Kunio Tada

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.

Realization of tensile strain on GaAs substrates for polarization independent optical modulation

The quantum-confined Stark effect of tensile-strained GaAs-InAlAs quantum wells grown on top of a related nonpseudomorphic InAlAs grid layer on GaAs substrates was studied. It was demonstrated by waveguide absorption measurements that polarization-independent optical modulation in tensile-strained GaAs wells is possible in a wavelength range of around 870 nm.<<ETX>>

Utilization of non-pseudomorphic growth to realize tensile strain on GaAs substrates

A novel structure consisting of a thick non-pseudomorphic InAlAs grid layer and InGaAs-GaAs strained-layer superlattice dislocation filter is proposed for realizing tensile strain on GaAs substrates. The structure is examined and characterized by photoluminescence, Nomarski etch-pit microscopy and double crystal X-ray diffraction and found to be effective in the incorporation of tensile strain in devices grown on the GaAs substrate.

Quantum Well with Mass-Dependent Width for Polarization-Insensitive Optical Modulation

The quantum confined Stark effect (QCSE) in semiconductor quantum well (QW) structures is attractive as it gives large change in optical constants and makes efficient optical modulation possible. However, QCSE depends on polarization of the incident guided optical wave (namely, TE or TM), which is a significant problem for applications. This is mainly due to the mass difference between heavy and light holes as described in the following section.

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>>

Enhancement of field‐induced optical absorption by potential modification of coupled quantum well structures

A novel structure, in which the potential profile of the coupled quantum well system is slightly inclined, is proposed for lowering the electric field required to transform the coupled system to a set of uncoupled single quantum wells. Numerical calculations of absorption spectra of graded coupled quantum wells under an electric field and room‐temperature photocurrent spectra of fabricated p‐i‐n diode samples confirm the above enhanced effect.

Design and Analysis of the X-Waveguide Optical Switch in a MESFET Geometry

Theoretical analysis of the change in refractive index, in the channel of a GaAs MESFET (metal oxide semiconductor field effect transistor) structure, has been presented for the first time. The analysis shows great potential applications, to use the MESFET geometries, in the design of optical modulators, and switches for monolithic integration. A X-waveguide-type optical switch has been incorporated into the MESFET geometry, and its switching characteristics have been studied by using the beam propagation method. No switching region has been deliberately built into the waveguide. Instead, the gate and drain voltages of the MESFET automatically form and control the switching region. A light wave propagated through a waveguide is reflected by the depletion edge due to the large change of refractive index. An extinction ratio better than 8.75 dB has been obtained at the gate voltage of 7 V.

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.

Low-Concentration Cadmium Diffusion into GaAs

Experimental results to obtain low p-type surface concentration into GaAs at high temperature using Ga-Cd alloy as a diffusion source, for the fabrication of a bipolar transistor optical switch, have been presented for the first time. The values of the diffusion coefficient of different proportions of cadmium into the alloy source are measured. The diffusion constant, D0, and the activation energy, e, for pure cadmium and 1% cadmium into the Ga-Cd alloy as a source have also been measured. The modified values of these parameters for pure cadmium seem to be different as compared to the data reported earlier due to the SiO2 thin film deposited on the surface of the substrate to prevent arsenic loss at high temperature. The experimental results show a continuous reduction in surface concentration and diffusion coefficient by reducing the percentage quantity of cadmium into the Ga-Cd alloy diffusion source.

Open-tube diffusion in GaAs using zinc- and tin-doped spin-on silica films

A simple technique for controlled p- and n-type open-tube diffusion using Zn (zinc)- and Sn (tin)-doped silica films is investigated. This paper first describes the impurity diffusion profile and mechanism study of both types of diffusion based on several different characterization methods. Furthermore, silica films are analyzed by X-ray photoelectron spectroscopy, and impurities in the films are determined. Next, the surface condition after high-temperature diffusion is observed by atomic force microscope. Many surface defects are detected and the image is in good agreement with the result obtained by scanning electron microscope. Finally, electrical characteristics of diffused layers are discussed by examining ohmic contacts with metals and pn junctions. It is concluded that this technique is quite useful for the fabrication of III-V compound semiconductor devices.