plasmons are expressed by the relative value of the mode energy of the plasmons.

Diffusion processes and interactions of hydrogen atoms in Pd

Abstract On the basis of the total energy obtained with the recursion method in the tight-binding framework, we discussed the adsorption and diffusion mechanism in the system of hydrogen atoms in Pd. Considering the relaxations of a few surface layers, we calculated the HH interaction energies, the metal-H binding energies per H atom, etc., inside and on the surface of palladium (0 0 1), which are in good agreement with the experimental data of Nernst and Brodowsky and of Harada. It was also shown that, in the HH bonding on the surface, the bonding parallel to the (0 0 1) surface is more repulsive than that normal to the surface. Two H atoms occupying the two neighboring octahedral sites in Pd is shown to be the most stable configuration.

Collective excitation of a multiple quantum well in HgTe/CdTe superlattice

A finite HgTe/CdTe superlattice with different dielectric media on either side of the surfaces is investigated by taking into account the wave function overlap not only between the interface states and plasmons but also between the interface states. The unit cell of the finite HgTe/CdTe superlattice consists of two electron-like states and a heavy-hole-like state in HgTe and two light-hole-like states in CdTe. Using the random phase approximation added with some assumptions, we have studied the density-density correlation function considering the wave functions of three different interface states overlapping with the electron-like states, the light-hole-like states, and the heavy-hole-like states. We have calculated the collective excitation spectra of the intrasubband for both the bulk plasmons and the surface plasmons of interface states. Raman intensities due to bulk and surface plasmons are expressed by the relative value of mode energy of the plasmons and as a function of the wave vector.

Hydrogen uptake kinetics and chemisorption processes on niobium surfaces

Abstract On the basis of the two-state model, the kinetics of hydrogen adsorption on a niobium surface was studied as a function of temperature and time. We calculated the normalized surface fraction in terms of the initial sticking coefficients at a fixed hydrogen pressure. We also obtained the normalized bulk fraction for various temperatures and initial sticking coefficients.