Yasuo Seki

Epitaxial Growth of Semi-Insulating Gallium Arsenide

Semi-insulating gallium arsenide is grown epitaxially from the vapor phase by using chromyl chloride as a doping agent. The resistivity of the epitaxial layer is greater than 108 Ωcm at room temperature. The temperature dependence of the resistivity gives an activation energy of 0.63 eV. Multilayer structure consisting of semi-insulating layer and n-type layer is obtained without halting growth process by flowing and stopping the chromyl chloride vapor. No dislocation is observed at the boundary between semi-insulating layer and n-type layer, though many dislocations are observed at the epitaxial-substrate interface. It is found that the grown layer becomes semi-insulating only when the donor concentration of the layer is lower than 5×1015 cm-3.

Optical Absorption at Dark Lines in Degraded Double-Heterostructure Lasers

The local heating effect at the dark lines in a degraded double-heterostructure laser was investigated under cw lasing operation by measuring the thermal radiation of infrared. The dependence of the temperature rise on current I observed at the dark lines showed an abrupt change at the threshold current Ith. Above the threshold the temperature rose more rapidly with increasing current. This abrupt change came from the local heating due to the absorption of the coherent radiation at the dark lines. When operated above the threshold, almost all the radiation was directed along the axis of the cavity. Therefore, the dark lines can absorb radiations emitted at distant non-degraded regions. In the present experiment, the additional temperature rise of 2.4°C was observed at I=Ith+100 mA. The analysis based on a simplified model showed that the absorption coefficient of the dark line was about 170 cm-1.

Effect of Dy3+ on the Luminescence of Y3OCl7:Yb3+.Er3+

Conversion efficiency of infrared stimulable phosphor Y3OCl7:Yb3+.Er3+ is significantly influenced by the presence of a trace of Dy2O3 in the starting materials. In the present experiments, effects of Dy3+ on the luminescence of Y3OCl7:Yb3+.Er3+ were examined under infrared and ultraviolet excitations at room temperature. The experiments indicate that degradation of the conversion efficiency is primarily due to depopulation of 4I11/2(Er3+) and 2F5/2(Yb3+) as a result of energy transfer from Er3+ and Yb3+ to Dy3+.

Trap Density in Epitaxially Grown GaAs

Trap Density in Epitaxially Grown GaAs Yoshitaka Furukawa, Kenji Kajiyama, Yasuo Seki et al. Improved Device Performance of GaN/AlGaN High-Electron-Mobility Transistor Using PdO Gate Interlayer Ray-Ming Lin, Fu-Chuan Chu, Atanu Das et al. Thermally enhanced threshold voltage shifts in amorphous In–Ga–Zn–O thin-film transistor Takashi Kojiri, Tokiyoshi Matsuda and Mutsumi Kimura Interface States in AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors Feng Qian, Du Kai, Li Yu-Kun et al. Enhancement-Mode AlGaN/GaN High Electron Mobility Transistors Using a Nano-Channel Array Structure Liu Sheng-Hou, Cai Yong, Gong Ru-Min et al. Characterization of Interface Charge in NbAlO/AlGaN/GaN MOSHEMT with Different NbAlO Thicknesses Feng Qian, Du Kai, Dai Bo et al. X-ray Photoelectron Spectroscopy Study of the Origin of the Improved Device Performance by a Thin Al Layer Insertion between AlGaN and Schottky Gate on the AlGaN/GaN HighElectron-Mobility Transistor

Development of an Ultrasonic Applicator for SonoFiber Therapy: A Thermal Therapy Using Fused-Quartz Fibers (SoFT)

We describe a preliminary investigation for determining an efficient method to transmit high-intensity ultrasonic waves from a thin quartz fiber to tissue. We found that the tapered fused-quartz rod with the diameter of 3 mm at 2.447 MHz is suitable for an efficient conversion. This applicator is promising for endoscopic use.

Comparison of Optical Ringing Effects of Injection Lasers for the Internal Loss Modulation and the Direct Modulation

The internal loss modulation which is the modulation of the photon lifetime has two advantages. (1) There is no coupling loss for the modulator. (2) The modulation current is small. The optical ringing effect for the internal loss modulation is investigated theoretically, comparing it with that for the direct modulation. A small-signal operation is analysed by an appoximation using expansion in terms of the spontaneous emission coefficient Cs and τp/τs, and a large-signal operation is analysed numerically. As a result, if step currents are applied, the amplitude of the ringing oscillation for the former is larger than that for the latter. However, if the current changes smoothly, the amplitudes for both modulations can be of the same order with a risetime of 3 ns.

Effect of the Facet Radiator on the Coupling between a High Radiance LED and a Fiber

We investigated the problem of the coupling between a fiber and an edge emitting type LED having a similar structure as a DH laser by numerical calculation. To improve the efficiency ef of transmitting the light power generated in the diode to the light power coupled to the fiber, we introduced a radiator placed on the diode facet, and made a thorough study of the coupling, in which the fiber facet is close to the radiator head. The results obtained are as follows: (1) The efficiency ef is much higher when a semispherical radiator is employed than when the diode and the fiber are directly coupled. (2) A radiator having the shape of a truncated sphere or a sphere improves ef still more. (3) The binding agent between the diode and the radiator does not substantially decrease the effectiveness of radiators. (4) Radiators of optimum dimensions can be produced by polishing.