Akio Yoshikawa

Structural investigation of sapphire surface after nitridation

Abstract Nitridation process of a sapphire surface was investigated with atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and X-ray photoelectron spectroscopy (XPS) in order to reveal a general nitridation mechanism. It was found that the nitridation process consists of two steps. First, inter-mixing between nitrogen-related species and sapphire surface occurs forming hydrogenated Al oxynitride. This step does not change the surface morphology significantly. Second, crystalline AlN islands are gradually formed by further nitridation of hydrogenated Al oxynitride, resulting in a very rough surface.

LDH (laser/detector/hologram) unit for thin optical pick-up head of CD player

The LDH unit with the size of 4.8/spl times/8.2/spl times/4.3 mm for a thin optical pickup head of a CD player has been successfully developed. The technology to make the LDH unit thin and small is based upon (1) the most efficient optical combination of the low operation current laser diode with the 45/spl deg/ built-in micro-mirror which is formed on the Si photodetector substrate, (2) the thin plastic-molded flat package with good thermal dissipation, and (3) the plastic-molded holographic optical element (HOE) as a cap. The pick-up head using the LDH unit has shown good optical performance sufficient for the application to CD players.<<ETX>>

Direct MOVPE growth of InP on GaAs substrates

Abstract We have been able to develop a new useful method for the heteroepitaxial growth on a highly lattice-mismatched substrate by MOVPE successfully. We have found that the surface morphology of InP epitaxial layer grown on a GaAs substrate by MOVPE strongly depends on the supply rate of the group III material. Based on this experimental finding, an InP epitaxial layer with a uniform and mirror-smooth surface has been successfully grown on a 2-inch GaAs (100) substrate. In addition to the surface morphology, we have measured the RHEED pattern, the x-ray diffraction pattern, the Hall mobility, the carrier concentration, and the photoluminescence as functions of the flow rate of the group III carrier gas.

A self-aligned ridge substrate laser fabricated by single-step MOVPE

Abstract A novel self-aligned structure semiconductor laser, called a self-aligned ridge substrate (SAR) laser has been successfully fabricated by a single-step MOVPE process. The single-step process is attained by the epitaxial growth on a ridge formed along the 〈011〉 direction on a (100) substrate, taking advantage of the anisotropic and isolated growth on the ridged substrate. As a result the current-injection region and the active region are formed in a self-aligned manner. A CW operation under a threshold current of 46 mA typically has been obtained at room temperature in the SAR laser.

Low operating current and high-temperature operation of 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned structure

Low operating current and high-temperature operation has been demonstrated in 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned (RISA) structure. The RISA structure features an Al/sub 0.5/In/sub 0.5/P current blocking layer, which leads to small internal loss in the waveguide and substantially reduced operating carrier density. The resultant operating current for 50-mW continuous-wave at 70/spl deg/C is as low as 98 mA, which is almost a half of the lowest value ever reported.

Inheritance of zinc-blende structure from 3CSiCSi(001) substrate in growth of GaN by MOCVD

GaN has been grown on 3CSiCSi(001) at temperatures from 600 to 800°C by low pressure MOCVD using TMGa and NH3. The grown films were characterized by X-ray diffraction signals from the zinc-blende GaN(002) and the wurtzite GaN(0002) planes. An X-ray diffraction signal detected at 36.8° indicated the wurtzite GaN(1011) plane. The growth of the wurtzite GaN was suppressed by increasing the growth temperature. The annealing of the thick polycrystalline GaN in NH3 ambient at 800°C was found to cause the atomic displacement and sublimation. Zinc-blende GaN with suppressed wurtzite domain was obtained on 3CSiCSi(001) at 800°C by inserting a thin polycrystalline GaN buffer layer deposited at 600°C.

800 mW peak-power self-sustained pulsation GaAlAs laser diodes

We have developed a high-power self-sustained pulsation laser by using a multiple-quantum-well real refractive index guided self-aligned (RISA) structure that allows formation of large saturable absorbers outside a stripe region. As a result, a peak power as high as 800 mW was obtained in the laser with the stripe width of 1.0 /spl mu/m. >

Low-noise operation (−140 dB/Hz) in close-coupled Nd:YVO 4 second-harmonic lasers pumped by single-mode laser diodes

Very-low-noise operation (as low as -140 dB/Hz) of close-coupled Nd:YVO(4) lasers is reported. This low noise level has been attained by controlling the spectra of the pumping laser diodes into the single longitudinal mode by suppressing mode hopping. The small-packaged second-harmonic laser (Φ12 × 16 mm) showed highly stabilized 530 nm/6 mW operations with a relative intensity noise of -140 dB/Hz.

Periodic pulse oscillation in an intracavity-doubled Nd:YVO(4) laser.

We report periodic pulse oscillation in an intracavity-doubled Nd:YVO(4) laser. The pulse oscillation is caused by alternate oscillation between two modes. We investigated improvement of green-light power with the pulse oscillation in a Nd:YVO(4) laser. The pulse oscillation gave a factor of approximately 2 higher average green-light power than that for cw oscillation at the same pumping power.

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency.

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.