Tooru Katsumata

Interfacial reactions between thin rare-earth-metal oxide films and Si substrates

Rare-earth-metal oxide films (Ln2O3; Ln=Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, and Yb) between 20 and 30 nm thick were grown on Si substrates by using a pyrolysis method. We found that a silicate (LnSiO) layer and a silicon oxide layer were formed at the interface between oxides and substrate after postannealing. The infrared absorption of the Si–O–Ln bonds increased as the postannealing temperature rose. The Si–O–Ln bond formation strongly depended on the ion radii of the rare-earth elements. We conclude that an interfacial silicate layer can easily be formed by a reaction with Si atoms diffusing from the substrate for oxides with larger ion radii. This is because such oxides may have a larger space between atoms. The quantity of Si–O–Si bonds also increased after postannealing. The increase in the Si–O–Si bonds for Ln2O3 was independent of the elements, and almost the same as the increases for Ta2O5 and ZrO2.

Growth and characteristics of long persistent SrAl2O4- and CaAl2O4-based phosphor crystals by a floating zone technique

Abstract Long persistent phosphor crystals, SrAl 2 O 4 doped with Eu and Dy, and CaAl 2 O 4 doped with Eu and Nd have been grown by a floating zone technique. The intensities and the persistent times of the phosphorescences are found to depend on the growth atmosphere. SrAl 2 O 4 crystals grown under H 2 Ar atmosphere and CaAl 2 O 4 crystals grown under Ar atmosphere exhibit strong and long persistent green and blue phosphorescences peaking at λ = 520 and 450 nm, respectively. The spectra of the phosphorescence from the crystals are quite similar to those obtained with sintered powders used for luminous pigments. The persistent time of the phosphorescence from CaAl 2 O 4 crystals is similar to that obtained with sintered powders, while in SrAl 2 O 4 crystals, it is shorter than in sintered powders. The difference in the persistent time is found to be due to Dy and/or Nd dopant concentration added as auxiliary activators. In SrAl 2 O 4 , the Dy concentration in the crystal is about 1/100 of the sintered powders. The Dy 3+ ion is responsible to the persistent time of the phosphorescence.

Highly erbium-doped zinc–oxide thin film prepared by laser ablation and its 1.54 μm emission dynamics

Erbium-doped ZnO (ZnO:Er) thin films were fabricated by the KrF excimer laser ablation technique, which is a useful and simple technique to dope Er atoms on the order of 1020 cm−3 into a host material. As-prepared ZnO:Er films showing strong c-axis orientation with a hexagonal crystalline structure indicate a low electrical resistivity of 6.4×10−3 Ω cm. The sharp and intense photoluminescence (PL) at 1.54 μm originating from the intra-4f transition in the Er3+ ions as well as PL in UV region from the ZnO host were observed even at room temperature. Significant distinction arising from the different Er emission centers responsible for the 1.54 μm emission cannot be found in the temperature dependence between the ZnO:Er and Si:Er film as a reference, except for the PL spectrum feature and main PL peak position. This result suggests the existence of Er emission centers in ZnO:Er and Si:Er films that are different from each other. The details of Er-related 1.54 μm emission dynamics of ZnO:Er films have been i...

1.54 μm emission dynamics of erbium-doped zinc-oxide thin films

Erbium-related 1.54 μm emission dynamics of Er-doped ZnO thin films has been investigated for the different excitation conditions. The excitation was achieved either by exciting indirectly Er3+ ions due to an electron–hole-mediated process or exciting directly discrete energy levels of Er3+ ions. There is no change in the 1.54 μm emission spectrum feature in spite of the different excitation conditions, whereas dramatic change can be seen in the rise time of 1.54 μm emission. The shorter rise time of 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to direct excitation of Er3+ ions.

Thermally Stimulated and Photostimulated Luminescence from Long Duration Phosphorescent SrAl2 O 4 : Eu , Dy Crystals

The traps in long duration phosphorescent SrAl 2 O 4 :Eu 2+ , Dy 3+ crystals have been evaluated using thermally stimulated luminescence and photostimulated luminescence (PSL) techniques. Afterglow phosphorescence is not observed at temperatures lower than 150 K. Traps with depths of E t,TSL = 0.0024, 0.46, and 0.49 eV are detected by thermally stimulated luminescence techniques. Photostimulated luminescence evaluations reveal that trapped carriers can be released optically using an infrared laser with E t,PSL = 0.55 eV. The trap at E t,PSL = 0.55 eV detected by the PSL may be correlated with the shallow trap levels at E t,TSL = 0.0024 eV.

Change in photoluminescence from Er-doped TiO2 thin films induced by optically assisted reduction

Erbium-doped TiO2 (TiO2:Er) thin films with the anatase structure have been prepared on Si substrate by laser ablation. Sharp and intense Er-related emission in the visible region as well as in the IR region has been observed under over-band-gap excitation. The broad photoluminescence (PL) peaking at about 530 nm newly appears at low temperature. It has been understood that the broad PL is induced by an optically assisted reduction effect that is caused by both the H2O adsorption and the reduction process of TiO2 to Ti2O3 by UV illumination. In the IR region, Er-related emission consisted of one main peak located at 1.534 μm and many subpeaks located at around 1.54 μm can be observed even at room temperature. The drastic thermal quenching of the Er-related 1.54 μm emission is also considered due to the optically assisted reduction effect.

Fiber-optic thermometer using Cr-doped YAlO3 sensor head

Cr-doped YAlO3 phosphor crystals have been grown and characterized for applications in fiber-optic thermometers based on the temperature dependence of the photoluminescence (PL) lifetime. PL peaking at λ=735 nm is observed from the crystals following an excitation by a light-emitting diode at λ=520 nm. Long PL lifetime (τ=43.14 ms) is observed from Cr-doped YAlO3 at room temperature as compared with those of ruby (τ=4.2 ms) and spinel (τ=8.0 ms). The temperature coefficient (−0.075 ms/K) of Cr-doped YAlO3 is also larger than those of ruby (−0.010 ms/K) or spinel (−0.037 ms/K). YAlO3 is considered to be a sensitive sensor head material for fiber-optics thermometers.

Time response of 1.54 μm emission from highly Er-doped nanocrystalline Si thin films prepared by laser ablation

Er-doped nanocrystalline Si thin films have been controllably prepared over the Er density of 1019–1021 cm−3 using a prescribed amount of Er in a bulk target by laser ablation. Intense photoluminescence at 1.54 μm originating from intra-4f shell transitions in Er3+ ions has been observed. The increase of Er density cannot immediately result in a linear increase in Er3+-emission intensity. The time response measurement indicated that the change in the rise time of the Er3+ emission directly shows that Er3+ ions are excited by the energy transfer associated with the recombination of electron–hole pairs generated optically in the Si host. We found that the decrease of the excitation efficiency of Er3+ ions was responsible for the suppression of the Er3+-emission intensity in highly Er-doped nanocrystalline Si thin films.

Fibre-optic thermometer using sensor materials with long fluorescence lifetime

A fibre-optic thermometer using newly developed phosphor sensor materials is described. A highly sensitive fibre-optic fluorescence thermometer is developed using chromium ion (Cr3+) doped phosphor crystals with a longer lifetime than that of conventional ruby sensors. Long fluorescent europium doped strontium aluminate phosphors co-doped with various auxiliary activators (SrAl2O4:Eu2+, Ln3+, Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), were also grown and evaluated. In the fibre-optic thermometer using Cr3+ doped phosphor, PL lifetime decreased at temperatures ranging from 270 K to 750 K. The sensitivity of the fibre-optic thermometer varies with the host materials of Cr3+ ions. In the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ doped phosphor, the value and lifetime of afterglow phosphorescence from the SrAl2O4:Eu2+, Ln3+ phosphors varied greatly in the narrow temperature region because the afterglow phosphorescence was dominated by thermal excitation of trapped carriers. Since the trap depth and the trap density depend on auxiliary activator elements, the sensitivity and temperature range of the fibre-optic thermometer using SrAl2O4:Eu2+, Ln3+ phosphors varied with auxiliary activator elements of SrAl2O4:Eu2+, Ln3+ phosphors.

Electrical Resistivity of Undoped GaAs Single Crystals Grown by Magnetic Field Applied LEC Technique

We have grown undoped GaAs single crystals by magnetic field applied LEC (MLEC) technique and measured their electrical and optical properties. The most striking result was that the electrical resistivity of undoped GaAs changed from being semi-insulating (108 ohm-cm) to semi-conducting (10 ohm-cm) by increasing the magnetic field (0 to 1300 Oe). PL, DLTS and SIMS measurements suggest that the decrease of resistivity is mainly due to the decrease in concentration of the deep level defects.