Sohachiro Hayakawa

Optical Properties of Tetracene Evaporated Films

The optical properties of tetracene films evaporated onto the fused quartz plates have been measured. They are markedly dependent on the film thickness as well as the substrate temperature. Their absorption spectra and X-ray diffraction patterns suggest that the two kinds of layers with different structure exist in a film. One of them, being in contact with substrate, has an unusual crystal structure and the other has a normal crystal structure. The unusual structure seems to be caused by the difference of the thermal expansions between tetracene crystals and the fused quartz plates. The temperature dependence of fluorescence spectra of the films, which were prepared at a low temperature, is different from that of the single crystal. The difference is interpreted in terms of tetracene dimerization in a film.

Triplet Exciton Trapping by Dislocations in Anthracene Crystals

Trapping of triplet excitons at dislocations, produced in anthracene crystals by plastic bending, was investigated by means of the temperature and the radius-of-curvature dependence of the triplet decay rate. As a result of the kinetic analysis, the trapping depth of 0.3 eV was obtained. The trapping rate constant was found to be dependent on the square root of the curvature, the dependence being related to the diffusion of triplet excitons in the ab plane and to the distance between dislocations. This phenomenon was re-analyzed by applying a one dimensional random-walk model.

DLTS Study on the Gradation of the Trap Concentration Profiles in n-CdTe Crystals

DLTS measurements were carried out on three types of metal/n-CdTe junction: I. gold evaporated on an etched surface; II. gold evaporated on a cleaved surface; III. cadmium evaporated on a cleaved surface. Type I had a tellurium-rich layer and type III a cadmium-rich layer. The concentration of level E4 (energy depth EC-ET=0.33 eV), one of the six electron traps detected in this experiment, decreased rapidly near the interface in type I at room temperature, but increased in type III. A relationship was found to exist between level E4 and the non-stoichiometric layers. A model in which the gradation of this levels concentration was caused by the diffusion of defects, is proposed, and the diffusion coefficient is estimated to be about 10-17 cm2s-1 from the concentration profiles of level E4.

DLTS Study of Heat Treatments on n -CdTe Crystals

DLTS measurements were carried out on Schottky barriers formed on the surfaces of n-CdTe:In crystals heated in argon gas. Four electron trap levels were detected. The carrier concentration near the surface decreased to less than a tenth as a result of heat treatments at 350 or 400°C for 1 hour. A sample heated at 400°C showed a steep decrease in the carrier concentration near the surface. The concentration of an electron trap level (EC-ET=0.34 eV) was reduced by heat treatments. Heat treatments at temperatures higher than 350°C for 1 hour seemed to induce some defects related to cadmium vacancies or tellurium interstitials, which compensate donors and reduce the concentration of the trap level near the surfaces of CdTe.

Fluorescence of Solution Scintillators with Naphthalene as a Solvent

This paper indicates the following items about solution scintillators with naphthalene as a solvent: a) Solute concentration dependence of fluorescence intensity in these systems follows the formula given by Kallmann and Furst et al. with respect of energy transfer in liquid scintillators. b) Measurements of fluorescence under excitation of different radiations can discriminate between solvent and solute quenching by addition of carbon tetrachloride. c) Pulse distribution curves are also investigated at an optimum concentration of each system. Some of them seem worthy of practical use.

DLTS Study of Pulsed Ruby Laser Irradiation Effects on n-CdTe

DLTS measurements were carried out for Au/n-CdTe junctions which were formed on surfaces irradiated by a pulsed ruby laser. Electron trap levels at 0.65 eV and 0.83 eV below the conduction band were induced in high concentration by the irradiation of more than 0.8 J/cm2 energy density. The irradiation of 0.3 J/cm2 improved the characteristics of Schottky junction of Au/n-CdTe and ohmic contact of In/n-CdTe.

Photoelectronic Properties of p-Type CdTe in an Aqueous Solution

The current-potential relations under monochromatic irradiation and the quantum efficiency-wavelength relations of p-type CdTe with different acceptor concentrations have been measured in a 0.1 N NaOH solution. The photocurrent was observed under cathodic polarization and the photoresponse spectra agreed approximately with those estimated theoretically. The quantum efficiency was found to be strongly affected by surface recombination, and it had a maximum value of 0.85 with an Na acceptor concentration of about 1016 cm-3.

Thermally Stimulated Current of CdTe Doped with Zn

Three traps in p-type CdTe doped with Zn are found using the TSC (thermally stimulated current) method between 77 and 200 K. Their depths are estimated to be approximately 0.24, 0.25 and 0.40 eV. The TSC peak for the 0.24 eV trap is considerably quenched by the infrared rays. A quantitative analysis of the processes for the 0.24 eV trap is presented. It is proved that there are two different processes involved in the occupation of 0.40 eV trap at 77 K: one is a fast process and the other is slow. A model is proposed to explain qualitatively the complicated processes of the 0.40 eV trap.

IN-PILE CALORIMETRIC DOSIMETRY. I.

An adiabatic calorimeter has been designed for measuring energy absorbed in materials (C, Al, (CH2)n and Pb) placed in the experimental hole of a nuclear reactor.Details are given of the apparatus and procedures of the calorimeter, followed by an account of the results of calibration and of dosimetry in the TRIGA II type reactor of the Atomic Energy Research Laboratory of the Musashi Institute of Technology, The operating temperatures were 25 ?? -30 ?? at 100kW reactor power, and the energy deposition (or dose) rate was 0.2-2mW/g. The performance has been satisfactory in spite of the poor agreement between the observed and calculated rates of temperature rise. The ratio between energy absorption due to fast neutron scattering and that due to γ-rays obtained in the TRIGA II is consistent with the data obtained with BEPO. The experimental accuracy (standard error) at full reactor power is estimated to be within 2.1%.

Phosphorescence Decay of Benzophenone containing Naphthalene at 77°K

Phosphorescence decay curves have been measured for benzophenone in the presence of various concentrations of naphthalene, at 77°K. Decay time of the phosphorescence of benzophenone was changed from 5 msec to 3 msec by changing concentration of naphthalene. A kinetic analysis of energy transfer in solid solutions has been carried out using benzophenone as the donor and naphthalene as the acceptor. From the decay data of benzophenone, 2.4×10-20 cm3 sec-1 has been obtained as the rate of excitation energy transfer from the triplet state of benzophenone to that of naphthalene. The effectiveness of zone purification process of benzophenone has been studied as well as the effect of doping naphthalene.