Syoji Yamada

DC Drift Phenomena in LiNbO3 Optical Waveguide Devices

DC drift phenomena in LiNbO3 optical waveguide devices are investigated. From low speed switching experiments of a directional coupler modulator, two kinds of dc drift are found to exist in this device. One is a short time dc drift with several seconds relaxation time and the other is a long time one whose relaxation time is several hours. It is shown analytically that the former originates from current leakage through a buffer layer, and it is successfully suppressed by a separation of the layer. For the latter problem, photocurrents measuring experiments were carried out for various LiNbO3 samples. We may consider from the results that the main origin of the long time dc drift is a photorefractive effect due to the guided light itself. Some solutions for overcoming these effects are also given.

Narrow two‐dimensional electron gas channels in GaAs/AlGaAs sidewall interfaces by selective growth

Narrow two‐dimensional electron gas (2DEG) channels have been fabricated for the first time on GaAs/Al0.3Ga0.7As sidewall interfaces by selective growth using metalorganic chemical vapor deposition (MOCVD). The 4‐μm‐wide 2DEG channels are formed on the {111}A facets by controlling the facet formation in the selective growth layers only through MOCVD growth conditions. The angular dependence of Shubnikov–de Haas oscillations has confirmed the existence of 2DEG on the {111}A facets.

Nonparabolic subband structure of Ga0.47In0.53As‐InP quantum wells

Nonparabolic subband structure of GaInAs‐InP quantum wells is studied theoretically. The dispersion relations for electron, light hole, and split‐off hole subbands are obtained using the envelope function approximation taking into account band nonparabolicity. Reported experimental electron effective mass values and photoluminescence energies for wide quantum wells are resonably explained by the present theory. The experimental photoluminescence energies, for the well width less than 50 A, differ significantly from the calculated results.

Low Temperature High Electron Mobility in In0.75Ga0.25As/In0.75Al0.25As Modulation-Doped Hetrostructures Grown on GaAs Substrate

We have grown In0.75Ga0.25As/In0.75Al0.25As modulation-doped hetrostructure on GaAs substrate via InAlAs step-graded buffer by molecular beam epitaxy and obtained electron mobilities of 175,000 and 397,000 cm2/Vs at 77 and 4.2 K respectively, those of which are the highest ones ever reported in this materials system. Comparing the results with those of In0.75Ga0.25As/In0.66A0.34As and In0.5Ga0.5As/In0.5Al0.5As heterostructures grown similarly, it is concluded that the high mobility was attained due to the reduced alloy scattering under the strain-free interface condition and to the reduced electron effective mass (0.040m0 from far-infrared measurement).

Very high electron mobilities at low temperatures in InxGa1−xAs/InyAl1−yAs HEMTs grown lattice-mismatched on GaAs substrates

We have grown In x Ga 1-x As/In y Al 1 -y As HEMTs on GaAs substrate via InAlAs step-graded buffer and obtained electron mobilities of 397,000 cm2/Vs at 4.2 K, for x = y = 0.75. Comparing the results with those of x = y = 0.75 and x = y = 0.5 HEMTs, it is concluded that the high mobility was attained mainly due to the reduced alloy scattering with no interface strain condition.

Subband mobility of quasi‐two‐dimensional electrons in Si atomic layer doped GaAs

Subband mobility and conductivity of quasi‐two‐dimensional electrons in Si atomic layer doped GaAs are estimated for the first time. The oscillations for different subbands in low‐temperature magnetoresistance are separated from each other by using the reverse Fourier transform technique. The mobility for each subband is then determined by fitting the field dependence of the amplitudes with conventional theory. A large subband mobility difference up to 20:1 is found. This is mainly due to strong screening. Furthermore, a partial conductivity for each subband is calculated and the importance of the shallower subbands in total current transport is clarified.

Effective mass and ground state of AlAs quantum wells studied by magnetoresistance measurements

Transport properties of electrons confined in AlAs quantum wells were studied using low temperature magnetoresistance measurement. The structure used is quantum wells consisting of AlAs channel layers sandwiched by Si‐doped Al0.45Ga0.55As barrier layers. This structure enables us to confine electrons in the X valleys of AlAs. The electron confinement in the AlAs quantum wells were confirmed by a mobility enhancement at low temperatures. Magnetoresistance measurements showed clear Shubnikov–de Haas oscillations for the magnetic field parallel to the growth direction. A large effective mass of (0.55±0.05) m0, which is close to √mtml (mt:transverse effective mass, ml:longitudinal effective mass), was obtained from the temperature dependence of the oscillations. This indicates that the ground state is Xx, Xy, even though Xz was expected to be the ground state due to its large effective mass parallel to the growth direction. This changeover of AlAs X valley states can most likely be attributed to a strain‐indu...

Nano-Fabrication on GaAs Surface by Resist Process with Scanning Tunneling Microscope Lithography

Recently scanning tunneling microscopes (STMs) have become important tools for nanometer scale fabrication. In order to make a nanoscale structure on a semiconductor surface, we attempted to lithograph very fine lines (possibly less than 10 nm) by using a resist process with an STM as an electron beam source. A diluted positive electron beam resist, poly-methyl methacrylate (PMMA), was first coated on a S-doped GaAs substrate. Using the STM in air, we approached a W-tip to the surface at a bias voltage of 10 V and a tunneling current of 0.5 nA, and made a line using by scanning the tip with various scan times. We then analyzed those lines made by the STM tip using an atomic force microscope (AFM) in air. After these processes, we deposited a Ti membrane on the sample surface by vacuum evaporation, then lifted it off. As a result, we confirmed that it was possible to make fine Ti lines as narrow as 50 nm by this process.

Identification of acceptors and donors in high‐purity InP grown by metalorganic chemical vapor deposition

Photoluminescence at 1.9 K and far‐infrared photoconductivity at 4.2 K reveal that Si and S are the predominant residual donors and Zn is the predominant acceptor in high‐purity epitaxial InP grown by metalorganic chemical vapor deposition using triethylindium at temperatures from 575 to 700 °C. The epitaxial layers show 77 K electron Hall mobilities exceeding 100 000 cm2/V⋅s. Si accumulation near the epitaxial layer/substrate interface is revealed by far‐infrared photoconductivity measurements. Although no residual C acceptor is observed in any samples, an unidentified acceptor called A1 is observed in samples grown at 700 °C.

Far-Infrared Magneto-Absorption of the Nonequilibrium Electron System in Indium Phosphide

Measurements of the far-infrared cyclotron resonance of conduction electrons and the Zeeman absorption of donors in a MOCVD grown InP have been carried out at 4.2 K under photo- and electric field-excitations. It is found that the effective mass of the electron and the donor binding energy are m*=(0.0817±0.0004)m0 and Eb=7.6 meV, respectively. The electric field dependence of the resonance intensity can be explained in terms of impact ionization of the donor electrons. From time-resolved experiments, both the apparent lifetime and the scattering time of photoexcited electrons are also obtained.