Noriyoshi Hayashi

Examination of band bending at buckminsterfullerene (C60)/metal interfaces by the Kelvin probe method

This study addresses the question of band bending at the C60/metal interface. The change in the energy of the vacuum level upon the deposition of C60 on various metal substrates (Au, Cu, and Ag) was examined by the Kelvin probe method under ultrahigh vacuum as a function of C60 thickness d. We observed (1) an abrupt shift of the energy of the vacuum level relative to the Fermi level of the metal substrate evacF at d⩽1 nm, to a uniform value of about 4.65 eV at all of the interfaces examined and (2) a slower shift with further deposition of C60, which stopped at d∼500 nm at another common value. These abrupt and gradual shifts can be ascribed to the formation of an interfacial dipole layer and to band bending leading to Fermi level alignment, respectively. The value of evacF for the thick region is ascribed to the bulk work function of the specific specimen studied, and it is noted that the values reported for a few monolayers in the literature should not actually be regarded as the bulk work function. The...

Preparation of ramp-edge Josephson junctions with natural barriers

We have demonstrated ramp-edge Josephson junctions using high temperature superconductors without depositing artificial barriers. We use a surface barrier formed naturally during an etching process by an Ar ion beam. The resistivity of the barrier, which is evaluated in the Nb/Au/YBCO structures, changes over 4 orders in accordance with the gas pressure in the vacuum annealing executed before the deposition of the counter-electrode. All the junctions having YBCO/barrier/YBCO structure exhibit RSJ-like current-voltage characteristics over the entire temperature range of operation. Fraunhofer-like modulation patterns are observed with a very small amount of excess current even at low temperatures except for large junctions. The temperature dependence of J/sub c/ and R/sub u/A are similar to those in HTS grain boundary junctions. Since the junction parameters are controlled by the total pressure during the vacuum annealing, the junctions made through this procedure have a potential for circuit applications.

Characteristics of Interface-Modified Josephson Junctions Fabricated under Various Etching Conditions

We have studied the influence of process parameters on electrical properties for interface-modified junctions (IMJs) based on YBa2Cu3Ox. Tunnel barriers are produced by an electron cyclotron resonance (ECR) plasma etching process and subsequent vacuum annealing. We have found that time and accelerating voltage (Vacc) in the etching sequence, in which the ramp-edge geometry is defined, are very important parameters for the barrier formation. Increasing the etching time and accelerating voltage lead to the reduction of critical current densities. By setting these parameters at appropriate values, we can control the critical current of the IMJs. We have also found that the IMJs with high junction resistance, which are fabricated at higher accelerating voltages above 700 V, have transport mechanisms via localized states for quasi particles. The X-ray photo spectroscopy analysis suggests that accelerating voltages above 700 V enhance the thickness of the barrier, while those below 700 V affect both barrier resistivity and thickness.

Energy level alignment and band bending at TPD/metal interfaces studied by Kelvin probe method

In order to examine the validity of Mott-Schottky model at organic/metal interfaces, the position of the vacuum level (VL) of triphenyldiamine (TPD) film formed on five metal substrates was measured as a function of the film-thickness by Kelvin probe method in ultrahigh vacuum (UHV). At all the interfaces, sharp shifts of the VL were observed within 1 nm thickness. Further deposition of TPD up to 100 nm did not change the position of the VL indicating no band bending at these interfaces. These findings clearly demonstrate the Fermi level alignment between metal and bulk TPD solid in UHV is not established within a typical thickness of real devices.

Examination of band bending at C60/metal interfaces by the Kelvin probe method

This paper addresses the question of band bending at the C 60 /metal interface. The change in the energy of the vacuum level relative to the Fermi level of the metal substrate e vac F upon the deposition of C 60 on various metal substrates (Au, Cu, and Ag) was examined by the Kelvin probe method under ultrahigh vacuum as a function of C 60 thickness d. We observed (1) an abrupt shift of e vac F at d≤ I nm, to a uniform value of about 4.65 eV at all of the interfaces examined, and (2) a slower shift with further deposition of C 60 , which stopped at d ∼ 500 nm at another common value. These abrupt and gradual shifts were ascribed to the formation of an interfacial dipole layer and to band bending leading to Fermi level alignment, respectively. We also examined the effect of air-exposed metal substrate and purification of C 60 .

Fabrication of natural-barrier ramp-edge Josephson junctions

We have studied the effect of process parameters on the natural-barrier ramp-edge Josephson junctions using YBa2Cu3Ox electrodes, in order to improve uniformity and reproducibility of junctions. The natural barrier is formed during an etching process and an annealing process. The junction properties are controlled by an annealing temperature and an annealing pressure. Also the junction characteristics depend on the ramp-edge angle. After ramp structuring, a bulge of the underelectrode is observed by AFM. The IcRn products of the junctions with the bulge of the ramp surface reach 2 mV at 4.2 K. We speculate that the etching condition is a very important parameter in the junction fabrication.

Spontaneous buildup of giant surface potential by the deposition of Alq3

Large and persistent surface potential (SP) was obtained in the film of tris(8-hydroxyquinolinato) aluminum (III) (Alq 3 ) prepared on a Au substrate by vacuum deposition. Under experimental condition without light irradiation, the SP changed continuously with increasing the thickness of Alq 3 , after abrupt shift at the interface. The value of the SP reached 28 V at the 560 nm-deposition of Alq 3 . This phenomenon was also observed in the Alq 3 film on an Al substrate. Preferential orientation of the Alq 3 molecules with a permanent dipole moment is suggested as an origin of this giant surface potential of the films, which corresponds to the buildup of the dipole layers in the film. Results of the second harmonic generation measurements supported this model. When we deposited Alq, under a usual experimental condition with the irradiation of the visible light, only abrupt vacuum level shift was observed and there was no further change of the SP. The giant surface potential was easily recovered by irradiation of the visible light for 1hour. These results indicate that the light irradiation during the film growth would affect the film structure or performance of organic electronic devices.

Systematic investigation of ramp edge junction using Ca-doped and Ga-doped PBCO barrier

We have investigated systematically the characteristics of ramp edge junctions with Ca- and Ga-doped PBCO barriers. The localized state density g and/or the volume of the state v in the barriers are increased by Ca-doping and decreased by Ga-doping. Ca-doping reduces the I/sub c/R/sub n/ products, and Ga-doping enhances the I/sub c/R/sub n/ products, though the dominant transport mechanisms are direct tunneling for the junctions having I/sub c/R/sub n/ greater than 1 mV. The change in the I/sub c/R/sub n/ products can be interpreted in terms of the proximity effect at the interface states formed by hybridization between the localized states in the barrier and conduction electron states of electrodes. The ideal superconductor-insulator interface at the junction interfaces is required to enhance I/sub c/R/sub n/ products.

A cavity ring-down spectrometer for study of biomedical radiocarbon-labeled samples

A compact cavity ring-down spectrometer aimed at the detection of trace amounts of radiocarbon (14C) in biomedical samples is demonstrated. Rapid sampling, as required for large scale studies, is made possible by modifications to a CHNS elemental analyzer. After conversion of the carbon content of a sample into CO2, spectral analysis using a high-finesse cavity in conjunction with a quantum cascade laser system reveals the ratio of 14CO2 in relation to the stable isotopologues of 12C and 13C. Cooling and temperature stabilization of the cavity is achieved by a combination of liquid cooling and thermo-electric elements. The system is studied in terms of reproducibility, linearity, and sensitivity as well as contamination and memory effects of the sampling process. While the performance of the system is still limited mainly due to etaloning and other systematic effects, first spectra of biomedical samples, such as urine and feces, have been analyzed at 14C concentration levels above ten times natural abundance. Results are compared with those from a traditional liquid scintillation counter system. Possibilities for improvement of the sensitivity are discussed.A compact cavity ring-down spectrometer aimed at the detection of trace amounts of radiocarbon (14C) in biomedical samples is demonstrated. Rapid sampling, as required for large scale studies, is made possible by modifications to a CHNS elemental analyzer. After conversion of the carbon content of a sample into CO2, spectral analysis using a high-finesse cavity in conjunction with a quantum cascade laser system reveals the ratio of 14CO2 in relation to the stable isotopologues of 12C and 13C. Cooling and temperature stabilization of the cavity is achieved by a combination of liquid cooling and thermo-electric elements. The system is studied in terms of reproducibility, linearity, and sensitivity as well as contamination and memory effects of the sampling process. While the performance of the system is still limited mainly due to etaloning and other systematic effects, first spectra of biomedical samples, such as urine and feces, have been analyzed at 14C concentration levels above ten times natural abunda...

The Effect of Process Parameters on the Electrical Properties of Ramp-Edge Josephson Junctions with Modified Interfaces

We have studied the ramp-edge Josephson junctions with modified barrier. Modified barriers are produced in an ECR plasma etching and subsequent vacuum annealing. The electrical properties of interface modified junctions (IMJs) can be roughly controlled by process parameters. Increasing the etching time and etching voltage lead to the reduction in critical currents. On the other hand, the critical current of IMJs is increased by high temperature anneal or high pressure one. Furthermore, the deposition temperature of counter-electrodes has the strong influence on junction characteristics.