Atsushi Motogaito

Recent Progress in Selective Area Growth and Epitaxial Lateral Overgrowth of III-Nitrides: Effects of Reactor Pressure in MOVPE Growth

Effects of reactor pressure on the epitaxial lateral overgrowth (ELO) via low pressure MOVPE have been studied in relation to the growth temperature. For the ELO GaN on SiO2 stripes along the 〈11-00〉 direction of the underlying GaN, by decreasing reactor pressures from 500 to 40 Torr or by increasing growth temperatures from 950 to 1050 °C, the (0001) surfaces become broad and the side walls are varied from inclined {112-2} surfaces to vertical {112-0} surfaces. For stripes along the 〈112-0〉 direction, the shapes of ELO GaN are independent of the reactor pressures and the growth temperatures. The mechanism of the morphological change is discussed based on the stability of the surface atoms. Typical ELO GaN layers with two-step growth are demonstrated and characterized in their crystalline properties.

Effects of Reactor Pressure on Epitaxial Lateral Overgrowth of GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy

The effects of reactor pressure on epitaxial lateral overgrowth (ELO) via low-pressure metalorganic vapor phase epitaxy (MOVPE) have been studied in relation to growth temperature. For the ELO GaN on SiO2 stripes along the direction of the underlying GaN, on decreasing the reactor pressures from 500 to 40 Torr or increasing the growth temperatures from 950 to 1050°C, the (0001) surfaces become broad and the side walls changed from inclined {1122} surfaces to vertical {1120} surfaces. For the ELO GaN on the stripes along the direction, the shapes with {1101} facets are independent of the reactor pressure and growth temperature. The ELO of GaN is dominated by the diffusion-limited process.

Buried Tungsten Metal Structure Fabricated by Epitaxial-Lateral-Overgrown GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy

A buried tungsten (W) structure with GaN is successfully obtained by epitaxial lateral overgrowth (ELO) technique via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The selectivity of GaN growth on the window regions is excellent. GaN with a striped W metal pattern is easily decomposed above the low temperature of 500°C by the catalytic effect of W. It is difficult to bury the W mask because severe damage occurs in the GaN epilayer under the mask. It is found that employing an underlying AlGaN/GaN heterostructure with a narrow W stripe mask width (L/S=2/2 µm) leads the epilayer to be free from damage, resulting in a good W buried structure.

Characterization of GaN Based UV-VUV Detectors in the Range 3.4–25 eV by Using Synchrotron Radiation

The characterization of Schottky type ultraviolet (UV) detectors with transparent electrode between vacuum ultraviolet (VUV) and visible light region using synchrotron radiation is described. The responsivity spectrum of the detectors at 0 V bias was obtained in the wide range between 2 eV (563 nm) and 25 eV (50 nm). The photoemission current from Au electrode was able to be canceled by improving the measuring circuit, and thus we succeeded in operating the detectors without any photoemission current from Au and GaN. The responsivity of the detectors is about 0.15 A/W at 3.5 eV. These results show that these Schottky type detectors with the transparent electrode are effective to detect VUV-UV light (50-360 nm, 3.4-25 eV) without any photoemission.

Characterization of GaN-Based Schottky Barrier Ultraviolet (UV) Detectors in the UV and Vacuum Ultraviolet (VUV) Region Using Synchrotron Radiation

Characterization of GaN-based Schottky barrier ultraviolet (UV) detectors with a comb-shaped electrode using synchrotron radiation (hν=2.2–30 eV, λ=41–563 nm) is described. Below hν=8.0 eV (λ>155 nm), the detectors are available without any photoemission of GaN and Au electrode. Under application of reverse bias, the responsivity is increased to 0.05 A/W at -0.4 V. The photocurrent is controlled by reverse bias. On the other hand, above hν=8.0 eV (λ<155 nm), the responsivity spectra are dominated by photoemissions of Au and GaN. These results show that these Schottky type detectors with mesa structures are effective to detect vacuum ultraviolet (VUV)-UV light (155

GaN layer structures with buried tungsten nitrides (WNx) using epitaxial lateral overgrowth via MOVPE

Abstract A buried WNx structure with GaN by means of ELO technique has been investigated. To prevent dissolution of the underlying GaN layer due to the W catalytic effect, the WNx mask is employed instead of the W mask. The WNx mask is produced by nitrogenation of the W film using NH3 at higher temperatures than 600°C. Thermal stability of WNx is good and the WNx/n-GaN contact shows a Schottky type. The buried WNx structure with GaN is successfully obtained by two-steps ELO technique. The WNx is effective to the mask of the ELO GaN layer, as well as the Schottky contact on n-GaN.

Fabrication of Binary Diffractive Lens on Optical Films by Electron Beam Lithography

Two types of lenses can focus light: an optical lens using refraction phenomenon and a diffractive lens using diffraction phenomena. Table 1 shows the characteristics of each lens. The focal length of the diffractive lens is controlled by the structures of the lens, as mentioned in detail in Section 2.2. This suggests that the focal length of the diffractive lens is independent of refractive index and curvature. Thus, application of diffractive lenses to UV optical elements or thin optical elements is possible.

Detecting High-refractive-index (n>1.5) Media using Surface Plasmon Sensor with One-dimensional Au Diffraction Grating on Glass Substrate

We experimentally detect high-refractive-index media using a surface plasmon resonance (SPR) sensor with a diffraction grating. In this study, we used the rigorous coupled-wave analysis method to simulate the dependence of the reflectance on an incident angle for media with refractive index values up to 1.700. In the experiment, a medium (n = 1.660-1.700) was successfully detected using this grating. Under the conditions of the grating (period: 600 nm, Au thickness: 40 nm) using a red laser (: 635 nm), the extraordinary transmission enabled by SPR is occurred at the interface between Au and a medium. We concluded that this method can be applied to sensors that detect high-refractive-index media.

Detection of high-refractive index media by a surface plasmon sensor using a one-dimensional metal diffraction grating

We aim to detect high-refractive index media by substituting metal diffraction grating with a metal film. The advantages of this structure are miniaturization, wide-range detection, and versatility. The Au gratings were fabricated on a glass substrate by electron beam lithography and lift-off process. Using a 635 nm laser, we measured the dependence of reflectance on the light incident angle. At a grating period of 600 nm, the sensor detected 1-bromonaphthalene (refractive index n =1.65) and ethyl alcohol (n = 1.36).

Fabrication and optical characterization of a 2D metal periodic grating structure for cold filter application

Cold filters, which simultaneously reflect infrared light and transmit visible light, prevent overheating in charge-coupled device cameras, microscopes, and other heat-sensitive equipment. This study proposes a cold filter based on a two dimensional (2D) metal periodic grating structure. Conventional dielectric multilayer films with abrupt filtering characteristics are undesirably affected by incident angle, temperature, and polarization. To solve these problems, a 2D metal periodic grating structure, which does not depend on the polarization, was applied. The grating structure comprises an Au layer and an electron beam resist layer, and was fabricated by electron beam lithography. The optical characteristics of this structure in the visible light region were measured by a spectrometer, and the optical properties were related to structural parameters of the double-layer, 2D grating structure. In particular, the reflectance over the entire visible light spectrum decreased at periods of 800 nm and 1 μm. The wavelengths of minimum and maximum reflectance were shifted by changing the spacing between the upper and lower metal layers from 270 to 370 nm. Simulation results suggested that the interference between the upper and lower layers and the surface plasmon resonance between the metal and resist layers occur simultaneously. Therefore, in the visible light region, the reflectance and transmission spectra were controlled by altering the structure of the 2D metal periodic grating.