Yubin Hwang

The leakage current mechanism of PZT thin films deposited by in-situ sputtering

PZT thin film has been expected to replace conventional dielectric as a charge storage element in DRAM without use of a sophisticated structure such as a trench, because PZT has a dielectric constant much higher than silicon dioxide [1]. However, PZT thin film has relatively high leakage current [2]. To improve the high leakage current, the mechanism must be studied. Some studies have reported that the leakage current mechanism of PZT thin films on platinum showed Schottky emission [3, 4]. Their interpretation depends on the linearity of l n J versus E 1/2 (J: current density, E: electric field). This method is ,not able to distinguish Schottky emission from Poole-Frenkel emission, which may be one of the main mechanisms in PZT thin films. The equation for Poole-Frenkel emission predicts a straight line graph In J against E 1/2. That is, under most circumstances, experimental results can be plotted as a good straight line on either type of graph. Therefore, the mechanism cannot be separated by this means. To tell them apart, it must be determined that the leakage current results from a bulk-limited process or a barrier limited process, Poole-Frenkel emission being bulk limited and Schottky emission being barrier limited [5]. In this study, we measure J V curves for two different polarities using different electrodes, for example, upper electrode aluminium and bottom electrode platinum. In this case, if the leakage current depends on the barrier limited process, the J V curve will be asymmetrical when electrode polarity is reversed because the work functions of aluminium and platinum are different [6]. If leakage current depends on the bulk limited process, the J V curve shows symmetrical characteristics. Platinum film (250nm) with an intervening titanium (300 nm) layer as an adhesion layer were deposited as the bottom electrode by DC sputtering on thermally surface oxidized Si substrate, and then PZT films were deposited by RF magnetron sputtering, the films having a compositional Zr/Ti ratio of 50/50 and a thickness of 200 nm. The deposition conditions are shown in Table I. The aluminium top electrode of radius 100 ~m was deposited by thermal evaporation. X-ray diffractometry revealed that the film was of (1 1 1)-oriented perovskite phase. Leakage current at various voltages was measured using a HP 4145B semiconductor parameter measurement equipment.

Characteristics of SrTiO3 thin films deposited under various oxygen partial pressures

SrTiO3 thin films were deposited by rf-magnetron sputtering under various sputtering conditions followed by conventional furnace annealing at 600 and 700 °C. The amorphous SrTiO3 thin films crystallized into polycrystals at 600 °C. The leakage current of the SrTiO3 thin films decreased with increasing oxygen partial pressure in the sputtering gas. On the contrary, the dielectric constant increased with increasing the oxygen content in the sputtering gas. The leakage current and dielectric constant increased with increasing substrate temperature and post-annealing temperature. The ratio of Sr∶Ti approached 1∶1 with increasing oxygen content in the sputtering gas and substrate temperature during deposition. The oxygen content in the film decreased with increasing the substrate temperature. The capacitance-voltage (C-V) curves showed that the capacitance was nearly independent of the applied voltage. Scanning electron microscopy (SEM) micrographs showed that interdiffusion between the bottom electrode (Pt) and the buffer layer (Ti) occurred during post-annealing, but that the interface between SrTiO3 and Pt was stable.

Effects of BF2+ implants on titanium silicide formation by rapid thermal annealing

The formation of titanium silicides on Si implanted with different BF2+ dosages has been studied by secondary ion mass spectrometry and transmission electron microscopy measurements. The thickness of the silicide layer formed in the temperature ranging from 600 to 800 °C has been investigated as a function of the implanted BF2+ dosage up to 1×1016 cm−2. Annealing at 700 °C results in conversion of the titanium film into predominantly C49 TiSi2, and most of it is transformed into the C54 phase at 800 °C or higher, resulting in a lower sheet resistance (16 μΩ cm). The titanium silicide thickness formed after the rapid thermal annealing (RTA) treatment depends on the implanted BF2+ dosage, caused by the native oxide enhanced by increased damage. Boron is redistributed into the silicide layer up to the solid solubility limit during annealing, leading to an accumulation at the silicide/silicon interface. The lowest contact resistance (with a size of 0.7 μm×0.7 μm) of 35 Ω is obtained at the annealing temperatu...

Surface structures and magnetic anisotropies of a Fe/Pt (001) surface: An ab initio study

Using ab initio calculations, we obtained the surface phase diagram of a Fe/Pt(001) surface and the magnetic anisotropy energies of the equilibrium Fe/Pt(001) surface structures. From the obtained surface phase diagram, Fe-rich L12 B and perpendicular L10 B were found to be the most stable Fe–Pt surface phases. The calculated magnetic anisotropy energies of the Fe-rich L12 B and perpendicular L10 B Fe/Pt(001) structures revealed that the magnetic easy axes of the surface structures prefer to align in the [001] direction. Through systematic calculations, we showed that the magnetic anisotropy reduction in Fe/Pt(001) originates from the changed electron filling in the 3dz2 orbital of Fe atoms due to the surface formation.

Atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface during the early stage of epitaxial graphene growth

The understanding of the formation of graphene at the atomic scale on Si-terminated 3C-SiC for obtaining high-quality graphene sheets remains elusive, although epitaxial graphene growth has been shown to be a well-known method for economical mass production of graphene/SiC heterojunctions. In this paper, the atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface for the formation of graphene buffer layer during the early stage of epitaxial graphene growth was investigated using a molecular dynamics simulation. Observation of the behavior of the remaining carbon atoms on the Si-terminated 3C-SiC (111) surface after removal of the silicon atoms revealed that graphene clusters, which were formed by sp2-bonded carbon atoms, start to appear at annealing temperatures higher than 1300 K. Our simulations indicated that the structural stability of the whole system increased as the number of sp2-bonded carbon atoms on the Si-terminated 3C-SiC (111) surface increased. It was also found that the diffusi...

Interface-dependent magnetic anisotropy of Fe/BaTiO3: A first principles study

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2–terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.

Thermally stable ternary titanium‐tantalum silicide formation on polycrystalline silicon

A material for thermally stable self‐aligned silicide technologies has been developed using sequentially deposited Ti/Ta on polycrystalline silicon. At lower annealing temperatures below 1000 °C two separate phases were found by cross‐sectional transmission electron microscopy to exist in the form of bilayer TiSi2/TaSi2. The formation of a ternary phase (TiTa)Si2 has been observed at a higher temperature of 1000 °C. Consequently, the ternary (TiTa)Si2 layer could be kept extremely flat, with a sheet resistance of 5 Ω/⧠, even after 1000 °C, 30 min annealing. Cross‐sectional transmission electron micrographs of the structure clearly reveal that no agglomeration occurs during the heat treatment.

Configuration Dependency of Attached Epoxy Groups on Graphene Oxide Reduction: A Molecular Dynamics Simulation

The atomic behavior of epoxy groups on a graphene oxide sheet was observed during high thermal heat annealing using a reactive force-field based on molecular dynamics simulations. We found the oxygen-containing functional groups interplay with each other and desorbed from the graphene oxide sheet by a form of O2 gas if they were initially in close distance. Through comparing reduction results of graphene oxide with different densities of the nearest neighboring epoxy pairs, we confirmed that the amount of released O2 gas has a clear tendency to increase with a higher density of epoxy pairs in close distance on a graphene oxide sheet.

Atomic-Scale Simulations of Early Stage of Oxidation of Vicinal Si(001) Surfaces Using a Reactive Force-Field Potentials

The early stages of the oxidation process on vicinal Si(001) surfaces were studied at the atomic scale using reactive-force field-based molecular dynamics simulations. Oxygen molecules at step edges on the vicinal Si(001) surface showed higher reactivity than those on flat terraces. In macroscopic simulations of oxidation on vicinal Si(001) surfaces with different miscut angles (0°, 5.5°, 10.5°), we found that the initiation of oxidation with higher miscut angles was earlier than with lower angles. These results clearly show that a high density of step edges on the vicinal Si surface accelerates the initial oxidation.