Yusuke Suzuki

Room temperature negative differential resistance of CdF2∕CaF2 double-barrier resonant tunneling diode structures grown on Si(100) substrates

The authors have demonstrated the crystal growth of CaF2∕CdF2∕CaF2 multilayered heterostructures on Si(100) substrates as double-barrier resonant tunneling diode structures by a low-temperature growth technique. Current-voltage characteristics were investigated and the authors observed negative differential resistance (NDR) characteristics at room temperature. The peak-to-valley current ratio was 2–8, and 13 at maximum, and peak current density was 80–90A∕cm2. The quantum-well layer thickness dependence of NDR peak voltages is also discussed on the basis of qualitative analytical model using the Esaki-Tsu formula.

Effect of Sputtering Deposition Process on Magnetic Properties of Magnetic Multilayers

The energy distribution of energetic particles produced in magnetron sputtering plasma was measured by varying the sputtering gas species and pressure, and Co/Pt, Gd/Fe, and Co/Cu multilayers were fabricated using the same sputtering cathodes as the diagnosed one in order to discuss the relationship between the plasma parameters and the magnetic properties of the multilayers. When a heavy mass target such as W or Pt was used in Ar gas sputtering, the energy of the recoiled Ar atoms from the target was found to reach up to 150 eV. The energy and flux of the recoiled atoms were reduced by using Kr or Xe gas in place of Ar gas, whereas they are rather independent of the sputtering pressure in the range of 0.3–0.5 Pa. Low-pressure sputtering was effective for increasing the energy of sputtered atoms, which was at most 10 eV. Layered structures of the magnetic multilayers were significantly influenced by the sputtering conditions. The atomic intermixing at the interface, i.e., degradation of the interface sharpness, was controlled mainly by using a heavy sputtering gas, while the surface or interface flatness mainly depended on the sputtering pressure. The perpendicular magnetic anisotropy of Co/Pt and Gd/Fe multilayers was enhanced by suppressing atomic intermixing rather than by improving the interface flatness, and the use of a heavy inert gas such as Kr or Xe was found to be effective to enhance the anisotropy. When Kr or Xe gas was used, the monatomic layered structure of the Co (1 ML)/Pt (1 ML) (ML: monatomic layer) multilayer was found to survive, and positive perpendicular anisotropy was observed. In contrast, the magnetoresistance ratio of Co (1.5 nm)/Cu (t nm) multilayers (t~0.9 nm) increased to around 50% when they were sputtered in an inert gas at low pressure, while the use of Xe gas degraded their magnetoresistance ratio. In the Co/Cu multilayers, the interface flatness rather than the sharpness was critical for enhancing the magnetoresistance ratio.

Dependences of Giant Magnetoresistance in Co/Cu Multilayers on Sputtering Conditions

The dependences of magnetoresistance properties in Co/Cu multilayers on sputtering conditions were investigated by measuring the energy distributions of Ar+ ions and metal atoms. It was found that the incident Ar+ ion energy to the substrate increased with decreasing distance D between the target and the substrate. When D was changed from 5 to 11 cm, the interface roughness estimated from low-angle X-ray diffraction was improved and the magnetoresistance ratio at the first peak was increased from 31 to 42% at room temperature. These results suggest that the incident Ar+ energy affects the magnetoresistance through the modification of interface roughness.

Properties of LiNbO3 Thin Film Deposited by Chemical Vapor Deposition and Frequency Characteristics of Film Bulk Acoustic Wave Resonator

We have realized the fabrication of a film bulk acoustic wave resonator (FBAR) using a thin LiNbO3 film deposited by chemical vapor deposition (CVD) for the first time. As a result, a 2.9 GHz FBAR with an impedance ratio of 40 dB at resonant ( fr) and antiresonant ( fa) frequencies was realized. The LiNbO3 film has a mixed polarity. As a result of scanning nonlinear dielectric microscopy (SNDM), -c domains and +c domains occupy 82 and 18% of the LiNbO3 film, respectively. A shear mode was not excited on the fabricated FABR. According to a calculation, it is found that the shear wave mode can be suppressed by selecting the thickness of electrodes without any changes in the main responses of the longitudinal mode wave. It is important to deposit another oriented LiNbO3 film with a larger coupling factor to realize a wider-band device.

Magnetic Susceptibility of Oxygen Adsorbed by Type A Bead like Zeolite

The magnetic susceptibility of A-type bead like zeolite is measured as a function of temperature between liquid helium and room temperatures under low pressure of oxygen gas. The zeolite specimen employed in the present work has micropores, the diameter of which is 5 A. The magnetic susceptibility of oxygen molecules adsorbed by zeolite is evaluated from the results obtained before and after oxygen adsorption. The temperature dependence of the susceptibility of oxygen exhibits a sharp peak at about 50 K, suggesting that oxygen molecules incorporated into zeolite induce the formation of an antiferromagnetic state. The present results are compared with the magnetism of oxygen adsorbed on the surface of graphite and physisorbed into slit-shaped micropores in activated carbon fibers.

A Simulation of Energy-Filtered Backscattered Electron Signals from Subsurface Cu Interconnect Structures

The exit energy dependence of backscattered electron signals from subsurface Cu interconnect structures is studied using a Monte Carlo simulation of electron scattering. The exit energies of the backscattered electrons reflect the depth of the Cu layers accessible in the samples and the energy-filtered backscattered electron signals provide more depth information on the sample structures than the total backscattered electron signals. The effects of the sample geometry on the energy-filtered backscattered electron signals are discussed for several structures.