Toshihide Ito

Similarities in photoluminescence in hafnia and zirconia induced by ultraviolet photons

Photoluminescence (PL) spectra induced by ultraviolet photons were measured for amorphous hafnia and zirconia deposited by plasma-enhanced chemical-vapor deposition (PECVD), amorphous hafnia deposited by pulse laser deposition, and crystalline yttria-stabilized zirconia. Two kinds of samples were prepared for both hafnia and zirconia deposited by PECVD using different source alkoxides in different deposition chambers. A PL peak was observed around 2.8eV similarly in all hafnia and zirconia samples, irrespective of the difference in crystallinity, oxygen deficiency, source alkoxide, deposition method, or the substrate material. The decay profile of this PL is also similar in all the samples. These facts clearly show that neither impurities, oxygen vacancy, nor defects at the interface between the sample and the substrate are responsible for the PL. It is a luminescence inherent in hafnia and zirconia and is most likely due to radiative recombination between localized states at the band tails. When the samp...

Mechanisms of several photoluminescence bands in hafnium and zirconium silicates induced by ultraviolet photons

Two photoluminescence (PL) components with peaks around 2.8–3.0 and 3.8eV were induced in hafnium silicates by the irradiation of synchrotron radiation photons at 8.0eV, while two similar ones were induced in zirconium silicates around 2.7–3.0 and 3.8eV. By examining PL excitation spectra, PL decay characteristics, and vacuum-ultraviolet absorption spectra, it is assumed that the origin of the PL component around 2.7(2.8)–3.0eV is the same as that of the PL component around 2.7–2.9eV observed in hafnia and zirconia. In the band gaps of hafnium silicates, zirconium silicates, hafnia, and zirconia, luminescent centers responsible for the PL components around 2.7(2.8)–2.9(3.0)eV have their respective upper and lower states with a certain constant energy difference that does not change by the hafnium or zirconium content. Electrons (or holes) excited by ultraviolet photons to tail states at the band edges first relax to the upper state of the luminescent centers, and then they are deexcited to the lower state...

Defects in Yttria-Stabilized Zirconia Induced by Irradiation of Ultraviolet Photons

When single-crystal yttria-stabilized zirconia samples are exposed to ultraviolet photons with an energy higher than 4.0 eV, three paramagnetic centers are observed at g=2.006, 1.97–1.95, and 1.91–1.86. With irradiation by ultraviolet photons, an absorption band and a photoluminescence (PL) band are also induced, showing respective peak energies at around 3.3 and 2.8 eV. The onset of the PL-excitation spectrum occurs around 4.0 eV. The induced absorption band and paramagnetic centers disappear with thermal annealing. By doing a numerical analysis on the experimentally obtained angular dependence of the signal at g=1.91–1.86, it is determined that signals at g=2.006 and 1.91–1.86 are due to the F+ center and the T center, respectively, and that the signal at g=1.97–1.95 has some origin other than the two ESR centers. Since two ESR signals at g=2.006 and 1.97–1.95, the 3.3-eV absorption, and the 2.8-eV PL have the same onset energy at around 4.0 eV, the reactions that induce them are triggered by electrons excited into the conduction band tail.

Role of number of consecutive voltage zero-crossings in propagation of water trees in polyethylene

Effects of superposition of a low-frequency (0.1 to 50 Hz) voltage and a high-frequency 2 kHz voltage on the growth of water trees in polyethylene are examined. It has become clear that both the number of total voltage zero-crossings during the whole voltage application period and the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage play important roles in the length and shape of water trees. Namely, the water tree length becomes longer as the number of total zero-crossings increases. Furthermore, among the voltages with the same number of total zero-crossings, the length becomes even longer with a prolonged shape in the direction of electric field as the number of consecutive zero-crossings increases. Effects of superposition of a high-frequency voltage onto a dc voltage are also examined. In this case, the dc voltage seems not to have any influence to the tree growth. By assuming the presence of space charge around the water-tree tip, the effective zero-crossings become only sensitive to the high-frequency component. Therefore, this result also indicates that the number of voltage reversals at the tree tip plays a crucial role in the growth of water trees.

Energy band profile of hafnium silicates estimated by X-ray photoelectron spectroscopy

Amorphous hafnium silicate films with several composition ratios were deposited on Si substrates by plasma-enhanced chemical vapor deposition, and their energy band profiles were studied by X-ray photoelectron spectroscopy. The band gap energy estimated from the energy loss spectrum of O 1s electrons decreases monotonically and then approaches a constant value with an increase in hafnium content. The valence band offset and the conduction band offset were estimated using the sample sandwiched between an evaporated Au electrode and the Si substrate. Although the two offsets decrease until they become almost constant with an increase in hafnium content at both silicate/Si and silicate/Au interfaces, they hold values higher than that necessary for a high-k dielectric material to maintain a good insulating performance for all the deposited silicates.

Performance enhancement of blue light-emitting diodes with InGaN/GaN multi-quantum wells grown on Si substrates by inserting thin AlGaN interlayers

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0 < y < 0.3) interlayers on Si(111) substrates. It was found by high-resolution transmission electron microscopy observations and three-dimensional atom probe analysis that 1-nm-thick interlayers with an AlN mole fraction of less than y = 0.3 were continuously formed between GaN barriers and InGaN wells, and that the AlN mole fraction up to y = 0.15 could be consistently controlled. The external quantum efficiency of the blue LED was enhanced in the low-current-density region (≤45 A/cm2) but reduced in the high-current-density region by the insertion of the thin Al0.15Ga0.85N interlayers in the MQWs. We also found that reductions in both forward voltage and wavelength shift with current were achieved by inserting the interlayers even though the inserted AlGaN layers had potential higher than that of the GaN barriers. The obtained peak wall-plug efficiency was 83% at room temperature. We suggest...

Effect of post-nitriding on electrical properties of high-permittivity hafnium and zirconium silicate films

The effect of post-annealing in nitrogen monoxide (NO) on the electrical properties of hafnium and zirconium silicate films prepared by plasma-enhanced chemical vapor deposition was investigated. The leakage current measured in the post-annealed film decreases by two orders of magnitude as compared to the s-deposited film. From FT-IR, ESR, and XPS analyses, it is found that the post-annealing makes the structural disorder smaller and reduces the interface states by nitridation. The C-V hysteresis width, which reflects the number of interface states, also decreases. These tendencies are observed both the hafnium and zirconium silicate films. It is concluded that the NO post-annealing can effectively improve the electrical properties of the two films.

Photoluminescence in polymeric insulating materials (low density polyethylene and polypropylene) induced by ultraviolet photons

For the polymeric insulating materials such as polypropylene (PP) and low density polyethylene (LDPE), the surface oxidization and chain scission that deteriorate the insulation performance are induced by the irradiation of the ultraviolet photons. Photochemical reactions are responsible for such oxidization and bond scission. In this research, photochemical reactions in the polymers are examined by analyzing photoluminescence (PL) spectra. However, it is the case that the luminescence of the polymer becomes a complicated phenomenon accompanied with several kinds of mechanisms even when the clearest excitation means of the laser irradiation is used. Therefore, luminescence due to the oxidation that relates essentially to the degradation of PP or LDPE is examined in this research by comparing luminescence spectrum induced by the irradiation in vacuum and the one induced in oxygen.

Origins of photoluminescence bands induced by ultraviolet photons in polyethylene

Photoluminescence (PL) spectra in low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) induced by irradiation of ultraviolet photons are examined. The samples were pre-irradiated by the ultraviolet photons under different atmospheres (air, O/sub 2/, and vacuum) in order to induce photochemical reactions. Two PL bands appear around 3.6 and 4.2 eV in LDPE. Their intensities decrease with the progress of the pre-irradiation regardless of the irradiation atmosphere. The two PLs are considered to be due to conjugated double bonds present in the antioxidant and impurities. A new PL band appears at 2.9 eV in LDPE if the sample was pre-irradiated in vacuum. This PL is considered to be due to photoinduced conjugated double bonds. In XLPE, a PL band different from the ones observed in LDPE appears at 3.1 eV. It is considered that the crosslinking byproducts are responsible for this band.

Improvement in electrical properties of hafnium and zirconium silicates by postnitriding

Hafnium and zirconium silicate films were deposited on a silicon substrate and the effects of postannealing on their electrical properties were investigated. When the films are postannealed in nitrogen monoxide (NO), the leakage current becomes lower by more than one order of magnitude as compared with that of the as-deposited films. The capacitance-voltage (C-V) hysteresis width is also decreased drastically by the NO postannealing. From electron spin resonance spectroscopy, it is indicated that paramagnetic defects at the interface between the film and the substrate are responsible for the leakage current and the C-V hysteresis. It is also indicated by x-ray photoelectron spectroscopy that the postnitridation effectively terminates these interface defects and contributes to the improvement in electrical properties.