Isao Sakaguchi

Superconductivity in diamond thin films well above liquid helium temperature

We report unambiguous evidence for superconductivity in a heavily boron-doped diamond thin film grown by microwave plasma-assisted chemical vapor deposition (MPCVD). An advantage of the MPCVD-deposited diamond is that it can contain boron at high concentration, especially in (111)-oriented films. Superconducting transition temperatures are determined by transport measurements to be 7.4 K for TC onset and 4.2 K for zero resistance. The upper critical field is estimated to be 7 T. Magnetization as a function of magnetic fields shows typical type-II superconducting properties.

Effect of hydrogen doping on ultraviolet emission spectra of various types of ZnO

A pulse-modulated plasma irradiation technique was applied to hydrogenation of ZnO. Three kinds of ZnO samples were employed to investigate the electronic state of hydrogen in ZnO. Secondary-ion-mass-spectroscopy analysis using isotope tracer revealed that the surface layer to 100 nm was doped with hydrogen after the irradiation and its concentration was in the order of 1016 cm−3. The efficiency of band edge emission was increased by the hydrogenation. However, the the degree of the improvements depended on impurity and defect concentration in the original samples. It was concluded that hydrogen in ZnO passivates deep donor and acceptor states by electron transfer from hydrogen to the defects.

Surface modification of zinc oxide nanoparticles by aminopropyltriethoxysilane

Commercial zinc oxide nanoparticles (20–30 nm) were coated by aminopropyltriethoxysilane (APTES) under varying environments. Three different processes, acidic, basic and toluene were used. The effects of coating conditions (acidic, basic and toluene) on the grafting, structural and optical properties of these nanoparticles were studied. In the three cases, it was possible to control the coating and according to X-ray diffraction, BET, TEM and SEM results, it is clear that the APTES coating plays a role of growth inhibitor even at 800 °C...

Surface conditioning of chemical vapor deposited hexagonal boron nitride film for negative electron affinity

The surface conditions favoring a negative electron affinity (NEA) on hexagonal boron nitride (h-BN) grown by radio-frequency plasma-assisted chemical vapor deposition (CVD) have been investigated by ultraviolet photoelectron spectroscopy. The NEA condition on the h-BN film appears to be resistant to oxygen-plasma or in-vacuo atomic oxygen treatment. It is not certain whether the segregation of bulk hydrogen onto the surface helps to promote the NEA; the depth profile of the deposited film reveals about 0.01%–0.1% atomic concentration of hydrogen. High temperature annealing at 1100 °C results in a positive electron affinity surface (PEA). Reexposure of PEA surface to atomic hydrogen at room temperature regenerates the NEA condition. This is evident of the role of superficial hydrogen in promoting NEA on the h-BN film.

Passivation of active recombination centers in ZnO by hydrogen doping

The effect of hydrogen doping on luminescence properties of ZnO was investigated. Hydrogen was incorporated in the ZnO crystal by irradiation with an inductively coupled plasma (ICP), in particular, the pulse modulated mode operation of ICP, and the luminescence spectra and hydrogen concentration of the resultant samples were analyzed. A hydrogenated region of 20–100 nm was formed at the sample surface by the irradiation and the concentration of hydrogen was 1017–1018 cm−3. Hydrogen doping improved the ultraviolet emission efficiency of all the samples, and the degree of improvement depended on the initial state (impurity concentration) of the original samples. The most significant improvements were recorded for the sample lightly contaminated with Cu, Al, and Li. The correlation between impurity concentration and hydrogen doping effects is discussed from the viewpoint of charge transfer between hydrogen and the other impurities.

Yellowish-white luminescence in codoped zinc oxide

The broad-band visible emission of codoped ZnO was studied. The codoped ZnO specimens were intentionally and simultaneously doped with IIIa elements (donors) and Li (acceptor). Broad-band emission covering nearly the whole visible range was achieved. The emission was found to be yellowish white to the naked eye. The visible band was composed of two components, i.e., a green emission having a peak at 2.2 eV and a yellow emission having a peak at 2.0 eV. The peak at 2.2 eV was distinct from the nonstructured green emission at 2.45 eV. The 2.2 and 2.0 eV peaks were attributed to donor-acceptor pair transitions involving the zinc vacancy and lithium, respectively.

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.

Nitrogen Ion Behavior on Polar Surfaces of ZnO Single Crystals

Nitrogen radicals were irradiated on the (0001) and (0001) surfaces of the ZnO single crystals, and the stability and the states of N ions on the surfaces were investigated by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). On the (0001) surface, many N ions replaced the O ions of ZnO during annealing in oxygen gas or vacuum after nitrogen treatment. However, few N ions replaced the O ions of ZnO on the (0001) surface. These results suggest that the nitrogen doping on the (0001) surface is more effective than that on the (0001) surface.

Dielectric properties of (Ba, Sr)TiO3 thin films and their correlation with oxygen vacancy density

This paper describes the dielectric properties of (Ba, Sr)TiO3 (BST) thin films deposited by rf magnetron sputtering, focusing on their correlation with the oxygen vacancy density in the films. The dielectric properties specific to the BST films can be explained by considering the influence of the dielectric relaxation phenomenon following a power law dependence on time in the time-domain measurement and on frequency in the frequency-domain measurement. From an electrical comparison of the films with and without post-annealing in oxygen ambient, it is derived that charging/discharging electrons at oxygen vacancies in the interfacial Schottky depletion layer, whose width is modulated by the applied voltage, are responsible for the observed dielectric properties. Preliminary results of the oxygen vacancy density measurement using a gas-solid oxygen isotopic exchange reaction showed that the higher the post-annealing temperature, the lower the oxygen vacancy density.

Structural and magnetic properties of Mn-ion implanted ZnO films

Zinc oxide films doped with Mn (Mn:ZnO) were prepared by implanting Mn+ ions into ZnO films deposited by pulsed laser deposition, and their structure and magnetic properties were studied. The Raman spectra of the films indicated that Mn ions occupied the Zn site of ZnO after annealing, while the as-implanted films were amorphous like the ones with very low crystallinity. Magnetic measurements revealed that neither as-implanted nor annealed Mn:ZnO films showed ferromagnetism. The Mn:ZnO films demonstrated paramagnetism that was likely due to Mn2+ ions at the substitutional Zn site.