Toshihide Ide

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.

Various low group velocity effects in photonic crystal line defect waveguides and their demonstration by laser oscillation

We investigated propagating modes in a two-dimensional photonic crystal slab waveguide with a line defect narrower than a single line missing hole structure from the low group velocity point of view. These modes showed low group velocities not due to the conventional distributed feedback (DFB) between a forward and a backward mode with the same lateral field distribution, but due to a DFB between modes with different lateral field distribution or property of a start point of a photonic-band-gap-guided mode. These low group velocities of over 40 were demonstrated as a Fabry-Perot lasing oscillation due to gain enhancement.

Room temperature continuous wave lasing in InAs quantum-dot microdisks with air cladding.

We demonstrated the first room temperature continuous wave lasing in InAs quantum-dot microdisk lasers with a standard air-cladding optical confinement structure. The spectrum shows the single strong lasing peak at a wavelength of 1280 nm. The threshold pump power is 410 muW, and the corresponding effective threshold obtained by considering the absorption efficiency is 81 muW. This achievement is mainly attributed to the increase in Q factor by the improved disk shape.

Lasing characteristics of InAs quantum-dot microdisk from 3 K to room temperature

We fabricated a microdisk laser with five-stacked InAs quantum-dot (QD) active region, and demonstrated the lasing operation from 3K to room temperature by femtosecond pulsed photopumping. At room temperature, the threshold power was estimated to be 0.75mW, when the influence of the surface recombination at the disk edge was neglected. The lasing wavelength was 1.2–1.3μm, which corresponded to excited states of the QDs. The temperature dependence of the threshold, slope efficiency, lasing wavelength, and linewidth are explained by the rapid increase in nonradiative recombination and internal absorption at critical temperatures of 200–230K.

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.

Stability of N- and Ga-polarity GaN surfaces during the growth interruption studied by reflection high-energy electron diffraction

GaN films with N- and Ga-polarity were grown on sapphire (0001) substrates using different buffer layers by plasma-assisted molecular-beam epitaxy. The surface stability of each lattice-polarity film during the growth interruption was studied by reflection high-energy electron diffraction (RHEED). It was found that the surface of N-polarity film was unstable against the exposure to the nitrogen plasma flux, while that of Ga-polarity one was stable. This provides a method to clarify the lattice polarity by the in situ RHEED observation directly. A model is proposed to explain the observed phenomenon, where the origin of the phenomenon is mainly attributed to the differences in surface dynamics processes and morphologies between the two kinds of lattice-polarity films.

Realization of compressively strained GaN films grown on Si(110) substrates by inserting a thin AlN/GaN superlattice interlayer

We investigate the strain properties of GaN films grown by plasma-assisted molecular beam epitaxy on Si(110) substrates. It is found that the strain of the GaN film can be converted from a tensile to a compressive state simply by inserting a thin AlN/GaN superlattice structure (SLs) within the GaN film. The GaN layers seperated by the SLs can have different strain states, which indicates that the SLs plays a key role in the strain modulation during the growth and the cooling down processes. Using this simple technique, we grow a crack-free GaN film exceeding 2-μm-thick. The realization of the compressively strained GaN film makes it possible to grow thick GaN films without crack generation on Si substrates for optic and electronic device applications.

InP Etching by HI/Xe Inductively Coupled Plasma for Photonic-Crystal Device Fabrication

We investigated the inductively coupled plasma etching of InP-based materials for photonic-crystal (PC) device fabrication. By optimizing bias power and gas pressure, circular holes with a diameter of <0.2 µm, a depth of 2.0 µm and a maximum aspect ratio of 13 were formed with an e-beam resist mask. This technique was applied to the fabrication of PC-slab devices, and a single-point-defect PC laser was demonstrated at room temperature.

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.

Advantages of AlN/GaN Metal Insulator Semiconductor Field Effect Transistor using Wet Chemical Etching with Hot Phosphoric Acid

An AlN/GaN metal insulator semiconductor field effect transistor (MISFET) was fabricated using a technique of wet chemical etching with hot phosphoric acid. This technique improved ohmic contact resistances for the source and drain, and provided a new and simple fabrication process for the MISFET. In our MISFET with a 3 µm gate length, a maximum transconductance (gm max) of 130 mS/mm and a maximum drain current (ID max) of over 600 mA/mm were obtained. We also fabricated the AlGaN/GaN heterostructure field effect transistor (HFET) with the conventional fabrication process and compared the operating characteristics.