Susumu Harako

Luminescence properties of Sm3+-doped TiO2 thin films prepared by laser ablation

Samarium-doped anatase TiO2 (A-TiO2 : Sm3+) and rutile TiO2 (R-TiO2 : Sm3+) single phase thin films were fabricated on silicon substrates by laser ablation, resulting from the control of O2 pressures during ablation. These thin films show the Sm3+-related emissions in the visible and in the infrared wavelength regions. Visible photoluminescence (PL) and cathodoluminescence originating from the 4G5/2 → 6HJ intra-4f transitions in Sm3+ ions were observed at room temperature. The results of the photoluminescence excitation spectroscopy measurement imply that the Sm3+ emissions are due to the indirect excitation of the Sm3+ ions through an energy transfer process from electron–hole pairs generated in the TiO2 hosts. Intense 1.2 µm PL emissions originating from 4G5/2 → 6F9/2 and 6F7/2 → 6H5/2 transitions were also observed. A clear difference in the fine structure of the PL spectra between the A-TiO2 : Sm3+ and the R-TiO2 : Sm3+ thin films was observed, resulting from the difference in the Sm3+ configuration in the host crystal matrices.

Charge propagation dynamics at trapping centers that induce the luminescence of rare-earth dopants in wide-gap materials

For rare-earth doped semiconductors, charge propagation from semiconductors to rare-earths is essential for excitation of optically active dopants. However, the qualitative model that has been widely accepted to describe this process is based upon indirect evidence; in this model, it is believed that trapping and subsequent recombination of the charges at some rare-earth-related defect excite the dopants. In this work, we observe the sequential process directly, and quantify the trapped charge density and relaxation frequency using a photoelectric measurement technique for samarium-doped titanium dioxide with intense visible luminescence under ultraviolet light.

Formation and structure analysis of very long ErSi2 nanowires formed on Si(110) substrates

Very long ErSi2 nanowires were formed on a Si(110) substrate by a self-assembly process. The wires were highly parallel and grew in the Si direction. Nanowires with lengths more than 50 µm were fabricated. It was observed by transmission electron microscopy that the wires were hexagonal crystal type, having their c-axis ( direction) perpendicular to the Si(112) plane. Then the nanowires were deeply buried in the Si(110) surface. Energy-dispersive X-ray spectroscopy measurements indicated that the nanowires had a ErSi2 chemical composition. Electrical conductivity measurement of the nanowires formed on high-resistivity Si substrates showed that the ErSi2 nanowires were good conductors at room temperature. The ErSi2 nanowire might be a promising candidate in nanometer-sized inter connection.

Effects of oxidization and deoxidization on charge-propagation dynamics in rare-earth-doped titanium dioxide with room-temperature luminescence

Anatase titanium dioxide (A-TiO2) with a wide band-gap energy of 3.2 eV can be used as a host semiconductor of rare-earth dopants for optical devices. However, the chemical activity of A-TiO2 strongly affects the luminescence properties of the devices. In this study, we analyzed oxidized and deoxidized samarium (Sm)-doped A-TiO2 (TiO2:Sm) by impedance spectroscopy and microscopic photoluminescence. Charge propagation analyses using dielectric relaxation (DR) revealed that different kinds of charge-trapping centers were formed by the oxidization and deoxidization. For oxidization, Sm-oxygen complexes incorporated in the A-TiO2 formed a trapping level that contributed to Sm excitation, while defective complexes at the A-TiO2 boundary formed other levels that dissipated the charges. For deoxidization using thermal treatment in a hydrogen (H) atmosphere, the number of profitable trapping centers in A-TiO2 was reduced but the remainder maintained the property of Sm excitation. It was also found that H adsorpti...

Photoluminescence and X-ray Absorption Fine Structure Analysis of Sm-Doped TiO2 Thin Films

The local structure of samarium-doped titanium dioxide (TiO2:Sm) thin films fabricated by laser ablation and post annealing has been investigated by Sm L III-edge X-ray absorption fine structure (XAFS) analysis using a synchrotron radiation. The TiO2:Sm samples at annealing temperatures lower than 1000 °C showed an anatase (A-)TiO2:Sm crystal structure and transferred into a rutile (R-)TiO2:Sm phase at annealing temperature of 1100 °C. All the A-TiO2:Sm phase samples showed intense Sm-related photoluminescence (PL) at room temperature. It was shown that the samples which showed intense PL have shorter coordination distance for the first nearest neighbor of Sm than that of the samples which did not show PL after the crystal phase transition.

Atomic-scale distortion of optically activated Sm dopants identified with site-selective X-ray absorption spectroscopy

The local structure of luminescent Sm dopants was investigated using an X-ray absorption fine-structure technique with X-ray-excited optical luminescence. Because this technique evaluates X-ray absorption from luminescence, only optically active sites are analyzed. The Sm L3 near-edge spectrum contains split 5d states and a shake-up transition that are specific to luminescent Sm. Theoretical calculations using cluster models identified an atomic-scale distortion that can reproduce the split 5d states. The model with C4v local symmetry and compressive bond length of Sm–O of a six-fold oxygen (SmO6) cluster is most consistent with the experimental results.

Charge Propagation Dynamics in Temperature Quenching of Sm-Doped TiO2: Impedance Spectroscopy of Release Processes of Trapped Charges Determining Luminescence Intensity

The mechanism of the temperature quenching of luminescence in samarium-doped titanium dioxide (TiO2:Sm) was investigated with electrical measurement techniques. Because electrical measurements are sensitive to charge dynamics, the indirect excitation processes of the Sm dopants, i.e., trapping and recombination of injected charges into the host TiO2, can be clarified. Complex impedance spectroscopy between 100 and 300 K revealed a correlation between the temperature quenching of TiO2:Sm and the trapping and recombination processes. Analyses using equivalent circuits revealed that the main factor determining the temperature quenching properties was delocalization of the trapped charges and decoupling of free charges in TiO2 from trapped charges. The delocalization and decoupling parameters were evaluated from the equivalent circuit constants, and a numerical model incorporating the determined values reproduced the experimentally observed temperature quenching of photoluminescence.

SYSTHESIS AND OPTICAL PROPERTIES OF RARE EARTHS DOPED NANO-SEMICONDUCTORS AND THEIR APPLICATIONS

Er and Yb have been doped into nanocrystalline Si and ZnO thin films. Sharp and intense photoluminescence (PL) lines related to intra-4f transitions in the rare earth ions were observed. The optical transition dynamics of the rare earth ions were investigated by time-resolved PL measurements. It was demonstrated that a nano-meter sizing of Si widened the energy bandgap and led to an increase of doping densities of the rare earths. It was also shown that the Er-related PL gave rise to different features under direct and indirect excitations indicating a strong interaction of electron-hole pairs in the host with 4f electrons in the rare earth ions. A co-doping effect of Yb and Er into nanocrystalline Si was presented also. Our results suggested a rare earth-rare earth coupling between the Yb3+ ions and the Er3+ ions, which transferred the energy from Yb to Er and enhanced the 1.54 μm PL of Er.

Sensitization effect of Al co-doping on Nd-related photoluminescence in TiO2 matrix

Nd and Al co-doped anatase phase TiO2 thin films were prepared on silicon substrates by laser ablation and post annealing. These thin films exhibited intense light emissions due to intra-4f shell transitions in the Nd3+ ions at room temperature. Furthermore, the intensity and line shape of the Nd3+-related luminescence were affected by the Al co-doping. The emission dynamics measurements showed that a slight Al co-doping led to an improvement of the Nd3+-emission lifetime. Our results indicated that Al is an effective sensitizer of enhancing Nd3+-related emission in the TiO2 matrix.

Visible Electroluminescence of a n+-Indium?Tin-Oxide/Sm-Doped i-TiO2/p-NiO/p+-Si Light-Emitting Diode

A n+-indium–tin-oxide (ITO)/Sm-doped i-TiO2/p-NiO/p+-Si light-emitting diode (LED) was fabricated via laser ablation and post annealing treatment. The LED emitted red light at room temperature under a relatively low threshold voltage of 12 V. The electroluminescence and photoluminescence spectra of the LED were compared, and a stable and single-type Sm3+ luminescent center was observed to give rise to red emission under different excitation conditions. These results suggest that there is a strong possibility of realizing LEDs based on rare-earth-doped metal oxide semiconductors.