Mitsuaki Shimizu

Impact of Vicinal Sapphire (0001) Substrates on the High-Quality AlN Films by Plasma-Assisted Molecular Beam Epitaxy

AlN films grown by plasma-assisted molecular beam epitaxy on vicinal sapphire (0001) substrates were investigated. High structural and optical qualities were confirmed by high-resolution X-ray diffraction and 77 K cathodoluminescence measurements. It was found that changing the vicinal angles of sapphire substrates can easily control the surface morphologies of AlN films. Spiral-growth features were greatly suppressed. Furthermore, well-ordered straight monatomic-layer steps and multi-atomic-layer macro-steps were clearly observed by atomic force microscopy. Surface diffusion and step incorporation kinetics during the growth are the key-factors in determining the surface morphologies.

Essential Change in Crystal Qualities of GaN Films by Controlling Lattice Polarity in Molecular Beam Epitaxy

GaN heteroepitaxial growth on sapphire (0001) substrates was carried out by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE). A Ga-polarity growth was achieved by using an AlN high-temperature buffer layer. The epilayer polarity was characterized directly by coaxial impact collision ion scattering spectra (CAICISS). It was found that the properties of the GaN films showing Ga-face polarity, including their structural and electrical properties, were dramatically improved compared to those of films with N-face polarity. This important conclusion is considered to be a breakthrough in the realization of high-quality III-nitride films by MBE for device applications.

Polarization Characteristics of MOCVD Grown GaAs/GaAlAs CBH Surface Emitting Lasers

Systematic measurements have been done for the polarization of a vertical cavity surface emitting (SE) laser. The light outputs of the tested devices were linearly polarized and the direction was along the specified crystal orientation. The polarization selectivity is discussed.

Continuous Output Beam Steering in Vertical-Cavity Surface-Emitting Lasers with Two p-Type Electrodes by Controlling Injection Current Profile.

Using vertical-cavity surface-emitting lasers with two p-type electrodes, continuous output beam steering was achieved for the first time by controlling the injection current profile. The far-field peak position can be shifted linearly by varying the ratio between injection currents into the two p-type electrodes. The deflection angle measured from the surface normal direction ranges from -1.3° to +1.0°. Also, we investigated operating characteristics that are important for optical processing such as optical fuzzy inference.

High breakdown voltage AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor with TiO2/SiN gate insulator

We report the fabrication of an AlGaN/GaN high-electron-mobility transistor (HEMT) with a high breakdown voltage by employing a metal–insulator–semiconductor (MIS) gate structure using TiO2/SiN insulators. We employed the TiO2/SiN gate insulator for the first time in a multilayered insulator structure MIS-HEMT. The gate leakage current was significantly reduced by employing the MIS structure, and the breakdown voltage characteristics of the fabricated MIS-HEMTs were 1.1 kV with an on-resistance of 15 mΩ cm2 for a gate-drain length of Lgd = 28 µm. The current collapse in the TiO2/SiN MIS-HEMT has been improved by employing a thin SiN film under the TiO2 insulator. AlGaN/GaN MIS-HEMTs are promising not only for high-speed applications but also for high-power switching applications.

High-Quality GaN Layers on c-Plane Sapphire Substrates by Plasma-Assisted Molecular-Beam Epitaxy Using Double-Step AlN Buffer Process.

High-structural-quality c-axis GaN layers were grown on (0001) sapphire substrates by N2-plasma-assisted molecular-beam epitaxy (MBE) using a double-step AlN buffer layer process. The introduction of double-step AlN buffer layers grown at two different high temperatures improved the quality of the GaN layers dramatically. The full-width at half maximum (FWHM) (ω scan) of GaN layers along the symmetric (0002) and asymmetric (1011) diffraction planes were 104 and 848 arcsec, respectively, whereas they were 532 and 1335 arcsec for the GaN layers grown by a single-step AlN buffer process. Double-step AlN buffers improve the structural quality of the layers and decrease the X-ray dislocation density. We have shown that the quality of the underlying AlN buffer layer is the determining factor in the successful growth of high-quality GaN epitaxial layers.

p-Type InGaN Cap Layer for Normally Off Operation in AlGaN/GaN Heterojunction Field Effect Transistors

The normally off operation of AlGaN/GaN heterojunction field effect transistor (HFET) devices with a p-type InGaN cap layer under a gate electrode was demonstrated. The threshold gate voltage VGth was 0.5 V, and the maximum transconductance gm was about 120 mS/mm. The maximum drain current IDmax was more than 210 mA/mm. RF characteristics were also measured, and it was found that fT is 2.1 GHz and fmax is 6.4 GHz when the gate length is 2 µm.

Equivalent circuit model for GaN-HEMTs in a switching simulation

An equivalent circuit model for gallium nitride-based high electron mobility transistors (GaN-HEMTs) in an exact circuit simulation is proposed. The equivalent circuit contains inherent GaN device properties, such as current-collapse and shot-channel effects. Base on the equivalent model, an power loss simulator was developed. The simulation accuracy was more than 93%. A converter optimum design method is discussed using the power loss simulator.

A method of polarization stabilization in surface emitting lasers

In this paper, we propose a surface emitting laser structure for controlling its output polarization. By preparing high refractive index films on the side walls of a dielectric multilayer reflector, we can provide a differential loss to the orthogonal polarizations. We estimate the reflectivity dependence on the polarization direction of this reflector and conclude that the control of polarization direction is possible by employing this structure.

High-Quality Growth of AlN Epitaxial Layer by Plasma-Assisted Molecular-Beam Epitaxy

We report the growth and characterization of high-quality h-AlN epitaxial layers on basal c-plane sapphire substrates by plasma-assisted molecular-beam epitaxy (MBE). The c-axis lattice constant has been found to vary with the layer thickness. AlN layers grown in an intermediate Al/N growth regime show a 2D streaky reflection high-energy electron diffraction (RHEED) pattern and the surface morphologies have few surface irregularities, and sufficient to speculate that the layers are free from large Al metal droplets. The full width at half maximum (FWHM) (ω scan) of the AlN layers along the symmetric (0002) and asymmetric (11–24) diffraction planes were 42 and 180 arcsec, respectively. Excitonic transitions (D°,X and A°,X), indicating high quality of the layers, have been found in all the samples by cathodoluminescence measurements, and a violet band has been attributed to either shallow or deep donor pair recombination.