Masashi Ishii

Static states and dynamic behaviour of charges: observation and control by scanning probe microscopy

This paper reviews charges that locally functionalize materials. Microscopic analyses and operation of charges using various scanning probe microscopyxa0(SPM) techniques have revealed static, quasi-static/quasi-dynamic and dynamic charge behaviours. Charge-sensitive SPM has allowed for the visualization of the distribution of functionalized charges in electronic devices. When used as bit data in a memory system, the charges can be operated by SPM. The behaviour of quasi-static/quasi-dynamic charges is discussed here. In the data-writing process, spatially dispersive charges rather than a fast injection rate are introduced, but the technical problems can be solved by using nanostructures. Careful charge operations using SPM should realize a memory with a larger density than Tbit/inch(2). Dynamic charges have been introduced in physical analyses and chemical processes. Although the observable timescale is limited by the SPM system response time of the order of several seconds, dynamics such as photon-induced charge redistributions and probe-assisted chemical reactions are observed.

Electron trapping at the Si (111) atomic step edge

We have investigated the charge distribution at the interface between the Si (111) wafer and its native oxide by Kelvin force microscopy (KFM) with excitation from a He–Cd laser source. Simultaneous imaging using KFM and atomic force microscopy revealed preferential electron trapping at the Si atomic step edge. No electron hopping (>3.5nm radius) to neighboring trapping centers was observed. We also found that the ultraviolet laser irradiation enhanced the electron trapping. The trapping probability under visible laser irradiation and that without irradiation were almost the same, viz. ∼40% of that under ultraviolet irradiation. These findings are explained in terms of incomplete bond termination.

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.

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...

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.

Surface dielectric relaxation: Probing technique and its application to thermal activation dynamics of polymer surface

For dynamic analyses of a polymer surface, a dielectric relaxation measurement technique with parallel electrodes placed away from the surface was developed. In this technique, a liquid heating medium was filled in the space between the polymer surface and the electrodes. The construction that maintains the surface can clarify the physical interactions between the liquid and the bare surface and controlling the temperature of the liquid reveals the thermal activation property of the surface. The dielectric relaxation spectrum of the surface convoluted into the bulk and liquid spectra can be obtained by a reactance analysis and the surface spectrum is expressed with an equivalent resistance-capacitance parallel circuit. On the basis of the electromechanical analogy, the electric elements can be converted into mechanical elements that indicate the viscoelasticity of the polymer surface. Using these measurement and analysis techniques, the electric and mechanical properties of the surface of a gelatinized chloroprene rubber sample were analyzed.