Yoshinobu Sekiguchi

Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory

We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% w...

Wide‐range wavelength tuning of an asymmetric dual quantum well laser with inhomogeneous current injection

A new asymmetric dual quantum well laser diode with a segmented contact is proposed. With inhomogeneous current injection, the lasing wavelength can be changed quasi‐continuously over 22 nm, which is much wider than any tuning range previously reported. To achieve such wide‐range tuning, it is found to be essential that the length of the cavity and each segment (and the mirror reflectivity) of the laser diode should be within an optimized range. In the asymmetric dual quantum well laser diode structure used in the present experiment, in particular, the cavity length should be such that light of a shorter wavelength lases under homogeneous excitation.

Low-Temperature Molecular Beam Epitaxy Growth of Single Quantum Well GaAs/AlGaAs Lasers

We report on the low-temperature Molecular Beam Epitaxy (MBE) growth of single quantum well (SQW) GaAs/AlGaAs lasers under a low flux ratio. Lasing action was observed at a substrate temperature as low as 300°C. A threshold current density of 600 A/cm2 was obtained at the substrate temperature of 375°C, which is the lowest threshold current density below 400°C reported so far.

Threshold Current Density of GaAs/AlGaAs Single-Quantum-Well Lasers Grown by Molecular Beam Epitaxy

We report on the effect of substrate temperature on the threshold current density in GaAs/AlGaAs single-quantum-well lasers grown by molecular beam epitaxy under various flux ratios. It is found that the threshold current density has W-shape dependence on the substrate temperature and exhibits minima of 600 A/cm2 and 400 A/cm2 at the substrate temperatures of 375°C and 650°C, respectively.

Photoluminescence from GaAs/AlGaAs Quantum Wells Grown at 350°C by Conventional Molecular Beam Epitaxy

Photoluminescence measurements have been carried out for GaAs/AlGaAs single quantum well (SQW) structures grown without interruption by conventional molecular beam epitaxy. The full widths at half-maximum (FWHM) from QWs having thin well width were reduced monotonously with the decrease of growth temperature until 350°C to one-half. This reduction of FWHM was explained by the transition of the top and bottom heterointerfaces which became close to the effective smooth interfaces at 350°C. The peak intensity had only a weak dependence on the growth temperature at the same temperature.

Continuous wave operation on extremely low‐temperature (375 °C)‐grown AlGaAs quantum‐well lasers

We report on the continuous wave (cw) operation of AlGaAs single‐quantum‐well (SQW) lasers grown at a low temperature (375 °C) by molecular‐beam epitaxy (MBE). The threshold current and the differential quantum efficiency of the ridge‐waveguide laser were 10 mA and 82%, respectively. During the life test, no obvious degradation was observed beyond 2300 h under 8 mW cw operation at 50 °C. The cw operation of SQW lasers grown at low temperatures was achieved by the long baking of the MBE system and the reduction of the V/III flux ratio.

Polarization switching in a tensile-strained InGaAs/InGaAsP multiple quantum well distributed feedback laser diode

An inhomogeneously biased distributed feedback (DFB) laser diode (LD) with two electrodes switched its polarization mode by 3 mA change of the bias current, maintaining single longitudinal mode oscillation. In the active layer of the LD, 13 nm thick and 0.6% tensile-strained InGaAs multiple quantum well (MQW) equalized the transverse electric and the transverse magnetic modal optical gain at 1.55 μm. With various grating pitches on the same MQW active layer, polarization switching DFB LDs were realized in the wavelength range as wide as 18 nm. The linewidth characteristics during the polarization switching were confirmed to be narrow due to the small switching current.

High‐reliability GaAs/AlGaAs multiquantum well lasers grown at a low temperature (375 °C)

We report on high‐reliability GaAs/AlGaAs multiquantum well (MQW) lasers grown at a low temperature (375 °C) by molecular beam epitaxy (MBE). Typically, a threshold current (Ith) of 26 mA and a differential quantum efficiency of 62% were obtained during a continuous wave (cw) operation at room temperature. During the life test, a stable operation was observed beyond 5000 h under a 20‐mW cw operation at 70 °C. We also observed an improvement of laser characteristics in low‐temperature‐grown MQW lasers during a cw operation at room temperature. The threshold current of our MQW lasers was reduced from 26 to 18 mA, and this suggests that the point defects in the low‐temperature‐grown MQW laser were decreased.