Takuya Ishikawa

Quantum-Confined Stark Effect in a Parabolic-Potential Quantum Well

Quantum-confined Stark effect in parabolic quantum well (PQW) has been analyzed both analytically and numerically. Analytical studies have revealed that the fundamental absorption edge shift in PQWs is proportional to the square of the well width, and that the Stark shift in PQWs is independent of the particle mass. From numerical calculations, it has been confirmed that PQW structure enables us more easily than rectangular quantum well to obtain the absorption-type optical waveguide switches whose characteristics are nearly polarization-independent, and that PQW is also advantageous if we intend to obtain the polarization-independent changes in the refractive index with low field and over a wide wavelength region. In addition, an equivalent PQW structure has been fabricated, and the mass-independent Stark shift of the PQW has been observed for the first time.

Observation of Quantum-Confined Stark Effect in a Graded-Gap Quantum Well

A graded-gap quantum well structure where the band gap of the well is inclined along the growth direction has been fabricated, and its quantum-confined Stark effect has been observed experimentally. It has been confirmed that, in comparison with the conventional square-shaped quantum well, the graded-gap quantum well exhibits larger and nearly linear energy shift of the fundamental exciton absorption edge with applied field.

Polarization‐independent optical waveguide intensity switch with parabolic quantum well

In a parabolic quantum well, the shift in optical transition energy due to the quantum confined Stark effect is independent of the carrier effective mass. This fact enables us to realize polarization‐independent optical waveguide intensity switches with high on/off ratio. An absorption‐type switch with GaAs/Al0.3Ga0.7As equivalent parabolic quantum wells is fabricated with molecular beam epitaxy. Both transverse‐electric and transverse‐magnetic mode lights exhibit an on/off ratio of 27.6 dB at an applied voltage of 6.84 V at 850 nm wavelength. To our knowledge, this is the first polarization‐independent optical waveguide intensity switch based on the electric‐field‐induced effect in the semiconductor quantum well.

Optical intensity modulator for integrated optics by use of a heterojunction bipolar transistor waveguide structure

Device fabrication and basic experimental results obtained with a heterojunction bipolar transistor waveguide structure optical intensity modulator, whose device structure is particularly suitable for possible monolithic integrated circuits, are described for the first time. Common emitter dc current gain hFE obtained with a 0.25‐μm‐thick base/waveguide was 38, and the switching times have been determined as 1.5 ns. Free‐carrier injection modulates the absorption coefficient α in the waveguide mainly by the band filling and plasma dispersion effects and consequently, the output light intensity can be modulated. The optical on/off modulation ratio of up to 2.1:1, corresponding to a modulation efficiency of Δα/α∼89%, has been demonstrated at a base current of 3 mA for a device with an emitter/waveguide width of 7 μm and a length of 190 μm.

Improved Theory for Carrier Leakage and Diffusion in Multiquantum-well Semiconductor Lasers.

A model of carrier leakage lifetime is presented taking into account the density of states for quantum-wells and band nonparabolicity. Rate equations are also proposed including leakage of both types of carriers and carrier loss in both sides of optical confinement layers. The carrier loss coefficients extracted by adopting this model on the measured modulation bandwidth of 1.5 µm-wavelength multiquantum-well lasers coincided with reported values within their distributions. Measured temperature sensitivity of threshold current and that of K factor were also well explained with the improved model using those extracted carrier loss coefficients. The dominant causes of low characteristic temperature T0 of present compressive-strained multiquantum-well lasers were quantitatively considered and found to be attributed to 1) Auger carrier loss and 2) thermionic carrier leakage and diffusion delay effect. T0 over 150 K is expected by reducing the effect of those two factors. Possibilities of finding an actual method to reduce the effect of the above two factors are discussed.

Integrated GaInAsP laser diodes with monitoring photodiodes through semiconductor/air Bragg reflector (SABAR)

A 1.3-/spl mu/m GaInAsP laser diode (LD) is integrated with a monitoring photodiode (M-PD) through a semiconductor/air Bragg reflector (SABAR). Instead of conventional cleavage, the SABAR can provide not only Fabry-Perot resonance with high reflectivity, but also possibility of integration of laser with other functional devices. The design, fabrication, and some characteristics including threshold current, monitoring photocurrent, SABAR reflectivity as a function of the number of semiconductor/air pairs N are reported. The threshold current of ridge waveguide laser with SABAR (cavity length L=160 /spl mu/m, ridge width W=7 /spl mu/m, SABAR pairs N=3) is 20 mA. The threshold current is reduced by improving butt-coupled interface between active and passive waveguides employed in this laser and is expected 2 mA//spl mu/m. The monitoring photocurrent responds linearly with output power from the laser and 0.024 mA at laser output power of 5 mW. From the threshold characteristics, SABAR reflectivity is determined to >80%. The increase of photocurrent can be achieved by optimizing the number of SABAR pairs to N=1. We have obtained threshold current of 22 mA in the followed laser structure (L=270 /spl mu/m, W=7 /spl mu/m, N=1), and detector photocurrent of 1.13 mA (@5 mW). The experimental SABAR reflectivity is /spl sim/50%, which is estimated by threshold characteristics and efficiency of light output power. The laser has a mode field converter section, resulting in narrow beam divergence 11/spl deg/ along vertical axis. This integrated laser is very promising candidate for coming optical module in low-power consumption and low-cost access network systems.

Strained Layer Multiquantum Barriers with Improved Carrier Injection and Confinement.

The effects of using strained layer superlattice in multiquantum barriers (MQBs) were analyzed with focus on InP-based materials and their application to strained layer multiquantum well lasers (SL-MQW LDs). Tensile strained barriers are shown to largely increase the effective barrier height of MQBs. A new barrier material, AlInP, which is 1–1.5% tensile strained on InP, is shown to have inherent advantages of large conduction band edge discontinuity ΔEc and low aluminum content compared to conventional AlInAs on InP. Those advantages are confirmed for strain-compensated MQB with tensile barrier/compressive well in terms of increasing the effective barrier height. Some disadvantages of the strain-compensated MQB are also pointed out; the increase in density of holes injected into the MQB region under lasing conditions and the high p-doping concentration required. To solve those problems, an improved method of carrier injection through minibands with gradually enhanced energy levels toward the MQW region is presented, which is shown to enable elimination of the carrier transport effect and to result in marked improvement in laser performances.

Vertical multiple-quantum-well directional-coupler switch with low switching voltage

A novel device structure of the vertical directional-coupler optical switch with multiple quantum wells (MQWs) is proposed and fabricated. All the clad, guide, and separation layers are superlattices of Al/sub 0.3/Ga/sub 0.7/As and GaAs. This leads to a better control of the complete coupling length. The main feature of the proposed structure is that the refractive index modulation due to the quantum confined Stark effect (QCSE) is applied only to the undoped central region of the separation layer. It is found that this leads to reduction of the switching voltage. In an actual sample 138 mu m long, a switching voltage of 5 V has been obtained.<<ETX>>

Application of Modern Control Theory to Temperature Control of the MBE System

A molecular beam epitaxy control system, whereby one microprocessor manages all the objects, has been constructed. The model-following algorithm, which is one of the applications of modern control theory, has been adopted instead of the conventional PID algorithm in order to improve the dynamic response of the temperature. It has been confirmed by computer simulation and measurement that the model-following algorithm is significantly effective especially when the set temperature is to be changed as a function of time. An application of the model-following algorithm to device fabrication has also been demonstrated.

A Novel Device Structure of Vertical Multiple Quantum Well Directional Coupler Switch for Low Switching Voltage

The semiconductor directional coupler optical modulator/switch in vertical coupled-waveguide configuration [1] is one of useful components in integrated and guided-wave optics because of the less difficult fabrication process and small size. In recent years these devices with multiple quantum wells (MQW) were reported[2]-[5], where the large change in refractive index due to the quantum confined Stark effect(QCSE) were employed for reduction of the switching voltageVs and the device length l. In this paper we report on a new device with very small Vs of 5V and l of 138µm.